APP. v.] LUTTKE'S METHOD. 503 



Volhard's method of determining chlorine as applied to the 

 The principle present object depends upon the fact, firstly, that in 

 of Voinard's the presence of strong nitric acid, silver nitrate corn- 

 process for de- pletely precipitates the chlorine and sulphocyanogen 

 termining ci. which may be present in a solution. 



Secondly, that the precipitates which silver nitrate gives with 

 albuminous bodies, with some organic acids, &c. are insoluble in 

 strong nitric acid. If then to an organic mixture, such as the 

 gastric contents or pure gastric juice, we add a known quantity of a 

 standard solution containing silver nitrate in presence of a large 

 quantity of strong nitric acid, taking care that the silver added is 

 more than sufficient to precipitate all the chlorine, if we filter the 

 mixture and determine in the filtrate the quantity of silver remaining 

 (i.e. which has not combined with the chlorine), we shall at once 

 know how much chlorine the mixture contained. 



Thirdly, upon the fact that when a solution of ammonium 

 sulphocyanate is added to a strongly acid solution of silver, con- 

 taining some ferrous sulphate, a curdy precipitate of silver sulpho- 

 cyanate falls, the reaction being shewn in equation 1 : 



(1) AgNO 3 + NH 4 CNS = AgCNS + NH 4 N0 3 . 



This precipitate at once redissolves with the production of a 

 blood-red colour, due to the formation of sulphocyanate of iron, as 

 shewn by equation 2 : 



(2) Fe 2 (S0 4 ) 3 + 6NH 4 CNS = Fe 2 (CNS) 6 + 3NH 4 S0 4 . 



But no sooner has the red colour been observed than it disappears, 

 50 long as any silver remains in solution, the iron sulphocyanate 

 taking part in the reaction which is shewn in equation 3 : 



(3) Fe 2 (CNS) 6 + 6AgN0 3 = Fe 2 (N0 3 ) 6 + GAgCNS. 



It is only when the whole of the silver has been precipitated as 

 silver sulphocyanide, that the blood-red colouration due to ferric sul- 

 phocyanide persists ; the persistence of the red colouration indicates, 

 therefore, the termination of the process, and, if the strength of the 

 solution of ammonium sulphocyanide be known, the quantity of silver 

 in the solution can be at once calculated. 



(B) The standard solutions needed in Luttke's process. 



i. Deci-nor- 16'997 grms. of dry and pure AgN0 3 are dissolved 

 mai solution of in about 900 c.c. of dilute nitric acid containing 25 per 

 silver nitrate. cent of HN0 3 , 50 c.c. of the 'liquor ferri sulfurici oxy- 



Chemie, Vol. v. (1881), p. 285; Salkowski u. Leube, Die Lehre vom Harne, Berlin, 

 Hirschwald, 1882, see p. 168. 



