Question

The intracellular pH of a yeast cell is maintained in part by phosphate ion buffering. Phosphoric acid has three protons and therefore three pKa values, 2.1, 7.2, and 12.7. At pH= 7.0, how much of each species (H₃PO₄,H₂PO⁴⁻,HPO₄²⁻ , and PO₄³⁻) is present? Ignore trace amounts of less than 1 percent.

Answer 1

At pH = 7.0, the species present in phosphate ion buffering include phosphoric acid, dihydrogen phosphate ion, hydrogen phosphate ion, and phosphate ion. The majority of the species will be in the form of H2PO4-.

Step-by-step explanation:

The pH of a solution is determined by the concentration of hydrogen ions (H+) present. At pH = 7.0, the solution is considered neutral, meaning the concentration of H+ ions is equal to the concentration of hydroxide ions (OH-).

In the case of phosphate ion buffering, at pH = 7.0, there are four species present:

1. H3PO4 (phosphoric acid)
2. H2PO4- (dihydrogen phosphate ion)
3. HPO4^2- (hydrogen phosphate ion)
4. PO4^3- (phosphate ion)

These species exist in a dynamic equilibrium, determined by their respective pKa values. At pH = 7.0, all species are present to some extent, but the majority will be in the form of H2PO4-.

The intracellular pH of a yeast cell is maintained in part by a phosphate buffer, which consists of mixtures of different forms of phosphate ions at various pH levels. Phosphoric acid (H3PO4) has three dissociation constants (pKa values): 2.1, 7.2, and 12.7, corresponding to its three acidic protons. At a pH of 7.0, the species present are primarily the dihydrogen phosphate ion (H2PO4−) and the hydrogen phosphate ion (HPO42−), with the H2PO4− form being predominant because pH 7.0 is closer to its pKa of 7.2 than to the next pKa of 12.7. The species H3PO4 and PO43− will be present in insignificant amounts (<1%) and can be ignored because the pH is much more distant from their respective pKa values.