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117
Volumetric Analysis
2.6.1 Steps of Gravimetric Analysis
It involves following step
1. Sampling:
In this step one has to consider the properties of material with respect to
bulk, costing of test, value of product, etc.
2. Dissolution of the sample:
The substance to be estimated must have to be in the solution form. For
this, accurately weigh a suitable quantity of substance and dissolve it in
distilled water or dilute acid or alkali. Heat the solution, if necessary.
3. Precipitation:
This step involves reaction with a precipitant to give precipitate. The
precipitate formed should be easily filterable, have low solubility, stable,
and have constant weight after drying.
There may be some impurities present in the precipitate which may be
due to.
a. Coprecipitation:
The term coprecipitation means precipitating together. This is the
precipitation of unwanted substance along with the precipitate we
require. Therefore, coprecipitation is precipitation of any soluble or
extraneous substance along with the precipitate of the substance to be
analysed.
b. Surface adsorption:
In surface adsorption, the unwanted substance gets adsorbed on the
surface of the precipitate of the substance to be analyzed.
c. Occlusion:
In occlusion the impurities get trapped within the growing crystal.
d. Post-precipitation:
When the precipitate is allowed to stand for longer time with mother
liquor, the impurities get deposited on the surface of precipitate. This
is called post-precipitation.
Post-precipitation increases with time and temperature.
1. Sampling
2. Dissolution of the sample
3. Precipitation
4. Filtration
5. Washing
6. Drying
7. Ignition
118
Practical Pharmaceutical Analytical Chemistry
4. Filtration:
This is used for separation of precipitate from mother liquor by the use
of filter paper. The filter paper used should have negligible and known
weight of its ash.
Different types of filter paper are in use. Some of them include:
a. Quantitative filter paper
b. Circular filter paper
c. Whatman filter paper
d. Schleicher and Schull
5. Washing:
Post-filtration washing is required for the removal of unwanted soluble
substance or impurities.
The wash liquid should
a. have no or minimum solvent effect on the precipitate
b. dissolve foreign matter easily
c. not peptize precipitate even after all the electrolytes have been
removed
d. easily volatile
Water is generally used as wash liquid for most of the precipitate.
Ammonium chloride or nitrate is sometimes added to avoid formation of
colloidal solution of the precipitate.
6. Drying:
Drying is used to remove water or wash liquid used. It can be achieved
by heating at 110120qC in an oven. It is used to give a weighed form of
precipitate having constant composition.
7. Ignition:
Heating of the dried precipitate at higher temperature to get the ash is
called ignition. This can be done by heating it with appropriate burner or
by use of crucible placed in electrically heated muffle furnace.
Sometimes, during ignition a small fraction of the precipitate may get
reduced by the carbon present in the paper to a compound altogether
different from the compound in which determination is required. The ash
is treated with suitable reagents to get back the form in which it is finally
to be weighed. This is called ash treatment.
8. Weighing and calculation:
The residue after drying and ignition is weighed and the weight of the
precipitate is obtained using the formula:
Weight of the precipitate Weight of crucible along with precipitate –
Weight of empty crucible
119
Volumetric Analysis
From the weight of the precipitate one can determine percentage of
analyte present in the sample by using the formula:
Percentage of analyte or precipitate present in the sample
W
W
a
s
=×1000
where W
a
is the weight of the precipitate or analyte obtained and W
s
is the
weight of the sample given.
Experiment 1 To find out percentage purity of sodium sulphate in the
given sample.
IP followed: 1966
Synonym: Glauber’s salt
Principle: Gravimetric determination
Molecular formula: Na
2
SO
4
·7H
2
O
Molecular weight: 142 (anhydrous); 322 (hydrated form)
IP limit: It contains not less than 99.0% and not more than the equivalent of
100.5% of sodium sulphate, calculated with reference to the substance dried to
a constant weight at 105qC.
Loss on drying: Loses not less than 51.5% and not more than 57% of its weight
when dried to constant weight at 105°C.
