Closely related to the bisulphite method is the Sulphite method, which is based on the observation that certain aldehydes and ketones react with neutral sodium sulphite to form water soluble compounds with the simultaneous formation of sodium hydroxide. In as much as the latter tends to reverse the reaction, it should be neutralized with dilute acid from time to time as it is formed. It was Tiemann1) who, in connection with his studies on citral, first pointed to the application of this reaction for quantitative determinations. Somewhat later Sadtler'2) claimed that the method was generally applicable for the quantitative determination of saturated and unsaturated aldehydes both of the aliphatic and aromatic series, and was also applicable in the case of several ketones. According to his method the free alkali is titrated with half-normal hydrochloric acid. It suffers from the disadvantage that the sodium sulphite solution cannot be titrated sharply, hence the end of the reaction can be determined only approximately. Accurate results are, therefore, not obtainable.

Practical significance the sulphite method has acquired only through the modification suggested by Burgess.5) The process is as follows:

1) Berl. Berichte 31 (1898), 3317.

2) Americ. Journ. Pharm. 70 (1904), 84; Journ. Soc. chem. Industry 23 (1904), 303; Journ. Americ. chem. Soc. 27 (1905), 1321.

3) Analyst 29 (1904), 78.

To 5 cc. of the oil to be examined and contained in a cassia flask1) a freshly prepared saturated (40 p. c.) solution of neutral, crystalline sodium sulphite and a few drops of phenolphthalein are added. The mixture is heated in a water bath while frequently shaken. The sodium hydroxide set free is almost neutralized from time to time with dilute acetic acid (1:5) until even the addition of more sodium sulphite solution causes no further reddening while the mixture is being heated. Water is then added to drive the non-dissolved oil into the neck of the flask where, upon cooling, its volume is read off on the scale. Multiplication of the number of cc. of oil with 20 yields the volume percentage of aldehyde or ketone in the oil.

Although this method is by no means as applicable2) as claimed by Sadtler and Burgess, it is applicable in several cases where the bisulphite method fails. Its principal value lies in the possibility to assay carvone and pulegone, of which the former reacts smoothly and rapidly with neutral sodium sulphite. In the case of pulegone the reaction is much slower, several hours being required at times to bring the process to completion.

Satisfactory results are also obtained with citral and cinnamic aldehyde. It should be remembered, however, that the results may deviate somewhat from those obtained with the bisulphite method in the case other aldehydes are present. This is true e. g. in the case of lemongrass oil which gives higher results with the bisulphite method than with the sulphite method. This is due to the fact that neutral sodium sulphite reacts with citral only whereas the other aldehydes present in lemongrass oil, likewise a part of the methylheptenone react with the bisulphite. It is urged, therefore, that in connection with statements as to the aldehyde content of oils, the method be mentioned according to which the aldehyde assay was made.

Which of the two methods is to be preferred in individual cases depends altogether on the nature of the oil in question.

1) Burgess makes use of a flask of 200 cc. capacity which differs from the ordinary cassia flask in being furnished with a tube, attached to the side of the neck, that leads to the bottom of the flask and serves for the addition of the liquids. The common cassia flasks, however, are much more convenient and serviceable, but it is best to use one with 200 cc. capacity and to take 10 cc. of oil for the sake of greater accuracy.

2) Report of Schimmel & Co. April 1905, 103.

According to Burgess, one of the advantages of the sulphite method consists in the absence of crystalline deposits which are occasionally formed at the zone of contact between oil and water in the case of the bisulphite method and which disturbing factor at times prevents an accurate reading of the volume of the oily layer. Schimmel & Co., however, observed that such depositions are also produced in the case of the sulphite method.

For the assay of cinnamic aldehyde the gravimetric method of Hanus1) can be utilized. It yields very accurate results and is especially applicable when only small amounts of oil are available. In the case of the Ceylon cinnamon oil, the results deviate from those obtained with the bisulphite method by as much as 4 to 5 p. c, even if the latter are computed in percentage by weight. Presumably this difference is due to the presence of other aldehydes which are determined with the cinnamic aldehyde by the bisulphite method.2)

Hanus separates the cinnamic aldehyde as semioxamazone and proceeds according to the following method:

To 0,15 to 0,2 g. of oil, contained in a 250 cc. Erlenmeyer flask, 85 cc. of water are added and the oil finely divided by shaking. In order to make possible a fine division of the oil and thus to prevent a part of the aldehyde to be coated by semioxamazone thereby preventing it from taking part in the reaction, it is expedient to dissolve the oil in 10 cc. of alcohol (95 to 96 percent by volume) before adding the water. One and one-half times the amount of semioxamazide,3) dissolved in 15 cc. of hot water, are then added and the mixture shaken thoroughly for 5 minutes. It is then set aside for 24 hours with occasional shaking. Care should be taken to shake the mixture repeatedly during the first three hours. The flocculent precipitate of semioxamazone is filtered by means of a Gooch crucible (prepared with asbestos, dried and weighed), washed with cold water and dried at 105° until of constant weight. If a is the amount of cinnamic aldehyde semioxamazone, s the amount of oil used, the percentage of cinnamic aldehyde can be computed with the aid of the formula a .60,83 / s