ASTM D4952 Test Method for Qualitative Analysis for Active Sulfur Species in Fuels and Solvents (Doctor Test)
6. Reagents and Materials
6.1 Purity of Reagents - Reagent grade chemicals shall be used in all tests. Unless otherwise indicated, it is intended that all reagents conform to the specifications of the Committee on Analytical Reagents of the American Chemical Society where such specifications are available. Other grades may be used, provided it is first ascertained that the reagent is of sufficiently high purity to permit its use without lessening the accuracy of the determination.
6.2 Purity of Water - Unless otherwise indicated, references to water shall be understood to mean reagent water as defined by Types II or III of Specification D1193.
6.3 Doctor (Sodium Plumbite) Solution - (Warning - Poisonous and suspect carcinogen.) Dissolve approximately 125 g of sodium hydroxide (NaOH) in 1 L of reagent water. Add 60 g of lead monoxide (PbO) and shake vigorously for 15 min, or let stand with occasional shakings for at least one day. Allow to settle and decant or siphon off the clear liquid. If the solution does not settle clear, filter it through filter paper. Keep the solution in a tightly sealed bottle and refilter before use if not perfectly clear. As an alternative, the lab may use a commercially prepared solution that meets the requirements of the laboratory preparation.
NOTE 1 - Alternative volumes of the solution may be prepared or purchased, provided the final solution concentration is equivalent.
6.4 Sulfur - Pure, sublimed, stored in a closed container.
6.5 Potassium Iodide, approximately 100 g/L Solution - Dissolve approximately 1 g of potassium iodide in approximately 10 mL of water. Prepare fresh for each test.
6.6 Acetic Acid, approximately 100 g/L Solution - Add approximately 10 mL of glacial acetic acid to approximately 100 mL water.
6.7 Starch Indicator, approximately 5 g/L Indicator Solution - Prepare fresh each time of testing.
7. Sampling and Handling of Test Samples
7.1 Improper choice of clear glass sample bottles followed by exposure to sunlight or fluorescent lighting emitting UV wavelengths shorter than 550 nm in the laboratory can generate peroxides in cracked gasolines as well as finished batches of gasolines. Peroxides are generated in proportion to the head-space air volume and time of exposure. It has been reported that peroxides can interfere with the Doctor Test - see 5.1.
7.2 It is preferable to collect the field samples using dark brown/amber bottles or metal cans, or if using clear glass bottles to wrap them in foil or place in a light-tight box to prevent UV light exposure.
7.3 The samples should be tested without delay upon receipt in the laboratory before further chemical interactions take place.
