ASTM D4952 Test Method for Qualitative Analysis for Active Sulfur Species in Fuels and Solvents (Doctor Test)
8. Procedure
8.1 Shake vigorously together in a test tube 10 mL of the sample being tested and 5 mL of sodium plumbite solution for about 15 s. Add a small amount of pure, sublimed flowers of sulfur so that practically all of it floats on the interface between the sample and the sodium plumbite solution after shaking. Shake again for 15 s. Allow to settle and observe within 2 min.
NOTE 2 - It is important to avoid adding more sulfur than will just cover the interface. About 20 mg to 25 mg is the proper quantity, which can be estimated with a little practice. If too much sulfur is added, any possible discoloration will be masked by the excess of sulfur.
9. Interpretation of Results
9.1 If the solution is discolored or if the yellow color of the sulfur film is noticeably masked, report the test as positive and consider the sample as sour. If the sample remains unchanged in color and the sulfur film is bright yellow or only slightly discolored with gray or flecked with black, report the test as negative and consider the sample as sweet. If a brown precipitate slowly forms, peroxide is probably present. Proceed in accordance with 5.2.
NOTE 3 - This examination must be made with extreme care. Sometimes the sulfur layer will be only flecked with spots of gray or black, and if there is any change in the color of either the sample or the Doctor solution these spots will be difficult to detect.
NOTE 4 - Strictly speaking, the test will not reject the sample on the basis of mercaptans only. The primary criterion for rejection is the appearance of the sulfur layer after shaking, and small amounts of mercaptans will only discolor the sulfur. However, samples which contain mercaptans will also normally contain small amounts of sulfur in other forms which will discolor the layers at the interface. The addition of pure, sublimed flowers of sulfur is required to indicate that a reaction of mercaptan and sodium plumbite has occurred.
NOTE 5 - When the sample contains appreciable amounts of hydrogen sulfide, a heavy black precipitate may be formed during the initial shaking and before the addition of the sulfur. If such a precipitate is noted, the test may be stopped at that point and the sample reported as "Does not pass". However, if this observation is at all doubtful, the test should be continued. If the sample darkens before the addition of sulfur is made, this indicates that the sample contains mercaptans and an excess of elemental sulfur that is needed to drive the sample reaction with sodium plumbite.
9.1.1 The relationship between the appearance of the sample and the type of sulfur is shown in Table 1.
9.2 If the doctor test is positive, mercaptan content may be determined using Test Method D3227.
10. Precision and Bias
10.1 No justifiable values of repeatability, reproducibility, or bias for this test method can be stated here because the test detects only the presence or absence of active sulfur species, such as hydrogen sulfide or mercaptan.
11. Keywords
11.1 doctor test; hydrogen sulfide; kerosine; mercaptans; motor fuel; sulfur
