EVALUATION OF BUFFERING CAPACITY AND ACID NEUTRALIZING-PH TIME PROFILE OF ANTACIDS

The antacid properties of seven antacids listed in the hospital formul ary of a medical center were evaluated with in vitro tests. These incl uded not only the preliminary antacid test and acid-neutralizing capac ity test as described in the United States Pharmacopeia (USP XXIII), b ut also a buffering pH profile test. The preliminary antacid test meas ured the final pH of a 10-mL solution of 0.5 N HCl 10 minutes after ad dition of the minimum recommended dose of an antacid, while the neutra lizing capacity test measured the amount (mEq) of HCl neutralized by t he minimum recommended dose in 15 minutes. The buffering pH profile re corded the ph time course of dynamic simulated gastric fluid neutraliz ation by a dose of an antacid. In the preliminary antacid test, magnes ium oxide showed the highest pH (9.52 +/- 0.14, mean +/- standard devi ation, n = 3); aluminum phosphate gel yielded a final pH of 2.51 +/- 0 .01, thus failing to meet the criteria of an antacid (pH > 3.5). In th e acid-neutralizing capacity test, hydrotalcite had the highest neutra lizing capacity (28.26 +/- 0.3 mEq), while sodium bicarbonate had the lowest (7.40 +/- 0.12 mEq). In the buffering pH profile test, aluminum -magnesium hydroxide suspensions and hydrotalcite tablets maintained a steady optimum pH (3-5) for around 1.5 hours. One tablet of calcium c arbonate, sodium bicarbonate or magnesium oxide could not raise the ga stric pH to above 3, but two tablets increased the pH excessively (5.3 to 8.6). The higher dose (two tablets) of aluminum hydroxide hexitol complex could not raise the pH to the optimal level. These findings de monstrate that there is disparity in the antacid effectiveness estimat ed by the neutralizing capacity test and the buffering pH profile test and suggest that the efficacy of an antacid cannot be accurately pred icted from its acid-neutralizing capacity. The dose of antacids greatl y influences the neutralizing pH profiles. Aluminum-magnesium compound s appear to provide steadier buffering than carbonate compounds or mag nesium oxide.