Insoluble aconitate salts

The first indication that the recovery of insoluble aconitates from these sugar-containing juices was feasible was the initial work by Ventral33,76 and Ventre and Paine.77 These workers found that during the evaporation of sorghum juice an insoluble aconitate salt separated in amounts... 

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 procedure as outlined by Collier is very similar to that previously described. The molasses is neutralized with lime to a pH value of 6.8, heated, and the desired amount of an aqueous calcium chloride solution is added. This mixture is subjected to the usual precipitation conditions (90-95°, mechanical agitation) for approximately forty-five minutes and the soluble barium and/or strontium salts are then added. The heating is continued for another forty-five minutes and the insoluble aconitates are recovered from the molasses. The salts obtained from this latter process thus contain barium and/or strontium ions in addition to the calcium and magnesium cations usually present. 

From a consideration of these data, it would appear that precipitation of insoluble aconitates from molasses is an extremely complicated reaction and is a function not only of the relative amounts of acid and metallic ions present but also the status of the acid in solution. In this latter respect, it would seem that aconitic acid exists in molasses not only as free ionized acid but also as soluble complexes and the conversion of these soluble complexes to insoluble salts is an important yield-governing consideration. 

In large scale operations the aconitic acid is usually recovered from the crude calcium magnesium aconitates by acidification with a mineral acid followed by the crystallization of the aconitic acid from the liquors obtained. Thus Ventre, Henry and Gayle,88 acidified the crude salts with dilute sulfuric acid. The insoluble calcium sulfate was removed by filtration and the aconitic acid was separated from the magnesium sulfate by fractional crystallization. 

Ambler and Roberts,81-89 in subsequent work, found that if the calcium magnesium salts were heated to remove a portion of the water of hydration the magnesium content could be replaced by calcium by treating the dried salts with a hot concentrated calcium chloride solution. Thus the aconitates could be converted into tricalcium aconitate and subsequent treatment of this salt with sulfuric acid enabled the removal of the cations as insoluble calcium sulfate. The crystallization of aconitic acid from such a filtrate was therefore not complicated by the necessity of a fractional crystallization to separate the aconitic acid and the magnesium sulfate formerly obtained. 

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