Is a Calcium Hardness Test Different from a Total Hardness Test?

I have had a few discussions lately concerning calcium hardness testing, and there is one question in particular that seems to permeate the conversations: “Is the calcium hardness titration test really that different from a test for total hardness?” Interesting question.

The total hardness test uses a buffer reagent (mostly ammonia compounds) to adjust the pH, an indicator, and a titrant (chelant) to tie up the total hardness, which is what turns the indicator from red to blue at the endpoint of the reaction.

Total hardness consists of calcium and magnesium hardness, so how do we test for only calcium hardness? A calcium hardness test uses sodium hydroxide as the buffer. The sodium hydroxide raises the pH to a higher level, and the hydroxide ions react with magnesium to precipitate the magnesium as magnesium hydroxide. Since the magnesium is no longer reactive, the addition of the indicator and titrant only react with the calcium in the sample. Note: The “red floater” you might see in the sample before the titrant is added is actually the precipitated magnesium hydroxide. These floaters do not affect the test results.

Although there are many kits on the market for testing calcium and total hardness, there are currently no test strips that can test both. The strips available today are only able to test total hardness.