ENV-33

Carbonate alkalinity

Alk = [HCO3−] + 2[CO3²−] + [OH−] − [H+] in meq/L.

Reading speed

Water chemistryAPHA 2320

Governing equation

\mathrm{Alk}=[\mathrm{HCO}_3^-]+2[\mathrm{CO}_3^{2-}]+[\mathrm{OH}^-]-[\mathrm{H}^+]

$[HCO_3^-] \in \left[0, 8\right]$

where

[HCO_3^-]: Bicarbonate (mmol/L)
[CO_3^{2-}]: Carbonate (mmol/L)
[OH^-]: Hydroxide (µmol/L)
[H^+]: Hydrogen ion (µmol/L)
Alk: Alkalinity (meq/L)
Alk: As CaCO3 (mg/L as CaCO3)

The figure, part by part

Each numbered marker wears the color of its card. Hover a card to light the marker. Tap a marker to jump to that card. The slider walks a red point along the curve; Animate drawing traces it like a pen.

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①

Governing law — red

\mathrm{Alk}=[\mathrm{HCO}_3^-]+2[\mathrm{CO}_3^{2-}]+[\mathrm{OH}^-]-[\mathrm{H}^+]

$[HCO_3^-] \in \left[0, 8\right]$

$[HCO_3^-] = 0$ $Alk = 5.045$

Along the curve

②

Domain of the variable — blue

[HCO_3^-]_{\min} = 0,\quad [HCO_3^-]_{\max} = 8

$[HCO_3^-] \in \left[0, 8\right]$

$[HCO_3^-] = 0$ $[HCO_3^-] = 0$

Along the curve

③

Response curve — green

\mathrm{Alk}=[\mathrm{HCO}_3^-]+2[\mathrm{CO}_3^{2-}]+[\mathrm{OH}^-]-[\mathrm{H}^+]

$[HCO_3^-] \in \left[0, 8\right]$

$[HCO_3^-] = 0$ $Alk = 5.045$

Along the curve

④

Rate of change — amber

\frac{\mathrm{d}Alk}{\mathrm{d}[HCO_3^-]}

$[HCO_3^-] \in \left[0, 8\right]$

$[HCO_3^-] = 0.05$ $Alk^{\prime} = 50$

Along the curve

Lecture brief

Historical brief

Streeter–Phelps (1925) oxygen sag, settling theory and Guldberg–Waage kinetics made water and air quality a rate problem. The lab computes sag, overflow and a snapshot of reactor mass balance. This sheet (ENV-33 — Carbonate alkalinity) is the form associated with APHA 2320. Working symbols: $[HCO_3^-]$ , $[CO_3^{2-}]$ , $[OH^-]$ , $[H^+]$ $\rightarrow$ $Alk$ , $Alk$ . Alkalinity is acid-neutralising capacity. In natural waters the carbonate system dominates.

Purpose

Purpose: compute

Alk

Alk

from

[HCO_3^-]

[CO_3^{2-}]

[OH^-]

[H^+]

in Environmental via

\mathrm{Alk}=[\mathrm{HCO}_3^-]+2[\mathrm{CO}_3^{2-}]+[\mathrm{OH}^-]-[\mathrm{H}^+]

Alk = [HCO3−] + 2[CO3²−] + [OH−] − [H+] in meq/L. Use it when a real environmental question must be answered in SI before a code check.

Live realistic example

In symbols

Live case. Given

[HCO_3^-] = 2.400\,\mathrm{mmol/L}

[CO_3^{2-}] = 0.050\,\mathrm{mmol/L}

[OH^-] = 1.000\,\mathrm{\mu mol/L}

[H^+] = 0.100\,\mathrm{\mu mol/L}

, the governing relation

\mathrm{Alk}=[\mathrm{HCO}_3^-]+2[\mathrm{CO}_3^{2-}]+[\mathrm{OH}^-]-[\mathrm{H}^+]

yields

Alk = 2.501\,\mathrm{meq/L}

Alk = 125.0\,\mathrm{mg/L as CaCO3}

. A titration curve, an inflection, an Alk bar. Move a slider: the numbers are this situation, not a canned story.

Calculator

Inputs

Bicarbonate [HCO_3^-]2.4 mmol/LCarbonate [CO_3^{2-}]0.05 mmol/LHydroxide [OH^-]1 µmol/LHydrogen ion [H^+]0.1 µmol/L

Outputs

Alkalinity Alk2.501 meq/L
As CaCO3 Alk125.0 mg/L as CaCO3

Reading speed

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Narration of this film

A titration curve, an inflection, an Alk bar.

Alkalinity is acid-neutralising capacity. In natural waters the carbonate system dominates.

Reading speed

Watch on YouTube