  The concept of the partial osmotic pressure of ions in an electrolyte
solution is critically examined. In principle these can be defined by
introducing a solvent-permeable wall and measuring the force per unit area
which can certainly be attributed to individual ions. Here I demonstrate that
although the total wall force balances the bulk osmotic pressure as required by
mechanical equilibrium, the individual partial osmotic pressures are
extra-thermodynamic quantities dependent on the electrical structure at the
wall, and as such they resemble attempts to define individual ion activity
coefficients. The limiting case where the wall is a barrier to only one species
of ion is also considered, and with ions on both sides the classic Gibbs-Donnan
membrane equilibrium is recovered thus providing a unifying treatment. The
analysis can be extended to illustrate how the electrical state of the bulk is
affected by the nature of the walls and the sample handling history, thus
supporting the 'Gibbs-Guggenheim uncertainty principle' (the notion that the
electrical state is unmeasurable and usually accidentally determined). Since
this uncertainty is conferred also onto individual ion activities, it has
implications for the current (2002) IUPAC definition of pH.
