Effect of pH on the viscosity of grewia mucilage

Background: The stability and efficacy of liquid pharmaceutical preparations depend on the pH of the medium. Such liquid preparations may contain varied additives performing different functions. One of the qualities of oral liquid pharmaceutical preparations is appropriate viscosity for pumping and transfer during manufacture and dispensing to patients. Gums find use in such liquid preparations as thickening or suspending agents together with different additives that may influence the pH of the environment and hence the stability and quality of the preparation. Aim of the study: The purpose of this study was to determine the effect of pH on the viscosity of grewia gum obtained from Grewia mollis that is potential pharmaceutical excipient. Setting and Design: The study was based on experiments carried out in the laboratory setting and the conclusions were based on the observations made. Materials and Methods: Aqueous mucilage of grewia (2% w/v) was prepared and the pH was determined at different shear rates on Brookfield cone and plate rheometer at 25°C. Adjustment of pH was facilitated by the addition of 0.25 N solution of either hydrochloric acid or sodium hydroxide before the readings were taken. Results: The viscosity of the mucilage was characteristically pseudoplastic and it depended on pH of the medium and storage time. The viscosity ratio generally decreased from 2.046 to 1.470 as the pH of the medium increased from acidic to basic (2.18 to 13.10). The dynamic yield value of the dispersion at pH 2.55 and 5.08 were, respectively, 10.5 and 45. The viscosity of grewia gum dispersion changed with change in pH of the medium anomalously. Conclusion: Changes in the viscosity of grewia gum dispersion were observed with change in the pH in an unrelated fashion. This suggests that the use of grewia gum together with other additives in oral liquid preparations should be done with discretion.