Two New Chlorine Compounds G . P. VINCENT, E. G . FENRICH, JOHN F. SYNAN, and E. R. WOODWARD The Mathieson Alkali Works, Inc., New York City
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ODIUM chlorite, NaC102, and chlorine dioxide, CIOz, although they have been known to chemists for over a hundred years, have only recently become available for commercial use. Within this short time these newcomers to the chlorine family have created an important place for themselves in many industries, including paper and textile manufacture and water purification. PROPERTIES OF SODIUM CHLORITE
Sodium chlorite was commercially introduced by Mathieson in 1940. Technical sodium chlorite is a tan, flaked product, containing from 2 to 5 per cent water. purified analytical or reagent grade contains only a trace of water. The solid is not hygroscopic and is stable when stored under normal conditions. Sodium &lorite is very soluble in water. Its neutral solutions are stable a t room temperatures. Alkaline solutions decompose upon boiling to give chlorate: 3NaCIOz + 2NaCIOa
+ NaCl
Acid solutions liberate chlorine dioxide, together with the chlorate, in the ratio of 2 to 1: 4HClOp
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+ HClOs + HCI + H10
Chlorite is the intermediate member in the series hypochlorite, chlorite, chlorate. It is more stable than hypochlorite but less so than chlorate. Heating sodium chlorite in the neighborhood of 175'C. gives a mixture of chlorate and chloride. Like the chlorate, sodium chlorite should not be allowed to dry on the clothing, since a serious fire hazard results. Sodium chlorite reacts violently with sulfur, and rubber stoppers, unless they are known to be sulfurfree, should not be used. Solid sodium chlorite reacts vigorously with acid to give chlorine dioxide. Since the development of sodium chlorite, there has been considerable investigation of its chemical proper& (1,2). For commercial purposes. the two most important are its unusual oxidizing power and its reaction with chlorine to form chlorine dioxide. SODIUM CHLORITE AS AN OXIDANT
Sodium chlorite, on the other hand, destroys all coloring matter to give a very high white, hut is a milder oxidizer than hypochlorite and therefore does not attack the cellulose fiber (3). Thus, sodium chlorite makes i t possible to produce a strong white kraft paper for hooks and other purposes. The use of sodium chlo- *. rite also permits rag paper pulps for stationery to be made directly from the lowest grade of cotton fiber (which contains coloring matter that cannot be successfully destroyed with hypochlorite), instead of from expensive cotton clippings and rags. Due to present government regulations, the pulp and paper indus* has not been able to make full use of the product. After the war, this field is expected to be an important user of chlorite. In textile bleaching, chlorite is safe, even under acid conditions. In certain cases, i t simplifies the bleaching process, since an extra precleaning or "scouring" step may be omitted when rayons are bleached with chlorite in acid solution. In alkaline solution, sodium chlorite is a very weak oxidizer and must be "activated" by the addition of hypochlorite. It is interesting to note that such alkaline "activated" chlorite has the properties of the chlorite, and not of the activator. That is, it bleaches effectively, without weakening the fiber, even in the presence of hypochlorite. PREPARATION OF CHLORINE DIOXIDE
addition to its usefulness as an oxidizer, chlorite is the source of another valuable chlorine compound, chlorine dioxide, B~~~~~~chlorine dioxide is extremely reactive, it cannot he manufactured and shipped in bulk, but must be prepared where it is to be used an$ consumed immediately, The original method for preparing chlorine dioxide can be recognized as a familiar laboratory test for chlorate. A drop of acid is placed in the chlorate, and a yellow to red gas with an irritating odor is evolved, ~ h reaction , of ,.hlorate with acid is not practical mercially, however, since i t produces chlorine dioxide mixed with chlorine and is also very dificult to regulate. Sodium chlorite now provides a practical source of chlorine dioxide through its reaction with chlorine:
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NaCIOa Clt CIOl NaCl Sodium chlorite offers certain advantages over hypochlorite in the bleaching of cellulose materials, such as If the chlorine dioxide is canied off during the reaction, wood pulp or textiles. At present the chlorine corn- exceedingly high yields are obtained. pound most widely used for this purpose, hypochlorite, Several methods of using this reaction to obtain chlohas the disadvantage that, in addition to attacking the rine dioxide for commercial application have been decoloring matter, i t also weakens the fiber by oxidizing veloped (4). One of the most useful of these methods the cellulose. In hypochlorite pulp bleaching, there- employs dry sodium chlorite and a stream of chlorine, fore, when high-strength ( h a f t ) paper is desired, the as follows: Two towers, 3.5 feet high and 4 inches in dipulp is not completely bleached, leaving the familiar ameter, lined with stoneware or glass, are filled almost brown paper. When textiles are bleached with hypo- to the top with flaked sodium chlorite. A stream of air chlorite, very careful chemical control is necessary in and chlorine is fed into the bottom of the first cylinder, order to keep fiber degradation to a minimum. and chlorine dioxide is formed as the chlorine travels up 283
