Detection and determination of coumarin in hydrocarbon oils

Abstract

As part of changes in the system of control of rebated oils, the UK Revenue authorities and the petroleum industry have agreed that coumarin should be introduced as a statutory marker for kerosines, except for aviation fuels, at a concentration of 2 mg 1-l. Quinizarin and Solvent Red 24 will be retained for gas oils at the same concentrations as at present but furfuraldehyde will not be used as a marker.In this paper, a suitable qualitative roadside test, based on the present test for quinizarin, is described, together with a laboratory method for the quantitative determination of coumarin. ExperimentalIn alkaline solution at room temperature, coumarin hydrolyses fairly rapidly to (2)-3-(2hydroxyphenyl) acrylate (cis-form), which is converted into the fluorescent 2-hydroxycinnamate (trans-form) by ultraviolet light. The reactions are well known for analytical purposes1 and clearly coumarin dissolved in a hydrocarbon oil could be extracted by shaking with aqueous alkali with simultaneous hydrolysis. The methods which follow apply this principle with modifications to take account of the need to mark at low concentrations for economic reasons and to be sensitive enough to detect a 5% admixture of marked oil with DERV.In the current (1981) roadside test for quinizarin,2 1 ml of 10% m/V sodium hydroxidein water and 2 ml of butan-1-01 are shaken with 20 ml of oil sample. The mixture is allowed to separate into two phases and the colour of the lower phase is noted; a purpleblue colour indicates that quinzarin is present. If coumarin is present, ultraviolet light (mercury 365-nm line) directed upwards through the base of the tube will isomerise the cis-salt and a green fluorescence will develop over the course of about 1 min. In this form the test is not very sensitive because the natural fluorescence of the oil interferes, both from the upper oil phase and from material extracted into the aqueous phase. The fluoresence from the coumarin is not very strong and therefore a fairly high concentration of marker, at least 10 mg l-l, would have to be used, If ethanol is added, the intensity of coumarin derived fluorescence can be considerably increased. The other effects of ethanol addition are that the solubility of coumarin in the alkaline phase is increased so that extraction and hydrolysis are possible with less vigorous and prolonged shaking, more fluorescent material is extracted from the oil, some colouring matter extracted from the oil interferes with the detection of small amounts of quinizarin and three phases may appear.Fluorescence in the oil phase can be almost eliminated by absorbing the exciting light and quenching the fluorescent species. The most effective reagent for these functions is nitrobenzene, which is too toxic for use outside the laboratory. The oil-soluble yellow dyes do not quench but absorb both the exciting and emitted light. They are preferred on safety grounds.