194 QUANTITATIVE ANALYSIS. 



.and calculate it as urea. Not applicable to urines rich in 

 hippuric acid. 1 The heating with magnesia may also be 

 entirely omitted if the ammonia be determined separately 

 and, calculated as urea, be deducted from the amount of 

 urea found. 



(6) According to Freund and Topfer. 2 Five cubic centi- 

 meters of urine are treated with the same volume of alcohol 

 and evaporated on the water-bath to dryness, the residue 

 ground and extracted several times with absolute alcohol and 

 filtered into a Kjeldahl flask. The alcohol is then driven 

 off as completely as possible on the water-bath, about 70 

 cc. of a saturated ethereal solution of oxalic acid are poured 

 into the flask, and the precipitate formed is allowed to settle. 

 Filter the solution, leaving as much of the precipitate as 

 possible in the flask, and wash with 60 to 80 cc. of ether in 

 several portions. 



When the ether has evaporated from the filter the con- ' 

 tents of the filter are washed into the flask with distilled water 

 and the solution of the contents of the flask is titrated with 

 fifth-normal sodium hydroxide solution, using phenol phtha- 

 lein (two drops of a 1 per cent, solution) as an indicator. 

 Afterwards the determination of nitrogen according to 

 Kjeldahl is undertaken. 



VII. DETERMINATION OF OXALIC ACID. 



The determination of oxalic acid is made according to 

 the method given for the detection of this substance, page 

 104, but the extraction with ether must be made five times. 

 Collect the calcium oxalate without loss on an ash-free filter, 

 wash, dry, place in a weighed crucible, heat with the blast- 

 lamp, to convert the calcium oxalate into calcium oxide 



1 Salaskin and Zaleski, Zeitschr. f. physiol. Chemie, 28, 73. 



2 Wiener klin. Rundschau, 1899, No. 23. 



