pH and ORP electrodes are available with a single junction or double junction reference cell. The single junction reference electrode is made with a silver (Ag) wire coated with silver chloride (AgCl). This wire is in contact with the sample to be measured by a fill solution that leaks through a junction. In single junction electrode this fill solution is 3.5M KCl with AgCl. The HI7071 is for four bottles of 30 mL 3.5M KCl + AgCl electrolyte solution that ensures optimum measurement performance from your single junction pH or ORP electrode.