The ammonium Ion-Selective Electrode (ISE) is a liquid membrane electrode. The ISE is used without an Ionic Strength Adjuster (ISA). The activity coefficient is based on typical surface water conductivity values. Water with very high conductivity will have a higher ion concentration due to other minerals being present. The actual measurement will be influenced by a change in ion activity. The ammonium ISE is specific for ammonium and at pH’s greater than 8.25 there will begin to be a shift to ammonia, which is not measured.