What is a doctoral experiment, and what are the procedures for conducting it?

The doctor test is a procedure in which sodium leadite is reacted with light petroleum products in the presence of sublimed sulfur, in order to detect thiol or hydrogen sulfide in the oil. Copper sheet corrosion testing: GB/T 5096 – Test for copper sheet corrosion in petroleum products; this is currently the most common method for determining the corrosiveness of industrial lubricants, and it is equivalent to the ASTM D130-83 method. The test method involves immersing a polished copper sheet in a certain amount of the sample, heating it to a specified temperature as required by the product standards, and maintaining that temperature for a certain period of time. At the end of the testing period, the copper sheet is removed, washed, and then compared with a standard color chart to determine the degree of corrosion. The commonly used test conditions for industrial lubricants are 100°C (or 120°C) for 3 hours. 1. Place the test copper sheet in the oil sample in the test tube, maintain a temperature of 50 degrees Celsius ±1, and leave it there for 3 hours + 5 minutes to compare with the corrosion standard color chart. Grading (1a, 1b, 2a, 2b, 2c, 2d, 2e, 3a, 3b, 3c, 4a, 4b). 　　2. The indicator is the degree of erosion of the copper sheet’s surface by the solution under test when the sheet is placed in that solution at 100°C for 1.3 hours; the degree of corrosion is classified into four levels ； 1: Mild discoloration---------- Light orange, almost the same as a new copper sheet ； Deep Orange 2: Moderate discoloration ------------- Purplish-red ； light pink ； Multicolored with light purple or silver, or both, covering the purplish-red color respectively ； Silver ； Brass or golden yellow 3: Deep color change--------------A multicolored shade with magenta overlying the brass color ； A multi-colored version with red and green hues (peacock green), but without gray; 4 corrosion levels – transparent black, dark gray, or brown with only peacock green hues ； Graphite is black or dull black in color ； Shiny black or highly glossy black. Silver sheet corrosion: Not observed; it seems to be used for measuring jet fuel quality, and the measurement method is probably similar to that used for copper sheet corrosion. High-temperature aviation lubricants and fuel oils are also required to undergo testing in accordance with GJB496-88: copper and silver sheets are immersed in the samples, and after being held at 232°C for 50 hours, their weight loss is measured. Grades 90 and 93 are suitable as gasoline and comply with GB 484—93

