408 PHYSIOLOGICAL CHEMISTRY. 



tion of the filtrate with a 10 per cent solution of silver nitrate until 

 precipitation is complete. 1 Filter off this precipitate, wash it with 

 water and determine its nitrogen content by means of the Kjeldahl 

 method (see p. 381). This is the " purine nitrogen." Render an 

 aliquot portion of the filtrate faintly alkaline, 2 with a one per cent 

 solution of ammonium hydroxide and add 50-100 c.C. of a 10 per 

 cent solution of silver nitrate. If allantoin be present a white, floc- 

 culent precipitate will form and gradually sink to the bottom of the 

 solution. Filter, wash the precipitate free from ammonium hydrox- 

 ide by means of a one per cent solution of sodium sulphate and 

 determine its nitrogen content by the Kjeldahl method (see p. 381). 



XXI. Oxalic Acid. 



Salkowski-Autenrieth and Barth Method. Place the twenty- 

 four hour urine specimen in a precipitating jar, add an excess of 

 calcium chloride, render the urine strongly ammoniacal, stir it well 

 and allow it to stand 18-20 hours. Filter off the precipitate, wash 

 it with a small amount of water and dissolve it in about 30 c.c. of 

 a hot 15 per cent solution of hydrochloric acid. By means of a 

 separatory funnel extract the solution with 150 c.c. of ether which 

 contains 3 per cent of alcohol, repeating the extraction four or five 

 times with fresh portions of ether. Unite the ethereal extracts, 

 allow them to stand for an hour in a flask and then filter through 

 a dry filter paper. Add 5 c.c. of water to the filtrate, to prevent 

 the formation of diethyl oxalate when the solution is heated, and 

 distil off the ether. If necessary, decolorize the liquid with animal 

 charcoal and filter. Concentrate the filtrate to 3-5 c.c., add a little 

 calcium chloride solution, make it ammoniacal and after a few min- 

 utes render it slightly acid with acetic acid. Allow the acidified 

 solution to stand several hours, collect the precipitate of calcium 

 oxalate on a washed filter paper, 3 wash, incinerate strongly (to 

 CaO) and weigh in the usual manner. 



Calculation. Since 56 parts of CaO are equivalent to 90 parts 

 of oxalic acid, the quantity of oxalic acid in the volume of urine 

 taken may be determined by multiplying the weight of CaO by the 

 factor 1.6071. 



XXII. Total Solids. 



. i. Drying Method. Place 5 c.c. of urine in a weighed shallow 

 dish, acidify very slightly with acetic acid (1-3 drops) and dry it 



1 Ordinarily from 20-30 c.c. is required. 



2 Using litmus as the indicator. 



8 Schleicher and Schull, No. 589, is satisfactory. 



