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.

    27/776 page 15
    which steam passes; in this case the vessels are usually wooden, and lined with lead, but some¬ times iron pans are used, the evaporation being conducted over a fire. As the liquid evaporates, tarry impurities rise to the surface, and are re¬ moved with a skimmer. As the acetate forms, it is withdrawn and drained in wicker baskets suspended over the pans. The proper drying of the salt is necessary to the formation of a good product. In large works a drying house is used, which is usually a wind furnace 7 or 8 feet long, 4^ feet broad. It is first heated from 75° to 115°, and the fire slackened; the salt is then spread over the bottom to the depth of about 2 inches, and when somewhat dry an equal quantity is spread above it, the salt is repeatedly turned, and the heat continued for about 24 hours. When apparently dry, the heat is increased to about 125°, and the last traces of moisture driven off. Care must be taken that the heat is not too high, or the salt becomes decomposed. As in the case of sodium acetate, no sparks must touch the mass, or it may burn away like tinder. The product, when prepared from the brown liquor, is dark-coloured and contains char¬ coal and decomposed tarry matters ; it may be dissolved in 3 parts hot water, filtered through animal charcoal, and again evaporated and crystallised yielding a nearly colourless product. Pure calcium acetate crystallises in silky needles or prisms containing two molecules of water. At the ordinary temperature the crystals effloresce, and at 100° become anhydrous, form¬ ing a white powder of saline taste, very soluble in water. Calcium acetate is used in the preparation of other acetates, and of acetic acid, and in calico- printing. The pure salt is completely soluble in water and proof spirit. The commercial article usually contains 62 to 67 p.c. of real acetate, and 1 to 8 p.c. of matters insoluble in water. The impurities are hydrate, carbonate, and sul¬ phate of lime and tarry matters ; formate and other salts of lime with fatty acids also occur. Many methods of assay for this substance are in use, varying considerably in accuracy. The most trustworthy method is to distil with pure phosphoric acid and titrate the distil¬ late as already described under acetic acid (see Stillwell, J. Soc. Chem. Ind. 1904, 305; Grosvenor, ibid. 530; Gladding, J. Ind. Eng. Chem. 1909, 250). Calcium aceto-chloride CaC2H302Clf5H20 has already been described as used in Condy’s pro¬ cess for the preparation of pure acetic acid. Copper acetates. The normal copper acetate is prepared by dissolving cupric oxide or verdigris in acetic acid; or by the action of copper sulphate on the ace¬ tates of lead, calcium, or barium. It crystallises in prisms; soluble in 13 parts cold or 5 parts hot water, and in 14 parts al¬ cohol. In commerce it usually occurs in bunches of deep-green coloured opaque crystals known as ‘grappes.’ Copper acetate is used in the manufacture of pigments ; as an oxidising agent in the indigo vat; and, to a slight extent, for the preparation of acetic acid. Basic copper acetate. Verdigris. Vert-de-gris. Vert de Montpellier. Griinspan. This substance consists of a mixture of mono-, di-, and tri-basic acetates of copper, which are present in different proportions in different varieties of verdigris. At Grenoble and Montpellier the following process is used : The 4 marcs ’ or residues from the wine factories, consisting of the skins and stems of grapes, are loosely placed in earthen vessels, about 16 inches high, 14 inches in diameter at the widest part, and 12 inches at the mouth, covered and allowed to ferment, until on inserting a piece of copper (previously moistened with verdigris and dried), it becomes uniformly coated green in 24 hours. The fermentation should not proceed too far, or decomposition may ensue. The copper used is in sheets inch thick, 4 to 6 inches long, and 3 to 4 broad, each weighing about 4 oz.; they are freed from scales, if neces¬ sary, rubbed with a solution of verdigris and dried ; unless this precaution be adopted, the first coating produced by the marcs will be black instead of green. They are heated over a charcoal fire until as hot as the hand can bear, and placed in an earthenware vessel in layers with the marcs. 30 to 40 lbs. of copper are used for each vessel. In from ten to twenty days, according to the temperature, the covers are removed, when, if the process has pro¬ gressed favourably, the marcs will be whitish and the copper covered with fine, glossy, green crystals. The plates are then removed and placed on end one against another. After two or three days they are moistened by immersion in water or damaged wine, and again placed on end for about a week. This alternate moisten¬ ing and exposure to the air is continued for about six or eight weeks. The plates thus be¬ come covered with increasing coatings of the verdigris, which is detached and the plates are again used until entirely eaten away. The verdigris is kneaded with a little water into leather bags, pressed into rectangular cakes and dried. This substance is known as blue verdigris, and consists principally of the basic acetate (C2H302)2Cu20. It should be dry, of a fine bluish colour, and soluble in dilute acids and ammonia. Green verdigris contains as a principal consti¬ tuent the sesquibasic acetate, Cu0*2Cu(sC2H302)2, and is prepared by placing the copper plates alternately with cloths moistened every two or three days with pyroligneous acid or acetic acid until the plates show green crystals. The plates are arranged so as to allow free access of air and occasionally moistened, for five or six weeks. Large quantities of verdigris are manufactured in England by this process from pyroligneous acid. The imports of verdigris are very small. The various forms of verdigris are used as oil and water colours. With white lead it is used in Russia and Holland as an oil paint, which by double decomposition produces a peculiar green. The paint is considered a good preservative. Verdigris is used in dyeing and calico-printing, and for the preparation of Schweinfurth green and other copper paints. Verdigris is frequently adulterated with chalk, sand, clay, pumice, and sulphates of copper, barium, and calcium. When brass sheets have
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