to the top. Complete reaction is assured by passing the gas mixture up through the second tower. The air serves both to carry the chlorine and to dilute the chlorine dioxide, a very important safety measure. When the first tower is exhausted, the second tower may be used alone while the first is recharged and then put into the system, this time as the second tower. Thus the process can be carried out continuously. Various control and measuring devices make it possible to produce exactly the quantity of chlorine dioxide required, a t the proper rate. Chlorine dioxide is a yellow to red gas, with an i m tating odor, and a density of 2.4. Because it decomposes readily, with explosive violence, i t is necessary to make certain that the gas is always diluted with air to a partial pressure of 30 mm. Hg or less. Chlorine dioxide has 2'/2 times the oxidizing power of chlorine:
cient, as it will oxidize the "carotene" in flour to an extent that usually requires about twice as much nitrogen trichloride, a t present widely used for flour bleaching. It also exerts a maturing action on certain flours, producing a dry, easily handled dough and improving the grain and the loaf volume of the bread. Its bleaching action is instantaneous. The amount of chlorine dioxide to produce optimum baking results varies with the type of flour. With patent flour, 0.3 to 1.5 grams per barrel usually suffices, but a somewhat larger quantity is needed with first clear flour. CHLORINE DIOXIDE
IN WATER TREATMENT
One of the most important applications of chlorine dioxide is its use in water purification (6). Most large public water supplies 'are now disinfected with gaseous chlorine. The "raw" water may be treated with an excess of chlorine, then carbon is added which, in addition CIOl f 2l/sH1 HCI + 2H10 to adsorbing certain objectionable impurities, reduces Ch + H1 2HC1 the chlorine content to about 0.2 parts of chlorine per Its remarkable oxidizing power makes i t an excellent 1,000,000parts of water, and the water is filtered. This bleach for flour, fats and oils, starch, paper and textiles, remaining chlorine, or chlorine residual, is left to proand many other potential uses are being investigated. tect the water against contamination as it goes through For bleaching flour (5) chlorine dioxide is very effi- the main and the distribution system.
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Frequently, this purification method results in a disinfect. After the water has been settled and filtered, water which is very unpalatable. If the water is con- chlorine dioxide is added to destroy unpleasant tastes, taminated with industrial wastes containing phenolic odors, and slow-acting organic material. This is accompounds, for instance, these will be chlorinated to complished by a special modification of the reaction of give "chlorphenols," with their characteristic unpleas- chlorite with chlorine. A solution of sodium chlorite is ant taste and odor. Waters containing algae growth passed into a mixing chamber, into which also passes the chlorine water produced by the chlorinator. The present a similar difficulty. Chlorine dioxide has been found to solve this problem. chlorine is thoroughly mixed with the chlorite solution Since it is a much stronger oxidizer than chlorine, it to produce chlorine dioxide which is then fed into the does not chlorinate contaminating chemicals, but in- water system. This method of water purification has been adopted stead oxidizes them. Thus, in many cases where other methods have failed, treatment with chlorine dioxide in several localities. includiup " Nianara Falls. New York. Greenwood, South Carolina, and Bangor, Maine. It has given an odorless, palatable water. Often water supply plants also find difficulty in main- has been found to reduce chemical costs, in some cases, taining a chlorine residual. If the water contains or- since the use of carbon may be eliminated in this ganic compounds which react only slowly with chlorine, method, and in some cases it has also simplified operathese compounds will pass with the chlorine into the tions. I n each case it has produced a palatable and main and react with it there, thus destroying the chlo- safe water where i t was not previously possible to do so. rine which should remain to protect the water in the distribution system. LITERATURE CITED The addition of a small amount of chlorine dioxide (1) WHITE,J. F., M. C. TAYLOR, AND G. P. VINCENT, Ind.Eng. (about % part per 1,000,000) will react rapidly with Chmn., 34, 782 (1943). such organic compounds, destroying them before the (2) Mathieeon Alkali Works, Inc., Ind. Etrg. Chem., 32, 899 (1V4V). water reaches the main. Thus a chlorine residual can (3) VINCENT, G.P., Chenz. Ind. (Sept., 1940). be maintained throughout the distribution system. (4) Woonwnno, E. R., Chem. Eng. Newr, 22,1092 (1944). Although chlorine dioxide is a powerful bactericide, (5) FERRARI,HUTCHINSON, Cnuzs, AND MECHAM,Cereal Chevr.. 18,699 (Nov.,1941). it is not economical for sterilizing purposes. I n water (6) SYNAN, AND G. P. VINCENT. JOHNF., J. D. MACMARON, treatment, therefore, chlorine is first added, as usual, to Water Wmks 6-f Sewerage. 91,423 (1944).
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