Anal. Chem. 1989, 6 1 , 1467-1469
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TECHNICAL NOTES Determination of Antioxidants in Lubricating Oils Using Ultramicroelectrodes G r a h a m T. Cheek* Chemistry Department, United States Naval Academy, Annapolis, Maryland 21402 Robert Mowery Chemistry Division, Code 61 70, Naval Research Laboratory, Washington, D.C. 20375 INTRODUCTION Recent studies have shown that the ester-based lubricants used in naval applications (1)degrade rapidly as antioxidant levels decrease (2),emphasizing the importance of monitoring antioxidant levels in the oils as operating time progresses. The methods presently used for antioxidant monitoring in these systems include thin-layer chromatography (3) and visible spectroscopy (4), both of which are used in a laboratory setting and can be time-consuming when used to acquire quantitative information. Considering that antioxidants are themselves oxidized in providing stability to oil formulations, it was decided t o employ a related phenomenon, electrochemical oxidation, t o determine antioxidant levels in these media. In recent years, the use of ultramicroelectrodes has revolutionized the practice of voltammetry in poorly conducting media (5-8) so that the application of this technology to the monitoring of antioxidants in lubricating oils seems very appropriate. Within this general strategy, several approaches can be taken, including addition of long-chain supporting electrolytes to the oils to provide conductivity and use of extremely small (submicrometer) electrodes so that addition of supporting electrolytes is not necessary. Given that such small electrodes are difficult to fabricate and that oil solutions are difficult to handle in voltammetric operations, it was deemed necessary to investigate other methods of voltammetric determination of antioxidants. We have found that a particularly convenient procedure involves the addition of the oil/antioxidant solution to acetonitrile, the resulting solution having sufficient conductivity to allow meaningful voltammetric measurements of antioxidant concentrations. The results of our investigations, including some considerations on the use of ultramicroelectrodes for these measurements, are presented here.
were used in this investigation (Figure 1): [l]Initially, a simple glass vial cell with a close-fitting top equipped with a platinum wire (0.5 mm diameter) counter electrode (Bioanalytical Systems) was used. The counter electrode wire could be positioned within 1-2 mm of the ultramicroelectrode. [2] The above cell was improved by using an inverted-configuration counter electrode (exposed platinum disk, 1.0 mm diameter), which allowed positioning of the counter electrode to within less than 1 mm of the ultramicroelectrode. [3] Most of the results in this study were obtained with a cell of fundamentally different design. A fritted glass tube (Ace Glass Co., Inc.; porosity E) was simply bent into a "U"configuration, with the ultramicroelectrode and a Ag/AgCl reference electrode (Bioanalytical Systems, Inc.) placed in the resulting arms of the cell. The solution under investigation was placed in the ultramicroelectrode arm of the cell and an aqueous 0.1 M NaC10, solution was placed in the other arm. This cell was found to be most satisfactory for this investigation and most of the results were obtained from its use. The instrumentation was similar to that described in other reports (7),involving application of a linear potential sweep to the cell with a simple triangular wave potential generator (Bioanalytical Systems, Model CV-1B) and measurement of the resulting current with a picoammeter (Keithley Model 617). The potential and current outputs were recorded on a Houston Model 2000 XY recorder. The analytical method finally adopted involved adding 5.00 mL of acetonitrile (Burdick and Jackson) to a weighed amount (0.250-1.000 g) of the antioxidant/oil solution whose antioxidant concentration ranged from 0.10% to 2.00%. The acetonitrile/ lubricant mixture was introduced into the cell, deoxygenated for 10 min with presaturated (acetonitrile)nitrogen, and then scanned in the positive direction from an initial potential of 0.00 V. Curves were recorded at a scan rate of 25 mV/s. The best results were obtained when the ultramicroelectrodes were polished (0.05 pm alumina, followed by ultrasonication) after each scan.
EXPERIMENTAL S E C T I O N Phenothiazine (PTZ), diphenylamine (DPA), and phenyl-1naphthylamine (PANA) were purified by sublimation. The oils investigated were Hercolube (pentaerythritol tetra-n-hexanoate, NRL preparation) and TMP (a trimethylolpropane-based lubricant with acyl chain length distribution of 1,1,5carbon atoms; Stauffer base stock 707) which were used as received. Standards were prepared on a weight percent basis, the manner customary in antioxidant/lubricating oil investigations. The cell system employed in this work was deliberately kept very simple and involved a two-electrode system composed of a platinum ultramicroelectrode and a suitable reference/counter electrode. The ultramicroelectrode was constructed by sealing closed a length of Pyrex capillary tubing (6 mm o.d., 1 mm i.d.), inserting a 1-2 cm length of platinum wire (10-50 pm diameter; Goodfellow Metals Ltd.) above the sealed area, and sealing the wire into the tubing under vacuum. Cutting the sealed end of the tube midway along the wire seal exposed a microdisk electrode. After the electrode was polished with successively finer grades of alumina (final polish, 0.05 pm), electrical contact with the microdisk was made by using a small amount of Ga/In eutectic mixture (Alfa Products, 89180) in the tube. Three types of cells
R E S U L T S A N D DISCUSSION To describe the method of choice in this investigation, it is of interest to mention the evolution of electrochemical cell design which led to the development of the final method. The results of previous approaches give some insight into the problems encountered in making electrochemical measurements in these highly resistive systems. The f i s t approach taken was perhaps the most obvious and involved the attempted measurement of phenothiazine concentration (1.00%) in Hercolube (a commercial ester-based lubricant) using cell configuration 1. Even with the counter electrode wire positioned 1 mm away from the ultramicroelectrode tip, very little (
