pH from Hydroxide Alkalinity

Estimates high range pH from hydroxide alkalinity without a meter, by converting CaCO3 equivalents to molarity (using the 50,000 mg/eq equivalent weight, not the 100,000 molecular weight) and applying pOH and the water ion balance.

$$[\mathrm{O}{\mathrm{H}}^{-}]=\frac{{\text{(mg/L as CaCO}}_3)}{50,000},\mathrm{pOH}=-{\log }_{10}[O{H}^{-}],\mathrm{pH}=14-\mathrm{pOH}$$

Variables

Symbol	Meaning	Unit
$[O{H}^{-}]$	hydroxide molar concentration	mol/L
$mg/LasCaC{O}_3$	hydroxide alkalinity as calcium carbonate	mg/L
50,000	equivalent weight of CaCO3	mg/eq
pOH	negative log of hydroxide concentration
pH	negative log of hydrogen ion concentration
14	pKw, the water ionization sum at 25 C

Worked example

For OH alkalinity of 300 ppm as CaCO3: 300 divided by 50,000 equals 0.006 eq/L, which equals 0.006 mol/L since hydroxide is monovalent. pOH equals minus log of 0.006 equals 2.22. pH equals 14 minus 2.22 equals 11.78. Using molecular weight 100 instead of 50 introduces about a 0.3 unit error.

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Part of The Blueprint of Water