Measurement of Physiological pH in Biological Specimens

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The scale of pH in common use today is designed to yield accurate differences of an acidity function between a standard of reference and other media of a comparable nature but differing acidity. These standards are dilute buffer solutions with typically ionic strengths near 0.1 m; that is, considerably more dilute than most media (I = 0.16 m) of physiological interest. Reliable interpretation of the results for other solutions of comparable composition can be expected, but others, such as those containing higher quantities of neutral salt may be suspect. Although standards covering the range of pH of most physiological media are available, it is presently difficult to choose a reference solution that matches closely in respect to other properties of the unknown media of concern. For ionic strength above I = 0.1 m, everyone is 'on his own' and differences of the results (calculation of the single ion activity coefficient) between various laboratories are likely to occur. A long-term goal of the research is to remove the present restriction of various conventions (the Bates-Guggenheim, valid up to I = 0.1 m) by use of the Pitzer ion-interaction approach to (i) calculate the required single ion activity of a single ionic species in the determination of pH values for the standard buffers at an ionic strength I = 0.16 m, similar to that found in biological specimens (blood, plasma, etc.); to (ii)compare them with those based on the simpler conventions; and to (iii) evaluate the effect produced in physiological media (I = 0.16m) by variations in the neutral salt content of either the standard or the medium of interest. If successful, the assignment of pH values to standard reference solutions matching closely the properties of physiological media should be possible, with a consequent improvement in the reliability and meaning of experimental pH data for these media. In this connection, twelve new biochemical buffer compounds (HEPPS, AMPSO, HEPPSO, PIPESO, CAPS, CAPSO, MOPSO, ACES, CHES, MES, TRICINE, and PIPES) will be investigated. Finally, this may establish a self-consistent, unified pH scale that is applicable to all cases (different ionic strengths, mixtures, etc.). Thus, the Pitzer convention and formulation fit the bills. In the past, some 600 students (about seventy students for the existing grant) have been challenged by this opportunity and many have been motivated toward careers in science and medicine. Consequently, it is planned that this opportunity will be offered to students once more to work on a more integrated, focused, and meritorious research proposal.