8. Procedure
8.1 The voltammetric analyzer used in this test method gives linear results between 2 to 50 mmol for hindered phenols using an oil sample size of 0.40 and 5.0 mL of the analysis solvent. The corresponding range of weight percents depends on the molecular weight of the hindered phenol and the density of the base oil. For instance, the weight % range of 0.044 to 1.1 is equal to 2 to 50 mmol/L for a hindered phenol containing one hydroxyl group and with a molecular weight of 220 g/mol (2,6-di-tert-butyl-4-methylphenol) and an oil density of 1 g/mL. Below 2 mmol, the noise to signal ratio becomes large decreasing the accuracy of the measurements. For measurements below 2 mmol or for fresh oils with high noise to signal ratios, the sample size should be increased to 0.60 mL and the volume of analysis solvent decreased to 3.0 mL.
8.2 General Voltammetric Test Procedure - The test procedure for voltammetric analysis will consist ofthe blank reading (calibration), followed by a standard reading and finally the test sample (in-service oil) reading.
8.2.1 Blank Reading - (0 mmol/L = 0 weight %).
8.2.1.1 Definition - The blank reading (voltammetric number) is a measurement of the analysis solution by itself. The blank measurement gives a reference number with no antioxidant present (the zero baseline).
8.2.2 Standard Reading - (30 to 150 mmol/L - weight % dependent on density of fresh oil and molecular weight of antioxidant).
8.2.2.1 Definition - The standard reading is a measurement of a fresh, unused oil (containing phenolic antioxidant) mixed with an appropriate analysis solvent. This measurement gives you a voltammetric reading (standard reading) that indicates the voltammetric response for the concentration hindered phenol antioxidant being analyzed for the oil being tested.
8.2.3 Test Sample (In-Service Oil) Reading:
8.2.3.1 Definition - The sample reading is a measurement of a fresh or in-service oil mixed with the same type of analysis solvent as the standard. This measurement will provide voltammetric readings that normally range between the blank and standard measurements, and reflect the concentration of hindered phenol antioxidant present (fresh oil) or remaining (in-service oil) in the oil sample. Voltammetric readings for in-service oil will decrease as hindered phenol antioxidants are depleted.
8.3 Voltammetric Reading - After the operator has selected the valleys before and after the antioxidant peaks (as shown in Fig. 1), the software (R-DMS) will automatically identify and calculate the area above the baseline between the two valley indicators. This calculated area is then used for the test sample reading (in-service oil), which will be established by comparing the in-service oil area to its standard (see Fig. 3) and make remaining antioxidant calculations (see Section 9). If peak shifting is occurring, it is advised to repeat the voltammetric test after performing the cleaning of the electrode. If after this second test the peak shifting remains persistent, it is advised to drag the valley indicators manually to their shifted locations.
8.4 Calibration (Blank Reading) Procedure - Pipette 5.0 mL of analysis solution into a 7–mL vial or other suitable container containing 1 g of sand. Insert the electrode of the voltammetric analyzer into the analysis solution to wet the bottom surface of the electrode, remove, and rub dry the bottom electrode surface with a lint–free paper towel. Insert the electrode into the vial so that the bottom of the electrode is submerged in the analysis solution without resting on the sand layer on the bottom of the vial. Place the vial/probe upright into the rack or foam block for testing. Perform the voltammetric analysis (see 5.1). Record the voltammetric reading in the voltage range of the phenols, 0.3 to 0.6 V (see Note 1) in basic solution and Fig. 1. Remove the combination electrode from the blank solution and rub dry the bottom surface of the electrode with a lint free paper towel. Run at least two tests of the analysis solution to assure the electrode is clean and the minimal blank value has been obtained.
8.4.1 Calibration Frequency - Recalibration with freshly prepared blank solution can be performed before each testing session, or with the use of a new batch of test solutions.
8.5 Standard and In-Service Oil Sample Preparation Procedures:
8.5.1 Preparing Solution Step - Remove the seal and cap of the test solution vial. Pipette 5.0 mL of analysis solution into a 7–mL vial or other suitable container containing 1 g of sand. Pipette 0.40 mL of the selected oil sample also into the 7–mL vial.
8.5.2 For measurements below 2 mmol or for fresh oils with high noise to signal ratios, the sample size should be increased to 0.60 mL and the volume of analysis solvent decreased to 3.0 mL.
8.5.3 Shaking Solution Step - Cap the vial and shake vigorously using a vortex mixer for 20 s or by hand (between 50 and 60 shaking cycles/min), until sand is thoroughly mixed. Place the prepared oil solution upright in a rack or perforated foam block for a minimum time of 30 s to allow the sand to settle on the bottom of the vial with the oil.
8.5.4 Cleaning Electrode Step - Prepare the electrode for analysis by cleaning the electrode. Use an alcohol-cleansing pad to wet the bottom surface of the electrode. The bottom of the electrode must be dried immediately with a clean lens tissue (lint-free paper towel). The glassy carbon surface should always have a polished look before running a test. A glazed or cloudy look indicates the presence of a chemical film. If the probe tip is not cleaned properly, voltammetric readings can be distorted.
8.5.5 Running Test Step - Insert the electrode into the vial so that the bottom of the electrode is submerged in the analysis solution without resting on the sand layer on the bottom of the vial. Place the vial/probe upright into the rack or foam block for testing. Perform the voltammetric analysis (see 5.1) for hindered phenolic antioxidants. Record the valley to valley antioxidant reading in the voltage range of the phenols, 0.3 to 0.6 V (see Note 1) in the basic test solution (Fig. 1). Remove the combination electrode from the oil solution and repeat the cleaning procedure of the electrode. Run at least two tests (cleaning the electrode and shaking the standard for 10 s between tests) of the standard or in-service oil sample to ensure the value is stable and repeatable.
8.5.6 Make all measurements within 5 min after the initial mixing of the analysis solution, selected sample, and sand.
8.6 When the manufacturer of the oil is known, and the uninhibited base oil is available, use it to prepare the standards (mmol or weight percent antioxidant calculations). Prepare a standard containing in the range of 30 to 150 mmol/L of oil (0.5 to 3.0 weight %) of the selected phenolic antioxidant dissolved in an uninhibited base oil. The concentration should be selected to span the expected concentrations of the new and in-service oil samples.
8.7 Standard readings should be updated whenever new batches of lubricants are stocked, and periodically to monitor the amount of natural oxidation occurring in the stock during storage.
8.8 For fresh or in-service oils of unknown origin, use a typical fresh turbine oil as the standard (100 % remaining antioxidant calculations).
8.9 The analysis solution and scan time should be the same for the blank, standard, and in-service oil sample.
9. Calculations
9.1 Percent Hindered Phenol Antioxidant Calculation - If the hindered phenol antioxidant present in the oil sample is known, then the percent hindered phenol antioxidant in the sample can be calculated as follows:
where:
reading = valley to valley area (see 8.3) for peak between 0.3 to 0.6 V (see Note 1).
9.2 Molar Concentration of Hindered Phenol Antioxidant Calculation - If the phenol hindered phenol antioxidant in the oil sample is unknown, then the millimoles of hindered phenol antioxidant in the sample can be calculated as follows:
where:
reading = valley to valley area (see 8.3) for peak between 0.3 to 0.6 V (see Note 1).
9.3 Percent Remaining Hindered Phenol Antioxidant Calculation - Calculate the percent remaining hindered phenol antioxidant in an in-service oil using the fresh oil as the 100 % standard with the following calculation:
where:
reading = valley to valley area (see 8.3) for peak between 0.3 to 0.6 V (see Note 1).