Republication or reproduction of this report or its storage and/or dissemination by electronic means is permitted without theAbstract: The definition of a "primary method of measurement" [1] has permitted a full consideration of the definition of primary standards for pH, determined by a primary method (cell without transference, Harned cell), of the definition of secondary standards by secondary methods, and of the question whether pH, as a conventional quantity, can be incorporated within the internationally accepted system of measurement, the International System of Units (SI, Système International d'Unités). This approach has enabled resolution of the previous compromise IUPAC 1985 Recommendations [2]. Furthermore, incorporation of the uncertainties for the primary method, and for all subsequent measurements, permits the uncertainties for all procedures to be linked to the primary standards by an unbroken chain of comparisons. Thus, a rational choice can be made by the analyst of the appropriate procedure to achieve the target uncertainty of sample pH. Accordingly, this document explains IUPAC recommended definitions, procedures, and terminology relating to pH measurements in dilute aqueous solutions in the temperature range 5-50 °C. Details are given of the primary and secondary methods for measuring pH and the rationale for the assignment of pH values with appropriate uncertainties to selected primary and secondary substances.

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The Hu ¨ckel equation is used to correlate the experimental activities of dilute NaCl and KCl solutions up to a molality of about 1.0 mol • kg -1 . The two parameters of this equation are dependent on the electrolyte and are B [which is simply related to the ion-size parameter (a*) in the Debye-Hu ¨ckel equation] and b 1 (which is the coefficient of the linear term in molality and related to hydration numbers of the ions of the electrolyte). In more concentrated solutions up the saturation for both electrolytes, an extended Hu ¨ckel equation was used, and it contains, additionally, a quadratic term with respect to the molality with parameter b 2 . The values of parameters B and b 1 for dilute KCl solutions were determined from the cell potential differences measured by Hornibrook et al. on concentration cells with transference (J. Am. Chem. Soc. 1942, 64, 513-516). With these values of KCl, the corresponding parameter values for NaCl solutions were determined from the isopiestic data of Robinson for NaCl and KCl solutions (Trans. R. Soc., N. Z., 1945, 75, 203-217). Only the data points for NaCl molalities less than 1.4 mol • kg -1 were included in this determination. The resulting parameter values were successfully tested with all reliable cell potential and isopiestic data in the literature for dilute NaCl and KCl solutions. For more concentrated solutions, new values of parameters b 1 and b 2 were determined for the extended Hu ¨ckel equations of NaCl and KCl, but the same values of parameter B were used as for dilute solutions. For these more concentrated NaCl solutions, the values of parameters b 1 and b 2 were determined from the vapor pressure data of Olynyk and Gordon (J. Am. Chem. Soc. 1943, 65, 224-226), which cover the molality range (2.3 to 6.1) mol • kg -1 . With these values for NaCl, the corresponding parameter values for more concentrated KCl solutions were determined from the isopiestic data of Robinson for this pair of electrolytes (see the citation above), where all experimental points were included in the determination. The resulting extended Hu ¨ckel equations were thoroughly tested with all reliable experimental data presented in the literature on the basis of electrochemical, isopiestic, and direct vapor pressure measurements. Most of these data can be reproduced within experimental error by means of the extended Hu ¨ckel equations up to the saturated solutions. Reliable activity and osmotic coefficients of NaCl and KCl can, therefore, be calculated by using the Hu ¨ckel and extended Hu ¨ckel equations determined in this study. The values obtained by these equations are probably the most accurate values available, and they have been tabulated at rounded molalities. The activity and osmotic coefficients obtained from the new equations were compared with the values suggested by Robinson (see citation above), with those calculated by using the Pitzer equations of Pitzer and Mayorga (J.

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The substance concentration of ionized calcium (c Ca2+) in blood, plasma or serum preanalytically may be affected by pH changes of the sample, calcium binding by heparin, and dilution by the anticoagulant solution. pH changes in whole blood can be minimized by anaerobic sampling to avoid loss of Co2, by measuring as soon as possible, or by storing the sample in iced water to avoid lactic acid formation. cCa 2+ and pH should be determined simultaneously. Plasma or serum: If centrifuged in a closed tube, and measured immediately, the pH of the sample will be close to the original value. If a delay has occurred between centrifugation and the measurement, causing substantial loss of Co2, equilibration of the sample with a gas mixture corresponding to pCO2= 5.3 kPa prior to the measurement is recommended. Conversion of the measured values to cCa 2+ (7.4) is only valid if the pH is in the range 7.2-7.6. Ca2+ binding by heparin can be minimized by using either of the following: (1) A final concentration of sodium or lithium heparinate of 15 IU/ml blood or less (2) Calcium titrated heparin with a final concentration of less than 50 IU/ml blood. Dilution effect can be avoided by use of dry heparin in capillaries or syringes. When heparin solutions are used, errors due to dilution or calcium binding can be reduced by using syringes with a heparin solution containing free calcium ions corresponding to the mean concentration of ionized calcium in normal plasma. Conditions for blood collection, storage, and transport to avoid preanalytical errors are described in this paper.