378 PHYSIOLOGICAL CHEMISTRY. 



fused mass in water, and remove the iodine or bromide by the addition of 

 dilute sulphuric acid and some nitrite, and thoroughly shake with carbon 

 disulphide. The liquid thus obtained may now be titrated with silver nitrate 

 according to VOLHARD'S method. The quantity of bromide or iodide is cal- 

 culated as the difference between the quantity of silver-nitrate solution used 

 for the titration of the solution of the fused mass and the quantity used for 

 the corresponding volume of the original urine. 



The otherwise beautiful titration method of MOHR, according 

 to which we titrate with silver nitrate in neutral liquids, using 

 neutral potassium chromate as an indicator, cannot be used directly 

 on the urine in careful work. Organic urinary constituents are 

 also precipitated by the silver-salt, and the results are therefore 

 somewhat high for the chlorine. If we wish to use this method, 

 the organic urinary constituents must first be destroyed. For this 

 purpose evaporate to dryness 5-10 c.c. of the urine, after the addi- 

 tion of 1 grm. of chlorine-free soda and 1-2 grms. chlorine-free 

 saltpetre, and carefully fuse. The mass is dissolved in water, 

 acidified faintly with nitric acid, and then neutralized exactly with 

 pure lime carbonate. This neutral solution is used for the titration. 



The silver-nitrate solution may be a solution. It is often 



made of such a strength that each c. c. corresponds to 0.006 grm. 

 Cl or 0.01 grm. NaCl. This last-mentioned solution contains 

 29.075 grms. AgN0 3 in 1 litre. 



VOLHARD'S METHOD. Instead of the preceding determination, 

 VOLHARD'S method, which can be performed directly on the urine, 

 may be employed. The principle is as follows : All the chlorine 

 from the urine acidified with nitric acid is precipitated by an excess 

 of silver nitrate, filtered, and in a measured part the quantity of 

 silver added in excess is determined by means of a sulphocyanide 

 solution. This excess of silver is completely precipitated by the 

 sulphocyanide, and a solution of some ferric salt, which, as is well 

 known, gives a blood-red reaction with the smallest quantity of 

 sulphocyanide, is used as an indicator. 



We require the following solutions for this titration : 1. A silver- 

 nitrate solution which contains 29.075 grms. AgN0 3 per litre and of 

 which each c. c. corresponds to 0.01 grm. NaCl or 0.00607 grm. 

 Cl ; 2. A saturated solution at the ordinary temperature of 

 chlorine-free iron alum or ferric sulphate; 3. Chlorine-free nitric 

 acid of a specific gravity of 1.2 ; 4. A potassium sulphocyanide 

 solution which contains 8.3 grms. KCNS per litre, and of which 

 2 c. c. corresponds to 1 c. c. of the silver-nitrate solution. 



About 9 grms. of potassium sulphocyanide are dissolved in water and 

 diluted to one litre. The amount of KCNS contained in this solution is 

 determined by the silver-nitrate solution in the following way : Measure 

 exactly 10 c. c. of the silver solution and treat with 5 c. c. of nitric acid and 



