D7318_9

Prepare the titrator by filling the titrator reservoir with lead titrant, 8.1 of the written standard. Follow the manufacturer's procedure for filling the burette. Perform titrations in monotonic titration mode, using either 25 microliter or 50 microliter titrant addition increments. Use a 10 millivolts per minute drift condition, or 20-second wait time between additions-- whichever is achieved first. Proper care of the lead selective electrode is essential for obtaining high-quality titration curves. Preparation of the lead electrode should be performed as specified by the manufacturer. A lead electrode utilizing a solid crystal sensor requires polishing when performance deteriorates. The voltage range for a 10 parts per million sulfate in ethanol titration should span at least 50 millivolts and produce an acceptably-shaped titration curve. See figure 1 of the written standard. If this level of electrode performance is not met, this indicates a need for lead electrode polishing. The lead electrode normally comes supplied with a polishing grit and a cloth rectangle. The polishing grit medium is placed on the cloth, wet with ethanol or water, moved over the surface of the cloth for a minute, rinsed with water to remove the polishing medium, and wiped dry with a tissue. The electrode is then soaked in lead titrant, 8.1 of the written standard, to re-equilibriate the electrode surface. The double junction reference electrode is filled with 1 molar lithium chloride in absolute ethanol, 8.7 of the written standard, in the outer chamber.