Thanks for ideas.
I have tried a complex solution pH calculator before and I found I couldn't work it- was too complex and more suited to preparation of buffer solutions in a lab than natural waters. Is this one user friendly enough in a kind of bucket chemistry approach?
Application is fertiliser solutions in agriculture with different borehole water starting alkalinity. Typically fertilisers are added at pretty low concentrations and resulting concentrations in fertigation water are mM or less. We usually use nitric, sulphuric, or phosphoric acid to reduce bicarb to 20-40ppm which generally gives acceptable pH. Usually the approach is to ignore anything but milliequivalents of bicarb, treat acids like H2SO4 as complete single H dissociation only, and work out a rough approximation. pH is usually in the right ballpark but never correct. I was wondering about the effect of ammonium and whether we should be trying to take account?
Or any different ideas about what might be throwing it off? Waters I am using typically have a fair bit of HCO3 which we do take account of but usually also some silicates which we do not. EC predictions we make are usually about right which makes me think we have most of the important ions involved.