DETECTION OF METHYL ALCOHOL

8. DETECTION OF METHYL ALCOHOL

The following procedure is based upon the fact that methyl alcohol may readily be oxidized to formaldehyde by potassium permanganate in the presence of dilute phosphoric acid. The resulting formaldehyde can then be detected by means of the reaction with chromotropic acid (1,8-dihydroxynaphthalene-3,6-disulfonic acid) which gives a violet color in the presence of sulfuric acid. The chemistry of this color reaction is unknown.
The following compounds give no reaction with chromotropic acid: acetaldehyde, aromatic aldehydes, butyraldehyde, chloralhydrate, crotonaldehyde, glyoxal, isobutyraldehyde, isovaleraldehyde, oenanthal, propionaldehyde. Fructose, furfural, glyceraldehyde, robinose and sucrose all give yellow colors. Other sugars, acetones and carboxylic acids do not react. High concentrations of furfural give red color.
This test is satisfactory for the detection of methyl alcohol in the presence of ethyl alcohol.
Procedure:195 Mix 2 drops of the alcohol in question in a test tube with 2 drops of 5% phosphoric acid and 2 drops of 5% potassium permanganate solution. After 1 min., add a little solid sodium bisulfite with shaking until the mixture is decolorized. If any brown precipitate of the oxide of manganese remains undissolved, add a further drop or two of phosphoric acid and a little more sodium bisulfite. When the solution is entirely colorless, add 8 cc. of 72% sulfuric acid and a small amount of finely powdered chromotropic acid. Shake the mixture well and then heat to 60 for 10 min. A violet color which deepens on cooling, indicates the presence of methyl alcohol.
According to Feigl the identification limit is 3.5 y methyl alcohol; the concentration limit, 1 : 13600.
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195 Feigl, "Laboratory Manual of Spot Tests," 193, published by Academic Press Inc., New York (1943).

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