Second pKₐ of Phosphoric Acid

That a pK_a should land at 7.2, within a fifth of a unit of the pH of cytoplasm, looks like a coincidence and is not. Phosphate's usefulness as biology's buffer, as the backbone of DNA and RNA, and as the energy currency in ATP all trace to the same property Frank Westheimer set out in his 1987 Science paper "Why Nature Chose Phosphates": at physiological pH a phosphate ester carries a negative charge, which keeps it inside the membrane and slows its hydrolysis to a rate life can control with enzymes.

For practical buffer-making, pH 7.2 is the sweet spot of the phosphate system, and Henderson–Hasselbalch gives the recipe directly. Note that phosphate buffers shift with dilution and with temperature — a buffer made at room temperature and used at 4 °C drifts by around 0.1 unit — so a careful protocol specifies the temperature at which the pH was set.