Procedure
Weigh accurately about 0.5 g of sodium sulphate and dissolve it in 100 mL of
water. Further add 1 mL of hydrochloric acid and heat the solution to boiling.
Add slowly an excess of hot solution of barium chloride and further heat the
mixture for another half an hour on a water bath. Filter the precipitate, wash
with water and ignite it to a constant weight.
Reaction involved
Na SO BaCl BaSO
Sodium sulphate
Barium chloride
Barium su
2 42 4
+⎯→⎯⎯⎯
llphate
Sodium chloride
NaCl+ 2
THEORY Sodium sulphate on treatment with barium chloride gives the precipitate
of barium sulphate. The precipitate is ignited to constant weight.
The precipitate of barium sulphate is easily reduced to sulphide by the action
of the carbon of the filter paper above 600qC.
The addition of few drops of sulphuric acid oxidizes the barium sulphide to
barium sulphate.
120
Practical Pharmaceutical Analytical Chemistry
Gravimetric factor
1BaSO
4
is produced from 1 Na
2
SO
4
i.e. {(1 u 137.4) (1 u 32) (4 u 16)} g of BaSO
4
{ {(2 u 23) (1 u 32)
(4 u 16)} g of Na
2
SO
4
or (137.4 32 64) g of BaSO
4
{ (46 32 64) g of Na
2
SO
4
or 233.4 g of BaSO
4
{ 142 g of sodium sulphate
or 1 g of BaSO
4
{ (142/233.4) g of sodium sulphate 0.6085 g of sodium
sulphate
Calculation
Weight of the sample of sodium sulphate a
Weight of the crucible (empty) w
1
Weight of the crucible with precipitate w
2
Weight of the precipitate (BaSO
4
) w
2
w
1
Percentage purity of
Sodium sulphate
Gravimetric factor Weight o
=
×
ff the precipitate
Weight of the sample of sodium sulphate
× 100
Category: Cathartic
Result The given sodium sulphate sample was found to be ________ %
pure.
Experiment 2 To find out percentage purity of zinc sulphate in the
given sample.
IP followed: 1966
Principle: Gravimetric determination
Molecular formula: ZnSO
4
·7H
2
O
Molecular weight: 287.39
IP limit: It contains not less than 55.6% and not more than the 61.0% of zinc
sulphate, corresponding to not less than 99.5% and not more than equivalent of
102.0% of the hydrated salt.
Loss on drying: Loses not less than 35.0% and not more than 39% of its
weight when dried to constant weight at 105qC.
121
Volumetric Analysis
Procedure
Weigh accurately about 1 g of zinc sulphate and dissolve it in 100 mL of water.
Heat the solution to about 90qC and add solution of sodium carbonate to
precipitate all of the zinc. Boil it for about 5 min and set aside to allow the
precipitate to subside. Collect the precipitate in a tared Gooch crucible and wash
with hot water until free from alkali. Dry the residue, ignite and weigh.
Reaction involved
ZnSO Na CO ZnCO
Zinc sulphate Sodium carbanate Zinc carba
423 3
+⎯→⎯⎯⎯
nnate Sodium sulphate
Na SO+
24
THEORY Zinc sulphate on treatment with sodium carbonate gives precipitate of
zinc carbonate. The precipitate is ignited to constant weight. On ignition pre-
cipitate of zinc carbonate gets converted to zinc oxide. The constant weight of
zinc oxide is correlated with the weight of the zinc sulphate.
