When using UniVer® 3, the sample turns blue and then keeps fading back to pink. What causes this?

Document ID

Document ID TE292

Published Date

Published Date 27/05/2017
Question
When using UniVer® 3, the sample turns blue and then keeps fading back to pink. What causes this?
Summary
When using UniVer® 3, the sample turns blue and then keeps fading back to pink
Answer
If the sample contains high levels of calcium (>200 mg/L as CaCO3), calcium carbonate will precipitate due to carbonate in the UniVer® 3 reagent. The sample will turn blue but then slowly fade back to pink as the calcium carbonate slowly redissolves.

Keep adding drops of the Hardness 3 solution until the sample remains blue. Alternatively, start over and add several drops of the Hardness 3 titrant before adding the UniVer® 3. Include the initial drops when counting the total number of drops.