Bleaching Powder, CaCl(OCl)

By the action of chlorine on slaked lime, a product is obtained which is of considerable commercial importance on account of its bleaching and oxidising power in the presence of a weak acid such as carbonic acid. This is the substance known as bleaching powder, CaCl(OCl).

In 1798, Charles Tennant, with a view to obtaining some cheaper bleaching agent than the expensive Eau de Javelle - made by the action of chlorine on caustic potash - passed chlorine into milk of lime. In the next year, however, he experimented successfully with dry slaked lime, patented the process, and began the manufacture immediately.

The method now employed for chlorinating the lime depends on the concentration of the chlorine gas available. Where the concentration of the gas is high, as in the case of the chlorine obtained by the Weldon process, considerable heat is generated by the reaction, and large leaden chambers, 8½ ft. high, 10 to 20 ft. wide, and about 100 ft. long, are used, to permit loss of heat by radiation. The best temperature for the operation is 30° to 40° C. The slaked lime should be of good quality (the best lime for the purpose is obtained from Buxton limestone) and, according to Lunge and Schappi, should contain 3.5 per cent, of water above that required for the formation of hydroxide. It is spread on the asphalt or flagstone floor of the chamber to a thickness of 3 to 4 inches and raked into furrows by wooden rakes. Chlorine is introduced and left for some time. The progress of absorption can be followed by observing the diminution in colour by means of two glass windows placed opposite to one another. The gas is also tested from time to time and the doors of the chamber must not be opened until the test shows less than 2½ grains of chlorine per cubic foot. The last of the chlorine may be removed by blowing in fine lime dust. Twelve to twenty-four hours are required for absorption. With a thick layer of lime it may be necessary to rake it over and re-chlorinate. The chambers may be arranged in series, so that the fresh gas comes into contact with nearly chlorinated lime, and fresh lime with nearly spent gas.

For dilute chlorine, such as that from the Deacon process containing as much as 90 per cent, of inert gases, the same precautions for keeping down the temperature are not necessary. The lime is therefore laid in layers, a little more than half an inch thick, on shelves of slate or sandstone 6 inches apart, set into the walls of the chambers in such a way that they are 6 inches short at alternate ends. The gas thus zig-zags through the chamber, passing over each shelf in succession. The chlorine is circulated through a battery of chambers.

In 1888 Hasenclever patented a process by which the lime is driven, by means of a worm screw, through a cylindrical leaden chamber, meeting on its way a counter-current of chlorine. After passing through a succession of these cylinders, the finished lime drops directly into the packing cask, thus greatly reducing the risk to workmen through coming into contact with chlorine and bleaching-powder dust.

Improvements in bleaching-powder manufacture lie mainly in the direction of the use of mechanical appliances in place of hand-work.

The final product, which should be a heavy powder, nearly white, and capable of being kneaded by the fingers into a sort of dough, must be carefully packed in hard-wood casks or mild steel drums in such a way as to prevent access of light, heat, moisture, or carbon dioxide, which all, especially light, cause rapid decomposition. The presence of free lime helps to stabilise the powder, and, for use in hot countries, it is apparently advantageous to mix it with 20 per cent, of quicklime.

Bleaching powder is a white powder smelling of chlorine and chlorine monoxide, owing to the gradual loss of these gases. This loss results in a diminution of available chlorine, which is also brought about by transformation into chlorate. A good commercial powder contains 36-38 per cent, of available chlorine. It is only slightly hygroscopic. On treating with water, chloride and hypochlorite are formed in solution and a residue of lime is left.

By heating to 100° C. about half the water which the powder contains is lost, most of the remainder is removed at 150° C., and the last traces only at red heat. By heating in the presence of moisture, chlorine is set free at 70° C. Above this temperature oxygen, along with a trace of chlorine monoxide, is obtained. At 190° C. all the bleaching chlorine is destroyed. Dilute acids liberate chlorine from it. If the liquid is very dilute, hypochlorous acid is obtained. On boiling a solution of bleaching powder, oxygen is given off and a small quantity of chlorate formed.

Ammonium salts are oxidised to nitrogen on boiling in solution with bleaching powder.

2NH₃ + 3CaOCl₂ = 3CaCl₂ + 3H₂O + N₂.

Certain metallic oxides, such as those of cobalt, nickel, iron, manganese, and copper, react catalytieally upon bleaching powder solution, causing a vigorous evolution of oxygen. The metals, iron, tin, copper, nickel, and cobalt, also have the same effect.

