PRACTICAL EXERCISES 509 



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. 



The standard solution of silver nitrate can be made by dissolving 

 29*063 grammes of pure fused silver. nitrate in distilled water and making 

 up the volume of the solution accurately to i litre. The solution should 

 be kept in the dark. One c.c. of this solution corresponds to 

 o-oi gramme NaCl or 0-00607 gramme Cl. 



The standard solution of ammonium sulphocyanide is prepared as 

 follows: Dissolve 13 grammes of pure ammonium sulphocyanide 

 (NH 4 CNS) in a litre of distilled water. Measure with a pipette into 

 a beaker 20 c.c. of the standard silver nitrate solution, and add 5 c.c. 

 of the iron alum solution and 4 c.c. of pure nitric acid (specific gravity 

 1-2). Fill a burette with the sulphocyanide solution, and run it into 

 the silver nitrate solution until a faint permanent red tinge is obtained. 

 Note the number of c.c. of the sulphocyanide solution required, and 

 then dilute the solution till 2 c.c. of the sulphocyanide solution corre- 

 spond 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. 



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 1-2), and 15 c.c. of the standard silver solution; 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 the 

 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. 



Suppose x c.c. of the sulphocyanide solution are required, then the 

 chlorine in 10 c.c. of urine evidently corresponds to (15 AT), 0*01 gramme 

 NaCl 



4. Phosphates (i) Qualitative Tests. (a) Render the urine alkaline 

 with ammonia. A precipitate of earthy phosphates (calcium and mag- 

 nesium phosphates) falls down. Filter. The nitrate contains the 

 alkaline phosphates. To the filtrate add magnesia mixture.* The 

 alkaline phosphates (sodium, potassium, or ammonium phosphates) 

 are precipitated as ammonio-magnesic or triple phosphate, (b) Add to 

 urine half its volume of nitric acid and a little molybdate of ammonium, 

 and heat. A yellow precipitate of ammonium phospho-molybdate 

 shows that phosphates are present. This test is given both by alkaline 

 and earthy phosphates. 



(2) Quantitative Estimation. The quantitative estimation of phos- 

 phoric acid in urine is best done volumetrically, by titration with a 

 standard solution of uranium nitrate, using ferrocyanide of potassium 

 as the indicator. Uranium nitrate gives with phosphates, in a solution 

 containing free acetic acid, a precipitate with a constant proportion of 



* Magnesium chloride no grammes, ammonium chloride 140 grammes, 

 ammonia (specific gravity 0*91) 250 c.c., and water 1,750 c.c. 



