Write the formula of the salts used to make a phosphate buffer
The correct answer and explanation is:
Correct Answer:
The salts commonly used to make a phosphate buffer are:
NaH₂PO₄ (sodium dihydrogen phosphate) and Na₂HPO₄ (disodium hydrogen phosphate).
Explanation (300 words):
A phosphate buffer is widely used in biological and chemical laboratories because it maintains a stable pH in the physiological range (around pH 6.8 to 7.4). This buffer is based on the equilibrium between two forms of phosphate ions:
- H₂PO₄⁻ (dihydrogen phosphate ion) — the weak acid
- HPO₄²⁻ (hydrogen phosphate ion) — the conjugate base
To prepare this buffer, two salts are typically used:
- Sodium dihydrogen phosphate (NaH₂PO₄) provides the H₂PO₄⁻ ion (acidic component).
- Disodium hydrogen phosphate (Na₂HPO₄) provides the HPO₄²⁻ ion (basic component).
These two components form a conjugate acid–base pair, which resists changes in pH by neutralizing added acids or bases. The key chemical equilibrium involved is: H2PO4−⇋H++HPO42−\text{H}_2\text{PO}_4^- \leftrightharpoons \text{H}^+ + \text{HPO}_4^{2-}
This system follows the Henderson-Hasselbalch equation, which is used to calculate the pH of the buffer: pH=pKa+log([base][acid])\text{pH} = \text{p}K_a + \log \left( \frac{[\text{base}]}{[\text{acid}]} \right)
For phosphate, the relevant pKa is approximately 7.2 (for the second dissociation of phosphoric acid). By adjusting the ratio of Na₂HPO₄ to NaH₂PO₄, you can create a buffer at a desired pH near this value.
Phosphate buffers are preferred in many biological applications because:
- They are non-toxic to cells.
- They maintain pH near physiological conditions.
- They have good buffering capacity in the pH 6–8 range.
However, phosphate can precipitate with divalent metal ions (e.g., Ca²⁺, Mg²⁺), which must be considered in biochemical experiments.
In summary, NaH₂PO₄ and Na₂HPO₄ are the salts used to create an effective phosphate buffer system that helps maintain a stable pH in biological and chemical environments.