Analytical chemistry could be divided into three main parts; qualitative, quantitative and applied.Qualitative analysis deals with detection and identification ofdifferent substances singly or in a mixture.This part deals with the qualitative analysis of anions,which could be defined as the negatively charged fragmentsof salt or compound. Alternatively anions refer to acid radical.An example is Nacl: NaCl Na + + Cl- Cation Anion Basic radical Acid radical
Anions are divided into six groups:1- Carbonates and Bicarbonates group2- Sulphur-containing anions3- Halides4- Cyanogen anions5- Arsinic and phosphorous containing anions6- Nitrogen- containing anions
Carbonates and Bicarbonates group CO32- HCO3- I. General characters 1- Parent acid: Carbonic acid (H2CO3) is a very weak volatile acid (stronger than HCN and boric acid) Heating of solution of H2CO3, CO2 will evolve. H2CO3 CO2 H2O Bicarbonates are considered to be the first step of ionizationof carbonic acid, while in the second step carbonates are formedH2 CO3 H+ + HCO3- H+ + CO32-
2-Solubility:All carbonated with the exception of those of the alkali metals (Na+ and K+) and of ammonium are insoluble in water.All bicarbonates are soluble in water. II. General Reactions 1- Dry Reactions a- Action of dilute HCl Decomposition with effervescence due to the evolution of CO2 gas, for both CO3 -- and HCO3- CO3-- + 2H+ CO2 + H2O NaHCO3+ H+ CO2 + H2O + Na+ This is a type of displacement reaction in which stronger acid liberates the very weak carbonic acid, which spontaneously decomposes to CO2 & H2O.
Test for CO2 gas:The solid substance is placed in a test tube, dilute HCl isadded, which immediately displaced the gas, which is evolved(upon warming) and passed into lime water or baryta water contained in another test tube.The production of a turbidity indicates the presence ofcarbonates or bicarbonates.CO2 + Ca(OH)2 CaCO3 + H2OCO2 + Ba (OH)2 BaCO3 + H2OWith prolonged passage of CO2, the turbidity formed dueto the insoluble carbonates, slowly disappears as a result of the formation of a soluble bicarbonate.CaCO3 + CO2 + H2O Ca (HCO3)2 Boiling
2- Wet Reactions In order to carry out the wet reactions, a solution of the substance in water must be done.Bicarbonates are mostly decomposed on heating with the liberation of CO2. 2HCO3- CO3-- + H2O + CO2 . a- Reaction with AgNO3 A white precipitate of silver carbonate is immediately formed. CO3 -- +2Ag+ Ag2CO3 The precipitate is soluble in mineral acids (nitric acid) and in ammonia. Ag2CO3 + 2H+ 2 Ag+ + CO2 + H2O Ag2CO3+4NH3 2[Ag (NH3)2]+ + CO32- The precipitate becomes yellow or brown if the mixture is boiled. Ag2CO3 Ag2O +CO2 boiling
b- Reactions with BaCl2, CaCl2 and MgSO4:White precipitates of BaCO3, CaCO3 and MgCO3 will be obtainedupon the addition of these reagents to samples of carbonate solution.BaCl2 + NaCO3 BaCO3 + 2 NaClCa++ + CO3 -- CaCO3Mg++ + CO3 -- MgCO3The precipitate is soluble in mineral acidsFor HCO3- ; No ppt. on cold since all bicarbonates are soluble in water Ba++ +2HCO3- Ba(HCO3)2 Soluble H2O + CO2 + BaCO3 Boiling
III. Mixture of CO32- & HCO-3 Both anions haves similar reactions, but CO32- form precipitates immediately on cold upon the addition of CaCl2, BaCl2 or MgSO4, while the bicarbonates of these metals are soluble. Separation: Add excess CaCl2 (BaCl2 or MgSO4) to a solution of the mixture CO32- /HCO3- a white ppt. indicates CO3-- , centrifuge or filter Contrifugate White ppt. May be HCO3- CaCO32-Confirmatory test: 1) Boil H+ 2) Add ammonia solution white ppt. CO2 + H2O Ca (HCO3)2 + 2 NH3 CaCO3+ (NH4)2 CO3
Sulphur-containing anions This group of anions, are; 1- Sulphide (S2-) 2- Sulphites (SO32-) 3- Thiosulphate (S2O32-) 4- Sulphates (SO42-) 5- Perasulphate (S2O82-).I. General characters 1- Parent Acids:a- Hydrogren sulphide or Hydrosulphuric acid (H2S)It is a gas with offensive rotten egg odour and poisonous. In solution it gives a weak acid, which ionizes in two steps;H 2S H++ HS- (hydrosulphide ion) HS- H++ S-- (sulphide ion) Both HS- and S-- ions give the same reactions.
b- Sulphurous acid:(H2SO3)This acid is only known in solution (like H2CO3). It has moderate strongacidity. Like H2CO3 in water; present in equilibrium as follows: heatH2O + SO2 H2SO3 H++ HSO3- H++ SO3-- Acid sulphitec- Thiosulphuric acid: (H2S2O3)It is not known in the free form, and decomposes to give, H2O, SO2 and S. Its more stronger than sulphurous acid in solutions.It consists of SO32- solution and S, which upon boiling gives S2O32-.d- Sulphuric acid: (H2SO4):Its a colourless oily liquiud (B.P. 3300C). General properties of H2SO4 1- Acid properties; It is one of the strongest acids, ionize in dilute solutions in two steps, H2SO4 H++ HSO4- (hydrogen sulphate) HSO4- H++ SO4-- (sulphate)
Metals can liberate hydrogen from H2SO4 solution. H2SO4+ Zno ZnSO4+ H2 Being a strong acid can replace weak acids like, boric acids, hydrocyanic acid and volatile acids or their decomposition products due to its high B.P. 2NaCl + H2SO4 Na2SO4+ 2HCl 2- Dehydrating properties; Conc. H2SO4 has a great tendency to combine with water to from stable hydrates H2SO4.x H2O. So it is used as a dehydrating agent for certain substance, and used mostly in the dissectors. It causes charring for certain organic substances as sugars due to the vigorous abstracting of water from theses substances. 3- Oxidizing properties: Its considered to be as moderately strong oxidizing agent when heated with most reducing agents H2SO4 heat H2O + SO2 + [O] It is reduced to SO2, while with active reducing agents it may be reduced to So or H2S.
2-Solubility:All Na+, K+ and NH4+ salts of sulphur containing anions are solublein water.Sulphides : Other sulphides are in-soluble except those of Ca++,Ba++, & Sr2+ dissolve due to hydrolysis.Sulphites: Other sulphites are all in-soluble.Thiosulphates: Most S2O32-are soluble, Ag+, Pb++, Hg2+ & Ba++ salts are slightly soluble.Sulphates: All sulphates are soluble except Pb++, Ba++ and Sr++. Ca++ & Mg++ salts are slightly soluble.
3-Complexing agent: Thiosulphate form complex with Fe3+ Fe3++ 2S2O3-- (Fe(S2O3)2)- purple color4-Reducing agent:Sulphides, sulphites and thiosulphates are reducing agents.They reduce solutions of I2, KMnO4 and K2Cr2O7 with varying activitiesin acidified solutions. H+ I2+S2- 2I-+So lodine (brown) Colourless 2KMnO4+ 5S2-+ 16H+ 2Mn+++ 5SO4--+ 8H2O +2K+
II. General Reactions 1- Dry Reactions a- Action of dilute HCl 1.Sulphide; S2-H2S gas; evolved upon adding dil. HCl to a solid sample.The gas evolved has its characteristic rotten egg odour, and could beidentified by 1- blackening of filter paper moistened with lead acetate sol. S-- + 2H+ H2S H2S+Pb++ PbS black 2- alternatively, a filter paper moistened with cadmium acetate solution, turns yellow H2S + Cd++ CdS Yellow H2S has reducing character, It reacts with l2 solution, acid KMnO4, acid K2Cr2O7
It bleaches the brown color of l2 solution, changes the pink color ofacid KMnO4 into colorless and changes the orange color of acidK2Cr2O7 into green. H2S + l2 2l- + 2H+ +So 5H2S + 6H+ +2 MnO4- 2Mn++ + 8H2O + 5So 3H2S + 8H+ + Cr2O7-- 2Cr3+ + 7H2O + 3So 2- Sulphite: SO32-Upon treatment of SO3-- with dil. HCl, SO2 gas will evolve, due to thedecomposition of the liberated unstable H2SO3 SO--3 + 2H+ H2SO3 SO2 +H2O The evolved SO2 gas has a characteristic bunt sulphur odor and turbid lime water (like CO2) due to the formation of the insoluble CaSO3 which is soluble upon prolonged passage of SO2 due to the formation of soluble calcium bisulphite. Ca (OH)2 +SO2 CaSO3 + H2O
CaSO3 + SO2 + H2O Ca(HSO3)2.SO2 like H2S has reducing character, bleaches the brown color of iodine,reacts with acid KMnO4 and acid K2Cr2O7. l2 + SO2 + H2O SO3 + 2H++ 2l- 2 MnO4- + 5 SO2 + 6H+ 2Mn++ + 5SO3 + 3H2O Cr2O72- +3 SO2 + 8H+ 2Cr3++ 3SO3 + 4H2O3- Thiosulphate; S2O32- No immediate change on cold, but on warming with dil. HCl or standing, the solution become turbid due to the liberated yellow colloidal sulphur with evolution of SO2 gas. This is due to the decomposition of the produced unstable thiosulphuric acid. S2O3-- + 2H+ H2S2O3 H2O + SO2 + So Thiosulphate has the same action of sulphite with HCl in addition to formation of yellow colloidal precipitate.
