How to derive "ppm (mg/L) Total Alkalinity (as CaCO3)" from mineral ion ppm's (mg/L)

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If you know your waters ppm (mg/L) ion concentration of CA++, Mg++, Na+, Cl-, and SO4--, then:

ppm (mg/L) Total Alkalinity (as CaCO3) ~= 50.0435(Ca++/20.039 + Mg++/12.153 + Na-/22.99 - Cl-/35.453 - SO4--/48.031)

 

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And if you leave off the CaCO3 'Eq_Wt' multiplier (50.0435) you have calculated the mEq/L value of your Total Alkalinity:

mEq/L Total Alkalinity ~= Ca++/20.039 + Mg++/12.153 + Na-/22.99 - Cl-/35.453 - SO4--/48.031

 

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Any slight error is mainly going to be due to not also considering within the above listed equation(s) such (generally minor) water ion constituents as iron, potassium, nitrate, manganese, fluoride, etc...

"Generally" not including these doesn't seem to throw things off by more than perhaps +/- 5%.

 

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mEq/L Cations ~= Ca++/20.039 + Mg++/12.153 + Na-/22.99

mEq/L Anions ~= Cl-/35.453 + SO4--/48.031+ Total_Alkalinity/50.0435

You can tell if/when water is "real" simply by seeing if its reported major Cations and Anions are in close agreement on a milliequivalents per liter (mEq/L) basis. Water averaged from two or more sources will most likely prove to be found impossible to exist (as averaged and analytically reported thereby) in the real world. Water profile analytical specs merely dreamed into existence on paper will also most likely prove to not possibly exist in the real world. For water to be factually real its Cations must equal its Anions. Error in the above given equations will exist such that Cation and Anion mEq/L's will deviate in value to the degree that minor ions such as those listed in my post directly above have not been included.

 

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Any single 'major' ion value that is missing from a water report can be "calculated" by rearranging and then solving the equation for the missing ion, just as initially was done for the case of total alkalinity. Likely in most cases (though clearly not all) to within about a ballpark of 5% accuracy.

In this it helps to understand that Cation mEq's must equal Anion mEq's.