Shi Hua You Ji Gong Ye Biao Zhun of the People’s Republic of China, SH/T 0174–92: Qualitative Test Method for Thiols in Aromatics and Light Petroleum Products. This standard is equivalently based on the international standard ISO 5275–1979: Qualitative Test Method for Thiols in Aromatics (Drabkin Test). Note: Aromatic hydrocarbons are toxic; avoid inhaling them, ingesting them through the digestive tract, or allowing them to be absorbed through the skin. Volatile aromatic hydrocarbons and light petroleum products are also flammable. Subject matter and scope: This standard specifies a method using Drab reagent for the qualitative detection of thiol in samples, and it can also be used for the qualitative detection of hydrogen sulfide. This standard applies to aromatics and light petroleum products. This standard does not apply to specimens with peroxide contents exceeding trace levels. Method summary: Shake the sample containing sodium hypolithiate solution and observe any changes in the appearance of the mixture to determine whether thiols, hydrogen sulfide, peroxides, or elemental sulfur are present in it. Then, by adding sulfur powder, shake the mixture and observe the final changes in its appearance. Further confirm the presence of thiol. Instrument: Measuring cylinder: 50ml, with scales and a ground glass stopper. Reagent: Sulfur powder: Sublimated and dried sulfur powder, stored in a sealed container. Lead acetate: analytically pure, colorless crystals or white powder. Molecular formula: Pb(CH3COO)2·3H2O. Sodium hydroxide: analytical grade, white granules or flakes. Cadmium chloride: analytically pure, colorless crystals or white powder. Hydrochloric acid: analytically pure, colorless and transparent liquid. Hydrogen chloride content: 36%–38% (m/m). Potassium iodide: analytically pure colorless crystals or white powder. Acetic acid: anhydrous, analytically pure, colorless and transparent liquid. Starch: analytically pure, soluble white amorphous powder. Distilled water or deionized water. Preparation steps: Preparation of reagents. Sodium leadite solution (Dr. Reagent): Dissolve 25 g of lead acetate in 200 mL of distilled water, filter, and add the filtrate to a solution of 100 mL of distilled water in which 60 g of sodium hydroxide is dissolved. Heat this mixture in a boiling water bath for 30 minutes; after cooling, dilute it to 1 L with distilled water. Seal this solution in a container. Before use, if it is not clear, it should be filtered. Cadmium chloride solution: Each liter of the solution contains 100 g of cadmium chloride and 10 mL of hydrochloric acid. Potassium iodide solution: freshly prepared, containing 100 g of potassium iodide per liter of solution. Acetic acid solution: Each liter of the solution contains 100 g or 100 mL of acetic acid. Starch solution: freshly prepared, containing 5 g of starch per liter of solution. Test procedure – Preliminary test: Pour 10 mL of the sample and 5 mL of sodium leadite solution into a graduated cylinder with a stopper, shake it vigorously for 15 seconds, and observe any changes in the appearance of the mixed solution. And continue the experiments as shown in the table below. “Table of changes in the \"doctor’s test\": Observe visual changes and make a judgment; proceed with the test as indicated below. Immediate formation of a black precipitate indicates the presence of hydrogen sulfide. 6.2 Slow formation of a brown precipitate suggests the possible presence of peroxides. 6.3 The mixture turns milky white during shaking, after which its color deepens; this indicates the presence of thiol and elemental sulfur. No change or a yellow color is observed. 6.4 Hydrogen sulfide is present; take a fresh sample and add a cadmium chloride solution accounting for 5% of the sample’s volume, then shake to remove hydrogen sulfide. Separate the treated samples and proceed with testing in accordance with Clause 6.1. If no black precipitate is formed, proceed with the test as specified in 6.4. If there is still black precipitate, reprocess the sample until no black precipitate remains. Then conduct the test in accordance with the provisions of 6.4. The presence of peroxides may be involved. To prove that it is the presence of a sufficient concentration of peroxides that interferes with the test, another sample was taken, to which a 20% potassium iodide solution, a few drops of acetic acid solution, and a few drops of starch solution were added, followed by vigorous shaking. If blue appears in the water layer, it indicates the presence of peroxides. In the final test, a small amount of sulfur powder was added to the mixture obtained according to the provisions in sections 6.1 or 6.2 (the amount added should not be excessive; just enough to cover the interface between the sample and the sodium hypophosphite solution). This mixture was shaken for 15 seconds and then left to stand for 1 minute. Immediately observe the mixture in the measuring cylinder; if, after adding sulfur powder and shaking, a brown (orange-red, brown) or black precipitate forms on the surface of the sulfur powder, it indicates the presence of thiol. Report: If the sample does not change color or turn milky white when shaken with sodium stannite solution. After adding sulfur powder, a brown (orange-red, brown) or black precipitate forms on the surface of the sulfur powder, indicating that the sample contains thiols; otherwise, it indicates that the sample does not contain thiols. If thiols are present in the sample, the test is reported as failed ； Report pass in the absence of thiol. If peroxides are present, this test is invalid. Note: Tests on samples with a high carbon disulfide content of 0.4% (m/m) are unreliable.

Reply to 3# chenhaidong5758: What are the things to keep in mind when conducting experiments? What issues should be noted? Thank you very much

1 The sodium leadite reagent used must be prepared correctly, as the laboratory technicians in our company often apply too much heat during preparation, which causes the reagent to decompose and makes it impossible to determine the endpoint. 2. After adding gasoline and shaking, it turns black; hydrogen sulfide must be removed, because its presence interferes with the ability to determine whether thiol sulfur is actually present after adding sulfur powder later. 3 Color change after adding sulfur powder and shaking. The final judgment for completion is based on whether the sulfur powder has changed color, rather than the color change of the solution. When sulfur powder changes from yellow to orange, brown, tan, or black, it indicates the presence of thiol sulfur. The problem is with identifying the orange color – while other color changes are obvious and can be recognized at a glance, orange is not easy to distinguish. From my experience, I prepare two parallel samples; if the color becomes darker compared to that of fresh sulfur powder, shades of orange-yellow or orange-red are considered a failure.