URINE 297 



that of Fehling's solution as a standard, at least a comparative, 

 if not a quantitative estimation of indican may be made. 



11. Allantoin. 



For details of the estimation of allantoin consult a larger 

 work. 



12. Oxalic Acid. 



Oxalic acid may be estimated by the isolation of its calcium 

 salt. 



13. Chlorides. 



Volhard Method. All of the chlorides of the urine are pre- 

 cipitated by adding excess (a known volume) of standard sil- 

 ver nitrate. The excess silver nitrate then is determined by 

 titration with thiocyanate, using a ferric salt as indicator. The 

 thiocyanate is made up so that 1 c.c. exactly corresponds to 1 

 c.c. of the silver nitrate. Silver thiocyanate (white) precipitates 

 first. When all the silver is precipitated, red ferric thiocyanate 

 is formed. 



Into a 100 c.c. volumetric flask measure accurately with a 

 pipette 10 c.c. urine and 20 c.c. standard silver nitrate solution. 

 Add about 4 c.c. cone, nitric acid and about 5 c.c. iron alum 

 solution. Add distilled water up to the 100 c.c. mark, stopper 

 and mix thoroughly. Filter through a dry filter into a dry 

 vessel. The above process will have precipitated all chlorides 

 as AgCl, and an excess of silver nitrate will be left over. 



Accurately measure 50 c.c. of the filtrate into an Erlenmeyer 

 or beaker and titrate the excess of silver with standard thio- 

 cyanate to a permanent faint pink or reddish-brown. The 

 amount of thiocyanate used corresponds to the excess of silver 

 remaining in the solution. 



One c.c. thiocyanate corresponds to 1 c.c. of the standard sil- 

 ver nitrate. 



One c.c. standard silver nitrate corresponds to 0.01 g NaCl. 

 Since only half the liquid was titrated, multiply the titration 



