Discussion
Total hardness by the EDTA method is determined at pH 10 and calcium hardness separately at
Answer: D
Correct answer: pH 12, where magnesium is precipitated as hydroxide
Raising the pH to 12 with sodium hydroxide precipitates the magnesium as its hydroxide, so it is no longer in solution to be titrated and the EDTA reacts only with calcium. Subtracting the calcium hardness from the total hardness measured at pH 10 gives the magnesium hardness, and a separate indicator such as murexide is used at the higher pH.
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