4- Sulphate: SO42- No reaction with dil. HCl.2- Wet Reactionsa- Reaction with BaCl2: Add BaCl2 reagent to neutral sample solution:1- S2- : No visible reaction2- SO32- : White ppt. of BaSO3 is formed which is soluble in dil. HCl. Ba+++ SO32- BaSO33- S2O3-- : No ppt. in dilute solution, but a ppt. is formed from very concentrated solution.4- SO4-- : A white ppt. of BaSO4 is formed which is insoluble in dil. HCl, even upon boiling. Ba+++ SO4-- BaSO4 White
b- Reaction with AgNO3: Add AgNO3 reagent to the neutral sample solution 1- S2- : a black ppt. of Ag2S is formed which is soluble in hot dil. HNO3, insoluble in ammonia and KCN solution 2 Ag++ S-- Ag2S black2- SO32-: A white crystalline ppt. of Ag2SO3 is formed, which on boiling with water undergoes self oxidation reduction with the production of grey ppt. of metallic silver. 2 Ag++ SO32- Ag2SO3 White 2 Ag2SO3 boil 2 Ago + Ag2SO4 + SO2
Silver sulphite is soluble in nitric acid, ammonia and in excess sulphite to give a complex salt, which on boiling gives a grey ppt. of metallic silver Ag2 SO3 + SO3-- 2(AgSO3)- 2(AgSO3)- boiling 2Ago+ SO4-- + SO23- S2O3-- : Forms white ppt. of silver thiosulphate which changes its color on standing to yellow, brown and finally black, due to the formation of Ag2S. Ag2S2O3 is soluble in excess S2O3-- to give a complex ion. 2 Ag+ + S2O3-- Ag2 S2O3 Ag2S2O3+ H2O Ag2S + H2SO4 Ag2S2O3+ 3S2O3-- 2(Ag(S2O3)2)3-
4- SO42- : No ppt. in dil solution, but a ppt. may be formed in a very concentrated solution.c- Reaction with FeCl3: Add FeCl3 reagent to the neutral sample solution1- S2- : a black ppt. of Fe2S3 is formed which is soluble in dil. HNO3 2Fe3++ 3S-- Fe2S3 black 2- SO3--: A drak red color of ferric sulphite is produced on cold. 2Fe3++ SO3-- Fe2(SO3)33- S2O32-: A purple color of complex ferric thiosulphate is produced which disappears on boiling as tetrathionate and Fe2+ are formed from the oxidation of S2O32- with Fe3+, even on cold Fe3++ 2S2O32- (Fe(S2O3)2)- 2 S2O3--+ 2Fe3+ 2Fe+++ S4O6-- 4-SO42- : do not react with FeCl3.
d- Reaction with lead acetate: Adding lead acetate reagent to the neutral sample solution. 1- S--: A black ppt. of PbS is produced Pb+++ S-- PbS 2- SO32-: A with ppt. of lead sulphite which is soluble in cold HNO3. On boiling oxidation to PbSO4 which is a white ppt. occurs. SO3--+ Pb++ PbSO3 3- S2O3--: A white ppt. of lead thiosulphate is formed which is soluble in cold HNO3, on boiling a black ppt. of PbS is formed. Pb+++S2O3-- PbS2O34- SO4--: A white ppt. lead suphate, which is insoluble in cold dil. mineral acids, but soluble in ammonium acetate and hydroxide solutions (Na+ and K+)
Pb+++ SO42- PbSO4 PbSO4+ 4 CH3 COO- (Pb (CH3COO)4)2-+ SO42- PbSO4+ 3OH- HPbO2-+ H2O +SO42- Plumbites III. Special Tests 1.Sulphide; S2- Cadmium carbonate test :The sulphide solution is shaken with CdCO3 powder, a canaryyellow ppt. of CdS is produced. S--+ CdCO3 CdS + CO32- This test could be used for the identification and separation of S2- when present in a mixture with other sulphur containing anions, or those anions which do not react with CdCO3.
2- Sulphite: SO32- Zinc nitroprusside test :Add to cold saturated ZnSO4 solution, equal volume of K4[Fe (CN)6]solution, add few drops of 1% sodium nitroprusside solution. This solution is added to the SO32-solution,a salmon-colored ppt. of zinc nitroprusside is formed Zn (Fe(CN)5 NO). The latter reacts with moist SO2 to give a red ppt. of Na5[Fe(CN)5 SO3]3- Thiosulphate; S2O32- Formation of thiocyanate : By boiling with KCN solution (poison), in the presence of NaOH, Cool, acidify and add FeCI3, a blood red color of ferric thiocyanate complex is produced. S2O3--+ CN- OH- SCN-+ SO3-- boil Fe3++ SCN- Cool Fe(SCN)2+
4- Sulphate: SO42- Hepar’s test Sulpate is reduced by carbon to sulphide by heating on a piece of charcoal in the presence of Na2CO3 in the reducing zone of the flame MSO4+ Na2CO3 Fusion Na2SO4+ MCO3 Na2SO4+ C Na2S + 4 COTransfer the fusion product to a silver coin and moisten with a littlewater, a brownish black stain of Ag2S results. S--+ 2H2O 2 OH-+ H2S H2S + 2 Ag Ag2S +H2
IV. Analysis of Mixtures 1- Mixture of S2-, SO32-, S2O32- and SO42- : Separation is carried first shaking the mixture solution with CdCO3 powder. The centrifugate is allowed to react with BaCl2 solution which will precipitate BaSO4 and BaSO3 leaving S2O32-as soluble centifugate. The precipitated BaSO4 and BaSO3 can be separated by the solubility of BaSO3 in excess dil. HCI. S2-, SO32-, S2O32- , & SO42- Solution + CdCO3 Yellow ppt Centrifugate S2- + BaCI2 White ppt Centrifugate BaSO3+BaSO4 S2O32- HCl Heat HCl White PPt Centrifugate SO32- SO2 + So SO42- confirm by reducing character
2- Mixture of CO32- and SO32- or S2O32- This type of mixtures are considered to be difficult, due to the interference occur upon the addition of dil. HCI which liberates CO2 and SO2 gases which turbides lime water and disappears on prolonged passage. SO2 can be detected by its reducing characters as discussed before, but CO2 has non reducing characters. Therefore SO32- or S2O32- ions must be firstly oxidized into SO42- by an oxidizing agent such as H2O2,K2Cr2O7 or KMnO4 and dil. H2SO4 and warm, CO2 will only evolve which can be test with lime water.3- Mixture of H2S and SO2 gases: In order to differentiate between these two gases which evolve upon the addition of dil. HCI to sulphides, sulphites and thiosulphates and having similar reducing properties. A paper moistened with lead acetate solution changes into black when exposed to H2S gas, SO2 can cause turbidity to lime water
Halides This group of anions, are; 1- Fluoride (F-) 2- Chloride (Cl-) 3- Bromide (Br-) 4- Iodide (I-)Fluorides, chlorides, bromides and iodides are known as halogens.They are characterized by their higher electronegativity As the ionic size increases, the tendency to loose electrons increases and therefore iodide ion is firstly and easily oxidized into free I2 by loosing readily an electron followed by Br - when present in a mixture. However its difficult to oxidize F- into F2, hence F- ions are highly stable to held strongly a proton. Therefore the order of stronger halogen acid is from HI HBr HCl HF.
