PH temperature increases. Pure water is neutral, pH 7 at (25 °C), being neither an acid nor a base. Contrary to popular belief, the pH value can be less than 0 or greater than 14 for very strong acids and bases respectively. Measurements of pH are important in agronomy, medicine, chemistry, water treatment, and many other applications. The pH scale is traceable to a set of standard solutions whose pH is established by international agreement. Primary pH standard values are determined using a concentration cell with transference, by measuring the potential difference between a hydrogen electrode and a standard electrode such
PH from aqueous pH values, because activities relate to different standard states. Hydrogen ion activity, "a", can be defined as: where "μ" is the chemical potential of the hydrogen ion, formula_9 is its chemical potential in the chosen standard state, "R" is the gas constant and "T" is the thermodynamic temperature. Therefore, pH values on the different scales cannot be compared directly due to different solvated proton ions such as lyonium ions, requiring an intersolvent scale which involves the transfer activity coefficient of hydronium/lyonium ion. pH is an example of an acidity function. Other acidity functions can be defined. For example,