The method of determining calcium in drinking water by chelatometry (Calcein as an indicator) could not obtained reliable results under the hitherto employed condition, pH at the titration or the concentration of indicator etc., because of the improvement of reagent-purity. It is necessary to adjust the pH of sample to more than 12.5 and to dilute the concentration of indicator as far as ten to fifty times of it. For the adjustment of pH in the sample, a solution of potassium hydroxide was more suitable than sodium hydroxide solution used.
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