Carbonate Species from P and M Alkalinity

Infers which alkalinity species (hydroxide, carbonate, bicarbonate) are present by comparing phenolphthalein (P) alkalinity to total methyl orange (M) alkalinity, both measured as CaCO3.

$$\begin{array}{rl}P=0& :\mathrm{HC}{\mathrm{O}}_3^{-}=M\\ P<\frac{1}{2}M& :\mathrm{C}{\mathrm{O}}_3^{2-}=2P,\mathrm{HC}{\mathrm{O}}_3^{-}=M-2P\\ P=\frac{1}{2}M& :\mathrm{C}{\mathrm{O}}_3^{2-}=2P\\ P>\frac{1}{2}M& :\mathrm{O}{\mathrm{H}}^{-}=2P-M,\mathrm{C}{\mathrm{O}}_3^{2-}=2(M-P)\\ P=M& :\mathrm{O}{\mathrm{H}}^{-}=M\end{array}$$

Variables

Symbol	Meaning	Unit
P	phenolphthalein alkalinity, titrated to pH 8.3	mg/L as CaCO3
M	total (methyl orange) alkalinity, titrated to pH 4.3	mg/L as CaCO3
$O{H}^{-}$	hydroxide alkalinity	mg/L as CaCO3
$C{O}_3^{2-}$	carbonate alkalinity	mg/L as CaCO3
$HC{O}_3^{-}$	bicarbonate alkalinity	mg/L as CaCO3

Worked example

If P equals 0, all alkalinity is bicarbonate (equal to M). If P equals one half of M, all alkalinity is carbonate. If P equals M, all alkalinity is hydroxide.

---

Part of The Blueprint of Water