Europaisches Patentamt 19 J) European Patent Office Office europeen des brevets © Publication number : 0 482 81 1 A1 EUROPEAN PATENT APPLICATION (2j) Application number : 91309468.6 © int. ci.5: C01B 11/08, A01N 25/22, A61L9/01 (22) Date of filing : 15.10.91 (30) Priority : 26.10.90 GB 9023387 (72) Inventor : Neil, Thomas Bruce McBride Hygiene Limited, Middleton Way Middleton, Manchester M24 4DP (GB) @ Date of publication of application : 29.04.92 Bulletin 92/18 (74) Representative : Ryan, Edward Terrence et al BP INTERNATIONAL LIMITED Patents & @ Designated Contracting States : Agreements Division Chertsey Road BE DE ES FR GB IT NL Sunbury-on-Thames Middlesex, TW16 7LN (GB) © Applicant : MCBRIDE HYGIENE LIMITED Middleton Way, Middleton Manchester M24 4DP (GB) @ Chlorous acid solutions. (57) An aqueous solution comprises a chlorite of an alkali metal or an alkaline earth metal and a buffer adapted to maintain the pH of the fluid substantially in the range from 3.5 to 6.0 and has sufficient acid reserve to convert substantially all the chlorite to chlorous acid. The solutions have valuable deodorizing and disinfecting properties. 00 CM 00 LU Jouve, 18, rue Saint-Denis, 75001 PARIS EP0 482 811 A1 This invention relates to aqueous solutions of chlorous acid. It has long been known that solutions of chlorous acid can bleach, deodorise and disinfect. The use of these solutions for the treatment of potable water and the bleaching of fabrics has been known for many years. More recently these solutions have been used for the disinfection of fruit, vegetables, salads and food preparation 5 surfaces. In all of these applications the chlorous acid is obtained via the acidification of a sodium chlorite sol- ution, the concentration of which determines the yield of chlorous acid. Existing methods for activating sodium chlorite solutions to give chlorous acid include acidifying to a given pH in the range between pH 2 and pH 4, or mixing sodium chlorite and acid in stoichiometric proportions. The method that is employed depends upon the recommendations of the supplier. Neither of these approaches is w desirable nor do they make the optimum use of the available chlorous acid. Chlorous acid tends to decompose to chlorine dioxide and chloric acid. Chlorine dioxide gas is explosive and it is necessary to avoid the release of significant amounts of free chlorine dioxide gas. US 3082146 seeks to minimise losses to chlorine dioxide and chlorate by the addition of a peroxide, but still requires activation by addition of acid. 15 For the disinfection of food preparation surfaces the F.D.A (USA Food and Drugs Administration) have approved the use of chlorine dioxide solutions containing 100 to 200 ppm of "available chlorine dioxide". These "chlorine dioxide solutions" are effectively solutions of chlorous acid. Addition of a stoichiometric quantity of an acid to a sodium chlorite solution of concentration sufficient to yield chlorous acid solution equivalent to 100 to 200 ppm of chlorine dioxide results in the evolution of chlorine dioxide into the atmosphere. In the head space 20 of sealed bottles in which the ratio of the head space: solution volumes were respectively 400 cm3: 100 cm3, levels of chlorine dioxide well above the T.L.V. (Threshold Limit Value) of 0.1 ppm were found. The generation of chlorine dioxide under these conditions is potentially hazardous. The addition of sufficient acid to bring the pH within the range from pH 2.0 to 2.5 was found to release all of the available chlorous acid immediately and to cause a fairly rapid build up of chlorine dioxide gas above the solution. In all of the instances described here, 25 the activated solutions rapidly developed a yellow concentration within a matter of minutes due to the high con- centration of chlorine dioxide in the solution. The chlorine dioxide solutions produced as described above were found to lose their activity progressively with time. Acidification to pH 4 is recommended by a major North American producer of chorite solutions. The benefit of this approach is that liberation of chlorine dioxide gas could not be detected from sodium chlorite solutions 30 containing the equivalent of 100 to 200 ppm available chlorine dioxide. However, this approach suffers from the disadvantage that at pH 4 there is only sufficient acid available to cause partial activation of the chlorite present. Typically, chlorous acid equivalent to 6 to 20 ppm of chlorine dioxide will be released from such a sol- ution. Hence this approach does not convert all of the sodium chlorite present into chlorous acid. The present invention has been made from a consideration of this. 35 According to a first aspect of the present invention there is provided an aqueous solution comprising a chlo- rite of an alkali metal or an alkaline earth metal and a buffer adapted to maintain the pH of the fluid substantially in the range from 3.5 to 6.0 and which has sufficient acid reserve to convert substantially all the chlorite to chlor- ous acid. The pH may for example be in the range 3.5 to 5.0. 