CHLORIDE OF LIME—BLEACH ING POWDER. Its composition is not yet fully determined. The chief agent in bleaching appears to be the hypochlor ite of lime, one of the constituents. It constitutes a large branch of chemical manufacture, which is carried on in con nection with that of carbonate of soda.
When hydrate of lime, very slightly moist, is exposed to chlorine gas, the lat ter is eagerly absorbed, and a compound produced which has attracted a great deal of attention: this is the bleaching pow der of commerce, now manufactured on an immense scale, for bleaching linen and cotton goods. It is requisite, in pre paring this substance, to avoid, with the greatest care, all elevation of temperature, which may be easily done by slowly sup plying the chlorine in the first instance. The product, when freshly and well pre pared, is a soft, white powder, which at tracts moisture from the air, and exhales an odor sensibly different from that of chlorine. It is soluble in about 10 parts of water, merely the unaltered hydrate being left behind ; the solution is highly alkaline, and bleaches feebly. When hy drate of lime is suspended in cold water, and chlorinegas transmitted through the mixture, the limo is gradually dissolved, and the same peculiar bleaching com pound produced ; the alkalies also, either caustic or carbonated, may, by similar means, be made to absorb a large quan tity of chlorine, and give rise to corres ponding compounds ; such are the "dis infecting solutions" of M. Labarraque.
The most consistent view of the con stitution of these curious compounds is that which supposes them to contain salts of hydrochlorous acid, a substance as remarkable for bleaching powers as chlorine itself; and this opinion seems borne out by a careful comparison of the properties of the bleaching salts with those of the true hypochlorfies. Hypo chlorous acid can be actually obtained from good bleaching powder, by distill ing it with dilute sulphuric or nitric acid, in quantity insufficient to decompose the whole ; when the acid is used in excess, chlorine is disengaged.
If this view be correct, chloride of cal cium must be formed simultaneously with the hypochlorite, as in the following dia gram : When the temperature of the hydrate of lime has risen during the absorption of the chlorine, or when the compound has been subsequently exposed to heat, its bleaching properties are impaired, or altogether destroyed ; it then contains chlorate of lime and chloride of calcium; oxygen, in variable quantity, is usually set free. The same change seems to en sue by long keeping, even at the common temperature of the air. In an open ves sel it is speedily destroyed by the carbonic acid of the atmosphere. Commercial bleaching powder thus constantly varies in value with its age, and with the care originally bestowed upon its preparation ; the best may contain about 30 per cent. of available chlorine, easily liberated by La acid, which is, however, far short of the theoretical quantity.
The general method in which this sub stance is employed for bleaching is the following : The goods are first immersed in a dilute solution of chloride of lime, and then transferred to a vat containing dilute sulphuric acid ; the chlorine or by pochlorous acid thus disengaged in con tact with the cloth, causes the destruction of the coloring matter. This process is often repeated, it being unsafe to use strong solutions. White patterns are, on this principle, imprinted on colored cloth, the figures being stamped with tartaric acid thickened with gum-water, and then the stuff immersed in the chloride bath, when the parts to which no acid has been applied remain unaltered, while the printed portions are bleached.
For purifying an offensive or infectious atmosphere, as an aid to proper ventilation, the bleaching is very convenient. The solution is exposed in shallow ves sels, or cloths steeped in it are suspended in the apartment, when the carbonic acid of the air slowly decomposes the chloride. An addition of a strong acid causes rapid disengagement of chlo rine.
The value of any sample of bleaching powder may be easily determined by the following method, in which the loosely combined chlorine is estimated by its ef fect in peroxidizing a proto-salt of iron, of whicli two equivalents require one of chlorine ; the latter acts by decomposing water and liberating a corresponding quantity of oxygen-78 grains of green sulphate of iron are dissolved in about two ounces of water, and acidulated by a few drops of sulphuric or hydrochloric acid ; this quantity will require for per oxidation exactly 10 grains of chlorine. Fifty grains of the chloride of lime to be examined are next rubbed up with a little tepid water, and the whole transferred to the alkalimeter before described, which is then filled up to 0 with water, after which the contents are well mixed by agitation. The liquid is next gradually poured into the solution of iron, with constant stirring until the latter has be come peroxidized, which may be known by a drop ceasing to give a deep blue precipitate with red ferrocyanide of pot assium. The number of grain-measures of the chloride solution employed may then be read off, and since these must contain 10 grains of serviceable chlorine, the quantity of the latter in the 50 grains may be easily reckoned. Thus, suppose 72 such measures have been taken, then Measures. Gros. chlorine. Measures. Gms. chlorine.
72 : 10 : : 100 : 13.89 The bleaching powder contains, therefore, 2718 per cent.
A party of Germans have erected in Steubenville, Ohio, an establishment for the manufacture of soda ash and chloride of lime. It is the only one of the kind in the United States ; it is estimated that during the first year it will produce be tween $40,000 and $50,000 worth of soda ash, and nearly $20,000 in value of chlor ide of lime. It is supposed that the amount of manufacture will be doubled the second year.