WO2019106387A1

WO2019106387A1 - Improvements relating to hypochlorous acid

Info

Publication number: WO2019106387A1
Application number: PCT/GB2018/053484
Authority: WO
Inventors: Kathleen Anne Hood; Luke IRELAND
Original assignee: Wcs Services Limited
Priority date: 2017-11-30
Filing date: 2018-11-30
Publication date: 2019-06-06
Also published as: GB2568922A; GB201719987D0

Abstract

A method of producing an acidified solution comprising hypochlorous acid, the method comprising adding a substantially chloride-free acidifier to an aqueous solution comprising hypochlorous acid and/or hypochlorite anions to form said acidified solution. The method comprises counteracting localised regions of low pH by one or more of: (i) addition of the acidifier in diluted form; (ii) addition of the acidifier over an extended time period; (iii) dispersed addition of the acidifier to a plurality of regions of the aqueous solution; and (iv) substantially immediate mixing of the acidifier and aqueous solution on addition of the acidifier.

Description

IMPROVEMENTS RELATING TO HYPOCHLOROUS ACID

TECHNICAL FIELD

This invention relates to the production of acidified solutions comprising hypochlorous acid. In particular, though not exclusively, this invention relates to methods and apparatus for producing acidified hypochlorous acid solutions.

BACKGROUND

Adding chlorine (CI₂) to water via the addition of chlorine gas or hypochlorites is a well-established method of treating water to reduce the level of microbes. Microbiologically contaminated surfaces and water remain a major source of infection resulting in a significant amount of death and illness, particularly in environments such as hospitals where patients are vulnerable due to lowered or compromised immunity. The use of sterilising and disinfecting solutions which can help to reduce of pathogens on surfaces, or help to provide water free from harmful pathogens, can greatly reduce infection rates.

When dissolved in water, chlorine dissociates to an equilibrium of chlorine, hypochlorous acid (HOCI) and hydrochloric acid (HCI). In acidic solutions, the major species are chlorine and hypochlorous acid while in alkali solutions the equilibrium favours the hypochlorite ion (OCI ) .

The efficacy of hypochlorous acid is known to be greater than that of the hypochlorite ion, due to faster penetration though the microbial cell wall. Therefore, acidic solutions containing more hypochlorous acid provide greater sterilising efficacy than alkali solutions containing more hypochlorite ions.

Alkali hypochlorite solutions, which are commonly referred to as bleach, are widely used to disinfect surfaces due to their broad availability and low cost. However, the high pH required to maintain stability of these solutions results in predominantly hypochlorite ions, rather than hypochlorous acid, being present.

When used to chlorinate drinking water, the lower efficacy of alkali solutions may result in more hypochlorite being added to the water than would be necessary to obtain adequate disinfection if hypochlorous acid were present. The addition of such alkali products can also increase the pH and lead to increased scale generation.

Hypochlorous acid solutions may be produced by electrolytic processes, but the resulting solutions contain high levels of chloride ions, resulting in poor storage stability and pH fluctuations with time. This route therefore has limited use, and is most suited to on-site generation of low volumes of product.

A further known approach is to acidify solutions of hypochlorite to obtain solutions of hypochlorous acid. However, this is not straightforward. Solutions of hypochlorous acid produced by the acidification of hypochlorite solutions are inherently unstable due to the presence of chloride ions (Cl-) in solution.

Chloride ions adversely affect the stability of hypochlorous acid solutions as they react with hypochlorous acid to produce chlorine (see Handbook of Detergents, Part F: Production, edited by Uri Zoller and Paul Sosis, CRC Press, 12 Dec 2010). The presence of chlorine in solution results in the formation of hydrochloric acid, which will dissociate, resulting in the formation of further chloride ions in solution.

WO2012/123695 describes a process for preparing a stable aqueous solution of hypochlorous acid which involves adding calcium hypochlorite to water (preferably deionised), manipulating the chloride levels in the resulting solution to be at a maximum of 1 chloride : 3 hypochlorous acid and controlling the pH of the solution to between 3.5 to 7.0 by using phosphoric acid. The resulting solution is shown to have an 80% retention in HOCI over one month.

GB2521810 describes a process for preparing stable aqueous chlorine solutions having a pH between 3.0 and 6.5 and a stability such that after 6 weeks storage at 20 degrees Celsius the pH remains in the range 3.0 to 6.5 and the amount of chlorine lost from the solution is less than 10%. The process involves: (a) providing a source of water having an electrical conductivity at 20 degrees Celsius of no more than 50 (and preferably no more than 4.3) pScrrr¹; (b) reacting said water with solid calcium hypochlorite having a purity of at least 60%; and (c) adjusting the pH to between 3.0 and 6.5 using phosphoric acid.

