Carbonizable Substances Acid Test

Medicinal oil requires a test in which the reaction of the oil to hot, strong sulfuric acid is used to determine the presence of carbonizable substance in the oil. However, the test for carbonizable substances (ASTM D-565) 

In the test method (ASTM D-565), the mineral oil is treated with concentrated sulfuric acid under prescribed conditions and the resulting color is compared with a reference standard to determine whether it passes or fails the test. When the oil layer shows no change in color and when the acid layer is not darker than the reference standard colorimetric solution, the oil is reported as passing the test. A bluish haze or a slight pink or yellow color in the oil layer should not be interpreted as a change in color. This is a pass-fail, and the more fully refined the oil, the lighter the color of the acid layer. 

With the introduction of ultraviolet absorption procedures (ASTM D-2008, ASTM D-2269), the test finds less use but still provides a useful method to determine possible contamination of mineral oil with impurities transparent to both visible and ultraviolet light and hence not detectable by color or by ultraviolet absorption measurements. 

For technical mineral oils the test for unsulfonatable residue (ASTM D-483) may be applied, but the test is of less significance for mineral oil that is refined to medicinal standard. The other quality criteria are much more stringent, and the test method for unsulfonatable residue does not have the required sensitivity. 

Mineral oils are usually considered to have a high propensity for carbon formation and deposition under thermal conditions. Nevertheless, the tests that are applied to determine the carbon-forming propensity of fuel oil (and other petroleum products) are also available for application to mineral oils. The test methods for the carbon residue should not be confused with the test method for carbonizable substances (ASTM D-565). The former test methods are thermal in nature, whereas the latter test method involves the use of sulfuric acid in a search for specific chemical entities within the oil. 

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Colorimetric solutions are used in the preparation of colorimetric standards for certain chemicals and for the carbonization tests with sulfuric acid that are specified in several monographs. Directions for the preparation of the primary colorimetric solutions and Matching Fluids are given under the test for Readily Carbonizable Substances, Appendix IIB. Store the solutions in suitably resistant, tight containers. 

Readily Carbonizable Substances Transfer 1.00 + 0.01 g of finely powdered Citric Acid to a 150-mm x 18-mm (od) tube previously rinsed with 10 mL of 98% sulfuric acid at 90° or used exclusively for this test. Add 10 + 0.1 mL of 98% sulfuric acid, carefully agitate the tube until solution is complete, and immerse the tube in a water bath at 90° + 1° for 1 h. Occasionally remove the tube from the water bath and carefully agitate it to ensure that the Citric Acid is dissolved and gaseous decomposition products are allowed to escape to the atmosphere. Cool the tube to ambient temperature, carefully shake the tube to ensure that all gases are removed, and using an adequate spectrophotometer, measure the absorbance and transmission of the solution at 470 nm in a 1-cm cell. The absorbance does not exceed 0.52, and the transmission is equal to or exceeds 30%. 

The ash from 0.5 Gm. is negligible. Dissolve 0.1 Gm. of finely powdered Cocaine in 1 cc. of sulphuric acid not more than a slightly yellow tint is produced (readily carbonizable substances). Dissolve 0.3 Gm. of finely powdered Cocaine in 1 cc. of normal hydrochloric acid, gently warming, if necessary, to aid solution, and dilute with distilled water to 15 cc. 5 cc. portions of this solution do not respond to the tests for cinnamyl-cocaine and for isoatropyl-cocaine under Cocaince Hydrochloridum. ... 

To determine the presence of carbonizable substances in paraffin wax (ASTM D-612) 5 ml of concentrated sulfuric acid is placed in a graduated test tube and 5 ml of the melted wax are added. The sample is heated for 10 min at 70°C (158°F). During the last 5 min the tube is shaken periodically. The acid layer is compared with a standard reference solution, and the wax sample passes if the color is not darker than the standard color. 

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