A problem associated with the use of a calomel reference electrode in an ISE analytical system

It was discovered during the testing of a new diluted ISE analytical system that a dark deposit formed in the vicinity of the reference electrode junction and on the junction membrane over a 1-3 month period of use. The effect of the black deposit was to shorten the life of the restricted flow reference membrane and increase the time required to maintain the system in its optimum state. Elemental analysis of the deposit revealed the presence of both mercury and sulfur. The rate of deposit build-up was found to depend on the concentrations of both the buffer and the microbicide in the system's reagents. The cause was traced in part to the generation of sulfide ions as a breakdown product of the microbicide. The disproportionation of calomel, Hg2Cl2, in saturated KCl to give Hg2+chloro-complexes and their reaction with sulfide ions at the reference junction resulted in HgS being deposited. Design changes over previous systems contributed to the effect by increasing residence time of calibrant solution at the reference junction and decreasing the frequency of reference electrolyte and membrane changes. Adding complexing agents to the reference fill solution lessened but did not eliminate the problem. The problem was solved without reformulating the reagents by using a reducing agent proximate to the calomel mercury amalgam to reduce mercuric ions to metallic mercury. This also prevents discharge of environmentally undesirable mercuric ions into the waste solution.