crystals are then emptied ont, drained, and the mother liquor' run into the evaporating pan again, where it is boiled down to P30 and again allowed to crystallize.
The mothers are treated in this way till they yield no further crystals, or only show a slight tendency to cryatallize. They are then evaporated and calcined, and whatever acetate of soda is left is dissolved out by hot water and transferred to the evaporating pans. The successive crops of crystals obtained are sometimes re-dissolved in water, re-evaporated to a density of purified by skimming, and re-crystallized before torrefaction. More usually, however, they are at once transferred to a cast-iron pot heated by a fire underneath and fused at a temperature of about 200°. All the water of crystallization is driven off, and the liquid froths up in a thick oily mass and gradually aubsidee. It is then ladled out upon iron plates to cool and harden. The firing of the fusing pot must be very carefully regulated—the temperature kept between 200° and 232° (395° and 450° F.). If white fumes come off, it is a sign that the acetate is undergoing decomposition, and the fire must be immediately alackened. When aet into a hard, compact mass, the fused acetate is broken up into small fragments, dissolved in not too much hot water (one and a half to two times its own weight), and evaporated to a density of P50. After being allowed to settle for a short time, the solution is drawn off to shallow crystallizing pans of copper or wood lined with lead. After a lapse of three or four days the crystals are removed, washtd, allowed to drain, and set on shelves to dry. They are then fit for packing. The mother liquor and the washings are run off to the evaporating pan to be worked over again. If the final crystals are in any way coloured, they are usually re-dissolved and treated as the first crystals from the crude liquor. It should be noted that precautions should be used in fusing the acetate to prevent it coming in contact with the fire, as it is capable of burning like tinder.
By this process, with various modifications, the bulk of the acetate of soda of commerce is produce d. Distilled pyroligneous acid is sometimes employed instead of a solution of grey acetate
of lime, and the methods of dissolving and filtering are various. Very fine crystals may be obtained by evaporating the solution of rough acetate in cylindrical vessels, made of wood lined with lead, by the agency of steam circulating in a coil of lead piping. By a slow crystallization, surrounding the pans with some non-conducting material, &c., larger crystals are obtained than by allowing the natural and more rapid setting. Filtration through animal charcoal, moisteued with hydrochloric acid, is occasionally resorted to as a means of purifying the liquors in substitution for the process described.
Ono ton of grey acetate 82 per cent. should yield 21 cwt. of good acetate of soda. The most common impurities that are contracted in the grey acetate process are sulphate of soda, chloride of sodium, and acetate of lime. To prevent the appearance of the last-named salt, it must be moat carefully noted that perfect precipitation of the whole of the lime is effected by admixture with the sulphate of soda. As is the case throughout all the processes for the production of acetic acid and the acetates—more perhaps than in other branches of chemical industry, because the materials operated upon are unusually variable in constitution—careful and unceasing supervision by au experienced eye and hand can alone ensure a good result.
The presence of sulphate of soda may be readily detected by dissolving the acetate in water, acidifying with hydmchloric acid and adding a few drops of chloride of barium ; the heavy preci pitate of barium sulphate separating out if the impurity is present. Chloride of sodium, proceeding from the use in excess of au imperfectly worked sulphate of soda, can be detected by acidifying a solution of the acetate in water with a little nitric acid, warming the solution and adding a drop or two of nitrate of silver. The presence of the chloride is shown by a white precipitate.