Volume 1

A dictionary of applied chemistry / by Sir Edward Thorpe, assisted by eminent contributors.

  • Thomas Edward Thorpe
Date:
1912-1913
Catalogue details

Licence: Public Domain Mark

Credit: A dictionary of applied chemistry / by Sir Edward Thorpe, assisted by eminent contributors. Source: Wellcome Collection.

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    atoms of hydrogen from the alcohol without their replacement by an atom of oxygen :— CH3-CH2-0H+0=CH3-CH0 + H20. Alcohol. Aldehyde. In presence of excess of oxygen aldehyde forms acetic acid. Aldehyde is a very volatile liquid, and is liable to be lost before its conver¬ sion into acetic acid ; it is therefore necessary in all cases where acetic acid is produced by the oxidation of alcohol to allow free access of air. This method produces a very pure acetic acid, but on account of the initial cost of the platinum (which, however, is not in any way in¬ jured by use) it is not used on the manufacturing scale. Patents have been taken out in England for the acetification of alcoholic liquors by ozone and by nascent oxygen and also for the manufac¬ ture of acetic acid from alkali acetates by electrolysis (Plater-Syberg, Eng. Pat. 1898, 233). A French patent (360, 249, 1905) describes the preparation of acetic acid from acetylene. Acetylene is passed into a solution of a normal mercuric salt which precipitates mercury acety- lide. The liquid is then boiled, when aldehyde is formed and the mercuric salt reformed. The aldehyde is then oxidised to acetic acid. Several kinds of ferments are capable of converting fermented alcoholic liquors into acetic acid. The two chief are the Mycoderma aceti of Pasteur, also known as Mycoderma vini, ‘ mother of vinegar ’ and * the vinegar plant,’ and the Bacterium xylinum of Brown. Buchner and Meissenheimer have shown that the fer¬ mentation is due to enzymes in the plant-cell. They are white gelatinous nitrogenous plants, requiring for their healthy growth proteid substances and mineral salts, which are always present in wanes and other alcoholic liquids. In the absence of this food they have no action on pure alcohol, but Pasteur has shown that pure alcohol, to which alkaline phosphate and ammonium phosphate have been added, is slowly acetified by this ferment, the ammonia supply¬ ing the nitrogen. The concentrated or glacial acid is usually prepared by the distillation of a dry acetate with an equivalent quantity of strong sulphuric acid, or acid potassium or sodium sulphate. Sodium acetate is generally used. The anhydrous salt is fused on sheet-iron pans, 6 feet by 4 feet, care being taken that no sparks reach the dried salt, as it would then ignite and burn like tinder. The mass is cooled, broken into small lumps, and distilled with concentrated sulphuric acid. The first portion distilling con¬ tains the water, the later portion is collected and cooled; when crystals have formed the still liquid portion is removed, the crystals are melted and redistilled as before, producing the glacial acid. When a solution of calcium chloride is mixed with a solution of calcium acetate, crystals of calcium aceto-chloride CaC2H302Cl, 5H20gradu- ally separate, dhese crystals may be produced in comparative purity even when impure brown acetate of lime is used. To obtain acetic acid ordinary commercial or ‘ distilled ’ acetate of lime is mixed with the proper proportion of cal¬ cium chloride, and the solution is concentrated by evaporation until it crystallises; the mother liquor is poured from the crystals and concen¬ trated with the production of a second crop of crystals ; this is repeated until about four crops have been produced. The crystals are dissolved in water, filtered through animal charcoal, mixed with about 10 p.c. of calcium chloride, and recrystallised, The crystals are distilled with a mixture of 1 part sulphuric acid of sp.gr. 1-84 and 2 parts water, and the acetic acid concentrated in the usual way. The glacial acid may also be prepared by the distillation of di- or acid-acetate of potash, which, w-hen heated, decomposes into acetic acid and the normal acetate of potash. If ordinary acetic acid be heated with normal potassium acetate, the acid acetate is formed, and a weaker acid at first distils over ; as the temperature rises, the diacetate begins to decompose, and the distil¬ late increases in strength until the glacial acid passes over. When the temperature reaches 300° the distillate becomes coloured from the decomposition of the acid (Melsens, Annalen, 52, 274 ; Compt. rend. 19, 611). Scott and Henderson (Eng. Pat. 1896, 6711) purify the crude acetates by boiling with sodium hypochlorite until the colour is nearly gone. The solution is then cooled and allowed to settle. The clear liquid is decanted and crystallised. The crystals are of great purity. Scott (Eng. Pat. 1897, 12952) has patented a process which dispenses with lime in the manufacture of acetic acid. He distils the crude acid at 100°, and fractionally condenses the vapours. The acid collects mainly in the first portions. Crude acetates may be decomposed with H2S04 or HC1, and the acetic acid distilled in vacuo (Thompson, J. Soc. Chem. Ind. 1896,357); or crude acetic acid may be treated with oxygen under pressure, filtered through charcoal and distilled over pure sodium acetate (Schmidt, Eng. Pat. 1896, 25100). Hochstetter (J. Soc. Chem. Ind. 1902, 1469) prepares pure acetic acid by heating pure sodium acetate with dry HC1 at 120°. ‘Aromatic vinegar’ may be prepared by distilling crystallised diacetate of copper (dis¬ tilled verdigris). The acetate is dried at 160° and heated in earthenware retorts, when the glacial acid distils over. The verdigris produces about half its weight of the acid. The pleasant odour of aromatic vinegar is largely due to acetone, which is always produced Avhen acetates of heavy metals are distilled, but camphor and essential oils are frequently added to increase or modify the smell. Acetic acid for pharmaceutical and medicinal purposes should not decolourise a solution of potassium permanganate. To remove the organic matter which would have that effect, the acid is distilled with potassium permanganate or dichromate. Properties.—The strongest acid solidifies at 16-7° in tabular or prismatic glistening crystals. The glacial acid may be cooled to —10° with¬ out solidification, even when agitated, but on the addition of a crystal of the acid the whole solidifies and the temperature rises to 16-7°. The specific gravity of the crystals at 15°/4° is T0607 (Mendeleeff, j. 1860, 7). They melt to a mobile colourless liquid of sp.gr. P0543 at 16°/4° (Petterson, J. pr. Chem. [2] 24, 301), P0495 at 20°/4° (Briihl) which boils at 118‘5°at 760 mm.
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