Originally Posted by waterbear
TA is usually defined as the amount of acid required to lower the pH of the sample to the point where all of the bicarbonate [HCO3-] and carbonate [CO3--] could be converted to carbonic acid [H2CO3]. This is the carbonic acid equivalence point or the carbonic acid endpoint. These equations show what happens to carbonate and bicarbonate as acid is added:
(1) H+ + CO3 ==> HCO3-
(2) H+ + HCO3- ==> H2CO3
True. The equivalence point occurs at around 4.5 pH.
For practical purposes and at the pH encountered in pools the contirbution to alkalinty of OH- is probably about .1% while the contirbution of bicarbonates is around 90%, carbonates around 7% and the remaining 3% would be composed of borates, phosphates, OH-, etc. This is, of course, with no cyanurates present. Cyanurates will contribute around 1/3 of the alkalinity at normal pool pH. Thus, for practical purposes TA will be essentially equal to carbonate alkalinity.
Also true so long as pH does not rise significantly.
It would stand to reason (correct me if I am wrong) that if the amount of carbonic acid in the buffer system was reduced by forced gassing off CO2 this would result in the lowering of the TA since the reverse of the 2 reactions above would produce a lower level of both bicarbonate and carbonate.
When CO2 is outgassed, this lowers the amount of H2CO3 so that pulls on HCO3- which pulls on CO3(2-). The missing piece, however, is the affect on the ratios of these 3 components as the pH rises due to the outgassing (from the H+ getting consumed in the reactions you showed). The lower H+ concentration is "filled in" from water as OH- so this latter species increases in exact correspondence with the CO2 outgassing. For every CO2 that escapes, there are 2 H+ that are consumed and 2 OH- that are created (from water). So when you titrate to get to the equivalence point, you have to consume all of this new OH- just as you would have had to consume the original CO3(2-) or HCO3-. This is why the measured Total Alkalinity does not change.
Also, if we are truly interested in measuring TA and not carbonate alkalinity wouldn't it make sense to include any alkalinity contirbutred by cyanurates? The fact that we do correct for them indicates that we are interested in the carbonate alkalinity, since this is what is active in the formation of scale (calcium carbonate) in the water and is also what is used in calculating the LSI ( or any other SI applied to pools for that matter)
When you do the titration test, you are measuring TA because you are also titrating the CYA-, CYA(2-) and CYA(3-) species. The reason we correct for CYA is to go from our measured TA to the actual Carbonate Alkalinity, just as you said. When CO2 is outgassed, that does not directly affect the cyanurates since they are not carbonates, but it does indirectly affect their ratios of concentrations due to the increased pH. Here again, since there is less H+ the following reactions take place according to LeChatlier's principle:
CYA --> CYA- + H+
CYA- --> CYA(2-) + H+
CYA(2-) --> CYA(3-) + H+
So here there are even more species adding to alkalinity making up for the loss from the carbonates. So yes, technically I should have included these and not just focussed on OH-. The CYA acts like an additional mini-pH buffer. All this means is that when CYA is present, the outgassing of CO2 will make the pH move a little less so less OH- is produced, but this is made up for by more alkalinity from the cyanurates.
