Gravimetric methods of analysis [compatibility mode]
Dr. Jehad M DiabFaculty of pharmacyDamascus UniversityGravimetric Analysisاﻟوزﻧﻲ اﻟﺗﺣﻠﯾلPharma.analytical chemistry II
Gravimetric AnalysisGravimetry is the Quantitative measurement of theanalyte by weighing a pure solid form of theprecipitate.Gravimetric Analysis is one of the most accurateand precise methods of macro-quantitative analysis.Often required for high precisionObtaining pure solids from solutions containingan unknown amount of a metal ion is done byprecipitation. Dr.Jehad Diab
Electrogravimetric methods. analyte isprecipitated on cathode as metal or on anodeas metal oxide Dr.Jehad Diabاﻟﺗرﺳﯾب طراﺋقاﻟﺗطﺎﯾر طراﺋقاﻟﻛﮭرﺑﺎﺋﻲ اﻟوزﻧﻲ اﻟﺗﺣﻠﯾل طراﺋق
and washand calculateDry and igniteprecipitatingPrecipitation steps in gravimetryDr.Jehad Diabاﻟوزﻧﻲ اﻟﺗﺣﻠﯾل ﻣراﺣل
Mechanisms of precipitation1.Saturation: the amount of salt dissolved ismaximum under certain condition of pressure andtemperature2.Supersaturation: the amount of salt is larger than atsaturation step, turbid solution3.nucleation: when a small number of ions ,atoms,molecules initially unite either spontaneous orinduced resulting in very small aggregates of asolid during precipitation.4.Particle growth: the three dimensional growth ofparticle nucleus converted into a larger crystalDr.Jehad Diabاﻟﺗرﺳﯾب آﻟﯾﺎتاﻹﺷﺑﺎعاﻹﺷﺑﺎع ﻓوقاﻟﺗﻧوياﻟﺟزﯾﺊ اﻟﻧﻣو
Particle size and filterabilityVon Weimarn described an equation that can controlthe particle size of ppt; named Von Weimarn ratio• RSS= Q-S/S (Rss = Relative supersaturation, Q isconcentration of the solute at any instant. S is itsequilibrium solubility)Dr.Jehad Diab
Proportional to RSSInversely Proportional to RSSDr.Jehad Diab
pH control of precipitationCa2+ + C2O42- CaC2O4 (s)H2C2O4 2 H+ + C2O42-Feeder Reaction:Dr.Jehad DiabWeak basic medium
adsorbedadsorbedThe result is coagulation of colloidal precipitateof adsorbedDr.Jehad DiabColloids particles must collide with one another tocoalesce. However ,the negatively charged ionicatmospheres of particles repel one another. the particles,therefore ,must have enough kinetics energy to overcomeelectrostatic repulsion before the can coalesce. Heatpromotes coalescence by increasing the kinetic energy .And coagulation of colloidal precipitate is resulted in.
And coagulation of colloidal precipitate is resulted in.Dr.Jehad Diabincreasing electrolyte concentration (HNO3 for AgCl) decreasesthe volume of the ionic atmosphere and allows particles tocome closer together before electrostatic repulsion becomesignificant. For this reason .most gravimetric precipitations aredone in the presence of an electrolyte.
High Electrolyte Concentrationto Aid PrecipitationExcess charge on colloid creates ionicatmosphere around particleD.C. Harris, Quantitative Chemical Analysis, 6th Ed., p686
Dr.Jehad DiabWashing a colloid with water to remove excess counterion or trapped impurities can result in peptization.اﻟﺗﺷﺑﻐر او اﻟﺑﺑﺗزة ﻣﻌﺎﻟﺟﺔ
(inclusion)Sources of Coprecipitation(trapped impurities)adsorption(interferences)Post precipitation اﻟﺗﺎﻟﻲ :اﻟﺗرﺳﯾب Sometimes a precipitate standingin contact with the mother liquor becomes contaminated by theprecipitation of an impurity on top of the desired precipitate.Dr.Jehad Diabاﻟﻣﺷﺗرك اﻟﺗرﺳﯾباﻟﺳطﺣﻲ اﻹﻣﺗزازاﻹﺣﺗﺑﺎساﻹﺣﺗواء
adsorbedDr.Jehad Diabاﻟﺳطﺣﻲ اﻹﻣﺗزاز()lsilver and nitrate ions, soAgNO3 is coprecipitated with the AgCl.
Homogeneous precipitation. The best precipitates are obtained withhomogeneous precipitation, in which theprecipitating reagent is gradually generated in thesample solution, through a slow chemical reaction. In this way there is never a large excess ofreagent, so that nucleation is slow, giving allexisting nuclei plenty of time to grow.The precipitating reagent is usually formed bythe slow hydrolysis of an organic compound atelevated temperature, but even synthesis ispossible, as in the generation of dimethylglyoximeDr.Jehad Diabاﻟﻣﺗﺟﺎﻧس اﻟﺗرﺳﯾب
Methods have been worked out to generate avariety of precipitants, such as hydroxide, sulfide,sulfate, phosphate, oxalate, 8-hydroxyquinoline,and chromate.Both the temperature and the pH must becontrolled, because both usually affect the rates ofthe hydrolysis reaction. Moreover, as we saw in thepH is often crucial in the formation of theprecipitate.Homogeneous precipitationDr.Jehad DiabSolid formed by homogeneous precipitation are generally purer and moreeasily filtered than precipitate generated by direct addition of a reagent tothe analyte solution.
