Wohler’s synthetical method for preparative purposes usually assumes the following form. Powdered prussiate of potash (see Prussic Acid) is dehydrated by heating, the anhydrous salt mixed with half its weight of anhydrous binoxide of manganese, and the mixture heated in a flat iron pan until the deflagration, which soon sets in, is completed. The residue now consists of oxides of iron and manganese and of cyanate of potash, NCO. K, formed from the cyanide of potassium of the prussiate by oxidation. The cyanate of potash is extracted by means of the least sufficient quantity of cold water (hot water decomposes the salt, with the formation of ammonia and carbonate of potash), and the solution is mixed at once with the calculated weight of sulphate of ammonia, 1(NH 4 ) 2 SO 4 for 2NCO. K. The two salts decompose each other into sulphate of potash and cyanate of ammonia; the latter, however, passes spontaneously and almost instantaneously into urea :—

NCO. NH 4 =CO(NH 2 ) 2 .

Cyanate of ammonia. Urea.

The sulphate of potash is eliminated exactly as the nitrate of baryta is in the urine method. That urea is not cyanate of ammonia itself is proved, first by the fact that real cyanate of ammonia (preparable by the union of anhydrous cyanic acid with ammonia gas) is a different substance, and secondly by its not yielding any ammonia when dissolved in cold caustic potash ley. Urea behaves to certain strong acids like a feeble basis, uniting (CON 2 H 4 parts) with HNO 3 parts of nitric, ½H 2 SO 4 of sulphuric, and ½C 2 O 4 H 2 of oxalic acid into crystalline salts. As shown by its formula, urea is the amide of carbonic acid: i.e ., it is CO(OH) 2 - 2OH + 2NH 2 , and consequently an anhydride of carbonate of ammonia, CO(O. NH 4 ) 2 . An aqueous solution of urea, when heated to 200° in a sealed-up tube, breaks up into carbonic acid and ammonia; and dry carbonate of ammonia, when kept at a certain temperature, suffers partial conversion into urea and water.

Of the many methods for the quantitative determination of the urea in urine, the simplest is to treat a measured volume with excess of solution of hypobromite of soda (a solution of bromine in caustic soda ley) and to measure the volume of nitrogen evolved. By theory CON 2 H 4 + 3NaBrO = 3NaBr + CO 2 + 2H 2 O (the CO 2 is absorbed by the excess of alkali); but in practice, according to Dupre, through unknown causes only 91 per cent. of the nitrogen is actually evolved, which must be remembered in the calculation of the result.