CH. XIII.] SULPHATES. 357 



418. Inorganic sulphates by the benzidine method of 

 Rosenheim and Drummond.* 



Principle. The urine is acidified with hydrochloric acid and treated with 

 an excess of benzidine hydrochloride. The sulphates are precipitated quantir 

 tatively. The precipitate is filtered off under suction, washed free from acid 

 with water (or better with a saturated solution of benzidine sulphate) and 

 suspended in hot water. Phenol phthalein is added, and the mixture titrated 

 with standard soda. A pink colour does not develop until enough soda has 

 been added to combine with the whole of the benzidine sulphate to form 

 sodium sulphate. Benzidine sulphate, being the salt of a weak base with a 

 strong acid, suffers hydrolytic dissociation into the base and the acid. The 

 base is only very feebly ionised, whilst the strong acid is freely ionised, 

 the solution in hot water thus behaving like sulphuric acid, which can be 

 titrated with the standard soda. 



Solutions required. 



1. Benzidine hydrochloride. Rub up 4 grams, of pure benzidine with 

 about 10 cc. of distilled water. Transfer with about 500 cc. of water to a 2 

 litre flask. Add 5 cc. of concentrated hydrochloric acid and make up to 2 

 litres with distilled water. 



2. Hydrochloric acid. Dilute i volume of pure concentrated hydro- 

 chloric acid with 3 volumes of distilled water. 



3. Saturated benzidine sulphate. Prepare some benzidine sulphate by 

 adding a little sodium sulphate to 200 cc. of the benzidine hydrochloride. 

 Collect the precipitate as described below and wash it thoroughly with cold 

 water. Suspend it in a considerable volume of hot water and allow it to 

 stand over-night in a cool place. Filter from the benzidine sulphate till quite 

 clear. 



4. o-i N. sodium hydroxide. See appendix. The exact strengthens 

 immaterial, so long as it be accurately determined. 



5. Phenol phthalein. A saturated solution in alcohol. 



Method. Measure 25 cc. of the urine (filtered, if necessary) 

 into a 250 cc. Erlenmeyer flask, with a wide neck. Add 2 cc. of the 

 hydrochloric acid and 100 cc. of the benzidine hydrochloride. Mix 

 and allow to stand for 10 minutes. Filter through paper and paper 

 pulp, as described in Ex. 406. The filtrate must be crystal clear. 

 If it is cloudy it must be passed through the filter again. Wash out 

 the beaker with 10 cc. of the saturated benzidine sulphate and wash 

 the precipitate with this. Repeat this at least once more. Transfer 

 the precipitate, filter pulp and disc to the Erlenmeyer flask and wash 

 the funnel into the flask with a jet of boiling water, using about 

 50 cc. of water. Any lumps of the precipitate must be broken up 

 by use of a glass rod before the titration is commenced, or, if this is 



* Biochemical Journal, viii., p. 134. 



