ion activity of 5×10−6 (at that level, this is essentially the number of moles of hydrogen ions per litre of solution) the argument of the logarithm is 1/(5×10−6) = 2×105; thus such a solution has a pH of log10(2×105) = 5.3. Consider the following example: a quantity of 107 moles of pure water at 25 °C (pH = 7), or 180 metric tonnes (18×107 g), contains close to 18 milligrams of dissociated hydrogen ions. Note that pH depends on temperature. For instance at 0 °C the pH of pure water is about 7.47. At 25 °C it is 7.00, and at 100 °C it is 6.14. This definition was adopted because ion-selective electrodes, which are used to measure pH, respond to activity. Ideally, the electrode potential, E, follows the Nernst equation, which for the hydrogen ion can be written as where E is a measured potential, E0 is the standard electrode potential, R is the gas constant, T is the temperature in kelvins, F is the Faraday constant. For the number of electrons transferred is one. It follows that the electrode potential is proportional to pH when pH is defined in terms of activity. Precise measurement of pH is presented in International Standard ISO 31-8 as follows: A galvanic cell is set up to measure the electromotive force (e.m.f.) between a reference electrode and an electrode sensitive to the hydrogen ion activity when they are both immersed in the same aqueous solution. The reference electrode may be a silver chloride electrode or a calomel electrode. The hydrogen-ion selective electrode is a standard hydrogen electrode. Reference electrode | concentrated solution of KCl || test solution | H2 | Pt Firstly, the cell is filled with a solution of known hydrogen ion activity and the electromotive force, ES, is measured. Then the electromotive force, EX, of the same cell containing the solution of unknown pH is measured. The difference between the two measured electromotive force values is proportional to pH. This method of calibration avoids the need to know the standard electrode potential. The proportionality constant, 1/z, is ideally equal to , the "Nernstian slope". 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 (International Union of Pure and Applied Chemistry) has proposed