2[C_{ 6}H_{ 5}(CH_{ 2}Cl)] + Pb(NO_{ 3})2 = 2[C_{ 6}H_{ 5}(CHO)] + PbCl_{ 2} + N_{ 2}O_{ 3} + H_{ 2}O.
The crude benzaldehyde thus obtained is agitated with warm solution of acid sodium sulphite, the solution formed thereby is separated from undissolved oily particles and cooled, whereby a combination of benzaldehyde with acid sodium sulphate crystallizes out. This combination is separated from the remaining fluid, decomposed by acid and submitted to distillation, whereby benzaldehyde passes over. Large quantities of benzaldehyde are at present prepared according to this method. The identity of benzaldehyde with oil of bitter almonds has been established by Lippmann and Hawliczek.
Genuine oil of almonds is much adulterated, chiefly with alcohol, nitrobenzole, and various cheaper oils. An addition of 3 to 5 per cent. of alcohol is frequently made by Italian dealers in order to conceal a content of water, which at a low temperature is apt to render the oil turbid. To detect the presence of alcohol, moderately heat a sample of the oil in a distilling apparatus and compound the drops, first passing over with sodium carbonate solution and then with potassium iodide solution. In the presence of alcohol a yellowish crystalline precipitate of iodoform is formed.
An addition of synthetically composed oil might seem of no importance, since the natural oil does not differ from it. However, for very fine perfumery the natural oil cannot be replaced by the artificial, it having been thus far impossible to obtain the latter absolutely chemically pure. It always contains small quantities of undecomposed chlorine combinations which injure the taste and odor. To detect such oil in the natural oil, bring a few drops upon a tuft of cotton and ignite it. Over the burning flame invert a beaker moistened inside with water. On the moist sides of the beaker the soot and hydrochloric acid formed by the combustion of the chlorine combination are precipitated. When the flame is extinguished, the beaker is rinsed out with water, the fluid filtered and tested for chlorine with nitrate of silver. An addition of 10 per cent. artificial oil can in this manner be accurately determined.
If genuine oil of bitter almonds containing prussic acid, be heated with an excess of alcoholic potash lye, and the excess of the latter be neutralized with hydrochloric acid, benzoin amounting to 40 to 50 per cent. of the weight of oil of bitter almonds is, according to A. Kremel, separated. By subjecting artificial oil of bitter almonds to the same treatment, no benzoin is separated, so that the genuine oil can in this manner be distinguished from the artificial. Kremel further found that oil of bitter almonds prepared from apricot kernels, when treated in an analogous manner, yielded considerably less benzoin, and that cherry-laurel oil containing prussic acid, which has been considered identical with oil of bitter almonds, separated no benzoin whatever. Should further experiments prove the constancy of this phenomenon, this reaction would be a convenient means of distinguishing the four products.