Metrohm 781 pH/Ion Meter - Manual (Page 143)

8 Troubleshooting – Messages – Maintenance
780/781 pH/Ion Meter, Manual
133
Storage
• Combined glass electrodes with c(KCl) = 3 mol/L as reference
electrolyte should be stored in the Storage solution 6.2323.000
(also contained in the maintenance kit for pH electrodes
6.2325.000). If the electrode is stored in water, then AgCl will
precipitate out in the diaphragm! pH glass electrodes filled with
another reference electrolyte should be stored in that reference
electrolyte.
• Separate glass electrodes should be stored in distilled water.
Cleaning the diaphragm
• After measurements in solutions with low chloride concentration
(precipitated AgCl in the diaphragm which is colored dark
brown): place electrode overnight in a concentrated ammonia
solution, rinse with water and replace the reference electrolyte.
• After measurements in protein-containing solutions: place elec-
trode in a solution of 5% pepsin in c(HCl) = 0.1 mol/L for several
hours; the best effects are obtained at a temperature of 38 °C.
Then rinse thoroughly with water and replace the reference elec-
trolyte.
• After measurements in sulfide-containing solutions (Ag
2
S in dia-
phragm, which has a dark color): immerse electrode in freshly
prepared, slightly acidic 7% thiourea solution for several hours.
Then rinse with water and replace the reference electrolyte.
• Note: Ultrasonic cleaning can destroy the electrode!
Care of the glass membrane
• When carrying out measurements in non-aqueous media the
electrode should be hydrated between measurements.