Gravimetric analysis

1. Gravimetric Analysis
Dr. Vikas A. Thakur
Mr. Vishal A. Naik
Mahatma Phule Arts, Science & Commerce
College, Panvel

2.  Gravimetric analysis is one of the oldest & important technique
for Quantitative estimation in chemical analysis.
 This technique involves determination of a constituent by
weight.
 Gravimetric analysis is one of the most accurate analytical
methods available for quantitative estimation.
 In gravimetric analysis the constituent to be measured is
converted into other constituent of known composition which
is stable, can easily purified and weighted.
 E.g. for the determination of iron in the solution, iron is
converted to ferric hydroxide Fe(OH)2, dried & weighed as
ferric oxide.

3. Two Major types
1. Precipitation method
2. Volatilization method

4.  In this method, the constituent is converted into sparingly soluble
precipitate.
 The precipitate is filtered, washed free of impurities, converted into a
product of known composition by heating & the weighed.
 E.g. determination of calcium by oxalate method
a) Solution containing calcium ions is treated with oxalic acid
(H2C2O4) and then ammonia to precipitate all the calcium as
calcium oxalate
b) When conversion of all calcium oxalate to calcium oxide takes
place, precipitate is filtered, dried and weighed using crucible.
c) From the weight of residue i.e. calcium oxide amount of calcium
in the given sample can be calculated.
CaC2O4 + 2H+Ca2+
(aq) + H2C2O4 (aq)
CaC2O4 (s) CaO (s) + CO2

5.  In this method, the constituents are decomposed due to
volatilization at a suitable temperature.
 the residual product is then collected & weighed.
 The mass of the product is determined indirectly from the loss
in the weight of the sample
 E.g. in the determination of sodium bicarbonate NaHCO3, from
a mixture of NaHCO3 & Na2CO3, the weight sample is heated
strongly to convert NaHCO3 into stable Na2CO3. By the loss in
weight, the amount of sodium bicarbonate can be obtained.

6. Recently four new types of gravimetric analysis are considered as fundamental
types of gravimetry.
1. Physical gravimetry -
 Commonly used in environmental engineering
 Involves physical separation and classification of matter in environmental
samples based on volatility and particle size
2. Thermogravimetry -
 In this type sample is heated & with change in temperature changes in mass of the
sample are recorded.
 Example- volatile sample analysis
3. Precipitation gravimetry -
 It involves chemical precipitation of the constituents
 It is most commonly used type of gravimetry, having wide application in the
environmental field.
4. Electrodeposition gravimetry -
 It involves the electrochemical reduction and deposition of the metal ions at the
cathode

7. Precipitation
Gravimetric
Analysis

8. 1. Preparation of the test solution
2. Precipitation
3. Digestion of precipitate
4. Filtration of precipitate
5. Washing of precipitate
6. Drying and ignition of precipitate
7. Weighing
8. calculation
The precipitates are mainly three types
1. Crystalline precipitates - these precipitates are relatively pure and easily
filterable.
2. Curdy precipitates - these precipitate are of colloidal particle size but are
filterable.
3. Gelatinous precipitates – these precipitates are flocculates colloids. Particle size
is to smaller and difficult to filter.

9.  The completeness of a precipitate depends on equilibrium solubility of a
precipitate
 Precipitation is an ionic reaction of combining positive ions of one substance
with the negative ions of another substance to form sparingly soluble substance.
 The solubility of any substance is define as the amount of the substance
dissolved in its saturated solution in a given solvent at given temperature
(g/dm3 or mol/dm3)
 Solubility product is defined as the product of the concentration or activities of
the constituent ions raised to the appropriate power
 Both solubility and solubility products are temperature dependent.
 When ionic product is grater than solubility product, precipitation takes place.

10. 1. Common ion effect :
 The solubility of a salt is less in a solution containing ions in common
with that of the salt in water provided the equilibrium is not disturbed by
the formation of complex ions this is known as common ion effect.
 The solubility of precipitate is decreased by the presence of other ions in
the solution common with the precipitate.
 E.g. during the precipitation of Ag+ ions from an aqueous solution
containing excess of Cl- ions, even small quantity of Ag+ ions in the
solution can get readily precipitated as Cl-ions are present in excess.
 Solubility product, Ksp of AgCl is constant, Ksp = [Ag+] [Cl-]
 In some cases excess of precipitant decreases the solubility of precipitate
but in some cases it increases.
 In some gravimetric estimation, organic reagents are used as precipitant
which are prepared in non-polar organic solvents. E.g. dimethyl
glyoximate (DMG) for precipitation of nickel present in solution.

