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.
| || Vendor||Part # ||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
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.