11/6/2020 USP-NF Aluminum Monostearate

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Aluminum Monostearate

Aluminum, dihydroxy(octadecanoato-O-)-;
Dihydroxy(stearato)aluminum
[7047-84-9].

DEFINITION
Aluminum Monostearate is a compound of aluminum with a mixture of solid organic acids obtained from sources of vegetable or
animal origin, and consists mainly of variable proportions of aluminum monostearate (C18H37AlO4) and aluminum monopalmitate
(C16H33AlO4). The content of stearic acid in the fatty acid fraction is NLT 40.0% of the total content. The sum of stearic acid and
palmitic acid in the fatty acid fraction is NLT 90.0% of the total content. Aluminum Monostearate contains the equivalent of NLT 14.7%
and NMT 16.7% of aluminum oxide (Al2O3) on the dried basis. Additionally, it contains small amounts of the aluminum salts of other
fatty acids.

IDENTIFICATION
• A. IDENTIFICATION TESTS—GENERAL 〈191〉, Chemical Identification Tests, Aluminum
Sample: 1 g
Analysis: Heat the Sample with a mixture of 25 mL of water and 5 mL of hydrochloric acid for 1 h, replacing the water as it evaporates.
Acceptance criteria: Fatty acids are liberated, oating as an oily layer on the surface of the liquid, and the water layer meets the
requirements.
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• B. The retention times of the two major peaks of the Sample solution correspond to those of the Standard solution, as obtained in the
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Content of Stearic Acid and Palmitic Acid test in the Assay.

ASSAY
• CONTENT OF ALUMINUM
Sample: 5 g
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Analysis
Sample: Sample
Weigh the Sample in a covered platinum crucible that previously has been ignited for 20 min, cooled over anhydrous magnesium
perchlorate, and weighed. Heat the open crucible gently, without allowing the Sample to burst into ame, and gradually increase
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the heat until the ash is white. Ignite the ash for 20 min after the organic matter is removed, and cool. Add 15 mL of water, cover
the crucible with a small watch glass, and boil gently for 5 min, using a small stirring rod to break up any large lumps of ash.
Decant the solution through ashless lter paper, retaining most of the ash in the crucible. Repeat the extraction with water twice,
passing the solutions through the same lter. Transfer the ash to the lter by means of a ne stream of water, and wash the
crucible and the residue three times with warm water. Transfer the lter paper and the residue to the crucible, dry, and ignite for
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20 min after the lter paper has burned away. Following the ignition period, cover the crucible, cool over anhydrous magnesium
perchlorate for 15 min, and weigh the residue of aluminum oxide (Al2O3) rapidly. Repeat the ignition until constant weight is
attained, using 20-min ignition periods and 15-min cooling periods. From the weight of the residue remaining in the crucible,
calculate the content of aluminum oxide (Al2O3).
Acceptance criteria: 14.7%–16.7% on the dried basis
• CONTENT OF STEARIC ACID AND PALMITIC ACID
Boron tri uoride–methanol solution: 140 g/L of boron tri uoride in methanol
Sample solution: Dissolve 100 mg of Aluminum Monostearate in a small conical ask tted with a suitable re ux attachment with 5
mL of Boron tri uoride–methanol solution. Boil under re ux for 10 min. Add 4.0 mL of n-heptane through the condenser, and boil
again under re ux for 10 min. Allow to cool. Add 20 mL of a saturated solution of sodium chloride. Shake, and allow the layers to
separate. Dry the organic layer over 0.1 g of anhydrous sodium sulfate. Dilute 1.0 mL of this solution with n-heptane to 10.0 mL.
Standard solution: Prepare as directed in the Sample solution using 50 mg each of USP Stearic Acid RS and USP Palmitic Acid RS.
Chromatographic system
(See Chromatography 〈621〉, System Suitability.)
Mode: GC
Detector: Flame ionization
Column: 30-m × 0.32-mm; coated with a 0.5-µm lm of phase G16
Temperatures
Injection port: 220°
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Detector: 260°
Column: See Table 1.

