The following table gives the means of comparing degrees Tw. and Baumd with each other and with actual sp. gr.; and also gives the approximate percentages of caustic soda and caustic potash in solutions of that sp. gr. containing no other salts.
It may be noted here that for every 31 parts of pure soda required in any operation, 53 parts of pure dry carbonate of soda, or 143 parts crystals (the soda-crystals or " Scotch soda " of commerce), are required, and that the corresponding quantities of caustic and carbonate of potash necessary to do the same work are respectively 47 and 69 parts by weight. With this basis, a simple proportion sum will show in any given form the quantity of alkali required for any purpose.
lt cannot be too strongly impressed upon the young soap-maker, however, that tlae indications of the hydrometer as regards the alkaline strength of his ley-s are only reliable in the case of pure solutions. When the lcys contain much carbonate, sulphate, or other alkaline salts, and especially common salt,- as the " half-spent " leys always do, the quantity of alkali must be determined by a more scientific method. This operation consists in ascertaining how much acid of kaown streng,th is necessary to neutralize a given quantity of the leys, and is very fully described in books upon volumetric analysis or alkalimetry. It is also referred to in Alkali, pp. 301-2. The apparatus employed is chiefly a burette" (Fig. 1237), with pinch-cock to regulate the flow of liquid,—and graduated, usually, into cub. cent. The standard acid solution may be crystallized oxalic acid (63 grm. in 1 litre or 1000 cc.), sulphuric acid, or even nitric acid. If oxalic acid of the above strength be used, every cc. correspouds to 0-031 grin. of soda. A known weight of the leys (or ash) to be examined is taken in a glass flask, diluted with, or dissolved in, water, a, few drops of tincture of litmus added, the whole boiled, and the acid solution added little by little until, after a few seconds' boiling, a permanent faiut-red colour is obtained in the solution. The number of cc. used is then read off, and the proportion of soda (or potash) is calculated from this. It should be noted that, in this operation, all carbonate, aluminate, silicate, and sulphite of soda (or potash) test as " available alkali." The determination of carbonic acid may he effected in various ways, the principle common to nearly all of them being to expose a known weight of alkali to the action of an excess of acid in a flask, under such conditions that the carbonic acid escapes in a dry state, and the loss in weight indicates its quantity. For further details, as well as for methods
of completely analysing leys or solid alkali, works upon analysis should be consulted.
Lune.—It is very important that the lime used by the soap-boiler should be as pure as poasible, and it is highly desirable to use it when freshly burnt. When this cannot be obtained, it should be kept in well-closed casks, avoiding access of air, which contains carbonic acid, and therefore destmys its power of causticizing alkali. It should contain but little, if any, alumina, magnesia, or silica, and should "slack" readily with water ; in fact only what is technically called " fat " lime should be used. Since 28 parts of pure lime are equivalent to 31 parta of pure soda, the quantity of lime necessary for causticizing a given quantity of ash may be calculated, but in manu facturing operations, at least 10 per cent. excess of lime should be used. A clear solution of lime in water may be used for testing the causticity of leys, since any carbonated alkali shows ita presenee by a cloudy deposit of carbonate of lime.
Miter.—The purity of the water employed in the factory is a matter of great moment to the soap-boiler. A8 a rule, spring-water should be avoided, and river- or lake-water employed when ever possible. If it contains suspended impurities, these should invariably be removed by sub sidence or filtration. Organic impurities, if colourless, may be disregarded. The great enemies of the soap-maker are the soluble salts of lime of alkaline earths, and sometimes even of metals, occurring in natural waters, because all these bases form insoluble soaps in the soap-copper, and use up largo quantities of fat to no purpose, since these insoluble soaps are of no market value themselves, and if disseminated in a marketable soap, injure its appearance greatly. The harduess of water may be determined by " Clark's test," in which an alcoholic solution of pure soap is employed. The amount of soap wasted by hard waters may be ascertained from this table, in which the hardness is supposed to be caused simply by lime salts.