I recently received some lab results from a sample which contained a lot of CO2, which degassed before the total alkalinity was measured by titration to a pH 4.5 end point. The best analogue for this sample would be a carbonated (sparkling) mineral water, which was measured 5 min after opening the bottle.
Is the total alkalinity, which was reported from these conditions correct? From my understanding, it is not, as degassing of the CO2 leads to a higher pH, which leads to a speciation shift from CO2 to HCO3-. This actually means, that more acid is required for the titration in order to reach the end point.
I have the feeling that there is a flaw in my argumentation as this would actually mean that for many samples, the actual total alkalinity cannot be correctly determined, or that you would have to measure the amount of degassed CO2 and correct alkalinity with a speciation calculation, which is not typically done.