11. 2. Diverse Ion Effect or Salt Effect
 Solubility of a sparingly soluble salt increases in presence of
foreign ion, that is ions which are not common to those of the salt.
This effect is called diverse ion effect, salt effect or activity effect.
 E.g. Solubility of BaSO4 is increased by 70% in 0.01M solution of
potassium nitrate than in water.
 Potassium nitrate is strong electrolyte and highly soluble in water
and its solution contains a high conc. of ions. This results increase
of ionic strength of the solution.
 The activity ‘a’ is related to the molar concentration ‘c’ by the
activity coefficient ‘γ’ as
a = c * γ
e.g. solubility product Ksp for AgCl is written as
Ksp = a Ag+ * a Cl- = [cAg+ * γ Ag+] x [cCl- * γ Cl-]
II. Factor affecting Solubility of precipitate

12. 3. Effect of PH
 Solubility of substance depends on the pH of the solution from which the
salt is precipitate.
 Solubility of the precipitate of salts of strong acids such as BaSO4, AgCl
is unaffected, but that of salts of weak acids is affected considerably. E.g.
precipitate of oxalates, carbonates, phosphates dissolves in the presence
of excess of acid
4. Effect of temperature
 The dissolution of a solute in a solvent is an endothermic process.
Therefore, the solubility and solubility product increases with increase in
temperature.
 E.g. solubility of silver chloride in water increases by about 20 times when
there is rise in temperature from 298 k to 373 k
 The process of precipitation is carried out in hot solution. It helps to give a
precipitate of large, uniform size and easy to filtrate.

13. 5. Effect of nature of solvent
 In general, like dissolves like, Ionic salts dissolves more readily in polar
solvents like water than non-polar(organic) solvents like alcohol, benzene
and CCl4.etc.
 Inorganic compounds are held together by electrostatic force of attraction.
The energy required to separate them is called Lattice Energy.
 Ionic solutes dissolve in polar solvent like water due to high dielectric
constant.
 Non-polar solvents have either zero or low dielectric constant, Hence lattice
energy is not overcome.
 Inorganic solids are thus less soluble in non-polar solvents like organic
solvents.
 e,g.CaSO4 which is moderately soluble in water can be precipitated by a
mixture of ethanol and water as a solvent

14.  The purity and filterability depend on particle size of ppt.
 The particle size formed by precipitation varied from
colloidal suspension 1 nm to 103 nm in diameter to
crystalline suspensions of the order of 106 nm or more.
 During precipitation process, initially number of small
particles called nuclei are formed. Subsequent
precipitation on these nuclei takes place ,which grow in
size and settle down.
 Particle size is related to two process:
 1.Nucleation :formation of nuclei
 2.Growth of nuclei i.e. particle growth or crystal growth.

15.  In formation of precipitate by two ions, ions come together to form ion
pairs. where they are attracted by electrostatic forces.
 These ion pairs further grow by addition of ions or ion pairs to form ion
clusters.
 Ion pair and ion cluster formation takes place even when ionic concentration
does not exceed the solubility product.
 The ion clusters can be separated easily into ion pairs till they attain some
minimum size called as critical size.
 Once critical size attained, ion cluster becomes the nucleus and continues to
grow to form crystals.
 Below critical size ,separation of ion clusters into ion pairs is faster than
their growth.
 Duration of time required to form nuclei, sufficient to grow further is called
as Induction Period.
 Induction period varies with precipitate and conditions of precipitation.
e,g. AgCl < BaSO4 < MgNH4PO4.

16. Nucleation Processes are of two types.
A) Heterogeneous Nucleation Processes:
 In this nuclei originate by clustering of ions around very
fine suspended matter like dust particles.
 This nucleation is commonly found in solutions of low
degree of supersaturation.
B) Homogeneous Nucleation Processes:
 In this nuclei originate by clustering of ions by
themselves and arranging in a definite pattern.
 This nucleation is commonly found in solutions of high
degree of supersaturation.

17.  The particle size is influenced by many experimental variables.
 Precipitate solubility, temp., reactant concentration and the rate
at which reactants are mixed.
 The net effect of these variables are related with relative
supersaturation.
 The speed with which precipitation takes place is main factor
affecting particle size.
 The effect of rate of precipitation on particle size has been
studied by Von Weimann.
 According to him faster the precipitation smaller is particle
size.
 He found supersaturation plays an important part in
determining particle size of precipitate.

