<P> To apply this process in practice, a glass electrode is used rather than the cumbersome hydrogen electrode . A combined glass electrode has an in - built reference electrode . It is calibrated against buffer solutions of known hydrogen ion activity . IUPAC has proposed the use of a set of buffer solutions of known H activity . Two or more buffer solutions are used in order to accommodate the fact that the "slope" may differ slightly from ideal . To implement this approach to calibration, the electrode is first immersed in a standard solution and the reading on a pH meter is adjusted to be equal to the standard buffer's value . The reading from a second standard buffer solution is then adjusted, using the "slope" control, to be equal to the pH for that solution . Further details, are given in the IUPAC recommendations . When more than two buffer solutions are used the electrode is calibrated by fitting observed pH values to a straight line with respect to standard buffer values . Commercial standard buffer solutions usually come with information on the value at 25 ° C and a correction factor to be applied for other temperatures . </P> <P> The pH scale is logarithmic and therefore pH is a dimensionless quantity . </P> <P> This was the original definition of Sørensen, which was superseded in favor of pH in 1909 . However, it is possible to measure the concentration of hydrogen ions directly, if the electrode is calibrated in terms of hydrogen ion concentrations . One way to do this, which has been used extensively, is to titrate a solution of known concentration of a strong acid with a solution of known concentration of strong alkaline in the presence of a relatively high concentration of background electrolyte . Since the concentrations of acid and alkaline are known, it is easy to calculate the concentration of hydrogen ions so that the measured potential can be correlated with concentrations . The calibration is usually carried out using a Gran plot . The calibration yields a value for the standard electrode potential, E, and a slope factor, f, so that the Nernst equation in the form </P> <Dl> <Dd> E = E 0 + f 2.303 R T F log ⁡ (H +) (\ displaystyle E = E ^ (0) + f (\ frac (2.303 RT) (F)) \ log ((\ ce (H+)))) </Dd> </Dl>

Name the scientist who had given the ph scale