I. General characters 1- Parent Acids: a- Hydrofluoric acid; HF : Its coloress fuming highly corrosive and itching liquid (B.P. 19.4oC). Soluble in water producing the weakest acidic solution in the halogen acid series. b- Hydrochloric acid : HClColorless gas with irritating odor, fumes in moist air, extremely solublein water to form acidic solution. Concentrated HCI contains 37% of HCI gas. c- Hydrobromic acids : HBr Colorless gas with irritating odor, fumes in moist air and is extremely soluble in water forming very strongly acidic solution. On standing the solution becomes yellow due to the oxidation to bromine.d- Hydroiodic acid: HIColorless gas with irritating odor, fumes strongly in moist air, soluble inwater forming the strongest acidic solution of the haloacid series.the solution is colorless, becomes brown on standing due to the liberated iodine.
2-Solubility:All the salts of CI-, Br- and I- are soluble except Ag+, Hg22+, & Cu+ salts,their lead salts are slightly soluble in cold water, soluble in hot water.The alkali metal salts of fluorides, ammonium and silver salts are soluble,other salts are insoluble or sparingly soluble. 3-Reducing agent: Cl- has very weak reducing character. Br- and I- have reducing character, they can react with oxidizing agent like chlorine water to give Br2 or I2. I- has strong reducing power than Br- so it react with FeCl3, H2O2 and nitrite solutions. II. General Reactions 1- Dry Reactions a- Action of dilute HCl Hydrochloric acid shows no reaction upon treatment of the solid sample with it even on heating. This reaction can differentiate carbonate and sulphur group from halides.
b- Action of concentrated H2SO4: Decomposition of the halides occurs upon the addition of the strong non-volatile concentrated H2SO4 to the solid sample, this occurs in the cold, completely on warming with the evolution of HX which can be recognized by a) the fumes evolved. b) Confirmatory chemical test 2X-+ H2SO4 = 2 HX + SO42- X = may be CI-, I-, Br- and F-1- For Fluoride: Fluoride gives a characteristic reaction when treated with conc. H2SO4. Hydrofluoric acid is produced which is colorless and fumes with moist air. due to the corrosive and itching action of the gas on the glass in presence of H2O, the test tube or the glass rod subjected to the evolved HF gas acquire oily appearance due to the formation of silicic acid and hydrofluorosilicic acid.This test is considered to be specific for fluoride anion, even in the presence of other halides. 2 F-+ H2SO4 2H F + SO4-- 4HF + SiO2 SiF4+ 2H2O glass 3 SiF4+ 3H2O H2 SiO3+ 2 H2 SiF6 silicic acid hydrofluoro silicic acid
2- For chlorideHCI gas is evolved upon treatment with conc. H2SO4 which can beidentified by : 2CI-+ H2SO4 2 HCI + SO4--1- Formation of white fumes with moist air due the formation of droplets of hydrochloric acid.2- Pungent irritating odor.3- Changing a blue moistened litmus paper into red.4- Formation of white fumes of NH4CI when a glass rod moistened with ammonium hydroxide solution is exposed to the evolved gas. NH4OH + HCI NH4CI + H2O3- For Bromide:A mixture of HBr and Br2 may be formed which have characteristicbrown color especially on warming. At the same time sulphuric acidwill be reduced into SO2, H2S or S 2 Br-+ H2 SO4 2 HBr + SO4-- 2 HBr + H2SO4 Br2 + SO2+ 2 H2O
4- For iodide: Since HI is the most active reducing agent, so it is readily oxidized to iodine which appears as violet fumes. I2 can be detected by exposing the evolved gas to paper moistened with starch solution, it changes into blue. 2I-+ H2SO4 2 HI + SO42- 2HI + H2SO4 I2 + SO2 + 2H2O 6HI + H2SO4 3 I2 + S + 4H2O 8HI + H2SO4 4 I2 + H2S + 4H2O c- Action of concentrated H2SO4 and MnO2:If the solid halide is mixed with an equal quantity of precipitatedmanganese dioxide, concentrated H2SO4 added and the mixturegently warmed. Chlorine, bromine and iodine are evolved from CI-, Br-and I- but F- liberates HF since it has no reducing properties. 2X- + 4H++ MnO2 Mn+++ 2H2O +X2 X = may be CI-, Br- and I-
The free halogen, (X2) could be detected by: 1- Bleaching of a moistened colored litmus paper. 2- Suffocating, and irritating odor. 3- Characteristic color of Br2 (brown), I2 (violet) and CI2 gas (greenish tint). 4- I2 changes starch paper into blue, Br2 turns it orange. 5- CI2 and Br2 change a starch – KI into blue due to the oxidation of I- to I2 produce a blue adsorption complex. CI2+ 2KI 2KCI + I2 Br2+ 2KI 2KBr + I22- Wet Reactions a- Reaction with AgNO3: To 1ml of the salt solution add AgNO3 reagent. 1- Fluoride: No precipitate, since AgF is soluble in water. 2- Chloride: A white curdy ppt. of AgCI which is insoluble in nitric acid, soluble in KCN and Na2S2O3 as other silver halides. The precipitated AgCI is soluble in dil. ammonia solution to give the ammine complex.
Ag++ CI- AgCI AgCI + 2NH3 [Ag(NH3)2]CI Silver ammine chloride [Ag(NH3)2] CI + 2H+ 2 NH4++ AgCI AgCI is reprecipitated upon treatment of the ammine complex with acid. AgX + 2CN- [Ag (CN)2]- +X- Soluble complex AgX + 2 S2O3-- [Ag(S2O3)2]3-+X- 3- Bromide: A curdy, pale yellow precipitate of AgBr, sparingly soluble in dilute, but readily soluble in conc. ammonia solution Ag++ Br- AgBr AgBr + 2 NH3 [Ag(NH3)2]++ Br-4- Iodide: A curdy yellow ppt. of AgI is formed which is insoluble in dil. ammonia but very slightly soluble in conc. ammonia solution. Ag++ I - AgI
There is a periodicity in character of three silver halides. Since AgIis the most insoluble one, followed by AgBr and AgCI. Therefore AgCIwill be dissolved in dil. ammonia, followed by AgBr in conc. Ammonia solution but AgI does notThis is also attributed to that the conc. of silver ions (Ag+) produced formthe dissociation of silver ammine complex according to its instabilityconstant is insufficient to exceed the high solubility product of AgCI,approach that of AgBr (partially soluble) but exceeds that of AgI. [Ag(NH3)2]+ Ag++ 2NH3 Instability constant = (Ag+) (NH3)2 _________________ [Ag(NH3)2]+Therefore when Br- or iodide solutions are added to AgCI, yellow ppt. ofAgBr or AgI are formed. AgCI + Br- (or I-) AgBr (or AgI) + CI- AgBr + I- AgI + Br-
b- Reaction with BaCI2 solution:Only fluoride gives a white gelatinous ppt. when BaCI2 reagent is addedto sample solution. Ba+++ 2F- BaF2 The white gelatinous BaF2 ppt. is partially soluble in dil. HCI or HNO3 No ppt. is formed in case of other halides. c- Reaction with FeCI3: Add few drops of FeCI3 reagent to concentrated sample solution. 1- F- : a white crystalline ppt. of the complex salt, which is sparingly soluble in water Fe3++ 6 F- [FeF6]3- 2- CI -and Br -: do not react with FeCI33- lodide reacts with FeCI3, due to its strong reducing action with the liberation of I2.d- Reaction with lead acetatePrecipitates of Pbx2 are formed in cold solution when lead acetate reagentis added to sample solutions.
F-, Cl- and Br- form a white ppt with lead acetate, sparingly soluble in cold more soluble in hot water, crystallize on cooling Pb+++ 2 F- PbF2 Pb+++ 2 CI- PbCI2 Pb+++ 2 Br- PbBr2Iodide forms a bright yellow ppt of PbI2 which is soluble in hot waterand crystallizes on cooling as golden spangles. e- Chlorine water test: Chloride and Fluoride do not react with chlorine water . Chlorine water oxidizes I -and Br -into I2 and Br2 which can be extracted with chloroform or carbon tetrachloride as violet color or brown or yellow color of I2 and Br2, respectively.Iodide react first with chlorine water before bromide as it has morereducing character.
