The Alkalinity to Carbonate Component Correction

Suppose now that we have a carbonate alkalinity measurement - either of an unpolluted fresh water, or a measurement corrected for the presence of other anions. This quantity (defined by Equation (3.57)) is obviously somehow related to the quantity we need, which is mcos,total (Equation (3.51)), but there are two problems  

Essentially, we have three unknowns, mH2CO, mHCO-, and mco2-, and three equations relating them, Equations (3.53), (3.54), and (3.57). We do not count m0H- or mH+ as unknowns, because in those few cases where they become significant, we can 

Solving for the three unknowns, and letting A represent the (possibly corrected) alkalinity, we find they are 

we need estimates of the three activity coefficients, but they can be obtained from a preliminary speciation model, or from experience. The equations are presented in their complete form for reference, but in practice they can be greatly simplified. For example, in solutions having a pH less than about 8.5, Equation (3.70) can be ignored, and the other two simplify to 

Remember that, after calculating iH2co( the speciation model should be recalculated if in2co( contributes significantly to mco3,total- 

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