CHM-19

Buffer capacity snapshot

β ≈ 2.303 C Ka [H] / ([H] + Ka)². How much acid a buffer soaks.

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Acid–baseVan Slyke

Governing equation

\beta=2.303\,C\dfrac{K_a[H^+]}{([H^+]+K_a)^2}

where

C: Total buffer (mol/L)
K_a: Ka (—)
[H^+]: Proton concentration (mol/L)
\beta: Buffer capacity (mol/L)

Lecture brief

Historical brief

Ideal-gas law, van ’t Hoff, Nernst, Michaelis–Menten and Clausius–Clapeyron are physical chemistry’s working equations of equilibrium and rate. The lab is pressure, potential and kinetics. This sheet (CHM-19 — Buffer capacity snapshot) is the form associated with Van Slyke. Working symbols: $C$ , $K_a$ , $[H^+]$ $\rightarrow$ $\beta$ . Van Slyke defined β = dCb / d pH. Maximum at pH = pKa for a monoprotic pair.

Purpose

Purpose: compute

\beta

from

C

K_a

[H^+]

in Physical chemistry via

\beta=2.303\,C\dfrac{K_a[H^+]}{([H^+]+K_a)^2}

β ≈ 2.303 C Ka [H] / ([H] + Ka)². How much acid a buffer soaks. Use it when a real physical chemistry question must be answered in SI before a code check.

Live realistic example

In symbols

Live case. Given

C = 0.100\,\mathrm{mol/L}

K_a = 1.750e-5\,\mathrm{—}

[H^+] = 1.000e-5\,\mathrm{mol/L}

, the governing relation

\beta=2.303\,C\dfrac{K_a[H^+]}{([H^+]+K_a)^2}

yields

\beta = 0.05329\,\mathrm{mol/L}

. Monoprotic, dilute, water terms dropped. Move a slider: the numbers are this situation, not a canned story.

Calculator

Inputs

Total buffer C0.1 mol/LKa K_a0.0000175 —Proton concentration [H^+]0.00001 mol/L

Outputs

Buffer capacity \beta0.05329 mol/L

Reading speed

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Narration of this film

Monoprotic, dilute, water terms dropped.

Van Slyke defined β = dCb / d pH. Maximum at pH = pKa for a monoprotic pair.

Reading speed

Watch on YouTube