CH. VIII.] TOTAL CHLORIDES. 199 



244. Total Chlorides by'the Prout- Winter method, 



Principle of Volhard's process for the estimation of chlorides. A measured 

 amount of standard silver nitrate is added to a known amount of the solution, 

 or to a properly prepared extract of a known amount of material. The 

 amount of silver used must be more than enough to completely precipitate 

 the whole of the chlorides. Iron alum is added (to serve as an indicator), the 

 mixture is made acid with nitric acid (to prevent the precipitation of other 

 substances), and the mixture made up to a definite volume. The precipitate 

 of silver chloride is filtered off and the amount of silver in an aliquot portion 

 of the filtrate determined by titration with standard thiocyanate solution. 

 From the amounts of standard silver originally used and that found in the 

 filtrate the amount precipitated by the chlorides in the material taken can be 

 calculated. 



Solutions required, o-i N. Silver nitrate. Dissolve 16-99 grams, of pure 

 fused silver nitrate in distilled water and make the volume up to i litre. The 

 solution should be kept in the dark. 



i cc. = 0-00365 gram. HC1. 



o-i N. thiocyanate. Dissolve about 15 grams, of potassium, or about 

 10 grams, of ammonium thiocyanate, in a litre of distilled water and 

 mix thoroughly. Standardise this in the following way : Measure out 

 20 cc. of the standard silver nitrate into a 150 cc. beaker or Erlenmeyer 

 flask. Add about 60 cc. of distilled water, 5 cc. of pure nitric acid, 

 and 5 cc. of a cold saturated solution of iron alum. Titrate 

 with the thiocyanate from a burette. A white precipitate of silver 

 thiocyanate is formed. Continue to add the thiocyanate until a 

 faint permanent pink (due to ferric thiocyanate) is obtained. Let x cc. 

 be the amount of thiocyanate required. To i litre of the solution add 



cc. of distilled water. The mixture should now be o-i N. It 



is advisable to check the strength once more against the standard silver. 

 Iron alum. A cold saturated solution in distilled water. 

 Pure nitric acid, free from chlorides. 



Method, To 10 cc. of the filtered fluid in a porcelain or silica 

 crucible (not in an evaporating basin) add i cc. of a saturated solu- 

 tion of sodium carbonate. Evaporate to complete dry ness on a 

 boiling water bath. Support the crucible on a pipe clay triangle, 

 and cautiously heat with a Bunsen burner. Gradually raise the 

 temperature until the mass has completely carbonised. The heating 

 should be continued until as much as possible of the carbon has 

 disappeared and the whole has been raised to a dull red heat. 

 Remove the flame and allow the crucible to cool until it can be 

 handled. Add about 10 cc. of distilled water and carefully stir with 

 a glass rod. Pour the fluid, together with the little pieces of carbon, 

 into a 100 cc. graduated flask, using a small funnel. Repeat the 

 extraction with another 10 cc. of water : then with 5 cc. of pure 

 nitric acid and 5 cc. of water : then twice more with water. Wash 



