General Science

by Bertha May Clark

FIG. 158.—Preparing chlorine from hydrochloric acid and manganese dioxide.

218. An Artificial Bleaching Agent. While the sun's rays are effective as a bleaching agent, the process is slow; moreover, it would be impossible to expose to the sun's rays the vast quantity of fabrics used in the civilized world of to-day, and the huge and numerous bolts of material which daily come from our looms and factories must therefore be whitened by artificial means. The substance almost universally used as a rapid artificial bleaching agent is chlorine, best known to us as a constituent of common salt. Chlorine is never free in nature, but is found in combination with other substances, as, for example, in combination with sodium in salt, or with hydrogen in hydrochloric acid.

The best laboratory method of securing free chlorine is to heat in a water bath a mixture of hydrochloric acid and manganese dioxide, a compound containing one part of manganese and two parts of oxygen. The heat causes the manganese dioxide to give up its oxygen, which immediately combines with the hydrogen of the hydrochloric acid and forms water. The manganese itself combines with part of the chlorine originally in the acid, but not with all. There is thus some free chlorine left over from the acid, and this passes off as a gas and can be collected, as in Figure 158. Free chlorine is heavier than air, and hence when it leaves the exit tube it settles at the bottom of the jar, displacing the air, and finally filling the bottle.

Chlorine is a very active substance and combines readily with most substances, but especially with hydrogen; if chlorine comes in contact with steam, it abstracts the hydrogen and unites with it to form hydrochloric acid, but it leaves the oxygen free and uncombined. This tendency of chlorine to combine with hydrogen makes it valuable as a bleaching agent. In order to test the efficiency of chlorine as a bleaching agent, drop a wet piece of colored gingham or calico into the bottle of chlorine, and notice the rapid disappearance of color from the sample. If unbleached muslin is used, the moist strip loses its natural yellowish hue and becomes a clear, pure white. The explanation of the bleaching power of chlorine is that the chlorine combines with the hydrogen of the water and sets oxygen free; the uncombined free oxygen oxidizes the coloring matter in the cloth and destroys it.

Chlorine has no effect on dry material, as may be seen if we put dry gingham into the jar; in this case there is no water to furnish hydrogen for combination with the chlorine, and no oxygen to be set free.

219. Bleaching Powder. Chlorine gas has a very injurious effect on the human body, and hence cannot be used directly as a bleaching agent. It attacks the mucous membrane of the nose and lungs, and produces the effect of a severe cold or catarrh, and when inhaled, causes death. But certain compounds of chlorine are harmless, and can be used instead of chlorine for destroying either natural or artificial dyes. One of these compounds, namely, chloride of lime, is the almost universal bleaching agent of commerce. It comes in the form of powder, which can be dissolved in water to form the bleaching solution in which the colored fabrics are immersed. But fabrics immersed in a bleaching powder solution do not lose their color as would naturally be expected. The reason for this is that the chlorine gas is not free to do its work, but is restricted by its combination with the other substances. By experiment it has been found that the addition to the bleaching solution of an acid, such as vinegar or lemon juice or sulphuric acid, causes the liberation of the chlorine. The chlorine thus set free reacts with the water and liberates oxygen; this in turn destroys the coloring matter in the fibers, and transforms the material into a bleached product.

The acid used to liberate the chlorine from the bleaching powder, and the chlorine also, rot materials with which they remain in contact for any length of time. For this reason, fabrics should be removed from the bleaching solution as soon as possible, and should then be rinsed in some solution, such as ammonia, which is capable of neutralizing the harmful substances; finally the fabric should be thoroughly rinsed in water in order that all foreign matter may be removed. The reason home bleaching is so seldom satisfactory is that most amateurs fail to realize the necessity of immediate neutralization and rinsing, and allow the fabric to remain too long in the bleaching solution, and allow it to dry with traces of the bleaching substances present in the fibers. Material treated in this way is thoroughly bleached, but is at the same time rotten and worthless. Chloride of lime is frequently used in laundry work; the clothes are whiter than when cleaned with soap and simple washing powders, but they soon wear out unless the precaution has been taken to add an "antichlor" or neutralizer to the bleaching solution.

220. Commercial Bleaching. In commercial bleaching the material to be bleached is first moistened with a very weak solution of sulphuric acid or hydrochloric acid, and is then immersed in the bleaching powder solution. As the moist material is drawn through the bleaching solution, the acid on the fabric acts upon the solution and releases chlorine. The chlorine liberates oxygen from the water. The oxygen in turn attacks the coloring matter and destroys it.