INTRODUCTION

Many pharmacopeial articles include water within their matrices and are also susceptible to water exchange with the surrounding environment. Total water content is an important quality attribute, and several methods for determining water content are provided in Water Determination 〈921〉. Some of the water may be tightly bound and not available to participate in chemical, biochemical, or physicochemical reactions (e.g., as hydrate salts), whereas some of the water may be more freely available to participate in reactions such as hydrolysis or may provide an environment that can support microbiological growth. It is important to establish what fraction of the total water is available (active), and the determination of water activity (a_w) provides this information. This chapter outlines the recommended methods to qualify, calibrate, and use water activity meters to accurately measure the water activity (a_w) of raw materials and products.