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LaMotte calcium hardness test

This is an inexpensive colorimetric assay for measuring calcium in water.
Consists of two solutions, Calcium Hardness CH1 and CH2.
I bought both from Amazon: 
  • CH1 - LaMotte # 7042-G, $12.95 for 30 mL bottle
  • CH2 - LaMotte # 7041-G, $13.95 for 30 mL bottle
The method is straight-forward. Take a clean tube and fill with 5 mL of water sample.
Add 5 drops of CH1 and mix
Add 5 drops of CH2 and mix.
A purple/violet color is formed which is read with a colorimeter.

This assay appears to be based on the O-cresolphthalein complexone calcium colorimetric assay. From online research:
  • Calcium in alkaline solution reacts with o-cresolphthalein complexone to form a purple complex proportional to the calcium concentration. Magnesium ions, which also react, are removed by 8-quinolinol or 8- hydroxyquinoline.
  • The reaction of calcium with o-cresolphthalein produces a purple/red complex at pH 10-12 with an absorbance maximum at 575 nm. The intensity of the color, measured at 575 nm, is directly proportional to calcium concentration in the sample.
  • Ethanolamine is used to achieve a high-buffered pH