18.  Relative supersaturation = Q – S/S
 Where Q-S =Supersaturation when precipitation starts.
 Q = Molar Conc. of mixed reagents produced in soln.
 S = Molar solubility of a ppt. at equilibrium.
 Experimentally found that particle size varies inversely with relative
supersaturation.
 When Q – S/S is large, large number of nuclei are formed. Particle size
tends to be colloidal.
 When Q – S/S is small, crystalline solid is formed.
 The conditions for precipitation should be adjusted so that minimum value
of Q – S/S is maintained.
 This can be achieved by lowering the value of Q or increasing value of S.
 The Q value can be minimised by adding dilute solutions of substance
slowly with constant stirring of sub. to be precipitated
 The value of S can be increased by precipitation in hot solutions and by
adjusting proper pH of the solution during addition of precipitating agent.

19.  The upper dotted line represents
supersolubility.
 The lower line represents regular
solubiilty.
 The area above dotted line represents
labile region.
 Nucleation takes place immediately
when the concentrations are in labile
region.
 The area between supersolubility and
solubility is known as metastable
region.
 In this region no nulei are formed.
 But particles will grow till
concentration equals with saturated
soln.

20.  When the conc. of the substance at temp. T is increased slowly
and uniformly till it reaches the point P on upper curve,
precipitation takes place.
 At this point, particles will form and grow till concentration has
been lowered to a value at point S on the lower curve.
 But if the concentration again exceed the value at point P small
particles will be formed.
 The particle size depends on metastable region i,e. difference
between supersolubility and solubility.
 This is represented in figure.

21.  Greater the metastable
region[A-C],larger is the
particle size as in BaSO4.
 Smaller the metastable
region[A-B],smaller is the
particle size as in AgCl.
 When the metastable region is
very small[D-E],a gelatinous or
flocculent precipitate forms as
in ferric hydroxide.
 Thus factor affecting solubility
also affects particle size.

22. Thus the precipitation process occurs in three stages.
i) Collision of ions in supersaturated solution to yield primary nuclei.
ii) Aggregation of ions around primary nuclei forming colloidal
particles.
iii)Growth of colloidal particles to form particles which settle as
precipitate.
 The precipitate process can be represented as ,
 Ions ( 10 -1 nm) ------
 Nucleation (10 -1 to 1nm)------
 colloidal particles ( 1nm to 10 3nm)-----
 Precipitate (>10 3nm)---------
 Thus nucleation clusters pass through colloidal particle size before
precipitation.

23.  Colloidal state has particle size ranging from 1nm to 10 3 nm.
 It possess characteristic properties of colloidal state.
 i) Brownian movement.
 ii) Tyndal effect.
 iii) As they have smaller size they acquired large surface area. Ratio
of surface area to volume is large.
 iv) They can be separated by parchment membrane. Process is
known as Dialysis.
 v) They are electrically charged particles.
 Colloidal particles are smaller in size and hence surface attraction is
more as the ratio of area to volume is more.
 In colloidal solution there is greater attraction of oppositely charged
ions of the solute on surface of small particles. This is known as
Adsorption.

24.  Adsorption of ions common to crystal lattice is always preferred.
 Colloidal particles acquire similar charge on each particle and are
stable because the particles are either all positively or negatively
charged.
 Hence they repel each other and resist precipitation.
 The colloidal particles are charged.
 The charge on colloidal particles formed in gravimetric analysis is
determined by charge on lattice ion that is in excess.
 e,g.on addition of small amount of NaCl to AgNO3,small particles of
AgCl are formed.
 The solution contains small amount of Na +,H3O+,OH- and large
amount of Ag+ and NO3
- ions.
 The particles of AgCl preferentially adsorb Ag+ ions which are
common to crystal lattice and forms Primary adsorbed layer.

25.  This primary adsorbed layer then
attracts a closely held layer of
oppositely charged NO3
- and OH- to
form Secondary adsorbed layer.
 These ions are also called as counter
ions.
 This results in formation of silver
chloride precipitate in colloidal
state.(fig.)
 If a small amount of silver nitrate is
added to NaCl solution, it results in
formation of small silver chloride
particles.
 The solution contains small amount of
NO3 ,H3O+,OH- and large amount of
Cl- and Na+ ions.

26.  The particles of AgCl preferentially
adsorb Cl- ions which is common to
crystal lattice and form primary layer.
 This primary layer then attracts
oppositely charged Na+ ions and form
secondary layer.(fig.)
 Thus in colloidal particle, there is an
electrical double layer which stabilises
colloidal particles.
 The colloidal particles repel one another
due to charge acquisition.

