D664_8 (17a) 04-2018

When a silver/silver chloride reference electrode is used for the titration and it contains an electrolyte which is not 1 molar-3 molar lithium chloride in ethanol, replace the electrolyte. Drain the electrolyte from the electrode. Wash away all the salt, if present, with water, and then rinse with ethanol. Rinse several times with the lithium chloride electrolyte solution. Finally, replace the sleeve and fill the electrode with the lithium chloride electrolyte to the filling hole. When refitting the sleeve, ensure that there will be a free flow of electrolyte into the system. A combination electrode shall be prepared in the same manner. The electrolyte in a combination electrode can be removed with the aid of a vacuum suction. Test the meter electrode combination when first put into use or when new electrodes are installed and retest at intervals thereafter. Rinse the electrodes with solvent, then with water, and dip them into a pH 4 aqueous buffer solution. Read the millivolt value after stirring one minute. Remove the electrodes and rinse with water. Dip the electrodes into a pH 7 aqueous buffer. Read the millivolt value after stirring one minute. Calculate the millivolt difference. A good electrode system will have a difference of at least 162 millivolts, 20 degrees Celsius to 25 degrees Celsius. If the difference is less than 162 millivolts, lift the sleeve of the electrode and ensure electrolyte flow. Repeat the measurements. If the difference is still less than 162 millivolts, clean or replace the electrodes. When the sensing electrode and the reference electrode are separate, one pair of electrodes shall be considered as one unit. If one or the other is changed, it shall be considered as different pair and shall be retested. Maintenance and storage of electrodes-- cleaning the electrodes thoroughly, keeping the ground glass joint free of foreign materials, and regular testing of the electrodes are very important in obtaining repeatable potentials since contamination may introduce uncertain erratic and unnoticeable liquid contact potentials. While this is of secondary importance when the endpoints are chosen from inflection points in the titration curve, it may be quite serious when endpoints are chosen at arbitrarily fixed cell potentials. See appendix X1 of the written standard for a possible procedure to check the electrode performance. Clean the glass electrode at frequent intervals based on use and type of samples being analyzed, not less than once every week during continual use by immersing in non-chromium-containing, strongly-oxidizing cleaning solution. The reference electrode shall be cleaned periodically when in use or when a new electrode is installed. Drain the reference electrode at least once each week and refill with a fresh lithium chloride electrolyte as far as the filling hole. Ensure that there are no air bubbles in the electrode liquid. If air bubbles are observed, hold the electrode in a vertical position and gently tap it to release the bubbles. Maintain the electrolyte level in the reference electrode above that of the liquid in the titration beaker or vessel at all times. Prior to each titration, soak the prepared electrodes in water, pH 4.5 to 5.5, for at least 5 minutes. Rinse the electrodes with isopropyl alcohol immediately before use and then with the titration solvent. When not in use, immerse the lower half of the reference electrode in lithium chloride electrolyte. When the glass electrode is used, store it in water that has been acidified with hydrochloric acid to a pH of 4.5 to 5.5. Do not allow electrodes to remain immersed in titration solvent for any appreciable period of time between titrations. While the electrodes are not extremely fragile, handle them carefully at all times. Electrode life-- typically, electrode usage is limited to three to six months, depending upon usage. Electrodes have a limited shelf life and shall be tested before use. See 8.2 of the written standard.