ZnCO ZnO CO
Zinc carbanate
Zinc oxide Carban dioxide
32
⎯→⎯⎯⎯ +
Gravimetric factor
1 ZnO is produced from 1 ZnSO
4
·7H
2
O
i.e. {(1 u 65.39) (1 u 16)} g of ZnO { {(1 u 65.39) (1 u 32) (4 u 16)
(7 u 18)} g of ZnSO
4
or (65.39 16) g of ZnO { (65.39 32 64 126) g of ZnSO
4
or 81.39 g of ZnO { 287.39 g of zinc sulphate
or 1 g of ZnO { (287.39/81.39) g of zinc sulphate 1.984 g of zinc sulphate
Calculation
Weight of the sample of zinc sulphate a
Weight of the crucible (empty) w
1
Weight of the crucible with precipitate w
2
Weight of the precipitate (ZnCO
3
) w
2
w
1
Percentage purity of sodium sulphate =
Gravimetric factor Weight o×
ffthe
precipitate ZnCO 100
Weight of the sample of zinc sulphate
3
()
×
Category: Emetic; Astringent
Result The given zinc sulphate sample was found to be _______ % pure.
122
Practical Pharmaceutical Analytical Chemistry
Judge Yourself
1. Define the following
(i) Primary standard
(ii) Acid
(iii) Base
(iv) Indicators
2. Show benzenoidquinonoid theory in phenolphathalein.
3. How will you prepare
(i) 50 mL of 2 N hydrochloric acid
(ii) 250 mL of 8 N sodium hydroxide
4. Write the primary standard for
(i) Hydrochloric acid
(ii) Sodium hydroxide
(iii) Sulphuric acid
5. Derive the equivalent weight of
(i) Hydrochloric acid
(ii) Sodium hydroxide
(iii) Sulphuric acid
6. Write the normality of pure
(i) Hydrochloric acid
(ii) Nitric acid
(iii) Sulphuric acid
7. Write the common name for
(i) Sodium bicarbonate
(ii) Sodium carbonate
(iii) Borax
(iv) Ammonium chloride
(v) Lactic acid
8. Derive an equivalent factor for
(i) Lactic acid
(ii) Boric acid
(iii) Zinc oxide
9. Write the reaction involved in the assay of
(i) Zinc oxide
(ii) Aspirin
(iii) Indomethacin
10. Give reasons for the following:
123
Volumetric Analysis
(i) Potassium hydrogen phthalate is preferred over oxalic acid for
standardization of sodium hydroxide
(ii) Heating decreases the solubilization of sodium hydroxide
(iii) Phenolphathalein becomes colourless at pH 12
(iv) Effervescence is observed during standardization of
hydrochloric acid with sodium carbonate
(v) Ether is added in the assay of sodium benzoate
(vi) Sodium hydroxide is used in the assay of lactic acid
(vii) Ammonium chloride is added in the assay of zinc oxide
(viii) Glycerine is added in the assay of boric acid
(ix) Mannitol and sorbitol is preferred over glycerol in the assay
of boric acid
(x) Sodium hydroxide is used in the assay of lactic acid
(xi) Formaldehyde is added in the assay of ammonium chloride
11. Write the principle involved in the assay of
(i) Borax
(ii) Boric acid
(iii) Ibuprofen
12. Write the IP limit of
(i) Zinc oxide
(ii) Lactic acid
(iii) Ammonia
13. Write the pharmaceutical use of
(i) Indomethacin
(ii) Sodium benzoate
(iii) Aspirin
14. Define the following
(i) Reducing agent
(ii) Oxidation
(iii) Oxidation number
15. How will you classify indicators in redox titrimetry? Give example of
each class
16. How will you prepare
(i) 50 mL of 2 N potassium permanganate solution
(ii) 250 mL of 1 N sodium thiosulphate solution
17. Write the primary standard for
(i) Iodine
(ii) Potassium iodate
124
Practical Pharmaceutical Analytical Chemistry
18. Determine equivalent weight of potassium permanganate by
(i) Available oxygen method
(ii) Oxidation number method
19. Differentiate between iodimetric and iodometric titration.
20. Write the common name for
(i) Sodium bicarbonate
(ii) Sodium carbonate
(iii) Borax
(iv) Ammonium chloride
(v) Lactic acid
21. Derive an equivalent factor for
(i) Ascorbic acid
(ii) Ferrous sulphate
