I wrote a post a few weeks ago on ocean acidification. The message was that there is more to acidification than pH, and that it's basically an issue of equilibrium between carbonate species. Difficulties of measuring pH can be avoided.
In the course of discussion, I said I would try to develop an active calculator to display some of the principles.
It calculates equilibrium adjustments, and I expect that is a rather specialised interest. I'm putting it forward to demonstrate
- that any two of the species are sufficient to determine the others - pH is not essential and can be treated as just a following variable
- CO2 when dissolved is almost completely reacted (at usual pH)
The gadgetThere are four species (H+,CO2,HCO3- and CO3--). For details of the chemistry, see my previous post. I also have two combinations, dissolved inorganic carbon (DIC) and total alkalinity (TA). These are conserved during the reaction as total C and nett charge. There are six bars, four in gold for the species, and two for the combinations. They are scaled on a negative log scale (like pH). You can click on each bar to vary each constituent.
But first you need to set a constraint using the radio buttons at the top. The default setting is marked "Add". Here you can only vary the species (gold). Your click will indicate what the concentration would have been if the change (+ or -) happened without reaction. But in fact your addition is added appropriately to DIC and TA, and then the equilibrium adjusts with these held constant. Sometimes almost all what you have added reacts, and you see only a small change in that variable.
If you click another radio button, you move to "set" mode. The column you have marked is held fixed, and the column you click (a different one) will move to the value you have set (if possible - see below). "Set" is a slight misnomer - it's better seen as the values the other components are known to have when the "set" values have been measured. The blue bars show the current state, with log values beside. The absolute values show in the table on the right, with the pK values. The intent,
Update: You can now vary pK1 and pK2 (but see Notes below). Modify the text box and click the new "Accept Data". You can use this to look at totally different equilibria if you want. To make it monoprotic, just set pK2 to, say, 15.