Instead of bleaching powder, bleach liquor, as first obtained by Tennant by the action of chlorine on milk of lime, is sometimes prepared by alkali works for use in bleaching works which are not too far away. It may also be made by extracting bleaching powder with water.

Bleaching powder solution has valuable germicidal properties, and has long been used as a disinfectant. One part of bleaching powder in 2000 parts of sewage will free it from typhoid bacilli and cholera spores in two hours. Anthrax spores require a 1 per cent, solution.

Apart from the value of its bleaching and antiseptic properties, bleaching powder is also a useful chlorinating agent in the laboratory and in industry, for example in the production of chloroform from alcohol or acetone.

It may be used in the laboratory as a source of chlorine or oxygen.

The composition of bleaching powder corresponds approximately to a mixture of equivalent proportions of calcium chloride and hypochlorite with a certain amount of water and free lime; but whether it is actually to be regarded as consisting mainly of a compound of formula CaCl(OCl) or CaCl₂.Ca(OCl)₂, or simply as a mixture, is still a vexed question. The literature on the subject is very extensive.

The first suggestion put forward is indicated by the popular name "chloride of lime." Berthollet regarded bleaching powder as formed by the direct combination of chlorine with lime. Berzelius considered it to be a compound of an oxygen acid of chlorine. This view was soon abandoned for that adopted by Balard and Gay-Lussac, namely, that it is a mixture of chloride and hypochlorite. Fresenius modified this to account for the lime, and suggested that a basic chloride is present. Other variants of the mixture formula were offered. The great drawback to these was the fact that they generally allowed less bleaching chlorine than can be obtained from a good powder.

According to Kraut, a mixture of chloride and pure hypochlorite (prepared by the action of chlorine monoxide on calcium oxide) has the same properties as chloride of lime, notably carbon dioxide sets free all the chlorine, the hypochlorous acid first formed acting upon the calcium chloride. He therefore favoured the Balard and Gay-Lussac formula. Schwarz, on the contrary, asserted much later that the synthetic bleaching powder only gives up, under the influence of a dilute acid, an amount of chlorine corresponding to the hypochlorite present.

Von Tiesenholt supported the mixture hypothesis on the grounds of the reversibility of the equation

2Ca(OH)₂ + 2Cl₂ ⇔ Ca(OCl)₂ + CaCl₂ + 2H₂O.

This reversibility also explained why it is impossible to prepare a bleaching powder containing the theoretical amount of available chlorine.

Winteler studied the formation of bleaching powder from the point of view of the mass action law. Dry chlorine does not react with dry lime, but the process is influenced by the reversible reaction in the liquid phase between chlorine and water,

Cl₂ + H₂O ⇔ HCl + HOCl.

Both acids are then supposed to act upon the lime, but whether to form a compound or a mixture of basic salts the author was apparently uncertain. A large concentration of hydroxyl ions in the liquid phase favours the evolution of oxygen, which is undesirable. This explains why calcium hydroxide, the least soluble of the alkaline hydroxides, is the most suitable for the formation of bleaching powder.

The properties of bleaching powder scarcely seem to be in harmony with the view that free calcium chloride is present. Carbon dioxide will not react with calcium chloride. Bleaching powder is not very deliquescent, and can be completely dehydrated at a lower temperature than is possible in the case of the chloride.

About the year 1860, Odling, in his Manual of Chemistry, put forward his theory of the chemical individuality of bleaching powder, and represented its constitution by the formula Cl-Ca-OCl. Kraut opposed this on the ground that a lithium bleaching powder could be obtained, whereas Odling's formula only permitted the existence of bleaching powders from divalent elements. The work of a number of investigators appeared to support Odling's formula.

A one-compound formula was also suggested by Tarugi. He assumed the intermediate formation of a small quantity of peroxide, CaO₂.H₂O₂, by the action of oxygen liberated by chlorine from water. This, with hydrochloric acid, gives CaO₂Cl₂, chloride of peroxide. A bleaching powder containing 44.1 per cent, of active chlorine might have the formula CaO₂Cl₂.H₂O.