3Preparation of NH4OH as precipitant by hydrolysis of ureaAl(OH)3,Fe(OH)3,Sn(OH)4,Bi(OH)3,Th(OH)4Dr.Jehad Diab∆
Problems: Calculate the mass of analyte interm of grams to each gram of ppt for thefollowing analytes:Analyte ppt P(31g) → Ag3po4 (711g ) K2HPO4(136g) → Ag3po4 (711g ) Bi2S3 (514 g) → BaSO4 (233 g)g analyte = g ppt × gfg p =g Ag3PO4 ×GF= 1 ×31 / 711=0.044 g p/1g pptDr.Jehad Diab
Problem : Determine the gravimetricfactors in term of symbols for thedetermination of:AnalyteAnalyte pptppt G.F.G.F.CaCaOO CaCaCOCO33 CaOCaO/CaCO/CaCO33FeFeSS BaBaSSOO44 FeSFeS/BaSO/BaSO44UUOO22(NO(NO33))22 UU33OO88 33UOUO22(NO(NO33))22/U/U33OO88CrCr22OO33 AgAg22CrCrOO44 CrCr22OO33//22AgAg22CrOCrO44Dr.Jehad Diab
Problem : Determine the gravimetric factors interm of symbols for the determination of:(a) Aluminum as its hydroxyquinolate,AI(C9H60N)3 (Al AI(C9H60N)3)(b) Phosphorus as phosphomolybdic anhydride,P2Mo24078 (P P2Mo24078 )(c) Potassium as its chloroplatinateK → K2PtCl6(d) Sulfur as barium sulfate,( S → BaS04)(e) Nickel as nickel dimethylglyoxime,( Ni → Ni(C4H702N2)2Dr.Jehad Diab
Problem : Determine the gravimetric factors for thedetermination of:1. In -> In2O3 GF2. HgO -> Hg5(IO6)2 GF3. K3PO4 -> K2PtCl6 GF4. K3PO4 -> Mg2P2O7 GF5. AgIO3 -> Ba(IO3)2 GF6. AgNO3 -> AgIO3 GF7. (NH4)2SO4 -> BaSO4 GF8. Mn3O4 -> MnO2 GF9. Cu2HgI4 -> Cu GFCr -> Cr203 GFDr.Jehad Diab10.
Example: in an organic sample (0.352g) phosphorouswas dissolved and converted to Mg2P2O7 precipitate(0.223 g). Calculate the percentage %P in the originalsample.Dr.Jehad Diab2P(gfw 31 g) → Mg2P2O7 (gfw 222.6)GF=2× 31 /222.6 =0.2783% analyte =(g ppt × GF / g sample) × 100%P =( 0.223 × 0.2783 / 0.352) ×100 = 17.1 %
Example: When an sample of impure potassiumchloride (0.4500g) was dissolved in water andtreated with an excess of silver nitrate, 0.8402 g ofsilver chloride was precipitated. Calculate thepercentage KCl in the original sample.Answer:KCl ( gfw =74.50) => AgCl ( gfw =143.50)GF= 74.50/143.50=0.519%KCl = (mass of AgCl *GF/mass of KCl) *100==( 0.8402*0.519/0.4500 )*100 = 96.90%Dr.Jehad Diab
Table: some of inorganic precipitating agentsprecipitants analyte and (formed precipitate, weighed)* NH3(aq) Be (BeO),Al (Al2O3),Cr(Cr2O3), Fe(Fe2O3)(NH4OH) Sn (SnO2),Zr (ZrO2) H2S Zn (ZnS→ZnO),As (As2S3 →As2O3 Or As2O5),Bi (Bi2S3 (NH4)2HPO4 Mg(Mg2P2o7),Zn(Zn2P2O7),Cd(Cd2P2O7) H2SO4 Sr,Cd,Pb,Ba (all as sulphate) H2PtCl6 K (K2PtCl6) HCl Ag (AgCl), Hg(Hg2Cl2)AgNO3 C l(AgCl), Br (AgBr), I (AgI)Dr.Jehad Diab
Table: some of inorganic precipitating agentsprecipitants analyte ( formed precipitate, weighed)* BaCl2 SO42- (BaSO4)( NH4)2S Hg (HgS) HNO3 Sn4+ (SnO2) H5IO6 Hg (Hg5(IO6)2 NaCl, Pb(NO3)2 F (PbClF) MgCl2,NH4Cl PO43- (Mg2P2O7)Ca2+ H2C2O4 CaCO3 or CaODr.Jehad Diab-------------------------------------------------------------------------------------------------------* After drying or ignitionFe3+ + OH-→Fe(OH)3 → Fe2O3weighed formppt
RevisionDiminishing importance of gravimetry, as a resultof the development of much faster (though usuallyless precise) instrumental methods.Consequently, gravimetry is used only when itssuperior precision is really needed. In that case,great care must be exercised to avoidcoprecipitation of other sample components,occlusion of solvent in the precipitate, andadsorption of excess reagent, otherwise the extraeffort is negated by an impure or otherwise poorlydefined precipitateDr.Jehad Diab
We have emphasized the factors that facilitate theformation of coarse, pure, easily filterableprecipitates, explained why precipitates arepreferably generated homogeneously, and why theyare usually washed with electrolyte solutions ratherthan with water. Ammonium salts are often used for this purpose,because they will readily volatilize upon subsequentheating of the precipitate.Dr.Jehad Diab