Standardization of pH Scale

As noted previously, the two most important chemical/electro-chemical properties of any aqueous system are the pH and the electrochemical (redox or corrosion) potential. The pH is defined 

It is also important to note that pH at ambient temperature is defined thermodynamically as the negative logarithm of the activity of hydrogen ion ), as noted above. The activity at ambient 

To overcome these difficulties, a practical pH scale for supercritical systems was defined somewhat arbitrarily, based upon available estimates of dissociation constants Ki) and activity coefficients (y) for solutions of some common 1-1 electrolytes, such as HCl and NaOH, in much the same way as has been done for aqueous systems at low subcritical systems. The concentration was chosen to be reasonably low, to allow for complete solnbility and to yield reliable model estimates for Ka and y, but at the same time the concentration of the buffering system should be much higher than the concentrations of potential impurities and corrosion products. As a compromise of all of these factors, 0.01 m solutions of HCl and NaOH were proposed as the primary pH standards.  

In the proposed standardization procedure, we assume that the contribution fiom the isothermal hquid junction potential (ITLJP) of the reference electrode to the cell potential, due to interfacing the internal reference electrode solution with the external solution, is negligible or has been calculated with sufficient accuracy The concentration of hydrogen ion in the HCl solution niH+), for example, may be estimated by considering the HCl dissociation reaction,  

For reactions (2) and (3), we establish the following system of four equations, which include the mass action constants together with the mass balance and charge balance constraints  

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