Microcrystalline Wax

This article is compiled based on the United States Pharmacopeia (USP) – 2025 Edition

Issued and maintained by the United States Pharmacopeial Convention (USP)

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C_30≤n≤120H_2n+2 with an average carbon chain length number of 50 (C₅₀). Branched-chain hydrocarbons predominate in the mixture. Molecular weight is from about 400 to 1700 with an average molecular weight of 700.

Petroleum wax

CAS RN^®: 63231-60-7. (NF 1-Aug-2021)

1 DEFINITION

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Microcrystalline Wax is a refined (NF 1-Aug-2021) mixture of solid saturated aliphatic (NF 1-Aug-2021) hydrocarbons, obtained by solvent fractionation of the still bottom fraction of petroleum by suitable dewaxing or deoiling means.

The branched-chain hydrocarbons predominate in the mixture, but the straight-chain and cyclic hydrocarbons are present. It may contain an antioxidant. (NF 1-Aug-2021)

2 IDENTIFICATION

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A. SPECTROSCOPIC IDENTIFICATION TESTS (197), Infrared Spectroscopy: 197A or 197F. Use a thin film of completely melted Microcrystalline Wax when performing 197F. (NF 1-Aug-2021)

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B. MELTING RANGE OR TEMPERATURE (741), Procedures, Procedure for Class ///: 54°-102° and falls within the range indicated by the Labeling (NF 1-Aug-2021)

3 IMPURITIES

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3.1 RESIDUE ON IGNITION (281)

Sample: 2 g

Analysis: Heat the Sample in an open porcelain or platinum dish. (NF 1-Aug-2021)

Acceptance criteria: It volatilizes (NF 1-Aug-2021) and on ignition yields NMT 0.1%.

4 SPECIFIC TESTS

4.1 COLOR

Standard solution: Mix 3.8 mL of ferric chloride CS and 1.2 mL of cobaltous chloride CS in a clear-glass, 16-x 150-mm bacteriological test tube.

Sample solution: Melt 10 g of Microcrystalline Wax on a steam bath, and pour 5 mL of the liquid into a clear-glass, 16-x 150-mm bacteriological test tube.

Analysis: Visually compare the contents of both tubes in reflected light against a white background, holding the tubes directly against the background at such an angle that there is no fluorescence.

Acceptance criteria: The Sample solution is not darker than the Standard solution.

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4.2 MELTING RANGE OR TEMPERATURE, Class III (741): 54°-102° (NF 1-Aug-2021)

4.3 CONSISTENCY

Sample: Microcrystalline Wax

Apparatus: Determine the consistency of the Sample by means of a penetrometer fitted with a polished metal needle weighing 2.5 ± 0.05 g and having a truncated symmetric tapered angle of 9°0' ± 15'. The needle is tapered, with a length of 25.4 mm, and the shaft attached to the needle is 58 mm in length and 3.17 mm in diameter. The plunger that fits into the penetrometer and guides the path of the needle weighs 47.5 ± 0.05 g. An additional weight of 50 ± 0.05 g is added to the top of the plunger to give a total load of 100 g.

Analysis: The Sample is cast in a brass cylinder open at both ends. The cylinder has an inside diameter of 25.4 mm and is 31.8 mm in height. Place the cylinder on a brass plate wetted with an equal volume mixture of Glycerin and water, and place the plate on two corks. Pour the wax, melted at approximately 17° above its congealing point, into the cylinder. Continue pouring the wax until a convex meniscus is formed above the cylinder. Allow the specimen to cool for 1 h at approximately 24°. Shave excess wax from the top of the cylinder, and remove the plate. With the smooth wax surface in the up position, condition the specimen in a water bath at 25° for 1 h.

Arrange the penetrometer so that the wax specimen is completely immersed in the water bath while penetration is run. Lower the needle until the tip just touches the top surface of the specimen. Release the needle for 5 s, and read the depth of penetration in tenths of millimeters. Perform four determinations, and calculate the average value of the four readings.

Acceptance criteria: 3-100 (0.3-10.0 mm)

4.4 ORGANIC ACIDS

Sample solution: 20 g of Microcrystalline Wax in 100 mL of a mixture of neutralized alcohol and water (1:2). Agitate thoroughly, and heat to boiling.

Analysis: To the Sample solution add 1 mL of phenolphthalein TS, and titrate rapidly with 0.1 N sodium hydroxide VS, with vigorous agitation, to a sharp pink endpoint in the alcohol-water layer.

Acceptance criteria: NMT 0.4 mL of 0.1 N sodium hydroxide is required.

4.5 FIXED OILS, FATS, AND ROSIN

Sample: 10 g

Sample solution: Digest the Sample with 50 mL of sodium hydroxide solution (1 in 5) at 100° for 30 min. Separate the water layer, and acidify it with 2 N sulfuric acid.

Acceptance criteria: No oily or solid matter separates.

4.6 ALKALINITY

Sample: 35 g

Analysis: Introduce the Sample into a 250-mL separator, add 100 mL of boiling water, and shake vigorously for 5 min. Draw off the separated water into a casserole, wash further with two 50-mL portions of boiling water, and add the washings to the casserole. To the pooled washings add 1 drop of phenolphthalein TS, and boil.

Acceptance criteria: The solution does not acquire a pink color.

4.7 ACIDITY

Analysis: If the addition of phenolphthalein TS in the test for Alkalinity produces no pink color, add 0.1 mL of methyl orange TS.

Acceptance criteria: No red or pink color is produced.

5 ADDITIONAL REQUIREMENTS

PACKAGING AND STORAGE: Preserve in tight containers.

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LABELING: Label it to indicate the name and proportion of any added antioxidant. Label it to indicate the melting range. (NF 1-Aug-2021)

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USP REFERENCE STANDARDS (11)

USP Microcrystalline Wax RSA (NF 1-Aug-2021)