Questions about alkalinity calculations

[Alexandra]

Hi,

I have a few questions regarding alkalinity calculations using both SpecE8 and React:

1) Is the alkalinity calculated as total alkalinity (including species other than carbonate species), or is it only calculated as carbonate alkalinity?

2) If total alkalinity includes other species, which species are included?

3) Can other alkalinity units be used for the calculation (e.g., meq)?

I'm trying to calculate dissolved CO2(aq) concentrations in natural waters based on pH and Alkalinity measurements (have converted the values to mg/kg CaCO3) however the numbers obtained with GWB (v. 7.0 Professional) result in very different values for CO2(aq) compared to Minteq and Phreeqc for some of our samples.

Thanks,

Ale

[Tom Meuzelaar]

Hi Ale:

The alkalinity reported by GWB is carbonate alkalinity, computed as follows:

Alk =  50.0445 * 1000 * (m_HCO3 + 2 m_CO3) * (kg_solv/kg_soln)

For computing total alkalinity from the above, refer to Craig Bethke's Geochemical and Biogeochemical Reaction Modeling, page 220 (Chapter 15).

I hope that helps,

Tom Meuzelaar

RockWare, Inc.