Calcium Hardness


Test Method Technical Information


Tests For: Calcium Hardness in natural and treated waters
Reagent Chemistry Used: Murexide
Basis of Test Method: Standard Method 3500-Ca-B, US EPA Ref. 215.2
Method Detection Limit*: 9 mg/L (11 mg/L for salt calibration)
Limit of Quantification**: 30 mg/L (35 mg/L for salt calibration)

*The Method Detection Limit (MDL) is defined as the minimum measured concentration of a substance that can be reported with 99% confidence to be different from the method blank results.
**The Limit of Quantification (LOQ) is the smallest quantity that can be detected with reasonable certainty for a given analytical procedure.

Testing for Calcium Hardness

Calcium hardness is caused by the presence of calcium ions in the water. Calcium salts can be readily precipitated from water and high levels of calcium hardness tend to promote scale formation in water systems. Calcium hardness is an important control test in industrial water systems such as boilers and steam raising plants, and for swimming pool waters.

Saltwater swimming pools use a chlorine generator for sanitisation. Swimming pool chlorine generators create chlorine in the pool water directly. For swimming pools containing salt up to a concentration around 3500 mg/L, the calcium hardness salt test should be used.

The Palintest Calcicol test provides a simple method of determining calcium hardness over the range 0–500 mg/l CaCO3.

Reagent Chemistry

The Palintest Calcium Hardness test is based on the Calcicol indicator reagent method. Calcium ions react specifically with Calcicol indicator in alkaline solution to give an orange coloration. The reagent itself gives a violet colour in solution. Thus, at different calcium levels a distinctive range of colours from violet to orange is produced.

The reagents for the method are provided in the form of two tablets. The test is carried out simply by adding one of each tablet to a sample of the water. The colour produced is indicative of the calcium hardness and is measured using a Palintest Photometer.


Magnesium hardness (up to 200 mg/l as CaCO3) does not interfere with the test.

Iron at levels above 10 mg/l may cause low results. Zinc above 5 mg/l may cause high results.

The pH required in the test is closely controlled by a buffer mixture included in the tablet formulation. However, to avoid exceeding the buffer capacity, strongly acid or alkaline samples should be adjusted to within the pH range 4 to 10, prior to the start of the test.

Best Practice Advice for Testing

  • At higher concentrations, tablet crushing technique can influence the result. Thoroughly crush tablets for best results.

EPA, Definition and Procedure for the Determination of the Method Detection Limit, Revision 2, Dec 2016.
IUPAC. Compendium of Chemical Terminology, 2nd ed. (the “Gold Book”).