If the sample turns purple during the hardness test but never turns blue no matter how much EDTA is add, what does this indicate?
Sample turns purple during hardness test
This is an indication that the sample contains an interfering metal. Dilute the sample and repeat the test. This may dilute the interfering metal to a level at which it does not interfere. If this helps, multiply the result by the dilution factor. If this does not help, follow the suggestions in the interference section of the procedure, found in the Water Analysis Handbook.