Pharmaceuticals and Water Activity
The pharmaceuticals industry has been measuring moisture for decades. Why?
Because most people think water is the enemy of API stability. Turns out they're only partly right.
The Difference Between Moisture and aw
Moisture is how much water is in a product. Water activity (aw) measures the water's energy level. The energy level is much more important to reactions that hurt pharma products. Let's take three famous ones: API hydrolysis, crystallization that affects dissolution rates, and caking/clumping of powders. All of these are time-dependent reactions with rates influenced by water activity. For example, USP 1112 specifically states that water activity can be used to reduce degradation of API formulations susceptible to hydrolysis.
Migrating Moisture
Moisture migration is another quality problem that crops up in the pharmaceutical industry. This typically happens when gelcaps are filled. Sometimes the capsules crack while sitting in packaging, rendering the product unsuitable. This happens because the water activity of the powder and capsule are different, so moisture leaves the gelcap and enters the powder. Note that water activity is the driving force for migration - moisture content is not.
See how moisture migration really works >>
Water Activity as Just a Better Way to Measure Moisture
For a particular product, water activity and moisture are linked: if one goes up, so does the other. So even if you just want a better way to measure moisture, water activity can give it to you. Why?
- Water activity meters are much more sensitive to changes in moisture than most moisture content meters. This is because on dry pharma products, a tiny change in moisture creates a large change in water activity. When compared with AOAC approved moisture methods like oven drying, water activity gives you about 10x more precision.
- Karl Fischer titrations are renowned for their precision, but lack speed, ease of use, and affordability. A water activity test gives you the precision of Karl Fischer without the costly reagents, glassware, and specialized training.
>>Learn More About Water Activity Meters