BackgroundCalcium 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.
Materials | | 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 |
Method- 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 ResultsExample 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.
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