Hardness is a measure of the concentration of divalent metal ions such as calcium and magnesium (Ca2+, Mg2+) per volume of water. It is common to use as unit for this parameter CaCO3-equivalents in ppm (mg/L). Regardless of the unit chosen the parameter is always the sum of the concentration of the two metal ions.

In most water there is more calcium than magnesium.

Thanks Henrik. But if I have Ca hardness value ONLY, how do I have an idea of what the Mg hardness content is?

The parameter "hardness" is the sum of the concentration of Ca and Mg even it is given as CaCO3-equivalents. From just this parameter you cannot find out how much there is of each. If you are making the analysis yourself I can explain how you can seperate the two ions by changing the analytical procedure.

I was able to determine the Ca hardness content using HACH reagents. If you can explain the way to get Mg hardness measured without the need for any further reagents, I'll be interested. Awaiting your response please.

If you are not doing the real titration analysis with the standard method it is not so easy. The parameter you have determined is the Ca+Mg concentration.

If you were using a the standard method you could raise the pH to precipate Mg. Remove the precipate by filtering and then do the analysis again. This time you will only determine the hardness from Ca and then you can determine the Mg as the difference between the two results.

If your raw water resource is not change in some time interval, may be you can a couple test for determining Ca and Mg hardness separately. YOu can analize Cam and Mg hardness seperately with using standard methods (titration methods). Or you can measure Ca and Mg ion concentration with using ion chromatography. YOu can use this ration for the ca hardness and mg hardness.

Thanks. I have the standard method for the determination of total hardness, Ca, and Mg hardness and the all the reagents are on ground. I am just curious to find out the relationship between those two hardness types - Ca and Mg - should there be a situation whereby you could only determine one and use the relationship between those two to play around the other and have an idea of what could be the content.

You can determine the total and Ca hardness and the difference will be mostly due to Mg. If you can carry out the analysis for the associated anions, you can develop a bar diagram and see the exact amount of carbonate, non-carbonate hardness and the amount of neutral salts, if any. If the quality of water does not change over time, you can establish some relationship as well. However, occassional checks should be done to make sure that there is no drastic changes in the composition. Removal of Mg is usually more expensive than removing Ca.

Generally hardness will express as CaCO3 scale. Because the molecular weight of CaCO3 is 100. Therefore,it is very easy to represent hardness.

Both the hardness are important. Normally water contains more amount of Ca than Mg. By measuring Ca harness and total harness you can calculate Mg harness.

Henrik Andersen

Hardness is not a direct measurement of Ca and Mg. It is an indirect and approximate method. Please refer the book

Process chemistry for water and wastewater treatment by Benfield, Judkins and Weand

I am quite aware of the definition of hardness and the many units it can be expressed in.

In practice the hardness of natural freshwater comes from Ca and Mg unless it is anaerobic. If it it anaerobic Fe and Mn can also contribute.

I disagree with your labeling of the hardness parameter as "indirect" and "approximate" as the parameter is defined by the analytic method (titration by EDTA) and not an equation. Hence the results of the titration is the true value. Also it is quite repeatable and exact if it is performed by a skilled analyst.