Moisture content in chemicals is measured by Karl Fischer titration for water alone, or by loss on drying for everything that evaporates. A salt purity test is normally a titration of the chloride content, reported as sodium chloride. The method named on the certificate matters as much as the number.
Two labs can report different moisture figures for the same sample if they use different methods, so write the method into the contract.
How is moisture measured?
Karl Fischer titration reacts with water directly, in a volumetric or coulometric instrument, and is specific to water. It suits solvents, oils, organic chemicals and low-moisture powders, where accuracy at small levels matters.
Loss on drying heats a weighed sample in an oven, often at 105 degrees Celsius, and weighs it again. It is simple and common for salts, minerals and many powders, but it counts anything that leaves, including solvents and volatile impurities, not just water. A quick moisture balance gives a similar result with a heated pan.
Which method should the contract name?
- Use Karl Fischer where water must be known precisely, as in solvents and moisture-sensitive products.
- Use loss on drying for salts and mineral powders where the drying temperature and time are set in the specification.
- Always state the temperature, time and sample size, because they change the result.
- State whether the figure is as shipped or on a dry basis.
How is salt purity tested?
Most labs titrate the chloride in a dissolved sample against silver nitrate and report the result as sodium chloride. Other tests measure insolubles, calcium, magnesium and sulphate, which affect industrial uses such as chlor-alkali. Some reports give purity by difference, subtracting the impurities from the total.
Purity can be quoted on a dry basis or on an as-is basis, and the two differ by the moisture content. A 99% figure on a dry basis can be lower as shipped. See what purity industrial salt should have and the tests on a bentonite or salt COA.
How do you make sure the sample is representative?
A good method on a bad sample gives a bad result. Moisture changes as soon as a sample is exposed to air, so the sample should go into a sealed container straight away. Take increments across the lot, mix them and divide, rather than scooping from the top. Our guides to drawing a chemical sample and checking moisture and purity at loading cover the practice.
Keep a retention sample so that a dispute can be settled by a referee lab. The agreed method and the lab should be written down before the order, not after the argument starts. Many disputes about moisture are really disputes about timing, because a sample tested at the factory and again weeks later in a humid port will differ. Say when the test counts: at loading, at discharge or on a dry basis, and which party bears the difference.
Next step
We have lots tested by independent labs against your specification, using the method you name. To set the test plan for your product, book a free sourcing review.