Assay of Sodium chloride IP Pharmaceutical Analysis I Practical
Pharmaceutical Analysis I Practical: Limit test for (1) Chloride (2) Sulphate (3) Iron (4) Arsenic Preparation and standardisation of (1) Sodium hydroxide (2) Sulphuric acid (3) Sodium thiosulfate (4) Potassium permanganate (5) Ceric ammonium sulphate Assay (1) Ammonium chloride (2) Ferrous sulphate(3) Copper sulphate (4) Calcium gluconate (5) Hydrogen peroxide (6) Sodium benzoate (7) Sodium Chloride Determination of Normality (1) Conductometric titration of strong acid against a strong base (2) Conductometric titration of strong acid and weak acid against a strong base (3) Potentiometric titration
To determine the percent purity of sodium chloride in given sample
Sodium chloride is determined by Volhard’s method, it is a back type of precipitation titration which is characterized by formation of soluble colored compound at the end point. Sodium chloride is first treated with a known excess of silver nitrate to form a precipitate of silver chloride and the remaining silver ions are titrated with thiocyanate in acid solution using ferric ion as an indicator. Initially excess Ag+ ions react with SCN– and silver thiocyanate is precipitated. After equivalence point, when no Ag+ is present, excess of thiocyanate added reacts with Fe3+ to give reddish brown ferric thiocyanate. This method is precise and most commonly used for determination of halides. Nitrobenzene is used to coagulate the precipitate of silver chloride.
Titration is carried out in an acidic medium to prevent hydrolysis of Fe3+ ions added as an indicator. As AgSCN has a tendency to adsorb Ag+, the endpoint will be reached earlier than the equivalence point. Hence to avoid this, the solution is stirred continuously till the indicator color change is permanent.
The solubility product of AgCl is lower than AgSCN.Hence AgCl is more soluble. Hence excess of SCN– reacts with AgCl when no Ag+ remains.
AgCl + NH4SCN AgSCN + NH4Cl
The above reaction takes place before reaction between Fe3+ and SCN– and thus endpoint will be observed after equivalence point which introduces titration error. To minimize this, reaction between AgCl and SCN– should be prevented in the following manner:
- AgCl ppt is filtered off before back titration. The ppt should be boiled for few minutes to coagulate it so as to remove adsorbed Ag+ ions from ppt and send it back to solution. The cold titrate is used for titration.
- KNO3 can be added as coagulant after addition of excess of AgNO3.
- Nitrobenzene is added to form coating over AgCl which prevents its reaction with thiocyanate.
Sodium chloride (Mol. wt.: 58.4) contains not less than 99.0 percent and not more than 100.5 percent of sodium chloride calculated on a dried basis.
Preparation of 0.1M silver nitrate
Dissolve g. in sufficient water to produce 1000ml
- Standardization of 0.1M silver nitrate
Weigh accurately about 0.1g of sodium chloride previously dried at 1100C for 2 hours and dissolve in 5ml of water. Add 5ml of acetic acid, 50 ml methanol and 0.15ml of eosin solution. Stir and titrate with silver nitrate solution
Preparation of 0.1M Ammonium thiocyanate
Dissolve 7.612g of Ammonium thiocyanate in sufficient water to produce 1000ml
- standardization of 0.1M Ammonium thiocyanate
pipette 30 ml of 0.1M silver nitrate into a glass stoppered flask ,dilute with 50ml of water, add 2ml of nitric acid and 2ml of ferric ammonium sulphate solution and titrate with ammonium thiocyanate soluition to first appearance of red-brown colour.
- Assay of sodium chloride
Dissolve 0.1 g in 50 ml of water in a glass stoppered flask. Add 50 ml of 0.1M silver nitrate, 5 ml of 2M nitric acid and 2 ml of dibutyl phthalate, shake well and titrate with 0.1M ammonium thiocynate using 2 ml of Ferric ammonium sulphate solution as indicator, until the colour becomes reddish yellow.
Indian Pharmacopoeia 2014; Vol. III; Pg. No. 2740-2741
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