206 ESSENTIALS OF CHEMICAL PHYSIOLOGY 



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

 compounds more easily precipitable than the chromate. 



To ob.viate such sources of error the following modification of the test, 

 us described by Button, may be used : 10 c.c. of urine are measured into a 

 thin porcelain capsule and 1 gramme of pure ammonium 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 silver 

 nitrate per litre) until the end reaction, a pink colour, appears. Each c.c. of 

 silver solution represents 0*005837 gr. of salt ; consequently, if 12*5 c.c. have 

 been used, the weight of salt in the 10 c.c. of urine is 0*07296 gr., or 0*7296 

 per cent. If 5*9 c.c. of urine are taken for titration, the number of c.c. of 

 silver solution used will represent the number of parts of salt per 1,000 parts 

 of urine. 



