472 PEACTICAL PHYSIOLOGY 



consists of the addition of a standardised solution of some well-known 

 precipitant of the body which is to be determined, to a measured 

 quantity of urine. A standardised solution is one of such a strength, 

 that each cubic centimetre of it corresponds to a definite amount of the 

 body which it is desired to determine. When sufficient of this solution 

 has been added to precipitate all the substance, and a slight excess of 

 it is accordingly present in the fluid this being determined by some 

 indicator the number of c.c. of the precipitant employed is read off, 

 and this result, multiplied by the standard of the solution, gives the 

 amount of substance in the number of c.c. of the urine employed. 



Estimation of Chlorides. The Standard Solution. Dissolve 29-075 

 grammes of fused argentic nitrate in a litre of distilled water : 1 c.c. = 

 0-01 gr. NaCL. 



The Indicator. A saturated solution of neutral potassium chromate. 

 This gives a red precipitate with AgN0 3 . 



Titration. Place 10 c.c. of urine, diluted to 100 c.c. water, in a 

 porcelain basin, and add a few drops of the potassium chromate 

 solution, till a distinct yellow colour is produced. The standard 

 solution is now run in from the burette. As this solution comes in 

 contact with the urine a red colour is produced, which disappears 

 on stirring, until a slight excess is present, when an orange tint 

 persists. From the number of c.c. of the standard solution employed 

 1 c.c. is subtracted, since the urine contains besides chlorides certain 

 substances which combine with silver nitrate before the chromate does. 

 A more accurate estimation of chlorides may be made by mixing 5-10 

 c.c. urine with 1 gr. Na 2 C0 3 (01. free) and 2 gr. KN0 3 (also Cl. free) ; 

 evaporating to dryness in a platinum crucible; incinerating the residue 

 till a white ash is formed; cooling and dissolving in water. This 

 watery solution is then titrated as above, but no deduction made from 

 the reading of the burette. By this method the organic constituents 

 of the urine are removed. 



Estimation of Phosphates. The Standard Solution. Dissolve 35 -5 gr. 

 of uranium nitrate in a litre of water: 1 c.c. = 0-005 gr. phosphoric acid 

 (P 2 5 ). On account of the nitric acid which is liberated when this 

 solution reacts with the phosphates, it 'is necessary to add to the urine 

 a solution of sodium acetate, which will absorb the free acid and prevent 

 it dissolving the uranium phosphate, for all phosphates are soluble in 

 free nitric acid. 



This solution is made by dissolving 100 gr. sodium acetate in 900 c.c. 

 water, and adding to this solution 100 c.c. glacial acetic acid. 1 



1 The acetic acid is added to make certain that all the phosphates are present 

 as acid salts, since alkaline calcium phosphate decomposes in boiling solution into 