Chlorine water reagent is added drop wise to a solution of iodide or bromide as excess chlorine water converts Br2 into yellow bromine monochloride or into colorless hypobromous acid or bromic acid and the organic layer turns pale yellow or colorless. Also, excess chlorine water oxidized I2 to colorless iodic acid. 2Br-+ CI2 Br2+ 2CI- Br2+ CI2 2 BrCI (yellow) bromine monochloride Br2+ CI2 (excess) + 2H2O 2HOBr+2HCI hypobromous acid Colorless Br2+ 5CI2 (excess) + 6H2O 2 HBrO3+10HCI bromic acid 2I- + CI2 I2+ 2CI- I2+ 5CI2 (excess) + 6H2O 2 HIO3+10HCI iodic acid
III. Special Tests 1- For Fluorides: Boron fluoride test: When fluoride is mixed with borax and moisten with conc. H2SO4. The formed HF and boric acid react to produce boronfluoride gas. If the mixture introduced into the flame tinged green by BF3 gas. Na2B4O7+ H2SO4+ 5H2O 4H3BO3+Na2SO4 Borax boric acid 2NaF+ H2SO4 2HF + Na2SO4 H3BO3+ 3HF BF3+ 3H2O 2- For chlorides: Chromyl chloride test:This test is a specific test for chloride even in the presence of otherhalides. Its classified as dry reactions test because, it is carried outon the solid sample:
The solid chloride is mixed with three times its weight of powderedpotassium dichromate in a tube, an equal bulk of concentrated sulphuricacid is added, the tube is attached to another tube by a pent tube,dipped into a NaOH solution. The deep red vapors of chromyl chlorideCrO2CI2 which are evolved are passed into sodium hydroxide solution.The resulting yellow solution in the test tube contains sodium chromate;this confirmed by perchromic acid test, which is carried out by acidifying with dil. H2SO4, adding 1-2 ml alcohol or ether, followed by a little H2O2solution. The organic layer is colored blue. 4CI-+ Cr2O7--+ 6H+ cond. 2CrO2 Cl2 + 3H2O CrO2CI2 + 4OH- CrO4-- + 2CI- + 2H2O 2 CrO4-- + 2H+ Cr2O7--+ H2O Cr2O7--+ 7H2O2 2 CrO83-+ 5H2O + 4H+ Blue in ether or amyl alcoholIt is possible to test for CrO4--also by lead acetate CrO4--+ Pb++ Pb CrO4 Yellow
N.B. 1- Some CI2 may also be liberated owing to the reacting. 6CI- + Cr2O7--+ 14H+ 3CI2+ 2Cr3++ 7H2O and this decreases the sensitivity of the test.2- Fluorides give rise to the volatile CrO2F2 which is decomposed by water, and hence should be absent or removed.3- Nitrites and nitrates interfere, as nitrosyl chloride may be formed.4- Bromides and iodides give rise to the free halogens, which yield colorless or pale yellow solution with NaOH. 6 Br-+ Cr2O7--+ 14H+ 2 Cr3++ 3Br2+ 7H2O 6 I-+ Cr2O7--+ 14H+ 2Cr3++ 3I2+ 7H2O Br2+ 2OH- OBr-+ Br-+ H2O (hypobromide) I2+ 2OH- OI-+ I-+ H2O (hypoiodide)
3- For iodides:A) lodide is readily oxidized in acid solution (dil. H2SO4) with nitrite solutionor H2O2 into free l2 2I-+ 2NO2-+ 4H+ I2+ 2NO + 2H2O 2I-+ H2O2+ 2H+ I2+ 2H2O B) I- reacts with Cu++ forming a whit ppt. of Cu2I2, the I- being oxidized to free I2. Thus a white ppt. in brown solution is formed on treating I- with CuSO4 solution. 2Cu+++ 4I- Cu2I2 +I2C) I- reacts with mercuric chloride solution mercuric iodide HgI2 will be precipitated as yellow-scarlet red ppt. which dissolves in excess iodide forming soluble colorless complex. HgCI2+ 2I- HgI2 + 2CI- Scarlet red HgI2+ 2I- (HgI4)2- Soluble complex Nesslers reagent
IV. Analysis of Mixtures1- Mixture of F-, Cl-, Br- and I- :a) The F- is separated by treating the mixture solution acidified with CH3COOH with Ba(NO3)2 or Ca (NO3)2 Centrifuge White PPt. Centrifugate BaF2 CI-, Br- and I- Confirmed by Conc.H2SO4 test b) for the centrifugate ( Cl-, Br- and I-), carry out chlorine water test for both I- and Br – ( or get rid of I- by oxidation to I2 using H2O2 or nitrite and extract I2 by chloroform then test for Br- in aqueous solution c) For CI-, carry out chromyl test on a solid sample.
2- Mixture of chlorine / chloride and Br2 / Br- : Chlorine is tested for by its smell, bleaching effect, while Br2 is tested by shaking with chloroform, it give brown color. CI -and Br-could be tested after removal of chlorine and bromine by shaking with metallic mercury (till the smell of CI2 disappears and the liquid doesnt bleach litmus paper). Insoluble Hg2CI2 and/or Hg2Br2 are formed. Test for CI -and or Br -in the clear supernatant (centrifugate CI2+ 2Hgo Hg2CI2 Br2+ 2Hgo Hg2Br23- Mixture of chloride and iodide : Add AgNO3 to the mixture, AgCl and AgI are precipitated. Add to precipitate dil ammonia solution and filter Filterate Cl- Precipitate Confirmed by chromyl chloride test Yellow ppt. I-
Cyanogen anions This group of anions, are; 1- Cyanide (CN-) 2- Thiocyanate (SCN-) 3- Ferrocyanide [Fe(CN)6]4- 4- Ferricyanide [Fe(CN)6]3-All cyanide containing anions are highly poisonous. In all experiments inwhich the gas is likely to be evolved or those in which cyanides areheated, should be carried out cautiously in the fume cupboard.I. General characters 1- Parent Acids: a) Hydrocyanic acid HCN Its very poisonous. Its colorless volatile liquid (B.P. 26.5oC). It has an odor of bitter almonds. It is not stable in solution due the formation of ammonium formate. Any dil. mineral acid can replace HCN in its solution.
On passing CO2 to CN- solution HCN is produced with HCO3-. CN-+ CO2+ H2O HCN + HCO3- b) Thiocyanic acid: HSCN It is colorless toxic liquid (B.P. 85oC) with unpleasant odor. It is as strong as HCI but unstable. It is soluble in ether after the addition of HCI to an aqueous solution of SCN-. On standing its aqueous solution is decomposed to HCN and yellow solid polymer. 3 HCNS HCN + H2N2C2S3 c) Ferrocyanic acid H4 [FeCN)6]Its white crystalline solid. Its aqueous solution is strongly acidic.The first two protons are nearly completely ionized. d) Ferricyanic acid: H3 [Fe(CN)6]Its browinish crystalline solid, soluble in water to give strongly acidssolution. The three protons are nearly completely ionized.
2-Solubility:CN-: All cyanides are water insoluble except alkali metals (Na+, K+), ammonium salt, alkaline earth metals ( Ba2+, Sr2+ and Ca2+) and mercuric cyanide.SCN-: All thiocyanates are water soluble except AgSCN, Hg2(SCN)2 & Cu2 (SCN)2. Pb (SCN)2 as PbCI2 is sparingly soluble in cold water, but soluble in hot water.Ferro and Ferricyanides:All are insoluble in water except those of alkali metals, ammonium salt andalkaline earth metals. 3-Complexing agent: Cyanide ion has strong tendency to the formation of complexes which may be double cyanides or complex cyanides. 1- Argentocyanide complexes: Double cyanides When a ppt. is formed upon reacting CN- with Ag+, at first white turbidity is formed which is AgCN. According to the medium, if CN- ions are present in excess a soluble complex is formed. AgCN + CN- (Ag (CN)2)-
2- Complex cyanides:Stable metallo-cyanogen complexes can be formed by reacting FeSO4with CN- in alkaline medium to give stable ferrocyanide complex.Similar complex is formed with Fe3+ to give ferricyanide.Therefore [Fe(CN)6]4- and [Fe(CN)6]3- are considered to be stable complexesfrom CN- ions. Also Co++ can form stable complexes with CN-. Fe2++ 6 CN- [Fe(CN)6]4- Fe3++ 6CN- [Fe(CN)6]3-When cyanides are heated with polysulphides (NH4)2Sx or thiosulphate (S2O3--)they give thiocyanate ion CN-+ (NH4)2Sx (NH4)2Sx-1+ SCN- CN-+ S2O32- SO3--+ SCN- 4-Oxidizing agent: Ferricyanides has oxidizing effect, they can oxidizes I- into I2 5-Reducing agent: Ferrocyanides has mild reducing effect, they can be oxidized to ferricyanide by oxidizing agents, such as MnO4-, NO3-, H2O2 and Cl2
II. General Reactions 1- Dry Reactions a- Action of dilute HCl a) CN-: HCN gas evolved with characteristic bitter almond odor and can be tested by: 1- Converting HCN evolved into SCN-, by exposing the evolved HCN gas to a paper moistened with ammonium polysulphide.The resulted SCN- can be tested by adding dil. HCI and a drop of FeCI3 solution, a blood red color is produced. 2- By passing the evolved gas into AgNO3 solution, a white ppt. of AgCN is formed insoluble in dil. HNO3, soluble in ammonia solution. HCN + AgNO3 AgCN + HNO3 AgCN + 2NH3 (Ag(NH3)2)CN3- Prussian blue test: The evolved HCN gas is passed into NaOH solution,add drops of FeSO4 solution, heat to boiling, the HCN is converted intoferrocyanide which can be tested by adding drops of FeCl3 solution to producea prussian blue ppt.
