PRACTICAL EXERCISES 



477 



1. Specific Gravity. Pour the urine into a glass cylinder, and 

 remove froth, if necessary, with filter-paper. Place a urinometer 

 (Fig. 177) in the urine, and see that it does not come in contact with 

 the side of the vessel. Read off on the graduated stem the division 

 which corresponds with the bottom of the meniscus. This gives the 

 specific gravity. 



2. Reaction. (a) 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 propor- 

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

 phosphate turns red litmus blue. 



(b) Titratable Acidity. To 25 c.c. of urine add 15 to 20 grammes 

 of powdered potassium oxalate, and one or two drops of a i per 

 cent, solution of phenolphthalein . Shake the mixture rapidly for 

 a minute or two, and then titrate with decinor- 

 mal sodium hydroxide at once (while still cold 

 from the solution of the oxalate) till a faint pink 

 colour remains permanent on. shaking. The 

 potassium oxalate is added to counteract the 

 tendency of the calcium present in urine to 

 form basic phosphates, which would be pre- 

 cipitated, and the acidity of the urine thus 

 increased (Folin). 



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 precipitate appears 

 to be incompletely soluble in ammonia, since 

 the ammonia brings down a small 'precipitate 

 of earthy phosphates. 



(b} Quantitative Estimation. The quantita- 

 tive estimation of the chlorine in urine with- 

 out previous evaporation and incineration is 

 best made by one of the modifications of 

 Volhard's method. It depends upon the com- 

 plete 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, which precipitates the silver 

 as insoluble silver sulphocyanide. A fairly strong 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 colour with a sulphocyanide so long as any silver in 

 the solution is uncombined with sulphocyanic acid. The iron- 

 ammonia-alum forms the red salt, ferric sulphocyanide, when any 

 excess of ammonium sulphocyanide is present, but it does not 

 react with silver sulphocyanide. ^ 



FIG. 177. URINO- 

 METER. 



