Treatise on general and industrial organic chemistry / Tr. from the 2d enl. and revised Italian edition by Thomas H. Pope.

  • Ettore Molinari
Date:
1913
Catalogue details

Licence: In copyright

Credit: Treatise on general and industrial organic chemistry / Tr. from the 2d enl. and revised Italian edition by Thomas H. Pope. Source: Wellcome Collection.

    28/798 page 6
    rr^r- o SCL o The melting-point of a fat can also he determinaci by drawing it in thè fuscd condition into a capillary tube blown out at thè middle into a bulb, whieh is half filled with thè fat (Fig. 11) ; thè upper end of thè tube is kept closed with thè fìnger until thè fat becomes solid, thè empty part of thè tube being then bent round as shown and attached, upside down, to a thermometer, thè whole being afterwards graclually heated in a beaker of water. When thè fat begins to melt it flows into thè lower part of thè bulb (Fig. 11 Ab, right-hand view), and when it is completely fused it becomes transparent. For fats and paraffins, or waxes in generai, and for soft fats (for example, lubricants) especially, an important determination is that of thè dropping-point, whieh is carried out, according to Ubbelohde’s method (1905), by filling with thè fat a glass capsule, e (Fig. 12, naturai size), 10 mm. long and 7 mm. wide, with an orifice 3 mm. in diameter in thè base ; a very small thermometer bulb is immersed in thè fat and thè capsule then affixed to thè thermometer with a metal sheath having an aperature at c, and three points, d, whieh determine thè position of thè capsule ; thè thermometer is then fixed in a test-tube 4 cm. in diameter, dipping into a beaker of water, whieh is heated so that thè temperature rises one degree per minute. At thè orifice of thè capsule a drop begins to form at a certain time, and when this falls thè temperature is read, and is usually eorrected by subtracting 0-5 to obtain thè reai instead of thè apparent dropping-point. This method lias been adopted for thè examination of lubricating oils supplied to thè Italian navy and railways. The specifìc gravity of liquids also serves to determine their purity, and thè various forms of apparatus used for measuring it are described in voi. i, p. 72. V fWl £ ' ■d Fio. 11. Fig. 12. ANALYSIS OF ORGANIC SUBSTANCES As will be seen later, rnany so-called organic substances are composed of carbon and hydrogen combined in various proportions ; a large nurnber of them also contain oxygen, while nitrogen is often present and sometimes sulphur, halogens, metalloids, and metals. Analysis of these compounds may be merely qualitative, when only a knowledge of thè constituent elements is required, or it may be quantitative when thè percentage amount of each of thè elements present is determined. QUALITATIVE COMPOSITION. When organic substances are heated on platinimi foil they either burn with a flame or leave a carbonaceous residue. The presence of carbon and hydrogen may be demonstrated by heating a little of thè substance, mixed with cupric oxide, in a test-tube fitteci with a delivery tube, thè gas evolved being passed into a clear solution of barium liydroxide : if thè latter becomes turbid, owing to thè formation of barium carbonate, thè presence of carbon is proveci, and if drops of water condense in thè colei upper part of thè tube thè substance must contain hydrogen. The presence of nitrogen can, in many cases, be shown by thè smeli of burning wool or nails developed when a little of thè substance is heated on platinum foil. A more generai and certain test is that devised by Lassaigne (1843) : 2-3 centigrams of thè sub- stance are fused with a piece of metallic potassium or sodium (0-2-03 gnu.) in a test-tube, whieh is broken by plunging it while stili hot into a beaker containing 10-12 e.c. of water. The alkaline solution of potassium cyanidc forinoci is filtered, mixed with a few drops of ferrous sulphate and ferric chloridc Solutions and boiled for two minutes, by whieh means potassium ferrous cyanidc is forinoti (when nitrogen is present in thè substance) ; thè liquid is acidified with hydrochloric acid, whieh clissolves thè ferrous and ferric oxitles,
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