Stabilization of ampicillin analogs in aqueous solution. I. Assay of ampicillin in solutions containing benzaldehyde by iodine colorimetry and the effect of benzaldehyde on the stability of ampicillin.

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Abstract
The interaction of ampicillin and benzaldehyde in aqueous solution was investigated. A modified iodine colorimetry (I2-colorimetry) was used for the assay of ampicillin in solutions containing ampicillin and benzaldehyde. Ampicillin, including bioactive ampicillin-like substances which are present in aqueous solution, could be determined by I2-colorimetry after diluting the solution and adjusting it to pH 3.5-4.0. The time courses of degradation of ampicillin with benzaldehyde in phosphate buffer (pH 8.00) at 35.degree. C were followed by this method and the reaction is pseudo-first-order. Benzaldehyde inhibited the degradation of ampicillin under these conditions. In view of the above results and the changes in the UV spectra of the solution, it is concluded that ampicillin and benzaldehyde form a complex.