b) SCN-: No reaction as SCN- is as strong as HClc) Ferrocyanide and Ferricyanide: With cold dil. HCI, no gases, but may be precipitation of hydro ferrocyanic and hydroferricyanic acid occur. (Fe(CN)6)4-+ 4H+ H4(Fe(CN)6) (Fe(CN)6)3-+ 3H+ H3(Fe(CN)6)b- Action of conc. H2SO4:a) CN- ; All cyanides are decomposed on heating. CN- 2H++ H2O NH4+ +COb) CNS-: Decomposition with evolution of carbonyl sulphide, which burns with a blue flame. SCN-+ 4H++ 2SO4--+ H2O NH4++ 2HSO4-+COS Carbonyl Sulphide
c) Ferrocyanide and Ferricyanide: On heating with conc. H2SO4, CO will be evolve which burns with a blue flame. SO2 is produced in case of ferrocyanide.(Fe(CN)6)4-+ 6H2O +22H++ 10 SO42- Fe2++6NH4++ 10 HSO4-+ 6 CO 2Fe2++ 4H++ SO4-- SO2+ 2H2O + 2Fe3+(Fe(CN)6)3-+ 6H2O + 22H++ 10 SO42- Fe3++ 6NH4++ 10 HSO4-+ 6CO 2- Wet Reactions a- Silver nitrate solution:1- CN- & SCN- : form white ppts. of silver cyanide and silver thiocyanate.AgCN is soluble in excess CN-, ammonia solution, but insoluble in dil. HNO3 Ag++ SCN- AgSCN H+ Ag++ CN- AgCN CN- (Ag(CN)2 )- HCN+ AgCN
2- Ferro- and Ferricyanides: Both [Fe(CN)6]4-and [Fe(CN)6]3- react with AgNO3 solution with the formation of a white ppt. and orange red ppt., respectively 4 Ag++ [Fe(CN)6]4- Ag4[Fe(CN)6] Insoluble in dil. ammonia Insoluble in dil. HNO3 3 Ag++ [Fe(CN)6]3- Ag3[Fe(CN)6] Orange red ppt. Insoluble in dil. HNO3 Soluble in dil. ammoniaThe solubility of silver ferricyanide ppt. can be used for the separation of ferrocyanide and ferricyanide when present in a mixture. Oxidation ofthe white ppt. of Ag4 [Fe(CN)6] by warming with few drops of conc. HNO3,leads to orange red ppt. of Ag3 [Fe(CN)6] which becomes soluble in dil. ammonia solution. b) Reaction with BaCI2: No observed reaction
c) Reaction with FeCI3: This reaction is very important, since it is diffrantiating reaction. The diluted sample solution is added to a 1ml of FeCI3 reagent. 1- CN-: iron (III) cyanide will be formed form dil. solution as a ppt. which is dissolved in excess cyanide forming ferricyanide. 3CN- Fe3++ 3 CN- Fe (CN)3 [Fe(CN)6]3- Ferricyanide 2- SCN-: This reaction is specific for iron(III) and SCN- in the absence of other interfering ions. A cold acidic solution of SCN- is treated with FeCI3 reagent, a blood red color is produced which is extractable with ether. The formed color is subjected to have the following structures: Fe3++ SCN- [Fe(SCN)]++ or Fe(SCN)3 or [Fe(SCN)6]3-In order to increase the sensitivity of the test the following precautions must be done: 1.Ensure the presence of iron in the Fe3+ state.
2- Acidification of the medium (dil. HCI is preferable). 3- Cooling of the solution befor testing. 4- Removal of intreferring ions by precipitation or complexation.F-, PO43- , oxalate and tartrate bleach the colour, therefore it must be absentF- for e.g, reacts with iron to form stable complex. 6 F-+ Fe3+ (FeF6)3- other ions which react with SCN- e.g, Hg2+ which form unionized Hg (SCN)2 which is colorless. Iodides also interferes by being oxidized by Fe3+ into the brown colour I2. 2I-+ 2Fe3+ H+ I2+ 2Fe2+ 3- Ferro and Ferricyanides: A Prussian blue characteristic ppt. is formed form acidic solution of [Fe(CN)6]4-, which is insoluble in dil. HCI, but soluble in alkali hydroxide. 3[Fe(CN)6)4-+ 4Fe3+ Fe4[Fe(CN)6]3 Prussian blueIn case of Ferricyanide, a brown color is formed of the non-ionised ferricyanide Fe3++ [Fe(CN)6]3- Fe[Fe(CN)6] Brown color This test can be used to differentiate between ferro and ferricyanide
d) Reaction with FeSO4 reagent: 1- CN-: Cyanide forms with FeSO4 solution a yellow brown ppt. at first which is then form ferrocyanide, this reaction is enhanced by heating or addition of alkali. 2CN-+ Fe2+ Fe(CN)2 4CN- [Fe(CN)6]4- 2- SCN-: No reaction.3- Ferri and Ferrocyanide: Ferricyanide forms with FeSO4 reagent a similar blue ppt. (turnbulls blue), as that of Prussian blue, but differ in the distribution of iron-different oxidation state is varied. [Fe(CN)6]3++ Fe2+ Fe3++ [Fe(CN)6]4- Turanbulls blue Prussian blue Ferrocyanide forms white ppt. of ferrous ferrocyanide. 2K++Fe+++ [Fe(CN)6]4- K2Fe[Fe(CN)6]
e- Reaction with CuSO4: To the sample solution, add CuSO4 reagent dropwise.1- CN-: In acidic medium, CN- likes I-, reacts with Cu++ which oxidizes CN- into cyanogens (CN)2 or cyanate CNO- (in alkaline medium). Cu+++ 2CN- Cu(CN)2 Greenish yellow 2CU (CN)2 Oxid-red Cu2 (CN)2 + (CN)2 white cyanide cyanogen Cu2(CN)2 + 4CN- 2 (Cu (CN)3)2- Excess cuprocyanide complex Soluble As a conclusion of this reaction, cupric ions react with excess cyanide to form soluble complex cuprocyanide and cyanogen. 2Cu+++ 8CN- 2[Cu (CN)3]2-+ (CN)2 In alkaline medium cyanogen is converted to CN- & cyanate CNO-. (CN)2+ 2OH- CN-+ CNO-+ H2O
2- SCN-: Thiocyanate reacts with CuSO4 reagent, to form a green color which changes into a black ppt Cu (SCN)2 with excess CuSO4 reagent Cu (SCN)2 decomposes gradually to white cuprous thiocyanate Cu2(SCN)2 and separation of thioyanogen as a gummy mass Cu+++ SCN- Cu (SCN)2 2 Cu (SCN)2 unstable Cu2 (SCN)2+ (SCN)2 decomposition white gummy mass 3- Ferro and Ferricyanides: Both ferro and ferricyanides form brown and green ppts. of copper ferro and copper ferricyanides, respectively. Both ppts. are insol. in dil. acids [Fe(CN)6]4-+ 2Cu++ Cu2[Fe(CN)6] Brown 2 [Fe(CN)6]3-+ 3Cu++ Cu3[Fe(CN)6]2 green
f- Reaction with Cobalt Nitrate: To the sample solution, add excess Co(NO3)2 reagent.1- CN-: A buff ppt., of cabaltous cyanide dihydrate is formed, which is soluble in excess CN- to form soluble complex, cobaltocyanide - Co2++ 2CN-+ 2H2O Co (CN)2. 2H2O 4CN [Co (CN)6]4- soluble complex 2- SCN-: Vogels Reaction The reaction of Co++ with SCN- to produce a characteristic blue color extractable with ether or amyl alcohol; known as vogels reaction. Other cyanogen anions form precipitates with Co (NO3)2 reagent. Co2++ 4SCN- [Co (SCN)4]2- Extractable with ether (blue) 3- Ferro and Ferricyanide: Both form greyish green and red ppts. of cobalt ferrocyanide and cobalt ferricyanide. 2 Co2++ [Fe(CN)6]4- Co2[Fe(CN)6] greyish green 3 Co2++ 2[Fe(CN)6]3- Co3[Fe(CN)6]2 red ppt.
