This is a battle I don't seem to be able to win. The lab that analyzes your water does not measure bicarbonate. It measures alkalinity. It then calculates bicarbonate and carbonate from alkalinity and pH. Many labs do this incorrectly.
In potable water the only dissociated acid should be carbonic. The 'state' of the water then has two degrees of freedom: pH and carbo. Know those and you can calculate everything else: alkalinity, carbonic, bicarbonate and carbonate. Know pH and any one of the others, you can calculate the rest. Because the lab measures alkalinity and, as part of doing that, pH, pH and alkalinity are the obvious choices for specification of the state of the water. Many of the spreadsheets and calculators use bicarbonate as a proxy for alkalinity. They assume that alkalinity = 50*bicarbonate/61. This is, in fact, approximately true over the pH range of perhaps 6 to 9 and the approximation is pretty good. This simplistic approach can lead to problems where acidification is concerned as phosphoric acid and lactic acid actually contribute appreciable alkalinity to the water. Again, the introduced errors are small in most cases.
Perhaps the most poignant question would be 'If I show you how to calculate bicarbonate (which I can certainly do) from alkalinity and pH, what would you do with that number?'. Your answer would doubtless be 'Plug it into my spreadsheet because that's what it wants'. A robust spreadsheet would immediately convert it back to alkalinity but most seem to just consider it a proxy for alkalinity. I bellyache about this a lot because people who post questions about their water will often say 'my bicarbonate is...'. That's not what I need to know. I need to know bicarbonate and pH (so I can calculate alkalinity) or alkalinity and pH so I can answer their question. I will, in that later case, not calculate bicarbonate because I don't need to know it to solve most of the problems. What I do need to know is the total carbo and the fractions of that which are bicarbonate and carbonate so that I can compute the total charge on carbo species. It is that which is the key to most water problems.
