Volhard’s method is a type of Argentometric titration of silver content determination.
The Volhard Method requires:
- a decinormal ammonium or potassium sulphocyanate solution,
- a decinormal silver nitrate solution,
- ferric indicator.
- A decinormal silver nitrate solution contains 16.988 grams of AgNO3 per litre, or 10.788 grams of Ag. It may be made up by direct weighing of the pure salt or if preferred by dissolving 10.788 grams of pure silver in nitric acid.
This may be a saturated solution of ferric alum or a 10% solution of pure ferric sulphate. 5 cc of either of these is used for a titration.
Decinormal Ammonium or Potassium Sulphocyanate
This salt being deliquescent cannot be weighed up correctly so an approximate solution is prepared by dissolving about 8 grams of the ammonium salt or 10 grams of the potassium salt in a litre of water. (Should the salt be unusually moist it will be better to increase the amounts.) The strength of this solution is then determined by titration with the decinormal silver nitrate, and a strictly decinormal solution of sulphocyanate prepared therefrom by adding the right amount of water. It should contain of the potassium salt 9.7 gram per litre or of the ammonium salt 7.6 grams per litre.
Method of Argentometry Procedure
Take 50 cc of the N/10 AgNO3 solution and place in a flask. Add 5 cc of ferric indicator and about 10 cc of pure nitric acid. The acid used for this purpose should be dilated with about one-fourth its volume of distilled water, and boiled until colorless to expel the lower oxides of nitrogen. The sulphocyanate is then added from a burette. At first a white precipitate will be thrown down, and a momentary red coloration will be produced which will instantly disappear on stirring or shaking. As soon as all the silver has been precipitated one extra drop of the sulphocyanate solution will produce a permanent faint reddish coloration which is to be taken as the endpoint. If the sulphocyanate solution is strictly decinormal it will need exactly 50 cc of it to neutralize the silver nitrate taken, or
1 cc N/10 KCNS = 0.010788 gm. Ag