In recent times a plausible combination of the two opposing theories has been made. Ditz found that it is possible to obtain a bleaching powder giving 48.74 per cent, of available chlorine by repeated additions of a little water during chlorination at ordinary temperatures. At lower temperatures, however, although the reaction takes place with evolution of heat, a product is obtained containing a lower percentage of active chlorine. Between -10° C. and -20° C. there is not more than 31.9 per cent, even under the most favourable circumstances. Ditz accounted for these facts as follows: He supposed an intermediate compound to be first formed. This, in the presence of a sufficient excess of water, is decomposed into bleaching powder and free lime. The latter, in contact with fresh chlorine, again forms the intermediate compound, to be again decomposed, and so on theoretically to infinity, the quantity of free lime becoming less each time. The process may be expressed by the following equations: -

2Ca(OH)₂ + Cl₂ = CaO.CaClOCl.H₂O + H₂O = Ca(OH)₂ + CaClOCl.H₂O
4Ca(OH)₂ + 3Cl₂ = CaO.CaClOCl.H₂O + 2CaClOCl.H₂O + H₂O

and so on, the composition of the final product being indicated by the right-hand side of the following general equation: -

2nCa(OH)₂ + (2n -1 )Cl₂ = CaO.CaClOCl.H₂O + (2n - 2)CaClOCl.H₂O + H₂O

where "n" is 1, 2, 2², 2³, etc. The larger the value of "n" the more complete the chlorination. At low temperatures the compound CaO.CaClOCl.H₂O is stable and can indeed be isolated by chlorinating at -15° C. It is also stable in dry air at 100° C. At a somewhat higher temperature it gives off oxygen but no chlorine. The reaction at low temperatures will tend to stop at this point, resulting in a bleaching powder of 32.4 per cent, of theoretically available chlorine. At higher temperatures hydrolysis will take place, the reaction continue with more or less readiness, and a bleaching powder containing a higher percentage of available chlorine be formed. The best theoretically possible would of course be one represented by the formula CaClOCl.H₂O, and containing 48.9 per cent, of available chlorine. In ordinary practice this is not obtained, so that commercial bleaching powder may be regarded as a mixture of this final product with a varying proportion of the intermediate basic compound.

The views of Ditz find support in the constitution of a crystalline bleaching powder obtained by Orton and Jones in the form of lustrous prisms 0.5 – 1.2 cm. long. These, on treatment with water, formed an alkaline solution containing both chloride and hypochlorite, and left a residue consisting mainly of hydroxide. The crystals could be kept in an atmosphere free from carbon dioxide without loss of available chlorine, but the lustre gradually disappeared. They did not deliquesce in air, but after exposure calcium carbonate was present and the hypochlorite had practically disappeared, chloride and chlorate taking its place. On heating at 100° C., the crystals reached a constant weight at the end of an hour, but there was no diminution in the proportion of active chlorine. To conform with the results of analysis the following formulae were suggested: -

2CaCl₂.2Ca(ClO)₂.9CaO(5H₂O).37H₂O,
or 2(Cl.CaO.CaCl).2(OCl.Ca.OCa.OCl).5Ca(OH)₂.37H₂O.

The ratio of calcium chloride to hypochlorite was constant in the crystals even when varied in the solution, but the proportion of calcium hydroxide and water was variable. The constancy of the chloride-hypochlorite ratio pointed to the existence of a compound of the two, and probably also of a basic salt. The authors supposed that the latter might be identical with the compound separated by Ditz at low temperatures. The variability of calcium hydroxide in the crystals seemed to indicate that association of the base with the compound was, partially at any rate, of the nature of an isomorphous mixture.

On the whole, the balance of evidence seems to be rather in favour of the view that bleaching powder consists essentially of the compound CaClOCl with an admixture of basic salts, but it must not be forgotten that the validity of the older chloride-hypochlorite mixture theory is still taken for granted by some authors, and that certain methods by which the problem might be attacked - for example, determination of heat of formation, decomposition pressure, and so on - are practically untried.

All the methods for the evaluation of bleaching powder consist essentially in the determination of the oxidising power by titration. It is most usual to titrate the bleaching-powder solution with a deci-normal arsenious oxide solution, using starch-iodide paper as an outside indicator.

2CaClOCl + As₂O₃ = As₂O₅ + 2CaCl₂.

The available chlorine may also be determined by oxidising an excess of potassium iodide solution with it and estimating the liberated iodine by thiosulphate. Acetic acid is used to set free the chlorine.

CaClOCl + 2KI + 2CH₃COOH = Ca(CH₃COO)₂ + 2KCl + I₂ + H₂O.

Lecomte estimated the active chlorine in bleaching powder or hypochlorites by titrating the same quantity of stannous chloride with deci-normal permanganate, first in the presence of, and then in the absence of hypochlorite solution.