346 REVIS: NOTE ON THE DETECTION OF BENZOIC ACID IN MILK NOTE ON THE DETECTION OF BENZOIC ACID IN MILK. BY CECIL REVIS. THE use of benzoates for the preservation of milk has shown a considerable and wide-spread increase this year, though they seem to be seldom detected. This is undoubtedly due to the fact that benzoic acid is not usually looked for in a routine manner, and also, that the quantity of milk taken for official purposes is, as a rule, too small to allow of the examination when other tests have been made.As a further difficulty arises from the unreliability of some of the methods employed, it may be of interest to note that the following method gives certain results. While not entirely original, it embodies the most necessary procedure in the detection of benzoic acid, vie., that the removal of fat, protein, etc.: shall be done in an alkaline medium, other- wise there is much lisbility of the benzoic acid being removed with the precipitate or coagulum.The method is as follows : One hundred C.C. (not less) of milk are diluted with an equal volume of water, and, after the addition of 5 C.C. of 10 per cent. sodium carbonate solution, heated in boiling- water for two to three minutes ; 10 C.C.of 20 per cent. calcium chloride solution are then added, and the heating continued, until coagulation of the casein, etc., is complete. The liquid is then cooled and filtered, and the filtrate, neutralised with hydrochloric acid to litmus-paper. Ten C.C. of Fehling copper sulphate solution (not mixed with the tartrate solution), followed by 10 C.C.of a solution of potassium hydrate (containing 31-18 grms. per litre) are now added, and the liquid again filtered. The filtrate is poured into a separating-funnel, acidified with hydrochloric acid, and extracted with about 50 C.C. of ether. The ether is then washed three times with a, little distilled water. About 10 C.C. of water are now added to the ether in the funnel, together with 1 drop of phenolphthalein solution, and then a saturated solution of barium hydrate added gradually, until, on violent shaking, the aqueous layer remains pink.This is then filtered off into a porcelain basin and evaporated to about 5 C.C. The contents of the basin are filtered into a test-tube and dilute (1 in 100) acetic acid dropped in until the pink colour is discharged, after which two more drops are added.The liquid is then tested with 1 drop of 10 per cent. neutral, freshly-prepared solution of ferric chloride, when, in the presence of benzoic acid, the usual characteristic precipitate is obtained. This method will detect 0.02 per cent. of benzoic acid. The test with ferric chloride is the most reliable and characteristic, but it ie necessary that everything else shall, as far as possible, have been removed, which the above method ensures. When examining cream, 50 C.C. should be diluted to 200 C.C. with water and the same treatment applied.