ASTM D6810 Standard Test Method for Measurement of Hindered Phenolic Antioxidant Content in Non-Zinc Turbine Oils by Linear Sweep Voltammetry
3. Summary of Test Method
3.1 A measured quantity of sample is dispensed into a vial containing a measured quantity of alcohol-based electrolyte solution and containing a layer of sand. When the vial is shaken, the hindered phenol antioxidants and other solution soluble oil components present in the sample are extracted into the solution and the remaining droplets suspended in the solution are agglomerated by the sand. The sand/droplet suspension is allowed to settle out and the hindered phenol antioxidants dissolved in the solution are quantified by voltammetric analysis. The results are calculated and reported as weight percent of antioxidant or as millimoles (mmol) of antioxidant per litre of sample for prepared and fresh oils and as a percent remaining antioxidant for used oils.
3.2 Voltammetric analysis is a technique that applies electro-analytic methods when a sample to be analyzed is mixed with an electrolyte and a solvent and placed within an electrolytic cell. Data is obtained by measuring the current passing through the cell as a function of the potential applied, and test results are based upon current, voltage and time relationships at the cell electrodes. The cell consists of a fluid container into which is mounted a small, easily polarized working electrode, and a large nonpolarizable reference electrode. The reference electrode should be massive relative to the working electrode so that its behavior remains essentially constant with the passage of small current; that is, it remains unpolarized during the analysis period. Additional electrodes, auxiliary electrodes, can be added to the electrode system to eliminate the effects of resistive drop for high resistance solutions. In performing a voltammetric analysis, the potential across the electrodes is varied linearly with time, and the resulting current is recorded as a function of the potential. As the increasing voltage is applied to the prepared sample within the cell, the various additive species under investigation within the oil are caused to electrochemically oxidize. The data recorded during this oxidation reaction can then be used to determine the remaining useful life of the oil type. A typical current-potential curve produced during the practice of the voltammetric test can be seen by reference to Fig. 1. Initially, the applied potential produces an electrochemical reaction having a rate so slow that virtually no current flows through the cell. As the voltage is increased, as shown in Fig. 1, the electro-active species (for example, substituted phenols) begin to oxidize at the working electrode surface, producing an anodic rise in the current. As the potential is further increased, the decrease in the electro-active species concentration at the electrode surface and the exponential increase of the oxidation rate lead to a maximum in the current-potential curve shown in Fig. 1.
4. Significance and Use
4.1 The quantitative determination of hindered phenol antioxidants in a new turbine oil measures the amount of this material that has been added to the oil as protection against oxidation. Beside phenols, turbine oils can be formulated with other antioxidants such as amines which can extend the oil life. In used oil, the determination measures the amount of original (phenolic) antioxidant remaining after oxidation have reduced its initial concentration. This test method is not designed or intended to detect all of the antioxidant intermediates formed during the thermal and oxidative stressing of the oils, which are recognized as having some contribution to the remaining useful life of the used or in-service oil. Nor does it measure the overall stability of an oil, which is determined by the total contribution of all species present. Before making final judgment on the remaining useful life of the used oil, which might result in the replacement of the oil reservoir, it is advised to perform additional analytical techniques (in accordance with Practices D6224 and D4378), having the capability of measuring remaining oxidative life of the used oil.
4.1.1 This test method is applicable to non-zinc turbine oils. These are refined mineral oils containing rust and oxidation inhibitors, but not antiwear additives. This test method has not yet been established with sufficient precision for antiwear oils.
4.2 This test method is also suitable for manufacturing control and specification acceptance.
4.3 When a voltammetric analysis is obtained for a turbine oil inhibited with a typical hindered phenol antioxidant, there is an increase in the current of the produced voltammogram between 3-5 s (or 0.3 to 0.6 V applied voltage) (see Note 1) in the basic test solution (Fig. 1 - x-axis 1 second = 0.1 V). Hindered phenol antioxidants detected by voltammetric analysis include, but are not limited to, 2,6-di-tert-butyl-4-methylphenol; 2,6-di-tert-butylphenol and 4,4'-methylenebis(2,6-di-tert-butylphenol).
NOTE 1 - Voltages listed with respect to reference electrode. The voltammograms shown in Figs. 1 and 2 were obtained with a platinum reference electrode and a voltage scan rate of 0.1 V/s.
4.4 For non-zinc turbine oils containing aromatic (aryl) amine compounds (antioxidants and corrosion inhibitors), there is an increase in the current of the produced voltammogram between 7-11 s (0.7 to 1.1 V applied voltage in Fig. 2) (see Note 1) which does not interfere with the hindered phenol measurement in the basic test solution. For the measurement of these aromatic amine antioxidants, refer to Test Method D6971, where the neutral test solution shall be used.
