How to Calculate Bicarbonate Concentration
- 1). Convert from total alkalinity to molarity. Total alkalinity is usually a measure of milligrams per liter of calcium carbonate; divide by 100,000 (approximately) grams per mole to find moles per liter or molarity. Molarity is the concentration of a substance in a solution.
- 2). Substitute the equation for carbonate concentration as a function of bicarbonate concentration and pH for the equation for total alkalinity. The expression for total alkalinity is 2 x total alkalinity = [HCO3-] + 2 [CO3 -2] + [OH-]. (Note that in chemistry, the brackets around a species refer to its concentration, so [HCO3-] is the concentration of bicarbonate). The equation for carbonate concentration is [CO3 -2] = K2 [HCO3-] / [H+], where K2 is the second dissociation constant for carbonic acid. Substituting this expression will give us 2 x total alkalinity = [HCO3-] + 2 x ( K2 [HCO3-] / [H+] ) + [OH-].
- 3). Rearrange this equation to solve for [HCO3-]. Since pH = -log [H+], [H+] = 10 to the negative pH. We can use this information and some algebra to rewrite the equation as [HCO3-] = ( 2 x total alkalinity ) - ( 10 to the (-14 + pH) ) / ( 1 + 2K2 x 10 to the pH).
- 4). Plug the value for moles per liter of calcium carbonate you found previously into the equation to find bicarbonate concentration.
Source...