Volume 1
A treatise on chemistry / by H.E. Roscoe and C. Schorlemmer.
- Henry Enfield Roscoe
- Date:
- 1877-1892
Licence: Public Domain Mark
Credit: A treatise on chemistry / by H.E. Roscoe and C. Schorlemmer. Source: Wellcome Collection.
667/792 page 651
![for this purpose is that proposed by Liebig. It depends upon the fact that when mercuric nitrate is added to a solution of urea, a white insoluble precipitate falls down, possessing the composition 2CO(NH2)2 + 3HgO + Hg(N03)2. It is convenient that each cbc. of the mercuric solution should precipitate 0 01 gram, of urea, and a solution of this strength is obtained by dissolving 66’666 grams, of metallic mercury in nitric acid, evaporating to drive off as much of the acid as possible, end diluting the solution to one liter. Before this solution is added to the urine it is necessary to precipitate the phosphates and sulphates which it contains in solution. For this purpose two volumes of urine are mixed with one volume of a mixture of equal volumes of baryta-solution and cold saturated solution of barium nitrate. The standard mercury solution is then added by a burette to fifteen cbc. of the filtrate, corresponding to ten cbc. of urine, until no further precipitate occurs. The end of the reaction is easily ascertained by adding a solution of sodium carbonate to a drop of the liquid, which assumes a yellow colour as soon as a slight excess of mercury is present. One cbc. of the mercury solution corresponds to 0 07 gram, of urea. According to Bunsen’s method, the urine is heated with a solution of barium chloride in dilute ammonia to a temperature of 230°, and the barium carbonate which separates out is weighed:— CO(NH2)2 + Ba(Jl2 + 2H20 = BaC03 + 2C1NH4. A third method for the determination of urine, known as the Lavy-Knop method, depends on the fact that urea when brought into contact with an alkaline hypochlorite or hypobromite evolves pure nitrogen:— CO(NH2)2 + 3NaOBr = C02 + FT2 + 3NaBr + 2H20. For this purpose a freshly-prepared alkaline solution of sodium hypobromite is prepared by dissolving bromine in excess of caustic soda, and this is mixed with a solution of urea, the nitrogen gas which is evolved by this reaction being collected in a graduated tube. For the details of this process reference must be made to the original papers.1 1 Htifner, Journ. Trad. Chem. [2], iii. Russell and West, Journ. Cham. Soc. xxvii. 249.](https://iiif.wellcomecollection.org/image/b28122409_0001_0669.jp2/full/800%2C/0/default.jpg)
