TTKEA AND CHLORIDES IN URINE 169 



of the total chlorides consists in their precipitation by a standard solution 

 of silver nitrate or mercuric nitrate. 



Mohr's' Method. — Precipitation by silver nitrate. 



The following solutions must be prepared : 



Standard silver nitrate solution. Dissolve 29-075 grammes of fused 

 nitrate of silver in a litre (1,000 c.c.) of distilled water ; 1 c.c. = OOl gramme 

 of sodium chloride. 



(a) Saturated solution of neutral potassium chromate. 



Analysis. — Take 10 c.c. of urine ; dilute with 100 c.c. of distilled water. 



Add to this a few drops of the potassium chromate solution. 



Drop into this mixture from a burette the standard silver nitrate solu- 

 tion ; the chlorine combines with the silver to form silver chloride, a white 

 precipitate. When all the chlorides are so precipitated, sUver chromate (red 

 in coloiu*) goes down, but not while any chloride remains in solution. The 

 silver nitrate must therefore be added until the precipitate has a pink tinge. 



From the amount of standard solution used, the quantity of sodium 

 chloride in 10 c.c. of urine, and thence the percentage, may be calculated. 



Sources of Error and Corrections.— A. high-coloured urine may give rise 

 to diflBculty in seeing the pink tinge of the silver chromate : this is overcome 

 by diluting the urine more than stated in the preceding paragraph. 



1 c.c. should always be subtracted from the total number of c.c. of the 

 silver nitrate solution used, as the mine contains small quantities of certain 

 compounds more easily precipitable than the chromate. 



(6) To obviate such sources of error the follo'wing modification of the 

 test, as described by Sutton,' may be used : 10 c.c. of urine are measured 

 into a thin porcelain capsule and 1 gramme of pure ammoniima nitrate 

 added ; the whole is then evaporated to dryness, and gradually heated over 

 a small spirit lamp to low redness till all vapours are dissipated and the 

 residue becomes white. It is then dissolved in a small quantity of water, 

 and the carbonates produced by the combustion of the organic matter 

 neutralised by dilute acetic acid ; a few grains of pure calcium carbonate to 

 remove all free acid are then added, and one or two drops of potassium 

 chromate. The mixture is then titrated with decinormal silver solution 

 (16-966 gr. of sUver nitrate per litre ) untU the end reaction, a pink colour, 

 appears. Each c.c. of silver 'solution represents 0-005837 gr. of salt ; con- 

 sequently, if 12-5 c.c. have been used, the weight of salt in the 10 c.c. of 

 urine is 007296 gr., or 07296 per cent. If 5-9 c.c. of urine are taken for 

 titration, the number of c.c. of sUver solution used wiU represent the number 

 of parts of salt per 1,000 parts of urine. 



(c) L iehig's Method. — Precipitation by mercuric nitrate. 



The following solutions must be first prepared : 



i. Standard mercinnc nitrate solution : Dissolve 20 grammes of pure 

 mercury in boiling nitric acid ; then dilute to nearly a litre. To dilute this 

 to the right strength, preliminary experiments must be performed with a 

 standard solution of pure sodium chloride, 20 grammes to the Utre. Take 

 10 c.c. of the standard sodiiun chloride solution, add to this 2 c.c. of a 4-per- 

 cent, solution of urea and 5 c.c. of a saturated solution of sodimn sulphate. 



' Volumetric Analysis, p. 309. 



