ASTM D4739 for base number determination by potentiometric titration
8. Preparation of Electrode System
8.1 Maintenance of Electrodes - Clean the glass electrode (Note 5) at frequent intervals (not less than once every week during continual use) by immersing in cold chromic acid cleaning solution (Warning - Causes severe burns. Recognized carcinogen. Strong oxidizer). Drain the calomel electrode at least once each week and refill with fresh LiCl electrolyte as far as the filling hole. Make sure that crystallized LiCl is present in the solution. Maintain the electrolyte level in the reference electrode above that of the liquid in the titration beaker at all times. When not in use, immerse the lower halves of the electrodes in either water (glass) or the LiCl in isopropyl alcohol electrolyte (reference). Do not allow them 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.
NOTE 5 - Cleaning the electrodes thoroughly, keeping the ground-glass joint free of foreign materials, and regular testing of the electrodes are important in obtaining repeatable potentials, since contamination can introduce uncertain, erratic, and unnoticeable liquid contact potentials. While this is of secondary importance when end points are chosen from inflection points in the titration curve, it is quite serious when end points are chosen at artibrarily-fixed cell potentials (that is, the nonaqueous acidic buffer potential).
8.2 Preparation of Electrodes, Before and after using, blot-dry the glass electrode thoroughly with a clean cloth or a soft absorbent tissue and rinse with water. Wipe the reference electrode with a cloth or tissue, carefully remove the ground-glass sieve, and thoroughly wipe both ground surfaces. Replace the sleeve loosely and allow a few drops of electrolyte to drain through to flush the ground-glass joint (Note 5). Wet the ground surfaces thoroughly with electrolyte, set the sleeve firmly in place, and rinse the electrode with water. Prior to each titration, soak the prepared electrodes in water for at least 5 min immediately before use and touch the tips of the electrodes with a dry cloth or tissue to remove the excess water.
8.3 Testing of Electrodes - Test the meter-electrode combination (Note 6) when first put into use or when new electrodes are installed and retest at intervals thereafter by dipping the electrodes into a well-stirred mixture of 100 mL of the titration solvent and 1.0 to 1.5 mL of 0.1 M alcoholic KOH solution. For the meter-electrode combination to be suitable for use, the potential between the electrodes must change by more than 0.480 V from the potential between the same electrodes when dipped in the nonaqueous acidic buffer solution.
NOTE 6 - Considerably more sensitive electrodes are now available that will show a potential change of at least 0.590 V under these conditions, and their use is recommended.
9. Standardization of Apparatus
9.1 Determination of Meter Readings for the Nonaqueous Buffer Solution Corresponding to Base End Point - To ensure comparable selection of end points with the meter described in A1.1.1, determine daily for each electrode pair, the meter reading obtained with the freshly prepared nonaqueous acidic buffer solution to be used for the determination of base numbers, and with the freshly prepared nonaqueous basic buffer solution to be used for the determination of strong base numbers.
NOTE 7 - The response of different glass electrodes to hydrogen ion activity is not the same. Therefore, it is necessary to establish regularly for each electrode system the meter readings corresponding to the acidic and basic buffer solutions arbitrarily selected to represent the end point.
9.2 Prepare the electrodes as described in 8.2, immerse them in the appropriate nonaqueous buffer solution, and stir for 5 min, maintaining the temperature of the buffer solution at a temperature within 2°C of that at which the titrations are to be made. Read the cell voltage. The reading so obtained in the acidic buffer solution is taken as the end point for the base number if an inflection is not observed as specified in 12.1, and the reading obtained in the basic buffer solution is taken as the end point for the strong base number.
10. Preparation of Sample of Used Oil
10.1 Strict observation of the sampling procedure is necessary, since the sediment itself is acidic or basic or has absorbed acidic or basic material from the sample. Failure to obtain a representative sample negates a meaningful value obtained.
NOTE 8 - As used oils can change appreciably in storage, samples should be tested as soon as possible after removal from the lubricating system; and the dates of sampling and testing shall be noted.
10.2 Heat the sample (Note 9) of used oil to 60 more or less 5°C in the original container and agitate the sample until all of the sediment is homogeneously suspended in the oil. If the original container is a can, or if it is glass and more than three-fourths full, transfer the entire sample to a clear-glass bottle having a capacity at least one-third greater than the volume of the sample. Transfer all traces of sediment from the original container to the bottle by vigorous agitation of portions of the sample in the original container.
10.3 After complete suspension of all sediment, strain the sample or a convenient aliquot through a 150-µm (100-mesh) screen for the removal of large contaminated particles.
NOTE 9 - When samples are visibly free of sediment, the heating procedures described can be omitted.
