Determination of additive content in aviation and turbine oils

Abstract

We employed Silufol plates produced by Kavalier (Czechoslovakia), a layer of silica gel (starch as binder) applied to aluminum foil for TLC. The method was worked out with model mixtures of turbine and synthetic aviation oil bases and additives. The antioxidant content undergoes the greatest change in these oils during aging, and we therefore focused on its determination. The fact that antioxidants are reactive compounds is of considerable importance. It allows use of such readily available reagents as iodine vapor for development of the spots they form. The present study was conducted to select conditions for quantitation of antioxidants: N-phenyl-~-naphthylamine (PAN), 4,4-dioctyldiphe nylamine (DAT), and 2,6-di-tert-buty l-4methylphenol (ionol) in synthetic oils based on isoparaffin hydrocarbons (IPM), dioctyl sebacinate (DOS), and pentaerythritol esters with synthetic fatty acids (PEE). We also selected conditions for determination of ionol in turbine oils (regardless of the aromatic hydrocarbon content of the base oil) and of 2-mercaptobenzothiazole (Captax) in PEE. Determination of additive content includes five basic technical operations: preparation of reference and working solutions, application of specimens to plates, elution of plates bearing specimens, development of separated additivespots, determination of additive spot areas, and calcuation of concentrations. The concentration calculations are based on the so-called w S method [5], which runs as follows: a reference specimen with known additive concentration c I and a double-concentration reference specimen (with an additive concentration c 2 = 2c l) are applied to a plate together with the test specimen (having additive concentration c). A number of conditions must be observed in order to obtain reliable results: linearity of the dependence of V~(where S is the area of the developed spot for the additive under investigation) on log c, fulfillment of the relation c I ~ c ~ c 2, and exact specimen metering during application to the plate. Satisfaction of these requirements was achieved in the following manner. Exact weighed portions of the test oil and oil with a known concentration of the additive to be determined (0.1 g) wwere dissolved in 1 ml of isooctane shortly before the determinations were made. The resultant solutions were applied to Silufol plates in spot form, using a single capillary tube (30-40 mm long and having an inside diameter of 0.2-0.5 mm) and washing it between applications. The volume of solution applied was determined from the total internal volume of the capillary tube. Three starting points were applied to each plate: one for the test specimen, one for the reference specimen, and one for twice the amount of reference specimen (two capillary loads at the same point). The plate bearing the start points was placed in a beaker containing the appropriate solvent (or solvent mixture) and eluted over a travel distance of I0 cm the requisite number of times. The plates were dried and the spots were developed with iodine vapor; in order to fix the colors, the developed plates were flooded with water and dried in air. The additive spot areas were determined with a millimeter reticle on a polycarbonate base; each spot was evaluated no less than three times and the average area calculated.