III. Special Tests 1- For Cyanides: a) Prussian blue test: This test is specific for CN- which can be converted into ferrocyanide and allowed to react with Fe3+. b) Iron thiocyanate: This test for CN- depends on the direct combination of alkali cyanides with sulphur (ammonium polysulphide). A blood red coloration is produced upon addition of FeCI3 reagent. This blood red color is extractable with ether. This test is applicable to CN- in presence of S2- or SO32-; if SCN- is originally present, the CN- must be isolated first by precipitation e.g. as zinc cyanide. 2- For thiocyanate:a) Reduction Test: This reaction depends on the reduction of SCN- with metallic zinc and dil. acid into H2S and HCN which can tested for. Zno+ 2H+ 2 (H) + Zn2+ 2SCN- +4(H) 2 HCN + H2S+ S--b) Vogel’s reaction
3- For ferrocyanides: As mild reducing agents: It can be oxidized to ferricyanide by oxidising agents, such as, MnO4-, NO3-, H2O2 and CI2. 2[Fe(CN)6]4-+ CI2 2[Fe(CN)6]3-+ 2CI- 4- For Ferricyanides: As oxidizing agents: For example, [Fe(CN)6]3- can oxidizes I- into a brown colored I2 which identified by starch or CHCI3. 2[Fe(CN)6]3-+ 2I- 2[Fe(CN)6]4-+I2 IV. Analysis of Mixtures 1- Mixture of CN-, SCN-, [Fe(CN)6]4- & [Fe (CN)6]3-CN- must be tested at first, then removed from the mixture. This is donedepending on its strong affinity to protons, low ionization and volatilityof HCN.
The following procedure could be applied.a- Passing CO2 in the mixture solution using acetic acid or NaHCO3and heat, until no more HCN evolved which can be confirmed by:i- Passing in AgNO3 solution acidified dil. HNO3 which gives a white ppt.ii- Passing in NaOH, adding FeSO4 solution heating, followed by HCIthen FeCI3 solution (Prussian blue). b- To the remaining solution, after removal of CN-, acidify with dil. HCI, cool and add FeCI3 solution and centrifuge Deep blue ppt. Centrifugate [Fe (CN)6]4- blood red color brown solution extractable with ether SCN- SnCI2 blue ppt [Fe (CN)6]3-
2- Mixture of SCN-, CI-, Br- and I-SCN- is tested for by reacting with FeCI3, to give blood red color which isextractable with ether and removed. In presence of I-, I2 is also formed whichcan be extracted with CHCI3 (Violet color).The blue complex formed with Co2+ can also be used to detect and removeSCN- by extraction with ether or amyl alcohol.The halides are tested for in the usual way after the removal of SCN-,since it interferes with their precipitation.After testing for SCN-, it is removed by igniting the mixture till no moreblackening or no odor of burnt sulphur is observed.The residue will contain only CI-, Br-, I-, and test for CI- by chromyl chloridetest for I- and Br-, carry out chlorine water test.
Arsinic and phosphorous containing anions This group of anions, are; 1- Arsenate (AsO43-) 2- Arsenite (AsO33-) 3- Phosphate (PO43-) I. General characters 1- Parent Acids: a) Orthoarsenic acid H3AsO4Its aqueous solution is a moderately strong acid, slightly weaker thanphosphoric acid. It has the tendency for condensation and formation ofpyroarsenic acid, H4As2O7, and meta-arsenic acid, HASO3 by gentle heating. -H2O -H2O 2H3AsO4 H4As2O7 2HAsO3 H2O H2O(Orthoarsenic acid) (Pyro arsenic acid) (Meta arsenic acid)
Arsenic acid and arsenate ion are mild oxidizing agents. Three series of arsenates exist, the primary arsenate H2AsO4-, the secondary arsenate (HAsO42-) and the tertiary arsenate (AsO43-). b) Arseneous acid H3AsO3It exist in aqueous solutions, cannot be isolated as such becauseof thermal decomposition to the anhydride, As2O3, sometimes writtenas As4O6. The oxide is slightly soluble in water yielding ortho arsenious acidand meta arsenious acid. As4O6+ 6H2O 4H3AsO3 4HAsO2+ 4H2O (ortho arsenious acid) (meta arsenious acid)Two series of salts of arsenites exist, orthoarsenites H2AsO3-,meta arsenites AsO2-, both respond similarly to different reactions. **[Arsin-containing acids and salts are highly poisonous]**
Reduction of As5+ and As3+: Pentavalent arsenic salts, or anions containing, can be reduced first to the trivalent arsenous, or the corresponding anion containing it, and finally to the metalic form. As5++ 2e As3++ 3e AsoThe reduction can be made using reducing agents with lower redox-potentiale.g. saturated solution of stannous chloride, a powerful reducing agent in thepresence of conc. HCI. As5++ Sn2+ Sn4++ As3+ (H+) 2AS3++ 3Sn2+ 3Sn4++ 2Aso (OH-) c) Orthophosphoric acid H3PO4It is crystalline solid, its aqueous solution is acidic & ionises into: H3PO4 H++ H2PO4- [dihydrogen phosphate] H2PO4- H++ HPO42- [monohydrogen phosphate] HPO42- H++ PO43- [tribasic phosphate]
The intermolecular loss of water from two molecules of orthophosphoric acid, will give pyrophosphoric acid (H4P2O7) and metaphosphoric acids (HPO3). Orthophosphoric acid forms three series of salts in which one, two or three hydrogens are replaced by metals, for example, NaH2PO4, Na2HPO4 and Na3PO4. these salts are known respectively as primary, secondary and tertiary orthophosphates.The aqueous solution of the primary salt is acid, that of the secondary is slightly alkaline while in the case of the tertiary salt, the solution is strongly alkaline 2-Solubility:All their salts are insoluble in water except those of Na+, K+ and NH4+ besidethe alkali dihydrogen salts as Ba(H2AsO4)2 3- Redox-reaction with I2/I-: Aresnate has oxidizing effect and aresnite has reducing effect Arsenate (AsO43-) ions oxidises iodide into iodine; but the redox reaction is reversible due to the narrow difference in Eo values of the two redox systems
H AsO43- +2H++ 2I- AsO33- + H2O +I2 NaHCO3Arsenate oxidise iodide into iodine in acid medium, while arsenite(mild reducing agent) reduces iodine into iodide in alkaline medium.II. General Reactions 1- Dry Reactions a- Action of dilute HClNo visible reaction, since phosphates, arsenates and arsenite acid arenon volatile. b- Action of conc. HCl 1- PO43- : no visible reaction 2- AsO43-:On hot arsenate ion oxidises HCI into free CI2, while it will be reduced to arsenite 2CI-+ AsO43-+ 4H+ CI2 +AsO2- + 2H2O
3- AsO33- : Arsenite will react and vapour of arsenious chloride is evolved. AsO2-+ 3CI- + 4H+ AsCI3 + 2H2O c- Action of conc. H2SO4 1- PO43- and AsO43-: no visible reaction 2- AsO33- : Arsenite on heating, some reduction to SO2 may occur.2- Wet Reactionsa- Silver nitrate solution: 3Ag++ PO43- Ag3PO4 (yellow ppt) 3Ag++ AsO43- Ag3AsO4 (chocolate ppt.) 3Ag++ AsO33- Ag3AsO3 (yellow ppt.) All the precipitates are soluble in dil. HNO3 due to the fact that the corresponding acids (phosphoric, arsenic and aresnious acids) are weaker than nitric acid in the presence of which they yield lower concentration of their ions insufficient to precipitate their silver salts
All the precipitates are soluble in ammonia solution, due to theformation of the complex ion [Ag (NH3)2]+, which yields lowerconcentration of silver ions insufficient to precipitate their silver salts. 3Ag++ 6NH3 3[Ag(NH3)2]+These precipitates are insoluble in acetic acid.b) Reaction with BaCI2:White precipitates of the secondary salt (BaHPO4, BaHAsO4, BaHAsO3)from neutral medium, or of the more insoluble tertiary salt (Ba3(PO4)2,Ba3(ASO4)2 or Ba3(AsO3)2) from ammoniacal or dilute alkaline solutions.The precipitates are soluble in dilute acids including acetic acid.c) Reaction with Magensia Mixture:Magnesia mixture reagent is formed of MgCI2, NH4CI and NH4OH[Mg2+, the precipitating ions, NH4OH, to render the medium ammoniacal;NH4CI, to reduce OH- concentration by common ion effect to be insufficient to ppt. Mg (OH)2]. The reagent solution form white crystalline precipitate with phosphates and arsenates in neutral or ammoniacal solution The precipitate is soluble in acetic acid and in mineral acids.No precipitate is formed with arsenites.
