Analytical Estimation of Aconitic Acid

Three separate and distinct methods have been utilized for the determination of aconitic acid in sugar cane and/or sorghum products. One involves the extraction of aconitic acid from the sample with an organic solvent, the second is based upon the decarboxylation of aconitic acid while the third employs the polarographic technique.5  

The first of these, utilized by Yoder, McCalip and Seibert,34 and by Balch, Broeg and Ambler,37 provides for the extraction of the aconitic acid from the sample being investigated, usually with diethyl ether, and the subsequent isolation of the acid from the solvent. In dealing with solid samples, e.g. alkaline earth aconitates, evaporator scale, etc., the prescribed procedure is to dissolve the material in aqueous mineral acid and to extract the acid solution exhaustively with ether. The ether extract is then evaporated under reduced pressure, the dried residue titrated with standard alkali and the titratable acid calculated as aconitic acid. In dealing with such solid samples it is often necessary to make an additional determination for oxalic acid which otherwise would be assumed to be aconitic acid.37 The aconitic acid in liquid samples is usually precipitated as the insoluble lead salt which is separated and treated as any other solid sample. In some cases this procedure is unnecessary and the liquid samples are merely acidified with a mineral acid and then extracted with ether.37 This method for the determination of aconitic acid, however, requires a considerable amount of time and is further complicated by the interference of ether-soluble waxes and non-volatile acids. 

The second method, developed by Ambler and Roberts,30-33 involves the decarboxylation of aconitic acid. These workers found that aconitic 

A number of materials,10 water, certain organic acids, the most important of which is citric acid, and certain inorganic salts, interfere with the determination. The decarboxylation cannot be conducted in the presence of nitrates and when carbonates are present two separate determinations of carbon dioxide are necessary. The procedure is also inapplicable when oxidizing compounds which are soluble in the hot reagent are present. Another interfering substance, sulfur dioxide,01 can be eliminated by the use of a saturated, acidified (sulfuric acid) solution of potassium dichromate to wash the gases evolved by the decarboxylation procedure. 

Although polarographic studies of aconitic acid in pure solutions had been made by several investigators, the application of such techniques to the determination of aconitic acid in sugarhouse products had been neglected.64-69 Recently, however, a procedure has been developed 

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