ASTM D4952 Test Method for Qualitative Analysis for Active Sulfur Species in Fuels and Solvents (Doctor Test)
3. Summary of Test Method
3.1 The sample is shaken with sodium plumbite solution, a small quantity of powdered sulfur added, and the mixture shaken again. The presence of mercaptans or hydrogen sulfide or both is indicated by discoloration of the sulfur floating at the oil-water interface or by discoloration of either of the phases.
4. Significance and Use
4.1 Sulfur present as mercaptans or as hydrogen sulfide in distillate fuels and solvents can attack many metallic and non-metallic materials in fuel and other distribution systems. A negative result in the doctor test ensures that the concentration of these compounds is insufficient to cause such problems in normal use.
5. Interferences
5.1 This test cannot be used if there are more than trace amounts of peroxides in the test sample. Peroxides can give a false positive results where mercaptans are at low level or not even present.
5.2 To check if peroxides are present in sufficient concentration to invalidate the test, shake 10 mL +/- 0.5 mL of a fresh portion of the sample with approximately 2 mL of the potassium iodide solution, add two drops of the acetic acid solution, and two drops of the starch solution. If the aqueous layer turns a blue color, this confirms the presence of peroxides in sufficient quantity to invalidate the test, and the test on this sample should be discontinued. Proceed in accordance with 5.4.
5.3 Alternatively, one may choose to perform a preliminary Doctor Test. If a brown precipitate slowly forms, peroxide is probably present. Proceed in accordance with 5.2 to confirm presence of peroxides at sufficient quantity to invalidate the test.
5.4 If interference from peroxides is confirmed, proceed to re-sample and retest. Ensure that sampling and handling procedures for the new sample prevent UV light exposures as prescribed in 7.1. None of the normal refinery units or blending processes producing spark ignition motor fuels are known to create peroxides under normal operating conditions.
