PH PH In chemistry, pH () is a logarithmic scale used to specify the acidity or basicity of an aqueous solution. It is approximately the negative of the base 10 logarithm of the molar concentration, measured in units of moles per liter, of hydrogen ions. More precisely it is the negative of the base 10 logarithm of the activity of the hydrogen ion. At 25 °C, solutions with a pH less than 7 are acidic and solutions with a pH greater than 7 are basic. The neutral value of the pH depends on the temperature, being lower than 7 if the
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,