PO43-+Mg2++ NH4+ Mg (NH4) PO4 [magnesium ammonium phosphate] AsO43-+ Mg2++ NH4+ Mg(NH4)AsO4 [magnesium ammonium arsenate]If the white precipitates are treated with AgNO3 (in acetic acid medium),that of the phosphate will be transformed into yellow ppt. while that ofthe arsenate into chocolate ppt. due to the transformation to the lesssoluble Ag3PO4 and Ag3AsO4 respectively.d) Reaction with ammonium molybdate:The addition of a large excess (2-3ml) of this reagent in conc. HNO3to a small volume (0.5ml) of the test solution acidified with HNO3 andheat gradually, produces a canary yellow crystalline precipitates ofammonium phosphomolybdate (NH4)3PO4. 12MoO3 (on warming to 40oC)and of ammonium arsnomolybdate (NH4)3 AsO4. 12MoO3 (on boiling)in case of phosphates and arsenates respectively.No precipitate is formed with arsenites.The precipitates are soluble in ammonia or alkali hydroxides,in excess phosphates or arsenates respectively and on boiling withammonium acetate solution, insoluble in HNO3.MoO3 produced from the action of acid on ammonium molybdate.
(MoO42-)+ 2H+ H2MoO4 MoO3+ H2O 3 NH4++ 12 MoO3+ PO43- (NH4)3PO4.12MoO3 3NH4+ + 12 MoO3+ AsO43- (NH4)3ASO4.12MoO3Chloride and reducing agents, such as S2-, SO32-,[Fe(CN)6]4- and tartarates,seriously affect the reaction, and should be destroyed before carrying out the test. e) Reaction with H2S: Acidify the test solution with dilute HCI and pass H2S. No precipitate is formed in case of phosphate. Aresnites, produce immediate yellow ppt. of arsenious sulphide As2S3. The ppt. is soluble in HNO3 and alkali hydroxides insoluble in hot conc. HCI. 2HAsO2+ 3H2S As2S3 + 4H2O
Arsenates, not produce any immediate visible change, but after prolongedpassage of H2S, yellow ppt. of AS2S3 is produced. It is evident that the firstaction of H2S is to reduce the arsenate into arsenite through the formationof thioarsenate ion H2AsO3S- which decomposes slowly arsenious acidand suphhur. H2AsO4-+ H2S H2AsO3S-+ H2O H2AsO3S-+ H+ HAsO2+ H2O +S 2HAsO2+ 3H2S As2S3 + 4H2OIf the acid concentration is high and the strean of H2S is rapid,no preliminary reduction to arsenite occurs and arsenic pentasulphideprecipitate (As2S5) is produced. 2H2AsO4-+ 5H2S +2H+ As2S5 +8H2O However, if the solution is heated under the same conditions, mixture of As2S3 and As2S5 is formed.
f) Reaction with CuSO4 solution: Phosphates and arsenates form bluish-green ppt. of the cupric phosphate or arsenate, CuHPO4, or CuHAO4, respectively. On adding an excess of NaOH, the ppt. assumes a pale blue color but dose not dissolve, and on boiling no red ppt. is produced. The ppt. is soluble in mineral acids and in ammonia. Aresnites from yellowish green ppt. of copper arsenite CuHAsO3 from the sample solution just alkaline with NaOH. The ppt. is soluble in excess NaOH to give deep blue color of CuO.HAsO2. On boiling red ppt. is formed due to the reduction of CuO into cuprous oxide (Cu2O), the arsenious acid is simultaneously partially-oxidised to arsenic acid. Cu2++ AsO2-+ OH- CuHAsO3=[CuO.HAsO2] 2[CuO.HAsO2]+H2O Cu2O + H3AsO4+HAsO2g) Uranyl acetate solution:Light yellow, gelatinous precipitate of uranyl ammonium phosphateUo2(NH4) PO4 or arsenate UO2 (NH4) AsO4 in case of phosphates andarsenates repectively, in the presence of excess ammonium acetate.The precipitate is soluble in mineral acids, but insoluble in acetic acid.
This test provides an excellent method of distinction of phosphate andarsenate from arsenite, which does not give a precipitate with the reagent. PO43-+ UO22++ NH+4 UO2(NH4)PO4 AsO43-+ UO22++ NH4+ UO2(NH4) AsO4 III. Special Tests 1- For phosphate: Magnesium testIt depends on reduction of the stable phosphates into phosphide (P3-),by mixing with magnesium powder and heat in an ignition tube. Moisten thecold mass with water, phosphine gas (PH3) is produced which hasunpleasant odor and is inflammable. PO43++ 4Mg (heat) 4MgO + P3- P3-+ 3H2O PH3 + 3OH-
2- For arsenate:Potassium iodide test: To the test solution (2ml) add 1 ml of chloroform, 3ml KI solution and 5ml conc. HCI. Shake vigorously and allow to settle, a violet color of free iodine appears in the organic layer. H AsO43-+ 2I-+ 4H+ ASO2-+ I2+ 2H2O HCO3-The test can be used for the detection of arsenate in presence ofphosphate and arsenite (in absence of other oxidizing agents).3- For arsenite:a) Iodine test: Add 0.5 ml of saturated NaHCO3 solution to 3 ml of the sample solution.Add few drops of I2 solution. The brown color of I2 disappears immediatelydue to the reducing effect of arsenite. This reaction is the reverse of thatfor arsenate. In absence of other reducing agents this test can be used todistinguish arsenite from arsenate or phosphate.
b) Bettendorfs test: A few drops of the test solution are added to 4ml of conc. HCI, and 1 ml of saturated stannous chloride solution is added. The solution is gently warmed; it becomes drak brown and finally black ppt. of arsenic is formed. Strong reducing agents as SnCI2 reduce arsenite in presence of conc. HCI to elemental arsenic. 3 Sn2++ 8H++ 2AsO2- (heat) 2As +3Sn4++ 4H2OThis test is also positive with arsenates, being first reduced into arsenites.However, the test can be made use of to establish the presence ofarsenic-containing anions. c) Marshs reaction: [ for small amounts of arsenic.] In acidic solution arsenic (III) and (V) compounds are reduced by hydrogen to the poisonous hydrogen arsenide gas (H3As) with garlic like odor which when heated dissociates to elementary arsenic and hydrogen: AsO33-+ 3Zno+ 9H+ H3As + 3Zn2++ 3H2O (heat) 2H3As 2Aso + 3H2
IV. Analysis of Mixtures 1- Mixture of arsenite and arsenate : Ammoniacal solution of the mixture + magnesia mixture & filter Filtrate White PPt. 1- Acidify with dil. HCI & Pass H2S Mg(NH4)AsO4 immediate yellow ppt. of As2S3 Arsenite Wash with dil Ammonia Solution+ AgNO3 acidified (or) 2- Add 5-7ml of 30%H2O2 soI.+ magnesia With acetic acid mixture drop by drop (10ml) with stirring a white crystalline ppt. Chocolate brown of Mg(NH4) AsO4 produced by ppt. of Ag3AsO4 Oxidation of arsenite. Aresnate (or) 3- Addition of NaHCO3 sat. sol.+few drops of I2 sol. The brown colour of I2 disappears arsenite
2- Mixture of arsenite and phosphate:1- With magnesia mixture ( as the mixture of AsO33- and AsO43-) with theonly exception that when the ppt. of Mg (NH4) PO4 treated with AgNO3acidified with acetic acid, yellow ppt. of Ag3 PO4 is produced.OR2- Pass H2S in the solution of the mixture acidified with dil. HCI, immediateyellow ppt. of As2S3 indicates AsO33- filter. Drive off the excess H2S byboiling and test for phosphate by the general test (amm. molybdate).3- Mixture of arsenate and phosphate: Dissolve in conc. HCI (10 ml), boil, pass H2S for 5 minutes. Dilute with 25 ml H2O & filter.Yellow ppt. Filtrate, evaporate to dryness, dissolve of As2S5 in conc. HNO3 Arsenate add ammonium molybdate & warm canary yellow ppt. Phosphate.
