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Acetate Buffer Formula

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Acetate Buffer Formula

Complete Guide to Acetate Buffer Chemistry and Calculations

What is an Acetate Buffer?

An acetate buffer is a solution that resists changes in pH when small amounts of acid or base are added. It consists of a weak acid (acetic acid) and its conjugate base (acetate ion).

Buffer Range: pH 3.6 to 5.6 (most effective around pH 4.76)

Buffer Components

Weak Acid

Acetic Acid: \( \ce{CH_3COOH} \)

Conjugate Base

Sodium Acetate: \( \ce{CH_3COONa} \)

Chemical Equations

Buffer Equilibrium

\[\ce{CH_3COOH(aq) + H_2O(l) \rightleftharpoons H_3O^+(aq) + CH_3COO^-(aq)}\]

Buffer Action

When acid is added:

\[\ce{H_3O^+(aq) + CH_3COO^-(aq) \rightarrow CH_3COOH(aq) + H_2O(l)}\]

When base is added:

\[\ce{CH_3COOH(aq) + OH^-(aq) \rightarrow CH_3COO^-(aq) + H_2O(l)}\]

Henderson-Hasselbalch Equation

General Formula

\[\text{pH} = \text{p}K_a + \log\left(\frac{[\text{A}^-]}{[\text{HA}]}\right)\]

For Acetate Buffer

\[\text{pH} = 4.76 + \log\left(\frac{[\ce{CH_3COO^-}]}{[\ce{CH_3COOH}]}\right)\]

Where: pKa of acetic acid = 4.76 at 25°C

Buffer Preparation Calculations

Step-by-Step Process

  1. Determine desired pH and buffer concentration
  2. Calculate the ratio [A⁻]/[HA] using Henderson-Hasselbalch equation
  3. Calculate individual concentrations of acid and conjugate base
  4. Convert to masses or volumes needed

Key Relationships

Buffer Capacity:
[HA] + [A⁻] = Total Buffer Concentration

Ratio Calculation:
\( \frac{[\text{A}^-]}{[\text{HA}]} = 10^{(\text{pH} - \text{p}K_a)} \)

Example: pH 5.0 Buffer Preparation

Given Data

Desired pH: 5.0

pKa: 4.76

Buffer: 0.2 M

Calculation Steps

Step 1: Calculate the ratio

\[\log\left(\frac{[\text{A}^-]}{[\text{HA}]}\right) = 5.0 - 4.76 = 0.24\] \[\frac{[\text{A}^-]}{[\text{HA}]} = 10^{0.24} = 1.74\]

Step 2: Calculate individual concentrations

Let [HA] = x, then [A⁻] = 1.74x

Total concentration: x + 1.74x = 0.2 M

Therefore: 2.74x = 0.2 M

\([HA] = 0.074\,\text{M}\)     \([A^-] = 0.126\,\text{M}\)

For 1 L buffer solution:

Acetic Acid

4.44 g

Sodium Acetate

10.34 g

Key Properties & Constants

Acetic Acid

  • Ka = 1.8 × 10⁻⁵
  • pKa = 4.76
  • MW = 60.05 g/mol
  • Formula: \( \ce{CH_3COOH} \)

Sodium Acetate

  • Anhydrous MW = 82.03 g/mol
  • Trihydrate MW = 136.08 g/mol
  • Formula: \( \ce{CH_3COONa} \)
  • Highly soluble in water

Applications

Biochemistry

Enzyme assays and protein studies

Analytical Chemistry

pH control in titrations and separations

Cell Culture

Maintaining physiological pH conditions

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