Calcium hardness in water can be determined with EDTA titration as described in APHA Method 3500-Ca.
With this method the pH of the sample is raised by the addition of sodium hydroxide. Magnesium is largely precipitated. An indicator is used which combines with calcium only.

Erichrome: During titration the color changes from red through purple to bluish purple to a pure blue with no trace of purple tint.

  VendorPart # Price  Shelf life
 Eriochrome® Blue-Black R, Dry Form in NaCl, for Calcium  LabChem LC140609$26.40 for 100 g  36 months
 Sodium hydroxide, 1N (1M) LabChem LC243502 $17.80 for 1L 24 months
 0.01M EDTA titrant LabChem LC138501 $13.80 for 500 mL 24 months
 1,000 ppm calcium carbonate standard, 
(1 mL = 1 mg CaCO3)
 LabChem LC127007 $21.50 for 125 mL 24 months


  • To a 10 mL sample, add 0.4 mL 1M NaOH solution
  • Transfer 2 mL to a cuvette
  • Place in colorimeter with stirring
  • Add 40 mg indicator dye
  • In plotting program, select red led and calibrate
  • Titrate with 0.01M EDTA. Increments of 10-25 uL for a 2 mL sample
  • Optional: use endpoint detection program to report volume of titrant

Sample Results

Example from a 200 ppm calcium carbonate sample shown below. Endpoint program returned an EDTA titrant volume of 398.75 uL. Very close to expected value of 400 uL for a 200 ppm standard. 

Joanne Long,
Apr 8, 2013, 4:48 PM
Joanne Long,
Apr 8, 2013, 5:07 PM
Joanne Long,
Jul 19, 2014, 6:38 PM