27.  The destruction of electrical double layer makes the colloid unstable and
colloidal particles flocculate.
 The particles lose their colloidal nature and settle down or coagulate.
 To bring about coagulation or flocculation, ions having charge opposite
to those initially adsorbed are required.
 The minimum amount of an electrolyte required to cause flocculation is
called flocculation value or coagulation value(FV).It is expressed in
milllimoles per litre.
 e,g. Colloidal suspension of AgCl having adsorbed Ag+ ions as primary
layer coagulated by addition of solutions of NaCl,Na2SO4 or Na3 PO4.
 This coagulation value decreases with increase in the charge of the salt
bringing about coagulation process.
 In above example Na3PO4 is most effective and NaCl is least effective
in bringing about coagulation.

28.  During coagulation of a colloid, an electrolyte of oppositely charged ions is
added to bring about precipitation.
 Precipitate therefore contaminated by electrolyte ions due to surface
adsorption.
 Hence it should be washed.
 Washing of precipitate may remove some of adsorbed electrolyte.
 Due to this conc. of adsorbed electrolyte may fall below flocculation value
and creates problem.
 In such cases, the precipitate may pass back into colloidal state and pass
through the filter.
 This phenomenon is called as peptisation and must be avoided.
 Thus peptisation refers to the process by which a coagulated colloid reverts
back to its original colloidal state.
 This problem is resolved by washing the precipitate with a solution
containing an electrolyte which volatilises during drying and ignition.
 AgCl is readily washed with dilute Nitric acid to avoid peptisation.

29.  Once the precipitate is formed, it has to be treated as it can be contaminated.
 A precipitate is contaminated due to following reasons.
i) Simultaneous Precipitation :
 It is a contamination of a precipitate by the impurities normally insoluble in solution
media under the prevailing conditions of analysis.
 When silver ions are added to the solution containing large conc.of Cl- ions and
small conc.of Br- ions, Siver chloride will be get precipitated.
 Silver ions are contaminated by both Silver Chlroide and bromide because both are
insoluble in solution media.
 Thus prior knowledge of ions present in solution is absolutely necessary to avoid
simultaneous precipitation.
ii) Co-Precipitation :
 It is a contamination of a precipitate by the impurities normally soluble in solution
media under the prevailing conditions of analysis.

30.  Addition of barium chloride to a solution of sodium sulphate forms a precipitate of
barium sulphate contaminated with sodium sulphate though it is soluble in water.
 The contamination of a precipitate by substance which are normally soluble in
mother liquor is termed as co-precipitation.
 Co-precipitation occurs due to
i) Surface phenomenon(adsorption):
 Contamination due to surface adsorption is more prominent for gelatineous
and colloidal precipitates.
 It is less prominent for crystalline precipitates.
 Precipitates with ionic lattices follow Paneth-Fajans-Hahn adsorption rule
which states that “The ion that is most strongly adsorbed by an ionic
substance is that ion which forms the least soluble salts.’’
 e,g. For sparingly soluble sulphates, calcium ions are adsorbed
preferentially over magnesium ions because calcium sulphate is less soluble
than magnesium sulphate.

31. ii) Occlusion and mechanical entrapment :
 This type of co-precipitation occurs during building up of the
precipitate from primary particles due to occlusion.
 Occlusion is a type of co-precipitation in which an impurity is
trapped within a pocket formed with process during rapid crystal
growth.
 Occlusion takes place when the impurity has similar size and charge
and gets substituted in the crystal lattice of the precipitate.
 e,g. Pb2+ions in BaSO4 and SO4
2- ions in Barium chromate.
 Sometimes substitution of ions of similar size but different charges
takes place if precipitate and impurity have similar chemical
formulae.
 Such occluded contamination is known as mixed crystals.
 E,g.BaSO4 and KMNO4.

32.  It is an precipitation of an impurity which occurs on the surface
of the first precipitate after appreciable time.
 The primary precipitate becomes the nucleus for the post
precipitation of the impurities from their supersaturated
solution.
 e,g. In the precipitation of calcium as oxalate in the presence of
magnesium, calcium oxalate forms primary precipitate.
 The solution becomes supersaturated with respect to
magnesium ions which now precipitate on precipitate of
calcium oxalate.
 To remove contamination several steps has been carried out
like i) Digestion ii) Filtration and iii) washing.

