METHANOL
Prepared at the 28th JECFA (1984), published in FNP 31/2 (1984) and in
FNP 52 (1992). Metals and arsenic specifications revised at the 63rd
JECFA (2004). An ADI 'limited by GMP' was established at the 14th JECFA
(1970)
SYNONYMS Carbinol
DEFINITION
Chemical names Methanol, methyl alcohol
C.A.S. number 67-56-1
Chemical formula CH3OH
Structural formula
Formula weight 32.04
Assay Not less than 99.5%
DESCRIPTION Clear colourless, mobile liquid with a characteristic odour
FUNCTIONAL USES Extraction solvent
CHARACTERISTICS
IDENTIFICATION
Solubility (Vol. 4)
Miscible with water, ether and ethanol
Specific gravity (Vol. 4)
0.792 - 0.795
Refractive index (Vol. 4)
n (20, D): 1.328 - 1.330
Boiling point (Vol. 4)
About 65o
PURITY
Water (Vol. 4)
Not more than 0.1% (Karl Fischer Method)
Distillation range (Vol. 4)
64.5 - 65.5o
Non-volatile residue
(Vol. 4)
Not more than 3 mg/100 ml
Acidity Not more than 15 mg/kg as formic acid
To a mixture of 10 ml of ethanol and 25 ml of water add 0.5 ml of
phenolphthalein TS, and titrate with 0.02 N sodium hydroxide to the first
pink colour that persists for at least 30 sec. Add 19 ml (about 15 g) of the
sample, mix and titrate with 0.02 N sodium hydroxide until the pink colour is
restored. Not more than 0.25 ml is required.
Alkalinity Not more than 3 mg/kg as ammonia
Add 1 drop of methyl red TS to 25 ml of water, add 0.02 N sulfuric acid until
a red colour just appears, then add 29 ml (about 22.5 g) of the sample, and
mix. Not more than 0.2 ml of 0.02 N sulfuric acid is required to restore the
red colour.
Aldehydes and ketones Not more than 0.015% w/v as acetone
See description under TESTS
Lead (Vol. 4) Not more than 2 mg/kg
Determine using an atomic absorption technique appropriate to the
specified level. The selection of sample size and method of sample
preparation may be based on the principles of the method described in
Volume 4, “Instrumental Methods.”
TESTS
PURITY TESTS
Aldehydes and ketones Principle
The aldehydes and ketones present are converted with 2,4-
dinitrophenylhydrazine into the corresponding 2,4-dinitrophenylhydrazones.
In alkaline medium these have a red colour, which is determined
spectrophotometrically or visually.
Apparatus
- Photoelectric absorptionmeter or spectrophotometer, with 0.5-cm cells.
Alternatively flat bottom tubes, capacity about 20 ml
- Water bath, controlled at 60 ± 1o
Reagents
The reagents used shall be of a recognized analytical reagent quality.
Distilled water or water of at least equal purity shall be used throughout
- Carbonyl-free methanol: Reflux 1000 ml of methanol with 5 g of 2,4-
dinitrophenylhydrazine and 5 drops of concentrated hydrochloric acid (d =
1.18) for 2-3 h. Distil off the methanol using a 300 mm by 25 mm diameter
Widmer or other suitable distillation column. Reject the first 100 ml and
collect the next 800 ml, rejecting the remainder. If, in spite of the
precautions taken, the distillate is found to be coloured, then it should be re-
distilled.
- 2,4-Dinitrophenylhydrazine solution: Dissolve 0.03 g of 2,4-
dinitrophenylhydrazine in 40 ml of the carbonyl-free methanol containing 0.3
ml of concentrated hydrochloric acid (d = 1.18) and dilute to the mark in a
50-ml one-mark volumetric flask with the carbonyl-free methanol. Prepare
this solution fresh each day.
- Potassium hydroxide solution: Dissolve 10 g of potassium hydroxide in 10
ml of water, cool and dilute to the mark in a 50-ml one-mark volumetric flask
with carbonyl-free methanol. Prepare this solution fresh each day.
- Standard acetone solution: Weigh 1.00 g of acetone and dilute to the mark
in a one-mark 100 ml volumetric flask with carbonyl-free methanol. Dilute
1.0 ml of this solution to 100 ml with the carbonyl-free methanol. 1 ml of the
diluted solution contains 0.1 mg of acetone.
Procedure
Prepare five solutions by diluting 1.0, 2.0, 4.0, 8,0 and 10.0 ml portions of
the standard acetone solution to 25.0 ml with carbonyl-free methanol. To
1.0 ml of each of the solutions thus obtained (containing 0.004 - 0.04 mg of
acetone/ml), contained in a test tube fitted with a ground glass stopper, add
1.0 ml of the 2,4-dinitrophenylhydrazine solution. Stopper the tube and heat
for 50 min in the water bath at 60o, cool, add 8.0 ml of the potassium
hydroxide solution and after 5 to 15 min measure the optical density of each
solution at a wavelength of 430 nm using as a blank 1.0 ml of the carbonyl-
free methanol treated in the same way. Prepare a calibration chart by
plotting weights (in mg) of acetone against corresponding values of optical
density.
Dilute 5.0 ml of the sample to 25.0 ml with the carbonyl-free methanol.
Transfer 1.0 ml of this solution to a test tube fitted with a ground glass
stopper and add 1.0 ml of the 2,4-dinitrophenylhydrazine solution. Stopper
the tube and heat for 50 min in the water bath at 60o, cool, add 8.0 ml of the
potassium hydroxide solution and after 5 to 15 min measure the optical
density of the solution at the wavelength of 430 nm using as a blank 1.0 ml
of the carbonyl-free methanol treated in the same way. By reference to the
calibration chart prepared as described above, read the acetone content (in
mg) of the solution.
The content is not more than 0.03 mg.
METHOD OF
ASSAY
Using the procedures for Gas chromatography (see Volume 4), establish
the following conditions:
Column
- length: 1.8 m
- diameter: 4 mm
- packing: 120-150 mesh Porapak R, or equivalent
Carrier gas: Nitrogen
Flow rate: 25 ml/min
Detector: FID
Temperatures
- injection port: 200o
- column: 160o
- detector: 210o
Prepare a standard solution of 0.4% (v/v) methanol in dioxane. Adjust
column temperature and/or gas flow rate so that methanol retention time is
about 5-7 min. Adjust detector so that 8 µl of standard solution provides at
least one-half scale deflection. Inject 5-10 µl sample, obtain chromatogram
and determine methanol content by the method of area normalization.
METHANOL
SYNONYMS
DEFINITION
Chemical names
C.A.S. number
Chemical formula
Structural formula
Formula weight
Assay
DESCRIPTION
FUNCTIONAL USES
CHARACTERISTICS
IDENTIFICATION
PURITY
Acidity
Alkalinity
Aldehydes and ketones
Lead (Vol. 4)
TESTS
PURITY TESTS
Aldehydes and ketones
Principle
Apparatus
Reagents
Procedure
METHOD OF ASSAY
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