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WATER ANALYSIS—HYPOTHETICAL COMBINATIONSSuppose for a minute we analyze the total mineral content of a typical water. This one has, as we shall see, nine grains of minerals per gallon. It could well be the water which Chicago, Detroit, Cleveland or any of a number of other cities draw from the Great Lakes.
Diagrammed, these minerals would appear as shown on the chart below: Diagram of Mineral Concentration of Water with 9 Grains 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. EXPLANATIONThe bar at the left in the graph represents the cations of positive ions of the various minerals in solution. The bar at 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 analysts 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 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 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: Hardness Producing Compound Equivalent WeightMagnesium sulfate MgS04 60.187 Calcium bicarbonate Ca(HC03) 2 81.057 Calcium chloride CaCl2 55.493
To determine the equivalent weight of any mineral compound in terms of calcium carbonate: concentration equivalent weight of CaCO3,
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 WeightsCATIONS
ANIONS Hydroxide 17.007 Carbonate 30.005 Bicarbonate 61.017 Sulfate 48.031 Chloride 35.453 Nitrate 62.005 Phosphate 31.657 Fluoride 18.998 Sulfide 16.032 COMPOUNDS Aluminum sulfate 57.025 Calcium carbonate 50.045 Calcium bicarbonate 81.057 Calcium sulfate 68.071 Calcium chloride 55.493 Calcium hydroxide 37.047 Magnesium carbonate 42.161 Magnesium bicarbonate 73.173 Magnesium chloride 47.609 Magnesium sulfate 60.187 Sodium bicarbonate 84.007 Sodium carbonate 52.995 Sodium sulfate 71.021 (--> Next article)
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