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Water Analysis - Hypothetical Combinations

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Calcium bicarbonate Ca(HCO3)2 5.0 gpg as CaCO3
Magnesium bicarbonate Mg(HCO3)2 2.0 gpg as CaCO3
Magnesium sulfate MgSO4 0.5 gpg as CaCO3
Sodium sulfate Na2S04 0.5 gpg as CaCO3
Sodium chloride NaCI 1.0 gpg as CaCO3
Total minerals 9.0 gpg as CaC03

These hypothetical combinations shown above are one of the ways of describing dissolved minerals in water.

Of course, all of the compounds listed would separate into ions when dissolved in water. Thus the various ions, not the complete compounds, would actually be present. However, if a chemist wanted to prepare a water sample having the same chemical characteristics as the sample which was analyzed, he could simply weigh out the amounts of the compounds listed, and dissolve them in water.

When hypothetical combinations are calculated, the ions are combined in their order of increasing solubility. As calcium compounds are generally less soluble than other compounds, calcium is usually first on the list of cations. Magnesium is second and sodium or potassium is last.

Similarly, the anions are used in the following order: hydroxides, carbonates, bicarbonates, sulfates, chlorides, and nitrates.

Note that all the various hardness mineral compounds listed above are expressed in grains per gallon as calcium carbonate (CaC03).

In order to make the calculations as shown, the concentrations of the ions must be expressed in such a way that they can be added and subtracted directly. This is similar to the conversion of 1/3 and 1/4 to 4/12 and 3/12 when these fractions are to be used in the same addition or subtraction problem.

Calcium carbonate has a molecular weight very close to 100, (actually 100.089) and an equivalent weight of 50 (50.045). It is possible that this is the reason for its selection as the basic compound, for it certainly simplifies the calculations.

If it is stated that a water sample has invisible hardness minerals in the amount of 10 grains per gallon as CaCO3, this hardness may be due to calcium or magnesium carbonates, bicarbonates, sulfates or chlorides or any combination of these compounds. But in every case the combined concentration is chemically equivalent to 10 grains per gallon of calcium carbonate, and the various calculations involved can be made with ease.

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Related Articles:

How to Interpret Water Analyses
Water Analysis Example NO. 2
Water Analysis Example NO. 3


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