496 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Methyl, ethyl, propyl, and butyl paraben are widely used in many types of pharmaceutical preparations such as ophthalmic, oral, and topical dosage forms and in cosmetic creams and lotions. They are used singly or in combina- tion with one another. Mixtures of the parabens have been shown to be more effective as preservatives than the individual parabens (5, 6). Their hydrolysis yields p-hydroxybenzoic acid, which possesses little preservative activity. It was the purpose of this investigation to study the stability of the esters of p-hydroxybenzoic acid over pH range of 3-9 and various temperatures. EXPERIMENTAL Reagents and Apparatus Reagents used were p-hydroxybenzoic acid (mp 213-214øC), methyl paraben (top 126-128øC), ethyl paraben (mp 116-117øC), propyl para- ben (mp 95-97øC), and n-butyl paraben (mp 73øC), all recrystallized from hydroalcoholic solution also, sodium hydroxide, monobasic potassium phosphate, potassium chloride, phosphoric acid, and chloroform, all AR grade. All pH's were measured using a Beckman model H-2 pH meter* and spectra were obtained with a Beckman DU spectrophotometer* with 1-cm quartz cells. A constant temperature bath with a Haake thermoregulator, series ED,t with a thermometer calibrated to 0.1øC was used for all the studies. Procedure Preparation o[ Buffers The buffers used in this investigation were prepared from monobasic po- tassium phosphate solutions of desired molarity which were adjusted to the desired pH with sodium hydroxide pellets and phosphoric acid. Buffers were adjusted to constant ionic strength of either 0.3, 0.6, 0.9, or 1.2 by the addition of potassium chloride. All buffers were prepared at 25øC. Since the studies were conducted at elevated temperatures, the pH of the buffers at those temperatures would be different from the pH determined at 25øC. The pH values of the buffers were calculated using the ion product of water at the various temperatures of the study, 40 ø, 50 ø, and 70øC. For example, a buffer of pH 9 at 25øC would have a calculated pH of 8.24 at 70øC. The pKw of water at 70 ø C is 12.82 (7, 8 ). Calibration Curves for the Paraber• For each of the parabens studied, calibration curves were prepared using chloroform as the solvent. The optical density of each paraben dilution was -- *Beckman Instruments, Fullerton, Calif. ?Gebriider Haake, Berlin, Germany.

KINETICS OF DEGRADATION OF PARABENS 497 determined at 253 nm, the wavelength of maximum absorbance, using chloro- form as the blank. A Beeifs law relationship was obtained for methyl, ethyl, propyl, and n-butyl paraben in chloroform at 253 nm. Method of Analysis The following general procedure was developed and used for stud,ving the effects of various factors, i.e., pH, temperature, ionic strength, on the rate of hydrolysis of the parabens. An accurately weighed quantity of paraben was added to a 100-ml volumet- fie flask and dissoIved in a small volume of appropriate buffer solution with the aid of heat. The solution was diluted to volume with the buffer solution which had been preheated to the temperature at which the particular analysis was made. The flask was placed in a constant temperature eirculator bath that had been previously adjusted to the desired temperature (-0.1øC). The solution was allowed to thermally equilibrate for 10 min, then a 5-ml aliquot, representing the zero hour sample, was withdrawn and transferred to a separatory funnel which contained g0 ml of chloroform. At pH 9.0, p- hydroxybenzoie acid (pK• 4.48) exists as a salt which remained in the aqueous phase when the solution was extracted with chloroform. Complete extraction of the unclecomposed paraben was accomplished using 3 x 20 ml of chloroform extractions. The combined chloroform extracts were filtered into a 100-ml volumetric flask through filter paper that had been previously wetted with chloroform. The solution was diluted to volume with chloroform, and the absorbance was determined at 253 nm using chloroform as the blank. Samples were removed at definite time intervals and analyzed by this procedure. Since p-hydroxybenzoie acid is the major degradation product of hydrolysis of the parabens (1, 9), mixtures containing known concentrations of methyl, ethyl, propyl, and n-butyl paraben and p-hydroxybenzoic acid were prepared. The ester concentration in each mixture was determined using the extraction procedure previously described. The per cent recovery of the parabens alone and in combination with varying concentrations of p-hydroxybenzoic acid is shown in TabIe I. The initial concentration of each paraben in all subse- quent studies was 5 rag/1. pH and Temperature Effects In order to study the effect of pH and temperature on the rate of hydrolysis of methyl, ethyl, propyl, and n-butyl paraben, the reactions were carried out at three temperatures in buffer solution as follows: 70øC-pH 2.75, 3.66, 4.58, 5.49, 6.41, 7.33, 8.24, and 9.16 50øC-pH 9.16 40øC-pH 9.16. Reactions were carried out for a maximum of 160 hours, and all aliquots were analyzed using the extraction procedure described above.