You can assume solution is dominated by carbonate alkalinity and ignore presence of other bases
That is exactly what I was thinking - that is enough for me, because the water we are using is mostly from wells - and that water is hard (caused by Ca(HCO3)2 ?) The calculations do not have to be too precise, it is meant for large agriculture - irrigated fields, vineyards etc.
My thougt process was like this:
original pH = 8
target pH = 6
original alkalinity 200
ppm HCO3 (I calculated the concentration to 0.0033 M)
having the Hendersson equation ph = pKa + log([A]/[HA]) I wanted to calculate the [HA] of the water and since there is CaCO3 as acid I used the pKa 9
8 = 9 + log(0.0033/[HA]) -> [HA]=0.033
So now I suppose I have the concentration of the conjugate base.
If I use the same equation, but with my target pH, pKa of acidifier H3PO4 (7,21) and [HA] I just calculated, do I get the concentration of the acidifier I must add to get to the target pH?
Or am I getting it all wrong? :-)