(iii) Paracetamol
22. Write the reaction involved in the assay of
(i) Metamizole
(ii) Isoniazid
(iii) Dimercaprol
23. Write the reason behind use of following chemicals in standardization
of iodine
(i) Sodium hydroxide
(ii) Sodium bicarbonate
(iii) Hydrochloric acid
24. Write the principle involved in the assay of
(i) Ascorbic acid
(ii) Copper sulphate
25. Write the IP Limit of
(i) Isoniazid
(ii) Metamizole
26. Write the pharmaceutical use of
(i) Ferrous sulphate
(ii) Ascorbic acid
27. Differentiate between Mohr’s and Volhard’s method
28. Discuss the mechanism of adsorption indicator
29. How will you prepare
(i) 50 mL of 2 N silver nitrate
(ii) 250 mL of 8 N ammonium thiocyanate
125
Volumetric Analysis
30. Write the primary standard for
(i) Silver nitrate
(ii) Ammonium thiocyanate
31. Derive the equivalent weight of
(i) Hydrochloric acid
(ii) Sodium chloride
32. How the end point is detected in
(i) Mohr’s method
(ii) Modified Volhard method
33. Write the common name for sodium chloride.
34. Derive an equivalent factor for
(i) Ammonium chloride
(ii) Potassium iodide
35. Write the reaction involved in the assay of
(i) Sodium chloride by Mohr’s method
(ii) Sodium chloride by modified Volhard’s method
(iii) Chlorbutol
36. Give reasons for the following:
(i) Dibutyl phthalate is preferred over nitrobenzene for assay of
sodium chloride
(ii) Potassium chromate can be used in neutral solution only
(iii) Bromides and iodides cannot be assayed using Mohr’s
method
(iv) Starch is required for assay of potassium iodide
(v) Nitrobenzene is used in the assay of sodium chloride
(vi) Distilled water is required for precipitation titration
(vii) Nitric acid is added in the assay of ammonium chloride
(viii) Sodium hydroxide is used in the assay of chlorbutol
37. Write the principle involved in the assay of
(i) Potassium iodide
(ii) Chlorbutol
38. Write the IP Limit of
(i) Ammonium chloride
(ii) Sodium chloride
39. Write the pharmaceutical use of
(i) Ammonium chloride
(ii) Sodium chloride
40. Write the pharmaceutical use of
(i) Ammonium chloride
(ii) Sodium chloride
126
Practical Pharmaceutical Analytical Chemistry
41. Write the pharmaceutical use of
(i) Ammonium chloride
(ii) Sodium chloride
42. How can you improve rate of diazotization in slowly diazotizable com-
pound?
43. Why low temperature is required for diazotization reaction?
44. How will you prepare 0.5 M sodium nitrite solution?
45. Write the Primary standard for sodium nitrite.
46. How will you detect the end point in diazotization reaction?
47. Derive an equivalent factor for
(i) Dapsone
(ii) Sulphadiazine
48. Write the reaction involved in the assay of
(i) Sulphadimidine
(ii) Sulphadiazine
49. Write the IP Limit of
(i) Sulphacetamide
(ii) Sulphadimidine
50. What is the principle of EDTA titration?
51. Why pH 10 is maintained for complexometric titration?
52. What is the common name for magnesium sulphate?
53. Write the primary standard for EDTA standardization
54. Differentiate between monodentate and bidentate ligand.
55. Write the structure of mordant black II.
56. Explain the theory of indicator used in complexometry.
57. Derive an equivalent factor for
(i) Zinc sulphate
(ii) Magnesium sulphate
58. Write the reaction involved in the assay of
(i) Calcium lactate
(ii) Calcium carbonate
59. Write the principle involved in the assay of
(i) Magnesium sulphate
(ii) Zinc sulphate
60. Write the IP limit of
(i) Calcium gluconate
(ii) Magnesium sulphate
61. Write the pharmaceutical use of
(i) Calcium gluconate
(ii) Magnesium sulphate