The hardness concentration is less than 1.0 mg/L as CaCO3 using the colorimetric method for hardness, but a titration on the same sample indicates a concentration of more than 3 gpg or 50 mg/L as CaCO3. Which is correct?

Document ID

Document ID TE302

Published Date

Published Date 27/05/2017
Question
The hardness concentration is less than 1.0 mg/L as CaCO3 using the colorimetric method for hardness, but a titration on the same sample indicates a concentration of more than 3 gpg or 50 mg/L as CaCO3. Which is correct?
Summary
Colorimetric and titration methods for hardness give different results
Answer
The colorimetric hardness test using calmagite indicator, Hach Method 8030, was designed for ultra-pure water or water that has been softened to remove hardness. This method was designed to measure low levels of hardness using a colorimeter or spectrophotometer at concentrations less than 4 mg/L (as CaCO3).

When the hardness concentration is well above 4 mg/L, this method may indicate an incorrect concentration. To see if this is the case, measure the hardness in the sample by titration using a test kit or buret. If the titration indicates that the hardness is much higher than 4 mg/L, the result using the colorimetric test may be incorrect. In this case dilute the sample using good quality deionized water to obtain a correct result with the colorimetric method. Then multiply the displayed result by the dilution factor.