The Henderson–Hasselbach equation gives the pH of a solution relative to the pK of the acid–base pair. However the pK is dependent on ionic strength and temperature, and as it shifts so will the pH of a solution based on that acid–base pair. Because the doubly charged [HPO] is stabilized more by high ionic strength than is the singly-charged [HPO], their pK is somewhat dependent on ionic strength. The often-cited pK of ~7.2 is the value extrapolated to zero ionic strength, and is not applicable at physiological ionic strength.
Phillips et al. measured the pK at 10, 25, and 37 °C at various ionic strengths. For the latter two temperatures they report pK in Debye-Hückel equations (plotted in the accompanying figure for µ up to 0.5 M):<br>
at 25 °C: pKa = 	7.18 − 1.52 sqrt(µ) + 1.96 µ <br>
at 37 °C: pKa = 	7.15 − 1.56 sqrt(µ) + 1.22 µ
The pK is weakly dependent on temperature. Phillips et al. reported ∆H at 25 °C of 760 cal/mol (3180 J/mol) and a linear dependence of pK on 1/T (Van t Hoff equation). The positive ∆H results in an increase in K, and thus a decrease in pK with rising temperature, the change in pKa being 166 × the change in (1/T), which around 25 °C results in a change in pK of −0.00187 per degree. This applies strictly to the extrapolated thermodynamic pK' at infinite dilution, and as the figure shows, the temperature effect can be much larger at higher ionic strength.