40 The buffered materials release chlorous acid less rapidly than materials which have been acidified without buffering. However the chlorous acid releasing solution is usually produced by the addition of liquid concen- trates to water. Concentrated mixtures of chlorite and buffer tend to release high concentrations of chlorine dioxide into the atmosphere. Therefore the two separate concentrates of buffer solution and chlorite solution are mixed at the point of use via controlled dosing equipment. Alternatively the solution can be provided as a 45 ready mixed dilute solution which has a substantially longer shelf life than a mixture of the concentrates. According to a second aspect of the present invention there is provided a method of producing a chlorous acid releasing solution comprising mixing a chlorite of an alkali metal and/or an alkaline earth metal with a buffer adapted to maintain the pH of the mixture substantially in the range from 3.5 to 6.0. The aqueous solutions of the present invention have valuable disinfectant and deodorizing properties. so In a preferred embodiment of the invention the fluid is a dilute aqueous (active) solution comprising 25 to 300 ppm available chlorous acid, more preferably 50 to 250 ppm. The concentration of alkali metal chlorite, such as sodium chlorite, in the fluid is in the range from 20 to 350 and more preferably of the order of 180 ppm. In such a solution the concentration of chlorous acid is main- tained at a very low but constant level. Sufficient buffer is provided to overcome the alkalinity of both the chlorite 55 solution and of the water used which may arise from the presence of hardness salts in the water. There must also be sufficient residual acid (acid reserve) to convert substantially all of the available chlorite to chlorous acid. Preferred buffers comprise weak organic acids such as at least one of acetic acid and optionally a mineral acid such as phosphoric acid. The buffer also comprises a strong base, for example the hydroxide of an alkali 2 EP0 482 811 A1 metal such as sodium hydroxide. The amount of buffer in the dilute fluid may be in the range from 0.1 to 4.0% by weight with up to 360 ppm sodium chlorite present and more preferably in the order of 1% by weight. The fluid in accordance with the present invention provides a means of generating chlorous acid in a gradual and controlled manner with consequent reduced risk in reaching the T.L.V. (Threshold Limit Value) of free 5 chlorine dioxide gas. The sodium chlorite solution is alkaline as it comprises the salt of a weak acid and a strong base and there- fore undergoes hydrolysis in water. As stated above the buffer composition must provide sufficient acid to over- come the alkalinity of both the chlorite solution and of the water used. Also once the pH of the solution has been reduced to the desired pH sufficient residual acid is present to convert substantially all of the available w chlorite to chlorous acid. In order that the invention may be more readily understood specific embodiments thereof will now be des- cribed by way of example only. A weak organic acid was selected for use in a buffer for use in a fluid in accordance with the invention. Acetic acid was selected as being suitable for commercial, toxicological and stability reasons. Buffer solutions 15 were prepared comprising 1 .8 to 2.0% sodium hydroxide, 1 0 to 1 5% acetic acid and from 3.0 to 0% phosphoric acid, depending upon the pH required. It has been found that large amounts of acetic acid may be required in order to reduce the pH to the required acidity. In these circumstances it may be preferably to use a mineral acid such as phosphoric acid in combi- nation with the acetic acid. This reduces the amount of acetic acid required to bring about the required acidity. 20 In use the preferred concentration of the buffer will be substantially 1 % wt/wt. At this rate of use there is sufficient free acid to reduce all of the available chlorous acid. However, the buffer will continue to buffer the pH down to dilutions of 0.1 to 0.15% with up to 360 ppm of sodium chlorite, though the preferred concentration is in the order of 180 ppm. At these low dilutions only partial conversion of chlorite to chlorous acid occurs. Concentrations greater than 1 .0% may be needed but should be used in equal parts with the chlorite solution.