Water Analysis - Hypothetical Combinations

Suppose for a minute we analyze the total mineral content of a typical water sample which has nine grains of minerals per gallon. It could well be water collected from Chicago, Detroit, Cleveland or any of a number of other cities drawn from the Great Lakes.

Diagrammed, these minerals would appear as shown on the chart below:

Diagram of Mineral Concentration of Water with 9 Grains of Total Minerals

NOTE: Analysis for sodium is not usually made directly in a water analysis. Its concentration is estimated by the difference between the total of the anions and the total hardness.

EXPLANATIONS

The bar at the left in the graph represents the cations of positive ions of the various minerals in the solution.

The bar at the right represents the anions or negative ions.

Remember that in all compounds the sum of the positive charges equals the sum of the negative charges. As a water analysis report simply gives the total of various compounds, the same holds true.

Our sample shows positive ions as follows: 5.0 gpg calcium, 2.5 gpg magnesium, 1.5 gpg sodium for a total of 9.0 gpg. The compensating negative ions are: 7.0 gpg bicarbonate, 1.0 gpg sulfate and 1.0 gpg chloride.

A chemist making an analysis of this 9 grain water could report its dissolved minerals in the following manner.

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

The hardness as CaC03 of the mineral compounds in water can be determined if the chemical analysis of the water is known. The concentrations of each of the hardness-forming impurities are divided by the equivalent weight of the compound and multiplied by the equivalent weight of CaC03. Here are a few of these equivalent weights:

To determine the equivalent weight of any mineral compound in terms of calcium carbonate:

equals the concentration of that mineral as CaC03

For example:

10.0 gpg MgSO4 X equivalent wt CaCO₃, / equivalent wt MgSO₄, concentration of MgSO₄, as CaCO₃,

10.0 X 50.045 / 60.187 = 8.3 gpg as CaCO₃

Traces of elements or compounds are not normally considered in these calculations. Iron, for example, would not be included, unless present in extremely high concentrations.

In the example shown above, the calcium and bicarbonates are combined first. The excess bicarbonates are then combined with the magnesium. The analysis still does not balance, and the remaining magnesium is combined with part of the sulfates present. The remaining sulfates and all of the chlorides are expressed as sodium compounds. (Adding 5.0 gpg Ca ++ as CaCO3, to 5.0 gpg HC03- as CaCO3, produces 5.0 gpg Ca(HC03)2 as CaC03, not 10 gpg.)

Table of Equivalent Weights