TRACEABILITY AND THE pH SCALE AT NIST

摘要

The following overhead summarizes the chain of traceability upon which the multi-point pH scale is based. The emphasis of this approach is to maintain traceability of the practical realization of the pH scale to the thermodynamic basis of the pH scale. This traceability chain preserves the link between the set of standards used for practical calibration of pH measurements and thermodynamics to the maximum extent feasible. The top level of the pyramid of pH traceability is the notional definition, in which pH is defined as the negative decadic logarithm of the hydrogen ion activity. This is the fundamental definition of pH. It cannot be realized without extra-thermodynamic assumptions, as it is based upon a single ion activity. The second step of the pyramid illustrates the experimental actualization of this notional definition on the fundamental level. The potential of the junctionless electrochemical cell Pt, H_(2) |HCl| Ag,AgCl is measured. This potential is related to the activities of the constituent ions by the Nernst equation expression as shown on the second line of this level. Note that down to this point (starting with the notional definition), no extra-thermodynamic conventions have been invoked. The next level constitutes the first introduction of extra-thermodynamic assumptions in the traceability chain of the multi-point pH scale. The pH is calculated using the value for the extrapolated acidity function, log (a_(H)γ_(Cl))~(o), where the subscript ~(o) denotes the value at zero molality of added Cl~(-) ion. This calculation requires an extra-thermodynamic convention to estimate the value of γ~(o)_(Cl(at I)), the single-ion activity of the Cl~(-) ion extrapolated to zero molality of added Cl~(-) ion. Currently, the Bates-Guggenheim convention is used, comprising two separate assumptions. First, γ_(H) is assumed to equal γ_(Cl). Second, the ion size parameter for Cl~(-) is taken such that Ba=1.5 in the Debye-Huckel equation calculation of the value of γ_(Cl). This dual convention permits the calculation of the pH.