Total Alkalinity as CaCO₃ from Species

Alkalinity is the water's capacity to absorb acid, and it lives in three species: bicarbonate below about pH 8.3, carbonate between roughly 8.3 and 11, and free hydroxide above that. An ICP or ion-chromatography report gives them as their own ions, but every alkalinity limit, every dealkalizer sizing chart and every LSI calculation wants a single number as CaCO₃. The factors are equivalent-weight ratios: 50.04/61.02 = 0.8202 for bicarbonate, 50.04/30.00 = 1.6679 for carbonate, and 50.04/17.01 = 2.9425 for hydroxide. A groundwater with 122 mg/L HCO₃⁻ and 30 mg/L CO₃²⁻ has TA = 0.8202 × 122 + 1.6679 × 30 = 100.1 + 50.0 = 150 mg/L as CaCO₃.

In the field you rarely get the species: you get a two-stage titration. Titrating to pH 8.3 with phenolphthalein gives P alkalinity, then on to about pH 4.5 with methyl orange gives total M alkalinity, and the classic relations back out the split: if P = 0 it is all bicarbonate, if 2P < M you have bicarbonate plus carbonate, if 2P = M it is pure carbonate, and if 2P > M you have carbonate plus hydroxide. Note that carbonate's 1.6679 is roughly double bicarbonate's 0.8202: a water whose pH has drifted up into the carbonate range gains alkalinity as CaCO₃ without gaining a single milligram of carbon, which routinely confuses operators chasing a runaway boiler.