Moisture content (sometimes shown as "Loss on Drying") is a characteristic that gets overlooked next to assay percentage, but for hygroscopic powders it can matter just as much to your actual cost and process performance.
What the Test Measures
Moisture content or loss on drying is measured by weighing a sample before and after controlled heating (driving off water without decomposing the chemical itself), and expressing the weight loss as a percentage. It's a standard characteristic on the spec sheet for hygroscopic products like caustic soda flakes, calcium chloride and many of the salt/mineral powders we stock.
Why It Affects What You're Actually Paying For
For a hygroscopic powder, moisture content and active-chemical assay are directly linked — every percentage point of absorbed moisture is a percentage point that isn't active chemical, which is exactly why poor storage (see our guide on storing hygroscopic chemicals) doesn't just risk caking, it can genuinely dilute what you're dosing without changing the label.
What to Check
If your process is sensitive to moisture content specifically — not just overall assay — check the moisture/loss-on-drying spec and result on the Certificate of Analysis for your batch, not just the assay number. For most general industrial dosing, standard assay specification already accounts for this indirectly, but for precision applications it's worth checking both numbers separately.