HCO3- / CO3-2 distribution

[liakos]

Firstly, forgive me if I post this on the wrong thread, seemed relatively right.

I got these chemical analyses of waters that I want to study. Some of them have a pH value > or = 8.3. In some of those analyses, carbonic compounds are given as HCO3- solely, which seems not right to me for pH >8.3, whereas others are given as both HCO3 and CO3. Concentrations are given as mg/L. My goal is to run Phreeqc for samples with pH>8.3 which have 0 CO3 in order to correct the values.
- Is it right to express those mg/L of HCO3 as "Alkalinity as HCO3" at a 1:1 in input?
- Can I rely on results like this output I attach in order to insert a valid value for CO3?
- Could you propose some theoritical concepts in order to lighten my darkness?

Best regards,

[dlparkhurst]

For Alkalinity, PHREEQC expects a value for the total alkalinity in solution, including HCO3-, CO3-2, and possibly other species like H2BO3- or H3SiO4-.

If the input value is given in ppm, it is converted to eq/kgw using a gram equivalent weight--in your case it is about 61 as calculated from the formula "as HCO3" (plus a correction for conversion from per kg solution to per kilogram water).

The measurement of alkalinity is almost always a titration of the sample with acid. From the total amount of acid added to an endpoint (often the inflection point in the titration, but there are Gran methods, and other options), the number equivalents of alkalinity can be calculated. That is really the number that is required for PHREEQC. The lab may report the total alkalinity as ppm HCO3, mg/L CaCO3 (gram equivalent wt 50), or possibly other units. The lab may have some formula where they split the total alkalinity into contributions from HCO3- and CO3-2 (based either on pH, or on the titration curve), in which case the total alkalinity would be the sum of the two contributions (Alk(tot as HCO3) = Alk(HCO3-) + 2*61/60*Alk(CO3-2)). Sometimes the lab will report both total alkalinity and the HCO3- and CO3-2 components, in which case I would use the total.

Roughly, m(HCO3-) = m(CO3-2) at pH 10.33. At pH 8.3, M(HCO3-) = 100*M(CO3-2), so at this pH and lower, the CO3-2 is negligible. 

[liakos]

Dear David,

thanks for taking the time to explain.

Best regards,

Ilias