PRACTICAL EXERCISES 



421 



division which corresponds with the bottom of the meniscus. This 

 gives the specific gravity. 



2. Reaction. Test with litmus-paper. Generally the litmus is 

 reddened, but occasionally in health the urine first passed in the 

 morning is alkaline. Sometimes urine has an amphicroic reaction, 

 i.e., affects both red and blue litmus paper. This is the case when 

 there is such a relation between the bases and acids that both acid 

 and * neutral ' (dibasic) phosphates are present in certain proportions. 

 The acid phosphate reddens blue litmus, and the ' neutral ' phosphate 

 turns red litmus blue. 



3. Chlorides (a) Qualitative Test. Add a drop of nitric acid 

 and a drop or two of silver nitrate solution. The nitric acid is added 

 to prevent precipitation of silver phosphate. A white precipitate 

 soluble in ammonia shows the presence of chlorides. The precipi- 

 tate appears to be incompletely soluble in ammonia, since the 

 ammonia brings down a small precipitate of earthy phosphates. 



(b) Quantitative Estimation. The quantitative estimation of the 

 chlorine in urine without previous evaporation and incineration is 

 best made by one of the modifications of Volhard's method. It 

 depends upon the complete precipitation of the chlorine combined 

 with the alkaline metals, and also of sulphocyanic acid, by silver 

 from a solution containing nitric acid in excess ; and avoids the 

 error introduced into simpler methods, like Mohr's, by the union of 

 some of the silver with other substances than chlorine. A given 

 quantity of a standard solution of silver nitrate (more than sufficient 

 to combine with all the chlorine) is added to 

 a given volume of urine. The excess of silver 

 is now estimated by means of a standard solution 

 of ammonium sulphocyanide. A solution of the 

 double sulphate of iron and ammonium (known 

 as iron-ammonia-alum) is taken as the indicator, 

 since a ferric salt does not give the usual red colou r 

 with a sulphocyanide so long as any silver in the 

 solution is uncombined with sulphocyanic acid. 



To carry out the method, put 10 c.c. of urine, 

 which must be free from albumin, in a stoppered 

 flask, with a mark corresponding to 100 c.c, or a 

 graduated cylinder. Add 50 c.c. of water, 4 c.c. 

 of pure nitric acid (specific gravity i'2), and 15 

 c.c. of the standard silver solution (of which i 

 c.c. corresponds to - oi gramme NaCl, or -00607 

 gramme Cl) ; shake well, fill with water to the 

 mark, and again shake. After the precipitate has 

 settled, filter it off. Take 50 c.c. of the filtrate, 

 add 5 c.c. of a concentrated solution of iron- 

 ammonia-alum, and run in from a burette the standard solution of 

 ammonium sulphocyanide until a weak but permanent red coloration 

 appears. 2 c.c. of the sulphocyanide solution correspond exactly 

 to i c.c. of the silver solution, so as just to allow of the end reaction 

 with the iron solution being seen, and no more. 



FIG. 130. URINO- 



METER. 



