HOW TO CALCULATE BICARBONATE, CARBONATE AND HYDROXIDE FROM TOTAL ALKALINITY

Bicarbonate, carbonate and hydroxide alkalinity can be calculated from total alkalinity (TA) if the pH is known. These concentrations are often calculated, especially for carbonate and hydroxide ion concentrations which are generally low and difficult to measure. The ion concentrations must be given in molar (M) concentrations, which are indicated by parentheses in this article. For most natural situations Equation 5, which takes into account charge differences, is adequate for determination of the various sources of alkalinity.

2*TA* = (_{}) + 2(_{}) + _{} eq. 5

TA is commonly expressed as mg/L _{} and must be converted
to moles/L (M) in order to make the following calculations. The conversion of mg/L to M _{} is accomplished by use
of Equation 6

(TA moles/L _{}) = _{} eq. 6

Rearranging the second disassociation constant for carbonic acid (eq. 7) in terms of

(_{}) yields Equation 8

_{} eq. 7

_{} eq. 8

Substitution of Equation 8 for (_{}) in Equation 5 and expressing (OH_{}) in terms of _{} based on Equation 9
yields Equation 10.

(_{}) = _{}= _{} eq. 9

2*TA* = (_{}) + 2(_{}) +_{} eq. 10

Rearranging Equation 10 to solve for _{} (eq.
12) is carried out by the following step (eq. 11).

2*TA* - _{}= (_{}) (1 + _{}) eq. 11

_{}= _{} eq. 12

Once (_{}) has been calculated (or determined by titration), this
concentration can be substituted into Equation 8 and (_{}) can be calculated.
To determine the hydroxide alkalinity, requires that only the pH be
known (eq. 9).

To convert these molar alkalinity concentrations (eqs. 8, 9 and 12) to mg/L, the molar concentrations must be
multiplied by the molecular weight of _{} (100,000 mg/mole) and
in the case of (_{}) and (_{}) a factor of ½ is required because 1 mole of _{} is equivalent to 2
moles of _{} and 2 moles of _{}, whereas 1 mole of _{} is equivalent to 1
mole of (_{})

The equations necessary for calculation of bicarbonate,
carbonate and hydroxide alkalinities as _{} mg/L are:

Bicarbonate
alkalinity (_{} mg/L) = 50,000 (_{}) eq. 13

Carbonate
alkalinity (_{} mg/L) = 100,000 (_{} eq. 14

Hydroxide
alkalinity (_{} mg/L) = 50,000 (10_{}) eq. 15