PRACTICAL EXERCISES 423 



in order to break up the aromatic sulphates. The elements of water 

 are thus taken up by these salts ; and the sulphuric acid is able to 

 unite with the barium. Filter through an ash-free filter. Add a few 

 drops of barium chloride to the first portion of the filtrate which 

 comes through. If there is no precipitate,enough barium chloride to 

 combine with all the sulphuric acid is present, and the filtration can 

 be proceeded with. But if there is any precipitate, more barium 

 chloride must be added, and the liquid passed again through the 

 filter. Treat the precipitate on the filter as in (2). Its weight after 

 incineration gives the quantity of barium sulphate corresponding to 

 the sulphuric acid of the aromatic compounds. 



6. Indoxyl can be oxidized into indigo, and so estimated. 



A qualitative test is the following : Ten c.c. of horse's urine is 

 mixed with 10 c.c. of strong hydrochloric acid, and a dilute solution 

 of sodium hypochlorite added drop by drop ; a bluish colour appears 

 if, as is generally the case, indoxyl is present, indigo (C 16 H 10 N 2 O2) 

 being formed by the oxidizing action of the hypochlorite on the 

 indoxyl, the compound of which with sulphuric acid has been broken 

 up by the hydrochloric acid. The number of drops of the hypo- 

 chlorite required to give the maximum change of colour is deter- 

 mined. Then the experiment may be repeated by dropping this 

 quantity of hypochlorite into 10 c.c. of the hydrochloric acid, and 

 adding 10 c.c. of the urine. The urine must be free from albumin. 

 If too much hypochlorite be added, the indigo is itself oxidized 

 In performing the test in human urine, which contains a smaller 

 quantity of the indigo-forming substance, the urine should first 

 be concentrated on the water-bath. If the faint blue liquid be 

 shaken up with a few drops of chloroform, the latter takes up the 

 colour, which is thus rendered more evident. The skatoxyl of urine 

 can also be oxidized to indigo, but it is present in far smaller amount. 

 The average quantity of indigo obtained from a litre of horse's urine 

 is about 150 milligrammes; from a litre of human urine, not a 

 twentieth of that quantity. 



7. Urea (i) Preparation. Urea can be obtained from dog's 

 urine by evaporating it to a syrup, extracting with absolute alcohol, 

 evaporating most of the alcohol, and allowing the mass to crystallize. 

 Or human urine may be concentrated to a small bulk, cooled to o, 

 and mixed with excess of strong pure nitric acid. A mass of rhombic 

 or six-sided tabular crystals of nitrate of urea separates. From the 

 nitrate, after purification, urea itself is obtained by addition of barium 

 carbonate till carbon dioxide ceases to be given off. What regains 

 is a mixture of urea and barium nitrate, from the dry residue of 

 which urea can be dissolved out by alcohol (Hoppe-Seyler). 



(2) Decomposition of Urea. Heated dry in a test-tube, it gives off 

 ammonia. The residue contains biuret, which, when dissolved in 

 water, gives a rose colour with a trace of cupric sulphate and excess 

 of sodium hydrate (or of the hydrates of certain other metals of the 

 alkalies and alkaline earths, p. 20). Some proteids peptones and 

 albumoses in the presence of the same reagents, give a similar 

 colour, the so-called biuret reaction. 