Table 1

Hold Time at Final

Initial Temperature (°) Temperature Ramp (°/min) Final Temperature (°) Temperature (min)

70 — 70 2

70 5 240 5

Carrier gas: Helium, passed through a bed of molecular sieve for drying, if necessary
Flow rate: 2.4 mL/min
Injection volume: 1 µL
System suitability
Sample: Standard solution
[NOTE—The relative retention times for methyl palmitate and methyl stearate are about 0.9 and 1.0, respectively.]
Suitability requirements
Resolution: NLT 5.0 between the methyl palmitate and methyl stearate peaks
Relative standard deviation: NMT 3.0% for the methyl stearate and methyl palmitate peaks (from six replicate injections of the
Standard solution); NMT 1.0% for the ratio of the peak areas of methyl palmitate to the peak areas of methyl stearate (from six
replicate injections of the Standard solution)
Analysis
Sample: Sample solution
Calculate the percentage of stearic acid (C18H36O2) in the fatty acid fraction of the sample taken:

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Result = (rU1/rT) × 100
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rU1 = peak area due to methyl stearate

rT = sum of all the peak areas, excluding the solvent peak

Calculate the percentage of palmitic acid (C16H32O2) in the fatty acid fraction of the sample taken:
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Result = (rU2/rT) × 100

rU2 = peak area due to methyl palmitate

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rT = sum of all the peak areas, excluding the solvent peak

Acceptance criteria
Stearic acid: NLT 40.0%
Sum of stearic acid and palmitic acid: NLT 90.0%
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IMPURITIES
• ARSENIC 〈211〉, Procedures, Method I
Test preparation: To 3.75 g of Aluminum Monostearate add 12.5 mL of hydrochloric acid and 0.5 mL of bromine TS, and heat on a
steam bath until a transparent layer of melted fatty acid forms. Add 50 mL of water, heat on a hot plate until the volume is about 25
mL, and lter while hot. Cool, dilute the ltrate with water to 50 mL, and to a 10-mL aliquot of this solution add 2.5 mL of
hydrochloric acid, then dilute with water to 55 mL.
Acceptance criteria: NMT 4 µg/g; the Test preparation meets the requirements, omitting the addition of 20 mL of 7 N sulfuric acid
speci ed for Procedure.

SPECIFIC TESTS
• LOSS ON DRYING 〈731〉
Analysis: Dry at 105° for 3 h.
Acceptance criteria: NMT 3.0%
• ACIDITY AND ALKALINITY
Sample solution: To 1.0 g of Aluminum Monostearate add 20 mL of carbon dioxide-free water and boil on a steam bath for 1 min with
continuous shaking. Cool and lter. To 10 mL of the ltrate add 0.05 mL of bromothymol blue TS.
Analysis: Titrate with 0.01 N hydrochloric acid or 0.01 N sodium hydroxide.
Acceptance criteria: NMT 0.5 mL of 0.01 N hydrochloric acid or 0.01 N sodium hydroxide is required to change the color of the
indicator.
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Add the following:

▲• FATS AND FIXED OILS 〈401〉, Procedures, Acid Value, Method I

Sample: 1.0 g
Analysis: Dissolve the Sample in 50 mL of a mixture of equal volumes of alcohol and ether (which has been neutralized to
phenolphthalein with 0.1 N potassium hydroxide or 0.1 N sodium hydroxide). Pass the suspension through dry lter paper, wash the
vessel and the lter paper with a small amount of a mixture of neutralized ethanol and diethyl ether (1:1), and combine the ltrate
and the washings. Proceed as directed in the chapter starting with “Add 1 mL of phenolphthalein TS”.
Acceptance criteria: NMT 11▲ (NF 1-Dec-2019)

• BACTERIAL ENDOTOXINS TEST 〈85〉: If labeled for use in preparing parenteral dosage forms, it also meets the following requirements. The
level of bacterial endotoxins is such that the requirement in the relevant dosage form monograph(s) in which Aluminum Monostearate
is used can be met. Where the label states that Aluminum Monostearate must be subjected to further processing during the preparation
of injectable dosage forms, the level of bacterial endotoxins is such that the requirement in the relevant dosage form monograph(s) in
which Aluminum Monostearate is used can be met.

ADDITIONAL REQUIREMENTS
• PACKAGING AND STORAGE: Preserve in well-closed containers.
• LABELING: Label it to indicate the content of stearic acid and the sum of stearic acid and palmitic acid content in the fatty acid fraction
and to indicate the fatty acids used to produce Aluminum Monostearate are from sources of vegetable or animal origin. Where
Aluminum Monostearate must be subjected to further processing during the preparation of injectable dosage forms to ensure
acceptable levels of bacterial endotoxins, it is so labeled.
• USP REFERENCE STANDARDS 〈11〉
USP Palmitic Acid RS
USP Stearic Acid RS

Auxiliary Information- Please check for your question in the FAQs before contacting USP.

Topic/Question Contact L Expert Committee

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ALUMINUM MONOSTEARATE Galina Holloway CE2020 Complex Excipients
Senior Scienti c Liaison
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Chromatographic Database Information: Chromatographic Database

Most Recently Appeared In:

Pharmacopeial Forum: Volume No. 44(4)
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Page Information:

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USP42-NF37 2S - 9317
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