33.  When precipitation is carried out from a hot dilute solution, the primary
precipitate obtained is in crystalline form having perfect crystal lattice
structure.
 But when precipitation is carried out from a concentrated solutions, the
size of primary precipitate obtained is varies and have imperfect crystal
lattice structure.
 Crystalline precipitates are more easily filterable and purer than the
coagulated colloids.
 To get precipitate of uniform and larger size, the precipitate is subjected to
digestion.
 During digestion,the smaller particles dissolve and solution becomes
saturated with larger particles.
 The dissolved particles get redeposited on larger particles.
 This increases average particle size.
This process is known as digestion or ageing or Ostwald repairing.

34.  The larger particles grow at the expense of smaller ones.
 The co-precipitated impurities adsorbed on these particles pass
into solution.
 The surface area decreases due to larger particle size.
 The process of digestion helps to give
i) Particles of uniform size.
ii) Get the precipitate of perfect crystal structure.
iii) Reduce contamination or increase the purity by removing
the co precipitated or occluded impurities.
iv) It ensures complete precipitation.
v) It brings about coagulation of precipitates.

35.  This operation separates precipitate from mother liquor.
 Main objective is to obtain the precipitate free from solution.
 Filtration is carried out using filter papers(ordinary or whatmann) and
sintered glass crucibles.
 When filter papers are used they must have a very small ash content.
 This is achieved during manufacture process.
 The ash value of 11cm circular whatmann filter paper is nearly = 0.00001g.
 These filter papers are made up of various degree of porosity.
 Three textures of filter papers are generally made, for fine particles, second
for average precipitate of medium sized particles and third for gelatinous
and coarse particles.
 Precipitate collected on filter paper is further dried and ignited to a constant
weight.
 When precipitate formed is stable, filtration is carried out by counterpoise
method or by using sintered glass crucibles G3 or G4.
 After filtration crucible with the precipitate is dried to a constant weight in
electric oven.

36.  Most of precipitates obtained are contaminated in the presence of one or
more soluble compounds. As these are not volatile at the temp.at which the
precipitate is dried, it is necessary to wash the precipitate.
The characteristics of ideal washing solution are a
 i) It should have no action on the precipitate but dissolve foreign substances
only.
 ii) It should form no volatile or insoluble product with the precipitate.
 iii) It should be easily volatile at the temperature of drying of the precipitate.
 iv) It shold contain no substance which will interfere with subsequent
determinations in the filtrate
 Generally pure water is not used for washing unless it is certain that it will
not dissolve ppt or does not undergo peptisation.
 If the precipitate is soluble in water, a common ion is usually added to wash
the solution to replace washed ions.
 Calcium oxalate is washed with dilute ammonium oxalate solution.

37.  The wash solution is mostly employed in hot due to greater solubility of
foreign substance and increased speed of filtration.
 Besides choice of suitable wash liquid, the mode of washing is also
important.
 The precipitate is washed using a jet of wash liquid thoroughly on the filter
paper.
 Subsequently the edges of filter paper should be washed.
 A large number of small washing using small volume of wash solution is
better and more efficient to remove impurities than a small number of large
washings using large volume of wash solution.
 A mathematical expression can be used as
Xn = Xo [ u/u + v] n
Where Xn = Concentration of impurity before washings.
Xo = Concentration of impurity after n washings.
u = cm3 of wash solution retained by the precipitate.
v = cm3 of wash liquid used for each washings.
n = Number of washings.

38.  From above expression, it follows that
 i) ‘u’ should me minimum and allow the liquid to drain as far as possible.
 Ii) Use a relatively small volume of liquid and increase the number of washings.
 Hence more number of washings with small volumes of wash solution is
appropriate.
Drying and Ignition of the precipitate.
After precipitation, filtration and washing the precipitate is either dried or it is dried
and subsequently ignited to give a compound of known composition.
 The term drying is used when the temperature is about 500K.
 The term ignition is used when the temperature range is 500K to 1500K.
 In most of cases precipitate is free of contaminants except water and hence can be
dried in an oven at low temperature for about 373 K to 425 K.
 Precipitate of AgCl is dried at about 393K when water and the antipeptization
electrolyte are totally removed.
 Such a situation is observed only for few precipitates in case of organic precipitants.