4- Mixture of arsenite, arsenate and phosphate: Ammoniacal solution + magnesia mixture & filter White ppt of Filtrate Mg (NH4) PO4 test for AsO33- Mg (NH4) AsO4 as in mixture (1) Wash with dil ammonia Solution. Dissolve in conc. HCI. Boil & pass H2S; proceed exactly As mixture of PO43- and AsO43-
Nitrogen- containing anions This group of anions, are; 1- Nitrate (NO3-) 2- Nitrite (NO2-) I. General characters 1- Parent Acids: a) Nitric acid HNO3Colorless liquid (B.P. 83OC), decomposes on aging to nitrogen dioxide(NO2). Its solution in water are strongly acidic. 4HNO3 4NO2 + O2 +2H2O b) Nitrous acid HNO2 The pure acid has never been isolated, due to its thermal instability. 2HNO2 NO + NO2 + H2OHowever addition of a strong acid to a solid nitrite or its solution in the coldyields a transient pale-blue liquid (due to the presence of free HNO2 acid orits anhydride, N2O3) and the evolution of brown fumes of NO2.
2-Solubility:All nitrates are soluble in water. Also all nitrites are soluble in waterexcept AgNO2 which is slightly soluble3- Redox-reaction : The nitrate ion contains in its highest oxidation state of + 5, thus reactsonly as oxidizing agent, while nitrite ion contains nitrogen which hasoxidation number + 3, it can therefore act either as a reducing oras oxidizing agent.II. General Reactions 1- Dry Reactions a- Action of dilute HCl No reaction case of nitrates, with nitrites, brown fumes of nitrogen dioxide NO2 evolve and a transient pale blue liquid. 2NO2-+ 2H+ 2HNO2 NO + NO2 +H2O 2NO + O2 2NO2
H+ ions from dil. acids (including acetic acid) displace nitrous acid from its salts. The acid spontaneously decomposes to colorless monoxide NO & brownish NO2 gases. The brown fumes intensify when getting in contact the atmosphere due to combination of NO with O2 of air. b- Action of conc. H2SO4 Nitrate: Nitric acid is formed and some of it decomposed with evolution of brown fumes of NO2 with characteristic odor. NO3-+ H+ HNO3 4HNO3 4NO2 + O2 + 2H2OWhen copper turnings are added, and the mixture heated to boiling, thebrown fumes of NO2 are increased due to the reduction of HNO3 by Cuo metalwhich is oxidized to Cu2+ ions, which imparts a blue color to the solution. 2NO3-+ 4H++ Cuo 2NO2 + Cu2++ 2H2O Nitrite: The reaction is the same as with dil HCI, but it takes place with considerable violence. On adding Cuo metal, the same occurs as with nitrates.
2- Wet Reactions a) Reaction with Ag2SO4 solution:- Nitrate: No ppt. Nitrite: White crystalline ppt. of AgNO2 form concentrated solutions. NO2-+ Ag+ AgNO2b) Reaction with BaCI2 solution:No precipitate is formed with either NO3- nor NO2-c) Reaction with KI solution:Acidify the test solution (3 ml) dil. H2SO4, then add Kl solution and fewdrops starch solution.Nitrate: No reaction.Nitrite: I2 is liberated imparting blue color to the starch. 2NO2-+ 2I-+ 4H+ 2NO + I2+ 2H2O
d) Reaction with Fe SO4 solution. (Brown Ring Test): Acidify the test solution (5ml) with dil. H2SO4, add (1ml) freshly prepared FeSO4 solution. Nitrate: No visible change in case of using only dil. H2SO4, but on adding conc. H2SO4 cautiously down the sides of the test tube, a brown ring is formed at the interface. Nitrite: Brown colour in the whole solution if FeSO4 solution is not cautiously added or a brown ring at the junction of the two liquids, if cautiously added.FeSO4 reduces nitrate or nitrite ions to nitrogen monoxide, NO; nitrate ionis not reduced except in solutions containing a high H+ ion concentration,that is conc. H2SO4. The excess Fe2+ ions then combines with the NOproduced to form the unstable brownish-black complex ion [Fe (NO)]2+,readily decomposed by heat. 3Fe2++ NO3-+ 4H+ 3Fe3++ NO + 2H2O Fe2++ NO2-+ 2H+ Fe3++ NO + H2O Fe2++ NO [Fe (NO)]2+
This test differentiates NO3- ion from NO2- ion, since the latter givesthe brown ring in presence of dil. H2SO4 or even acetic acid, while NO3-ion dose not form the ring except in presence of conc. H2SO4.(NO2-, I- and Br- ions will interfere) III. Special Tests 1- For Nitrate: Ammonia test If solution of NO3- is boiled with Zno or Alo metals and NaOH solution, NH3 will be evolved which can identified by its odor or with red litmus paper (nitrites interfere). NO3-+ 4Zno+ 7 OH- 3NH3 + 4[ZnO2]2-+ 2H2O zincate ions 3NO3-+ 8Alo+ 5OH-+ 2H2O 3NH3 + 8 [AlO2]- In acidic solution ( CH3COOH), nitrate can be reduced with Zno to nitrite.
2- For Nitrite: a) Permanganate test When a dilute potassium permanganate solution is added to an acid solution of nitrite, its pink color is bleached. In this test, the permanganate is reduced by the nitrite into colorless manganous Salt and the nitrite is oxidized into nitrate. 2MnO4-+ 5NO2-+ 6H+ 2Mn2++ 5NO3-+ 3H2O Pink colorless b) Urea test: When a solution of a nitrite is treated with urea and the mixtureacidified with dilute HCl, the nitrite is decomposed, and N2 and CO2 areevolved. CO (NH2)2+ 2HNO2 2N2 + CO2 +3H2O c) Ammonium Chloride test: By boiling a solution of a nitrite with excess of the solid reagent, N2 is evoled and the nitrite is completely destroyed. NO2-+ NH+4 N2 + 2H2O
d) Thiourea test: When a dil. acetic acid solution of a nitrite is treated with a little thiourea, N2 is evolved and thiocyanic acid is produced. The latter may be identified by the red color produced with dil. HCl and FeCl3 solution. CS (NH2)2+ HNO2 N2+ H++ CNS-+ 2H2O N.B: Thiocyanates and iodides interfere, and if present must be removed either with Ag2SO4 (solid) or dil AgNO3 solution. IV. Analysis of Mixtures 1- Mixture of Nitrate and Nitrite :Nitrite can be tested for in presence of nitrate (by treatment with dil HCI,KI, KMnO4, FeSO4 in dil. H2SO4); and by the special tests for nitrite.Nitrate cannot be tested for in presence of nitrite, since nitrite gives allthe reactions of nitrate (conc. H2SO4, brown-ring test and ammonia test).Therefore nitrite be removed before testing for nitrate by:- 1- Decomposition of NO2- through its brown complex with FeSO4 formed in dil. H2SO4 or acetic acid by heat and shaking. [Fe (NO)]2+ heat NO + Fe2+
2- Decomposition of NO2- through its reduction to nitrogen by boiling with NH4CI or warming with urea and few drops of dilute H2SO4 or warming with little sulphamic acid. HO.SO2. NH2+ HNO2 N2 + H2SO4+ H2O 2- Mixture of Nitrate and Bromide or / and lodide: Br- and I- can be detected in presence of NO3- by chlorine water test. NO3- can be detected in presence of Br- and I- by the ammonia test. On the other, the brown ring test for nitrates cannot be applied in the presence of Br- and I-, since the liberation of free halogen with conc. H2SO4 will obscure the brown ring due to NO3-Therefore Br- and I- must be first removed by either: 1- Addition of saturated solution of silver sulphate, where AgBr and AgI are precipitated and then filtered off, the excess Ag+ is precipitated with Na2CO3 OR 2- Adding potassium persulphate and dil. H2SO4 and warming to about 80oC. The halogen is removed by boiling or extraction with organic solvent. S2O82-+ 2Br- Br2+ 2SO42- S2O82-+ 2I- I2 + 2SO42- The Halide-free solution is tested for NO3- by the brown ring test