Nitrogen may generally be detected in an organic body by the odour of ammonia which is evolved when a little of the substance is heated in a test-tube with hydrate of potash. It is estimated on the same principle by the following arrangement of apparatus. A combustion tube drawn out to a point, as already described, is charged with soda lime in the place of oxide of copper. Soda-lime is made by slaking lime with strong solution of caustic soda, evaporating to dryness, and heatidg to redness. The same precautions are observed as before detailed, but instead of a chloride of calcium tube and potash bulbs, a three-bulbed piece of apparatus, of the annexed form, is attached to the open end of the combustion-tube, and is charged with moderately strong hydrochloric acid. The acid absorbs the ammonia that is evolved on gra dually heating the tube to redness ; residual ammonia being driven into the acid by a current of air drawn through the whole arrangement in the manner already indicated. If nitro gen be present in an organic substance it is liable, when burnt with oxide of copper, to pass off in the state of acid-oxides absorbable by potash, hence the estimation of carbon and hydrogen must only be proceeded with after a small bundle of copper turnings have been placed just inside the combustion-tube; this, when heated to bright redness, effects the decomposition of the oxides into non-absorbable nitrogen and oxygen.
So much for the practical operations connected with ultimate organic analysis. The weights of the chloride of calcium tube and potash bulbs having been ascertained, both before and after the combustion, a few simple calculations suffice for ascertaining the percentage amounts of the elements present. These calculations will perhaps be most easily described by an illustration. Suppose that sugar were the
substance experimented on, then :— The percentage of nitrogen is calculated from somewhat different data. The acid liquid in the bulbs containing chloride of ammo nium, formed by the combination of the ammonia with the hydro chloric acid, is placed in a small capsule, bichleride of platinum added to it, the whole evaporated to dryness over a water-bath, the residue washed with a mixture of alcohol and ether, and the insoluble crystals of the double chloride of platimun and ammonium collected upon a tared filter and weighed 220.52 grains of such crystals contain 14 grains of nitrogen, their weight, therefore, and the amount of the substance operated on being known, the percentage quantity of nitrogen contained in the matter under examination is easily calculated.
To determine the equivalent orcombining proportion of an organic cont. pound necessarily forms an important part of the labours of an analyst, and cannot be deduced merely from the data yielded by the operations that have been described. It is, however, ascertained with tolerable ease, if the compound can be made to unite and form a definite salt with eome body, the combining proportion of which is already well known. An analytical operation having then shown how much of the well-known body is thus introduced, an easy calculation indicates how much of the organic compound would combine with one equi valent of the other matter, and this number is obviously the combining proportion of the organic compound. This point having been obtained, theoretical considerations and the data obtained from combustion conduct to a formula for the compound submitted to organic analysis.