39.  In most cases precipitate is ignited at elevated temperature to a constant
known composition due to following reasons.
i. To convert precipitate to some other form having a constant known
composition.
ii. To volatilise the antipeptization agent at higher temperature as it cannot
be volatilised at 373K to 425K.
iii. To remove strongly adsorbing or occluded water which is found in
gelatinous precipitate and hydrated oxides of aluminium, iron and silica.
 Thus ignition of precipitate is carried out to convert it into new compound
of definite known composition.
 It is necessary to know the temperature range at which such a form is
produced.
 The exact temp. range is carried out by use of thermobalance.
 A thermobalance helps to weigh precipitate accurately as it passes through
various stages of decomposition.

40.  The balance is sensitive to 0.2mg and
temp. between 274K to 1373K.
 The plot of precipitate weight against
temp. is known as pyrolysis curve or
thermogravimetric curve(tg) curve.
 The TG curve shows that the final
weighed form depends on ignition
temp.
 TG curve for calcium oxalate
monohydrate when heated pass
through CaC2O4 CaCO3 CaO
at different temp.
 During ignition precipitates are ignited
in porcelain or silica crucible with
filter paper or apart from filter paper.

41. A) Incineration of the filter paper in presence of a precipitate :
 A silica crucible ignited cooled to a constant weight.
 Filter paper with the precipitate is folded carefully to enclose precipitate
laced in the crucible supported on pipe clay triangle.
 It is ignited first on low flame to carbonise paper slowly.
 When filter paper is completely carbonised and there are no vapours the
crucible is heated on a strong flame to a constant weight
B) Incineration of the filter paper apart from a precipitate :
 Method is employed when precipitates are reduced by burning of filter
paper.e,g.BaSO4,PbSO4,Cu2O etc.
 In this filter paper along with residue is dried.
 Residue is then removed on a glazed paper, covered and filter paper is then
ignited as before.
 Crucible is cool, the precipitate is finally transferred into crucible and is
heated to a constant weight.

42. Advantages of organic precipitants over inorganic precipitants:
1. The precipitates formed are called organometallic compounds have high
molecular weight. Thus a small amount of metal ions yield a large amount
of precipitate. Hence percentage error is reduced.
2. By maintaining proper pH, one metal ion can be precipitated in presence
of other ion without interference.
3. Precipitate can be dried at suitable temp. and weighed. It is less tedious
than ignition and weighing carried out for inorganic precipitation.
4. Organometallic precipitates are less soluble than inorganic precipitates.
Thus accuracy of results is more.
5. The particle size is better than inorganic precipitate. Hence it is better
suited for filtration and washing of precipitates.
Examples.1) Dimethyl Glyoxime[C4H8O2N2]
2) 8-Hydroxy quinoline(Oxine)[C9H7ON]
3)Cupron (a-Benzoin oxime)
4) Salicylaldoxime[C7H7O2N]

43. Determination of Sulphur in Organic Compounds
 Sulphur containing organic compound is mixed with
excess of sodium peroxide and the mixture is heated in
nickel crucible till it fuses.
 This oxidizes organic sulphur to sulphate, SO42- ,
 the resultant mass is extracted with de-ionized water,
heated and treated with excess of barium chloride
solution to precipitate barium as barium sulphate.
 The precipitate is filtered, washed and dried.
 Ignition of precipitate is done and barium sulphate is
weighed.

44. Estimation of Nickel in Cu-Ni Alloy using
Dimethylglyoxime
 An alloy of Cu-Ni is opened with a suitable Acid/Mixture of acids
by evaporation the solution near to dryness & extract with water.
 Add 2 cm3 of conc. HCl & heat the solution.
 Pass H2S gas to the precipitate all copper as CuS.
 Filter the solution to remove H2S completely.
 Add 1:1 ammonia to raise the pH of the solution to neutralize.
 Nickel in the solution is precipitated by adding alcoholic
solution of DMG.
 Add 1:1 ammonia till the smell of ammonia persist.
 The precipitate of Ni-DMG is filtered through silica crucible,
washed with cold water, dried it at 383 K to 393 K.

45. Estimation of Aluminium as its Oxide.
 Aluminium is precipitated as hydrated aluminium oxide by means of
ammonia in presence of ammonium chlorede.
 The gelatinous precipitate is washed, dried, converted into its oxide by
ignition, and weighed as Al2O3.
 Al(OH)3 is amphoteric in nature.
 precipitation is starts at approximately pH 4 and is complete when pH
is in between 6.5 to 7.5.
 The precipitate cannot be washed with hot water as Al(OH)3 is readily
peptized. Hence, 2% solution of ammonium chloride or ammonium
nitrate used for washing the precipitate.
Al(OH)3 + 2H+
Al3+
+ 3 H2O
Al(OH)3 + OH-
AlO2
-
+ 2 H2O