ASTM D2440 for oxidation stability of mineral insulating oil
13. Procedure
13.1 Prepare two copper catalysts as described in Section 10. Insert one in each of two clean, dry oil receptacles prepared as described in Section 9.
13.2 Transfer 25 more or less 0.01 g of the conditioned oil test specimen into each of the two prepared oil receptacles by means of a clean dry pipet. Immediately place the head on the oil receptacle to protect the oil from contamination during the interval between placing the oil in the receptacle and the actual start of the oxidation.
13.3 Adjust the heating bath to maintain a temperature of 110 more or less 0.5°C during the oxidation of the sample.
14. Oxidation
14.1 Immerse the oil receptacle in the heating bath, which has been adjusted to the test temperature. Immerse the oil receptacle in the bath to a depth such that the surface of the liquid in the heating bath will be 3 to 5 cm higher than the surface of the test oil in the receptacle, after the test oil reaches test temperature. Attach the oxygen inlet tubes to the oxygen supply line. When more than one oil receptacle is fed from a single oxygen supply line, adjust the oxygen flow to each oil receptacle to deliver 1 more or less 0.1 L/h.
14.2 Oxidize the two oil test specimens for 72 and 164 h respectively, under carefully controlled conditions.
15. Treatment of Oxidized Oil
15.1 Sludge Formation - After the required period of oxidation, shut off the oxygen supply and remove the oil receptacle and test specimen from the oxidizing bath. Cool for 1 h to room temperature in a clean dark box or dark room, protecting the test specimen from dust, moisture, or other contamination and light. At the end of the cooling period, remove the oxygen delivery tube and the copper catalyst coil from the oil test receptacle and transfer the oxidized oil into a 500–mL Erlenmeyer flask fitted with a ground glass stopper. Separate 300 mL of n-Heptane into three equal aliquots to be used for the sequential washing of the oxygen delivery tube copper catalyst coil and the test receptacle to recover the adhering oil. Into the same flask, thoroughly rinse the catalyst coil, delivery tube, and test receptacle to recover adhering oil, using a total of 300 mL of n-Heptane.
15.2 Weighing Sludge - Allow the mixture to stand in the dark for 24 h, at a temperature of 20 more or less 2°C, before filtering through a glass filter or polymer membrane, previously dried to constant weight. To prevent sludge from passing through the filter, use a small pressure drop at the start of filtering. Pass cloudy filtrates through a second time. Remove all traces of oil from the sludge by repeated washing with n-Heptane. Wash the 500-mL Erlenmeyer flask with n-heptane to remove adherent oil, and empty each rinse into the filter. Use a total volume of 150 mL of n-heptane, divided into three aliquots for sequential washes, for washing the sludge. Dry the filter containing the sludge at 110°C to constant mass and weigh to the nearest 0.0001 g. Transfer sludge adhering to the catalyst, test receptacle, oxygen delivery tube, and the 500-mL Erlenmeyer flask by dissolving the sludge in small quantities of chloroform or acetone (a total of 30 mL) into a tared porcelain vessel. Dry at 110°C after the evaporation of the chloroform or acetone to constant mass. Add the weight of the residue to that of the sludge obtained by precipitation with n-heptane. Express the total sludge as a percentage of the initial weight of the oil as follows:
Sludge, % = (weight of sludge/weight of oil sample) x 100
15.3 Neutralization Value - Collect the n-heptane solution in the 500-mL volumetric flask after filtering the sludge. Rinse the vacuum flask three times with n-heptane, emptying the contents into the volumetric flask, and dilute to the mark. Make three determinations of the neutralization value on 100-mL aliquots of the n-heptane solution containing test specimens. Add 0.5 mL of p-naphtholbenzein indicator solution to 100 mL of titration solvent. Add this indicator solution (100 mL) while stirring to 100 mL of the heptane solution. Titrate this mixture with 0.1 N alcoholic potassium hydroxide solution at a temperature not exceeding 25°C. The orange color of the indicator changes to a green or green-brown as the end point is approached. Consider the end point definite if the color change persists for 15 s or if it reverses with 2 drops of 0.1 N HCl. A buret having a capacity of 10 mL and with calibrations of 0.01 mL is recommended for the purpose of accurately controlling the flow of titration solvent. Hypodermic needle tips have been used to provide finer control of the flow rate.
15.3.1 On dark colored oils, it is permissible to determine the neutralization value with Test Method D664. The precision and bias statements for this test method do not apply, however, when Test Method D664 is used. Moreover, the relationship between the values obtained with the colorimetric and potentiometric methods has not been proven. Values obtained by the potentiometric titration method cannot be used to meet the Test Method D2440 limits of Specification D3487.
15.4 Calculation - Calculate the neutralization value, in milligrams of KOH per gram of oil, as follows:
Neutralization value = (N2 - N1) 5.61/5
where:
N2 = millilitres of 0.1 N alcoholic potassium hydroxide solution necessary to neutralize the n-heptane solution,
N1 = millilitres of 0.1 N alcoholic potassium hydroxide solution necessary to neutralize 100 mL of n-heptane to which have been added 100 mL of the titrating solution,
5.61 = milligrams of KOH per millilitre of 0.1 N solution, and
5 = grams of oil in 100 mL of heptane.
