The simple speciation method reports zero carbonate but the advanced method reports a small amount of carbonate. What do I do?
This is a common occurrence for samples with pHs between 8.0 and 8.3. The simple speciation method does not take the chemistry of carbonic acid into account. It simply assumes that no carbonate is present if the titration does not detect a carbonate equivalence point. The chemistry of carbonic acid, however, tells us that a small amount of carbonate will be present even at pH values a bit less than 8.3. The advanced speciation method takes this into account and reports a more accurate carbonate concentration. Bear in mind, however, that the amount of alkalinity assigned to carbonate in these sorts of cases typically is a small fraction of the total sample alkalinity. So, whether the carbonate is reported as zero or some small concentration probably makes little difference.
