 1.  It is calibrated against buffer solutions of known hydrogen ion activity.

 2.  For example, pH is defined as the negative logarithm of the hydrogen ion activity.

 3.  Debye and H�ckel developed a theory with which single ion activity coefficients could be calculated.

 4.  Singleion activity coefficients can be calculated theoretically, for example by using the Debye H�ckel equation.

 5.  Expressions were derived for the variation of singleion activity coefficients as a function of ionic strength.

 6.  This led to the development of what is now called the freeion activity model ( FIAM ).

 7.  PH is defined as the decimal logarithm of the reciprocal of the hydrogen ion activity, " a"

 8.  Firstly, the cell is filled with a solution of known hydrogen ion activity and the emf, " E"

 9.  However, chemists have never been able to give up the idea of single ion activities, and by implication single ion activity coefficients.

 10.  However, chemists have never been able to give up the idea of single ion activities, and by implication single ion activity coefficients.
