Quinaldine test

Only aldehydes with an a-methylene group (i, e., R—CH2CHO) condense with 3,5-diaminobenzoic acid to form quinaldines.92 Consequently, in the carbohydrate series only 2-desoxysugars will react. The usual procedure adopted is to add to the unknown sugar solution an equal volume of a 1.3 percent solution of 3,5-diaminobenzoic acid hydrochloride in 50 percent aqueous perchloric acid. If the carbohydrate solution contains a 2-desoxysugar it will assume a yellow color with a green fluorescence.93 Use of an ester instead of a salt of 3,5-diaminobenzoic acid reduces the sensitivity of the test. 

Into a second test tube is placed 4-9 c.c. of a sodium hydroxide solution approximately 0.01 N. The author has found that ordinary tap water may be used for a-, j8-, and -y-dinitrophenol and for quinaldine red. Comparison solutions of the methoxytri-phenylcarbinols are prepared in 0.1-0.5 N hydrochloric acid. A quantity of the same indicator, sufficient to produce a color approximately the same as that in the first tube, is then added. As a rule the stock solution of indicator will thus be diluted ten or twenty fold. It is best to employ for this purpose a burette graduated to 0.01 c.c. Then the solution is diluted with solvent (0.01 N NaOH, water, sodium carbonate, or hydrochloric acid, as the case may be) to the total volume of the first tube. The ratio of the quantity of indicator in the comparison solution and in the solution being studied is equal to the degree of coloration F. 

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