To use Half cell ISEs a reference electrode is required.
Reference electrodes leach out small amounts of their filling solutions during measurements. The single junction references will therefore contaminate the sample and standards with Potassium and Chloride Ions. Ag/AgCl single Junction references will also leach out small amounts of Silver Ions.
To avoid this contamination it is advisable to use Double Junction reference Electrodes. The electrolyte in the outer chamber and its concentration can be selected to suit the application in question.