This section is from the book "Alcohol, Its Production, Properties, Chemistry, And Industrial Applications", by Charles Simmonds. Also available from Amazon: Alcohol: Its Production, Properties, Chemistry, And Industrial Applications.
1 "Identification of Pure Organic Compounds," Vol. I, p. 160.
2 Amer. Chem. J., 1900, 24, 444.
The method of oxidation proposed by Hinkel1 is somewhat less sensitive than the foregoing. About 5 per cent. of methyl alcohol in ethyl alcohol can be detected by it. Ammonium persulphate is used as the oxidising agent.
Add 0 8 gram of this salt to 1 c.c. of the alcohol, followed by 3 c.c. of sulphuric acid (20 per cent.); dilute the mixture with water to a bulk of 20 c.c. and distil it. Collect the distillate in fractions measuring 2 c.c. each, until five are obtained. The first two are rejected, as they contain acetaldehyde; the others are tested by adding a few drops of solution of morphine hydrochloride (1 in 200) to the liquid, and then carefully pouring a layer of strong sulphuric acid down the side of the tube. If formaldehyde is present, a violet ring is produced at the surface of contact of the acid and upper liquid. A blank experiment on pure ethyl alcohol should be done at the same time. Acetaldehyde gives a deep orange colour. Hinkel found that traces of formaldehyde, enough to give a faint violet ring with the morphine test, were always produced in oxidising ethyl alcohol, whether with persulphate, permanganate, or bichromate.
Phloroglucinol may also be used as a test for formaldehyde. It gives a colour varying from yellowish-red to violet-red, according to the relative concentrations of the formaldehyde and phloroglucinol. Acetaldehyde, however, unless in very small quantity, also gives a colour; and, moreover, ethyl alcohol may under certain conditions of oxidation yield traces of formaldehyde, as noted by Hinkel. Hence the acetaldehyde must be removed, and a comparative experiment should be made with ethyl alcohol. The acetaldehyde may be eliminated by distillation as described by Hinkel; or by boiling the liquid down to about one-half its bulk; or by oxidising with hydrogen peroxide, excess of the latter being removed with a little sodium thiosulphate solution.
Gallic acid also gives a delicate reaction with formaldehyde. The liquid to be tested is mixed with 0 2 c.c. of a saturated alcoholic
1 Analyst, 1908, 33, 417.
N solution of gallic acid, and poured carefully on the top of some strong sulphuric acid contained in a test-tube. A ring of blue colour, or green and blue, is produced at the surface of contact if formaldehyde is present.
Sangle-Ferriere and Cuniasse have applied both the phloroglucinol and the gallic acid reactions for the detection of methyl alcohol in absinthe.1 Fifty c.c. of the distillate are mixed with 1 c.c. of pure sulphuric acid and then treated with 5 c.c. of a saturated solution of potassium permanganate. After a few minutes the liquid should have a brown colour, and if any excess of permanganate has been used, one or two drops of a concentrated solution of tannin are to be added. Sodium carbonate solution is then added until a slight alkaline reaction is obtained, the liquid filtered, and a part of the clear filtrate treated with 2 c.c. of a 01 per cent. solution of phloroglucinol and 1 c.c. of a strong solution of potassium hydroxide A bright red colour is given if the absinthe contained methyl alcohol; any yellowish-rose or violet colour is disregarded. Then to confirm the result another part of the alkaline filtrate is acidified with dilute sulphuric acid, and shaken with a few centigrams of powdered gallic acid until the latter has dissolved, when a little strong sulphuric acid is cautiously poured down the side of the tube. After a few seconds a blue coloration appears at the zone of contact if formaldehyde is present.
H. Scudder considers the confirmatory part of the test (with gallic acid) as of no value, because the reaction is too delicate, and may be produced with traces of formaldehyde yielded in the oxidation of ethyl alcohol.
Two other processes depending upon colour reactions given by formaldehyde may be briefly mentioned.
G. Franceschi detects methyl alcohol in beverages and tinctures by distilling 20 c.c. and collecting 15 c.c. of distillate, which is again distilled and 8 c.c. collected. This is oxidised with a glowing copper spiral, and the solution is divided into two portions. One of these is treated with 1 drop of 10 per cent. solution of sodium nitroprusside and 1 drop of piperidine; a blue coloration indicates the presence of ethyl alcohol. The other portion is boiled for one minute, a drop of phenol is added, and the solution poured on the surface of strong sulphuric acid; a red zone develops if the sample contained methyl alcohol.2
U. Pazienti proceeds as follows: Five c.c. of the alcohol, diluted to 50 c.c, are distilled with 3 grams of sodium persulphate and 10 c.c. of 20 per cent. sulphuric acid from a 250 c.c. flask. The distillate is collected in fractions, each of about 2 c.c, and the fifth such fraction is tested with Schryver's reagent. This reagent consists of 2 c.c. of a freshly-prepared and filtered solution (1 per cent.) of phenylhydrazine hydrochloride, 1 c.c. of freshly-prepared potassium ferricyanide solution, and 5 c.c. of concentrated hydrochloric acid: it gives a red coloration with formaldehyde. It is claimed that 1 part of methyl alcohol in 1000 parts of ethyl alcohol may be detected in this manner. Too much formaldehyde diminishes the sensitiveness; hence if the colour i; not very distinct the distillate should be diluted and again tested.1
1 Ann. Chim. anal, 1903, 8, 82-3; Analyst, 1903, 28, 148.
2 J. Chem. Soc. (Abst.), 1915, 108, ii, 588.
Trillat's method,2 as improved by Wolff.3 - The principle of this method consists in the transformation of methyl alcohol into formaldehyde and methylal, followed by the condensation of these products with dimethylaniline. Tetramethyldiamino-diphenyl-methane is obtained: -
By oxidation of this with lead dioxide, in a solution slightly acid with acetic acid, a fine blue coloration is produced, intensified by heating.