Despite these prior art attempts to control chloride and enhance stability, a need remains for an improved process for preparing aqueous solutions comprising hypochlorous acid, for example resulting in more stable solutions and/or improved ease of production.

SUMMARY OF THE INVENTION

The invention is based upon the appreciation that, during the acidification of solutions of hypochlorite to obtain solutions of hypochlorous acid, the addition of an acid to the hypochlorite solution leads to a localised, short-lived drop in pH. This in turn is likely to accelerate formation of chlorine, and consequently chloride ions, which in turn will be detrimental to the stability of the hypochlorous acid solution produced.

From one aspect, the invention provides a method of producing an acidified solution comprising hypochlorous acid, the method comprising adding a substantially chloride-free acidifier to an aqueous solution comprising hypochlorous acid and/or hypochlorite anions to form said acidified solution, wherein the method comprises counteracting localised regions of low pH by one or more of: (i) addition of the acidifier over an extended time period; (ii) addition of the acidifier in diluted form; (iii) dispersed addition of the acidifier to a plurality of regions of the aqueous solution; and (iv) substantially immediate mixing of the acidifier and aqueous solution on addition of the acidifier. By counteracting localised regions of low pH, the method aims to provide more stable acidified solutions.

In various embodiments, the acidifier is added over an extended time period. This counteracts low pH because additional time is provided for the acidifier to disperse. The extended time period may suitably be at least 30 seconds, optionally at least 1 minute, or even at least 5 minutes or at least 30 minutes.

In various embodiments, the extended period may additionally or alternatively be based on the volume of aqueous solution to be acidified. Suitably, the extended period may be at least 1 second per litre of aqueous solution, or at least 5 seconds per litre of aqueous solution, or even at least 10 seconds per litre of aqueous solution.

Advantageously, the method may comprise restricting or controlling a rate of addition of the acidifier during the extended time period. Optionally, the method may comprise adding the acidifier at a substantially constant rate during the extended time period.

The acidifier may suitably be added in a single amount, or as a plurality of distinct, consecutive amounts.

The acidifier may advantageously be added in diluted form. For example, the acidifier may comprise an acid in aqueous solution at a concentration of at most 16M. Suitably, the acid may be at a concentration in the range of from 0.5 to 16 M, for example at a concentration in the range of from 1 to 16 M. optionally, the concentration may be at most 10M, at most 5M, or at most 3M.

In various embodiments, the method comprises dispersed addition of the acidifier to a plurality of regions of the aqueous solution. For example, the acidifier may optionally be added to at least two distinct regions of the aqueous solution. Conveniently, the acidifier may be spread from a dispenser onto at least two distinct regions of the aqueous solution. Suitably, the dispenser may comprise a nozzle or tap. The dispenser may spray the acidifier to facilitate dispersion. The acidifier may of course be added via a plurality of dispensers.

The method may advantageously comprise substantially immediate, or indeed immediate, mixing of the acidifier and aqueous solution on addition of the acidifier. Suitably, the method may comprise contacting the acidifier with a mixing means within 10 seconds of addition, optionally within 5 seconds of addition. The mixing means may suitably comprise a stirrer, paddle or the like which moves through the aqueous solution to effect mixing.

Suitably, the method may comprise substantially homogenising the acidifier with the aqueous solution within 60 seconds of addition, optionally within 10 seconds of addition. In various embodiments, the method may comprise agitating the aqueous solution prior to addition of the acidifier, such that the aqueous solution is in an agitated state on addition of the acidifier.

Optionally, the method may comprise mixing the aqueous solution with a mixing energy of at least 0.075 W/litre based on the volume of the aqueous solution, optionally at least 0.10 W/litre or even at least 0.15 W/litre. The mixing energy may be determined by the energy consumption of mixing means used to mix the aqueous solution.

In various embodiments, the method may comprise mixing the aqueous solution with a rotating paddle having a shaft speed of at least 1500 rpm, optionally at least 1750 rpm.

The method may comprise settling and removing a precipitate from the acidified solution and obtaining the acidified solution as a supernatant liquid. This may be of benefit, for example, where the acidifier is phosphoric acid and the aqueous solution comprises calcium ions.

Suitably, the acidified solution may have a pH in the range of from 3.0 to 7.0, for example in the range of from 3.5 to 6.5, or in the range of from 4.0 to 6.0.

The method may lead to reduced chloride ion formation. Suitably, the aqueous solution comprises chloride ions, and the increase in the concentration of chloride ions in the acidified solution may be no greater than 10%, preferably no greater than 5% as compared to the concentration of chloride ions in the aqueous solution.

As aforesaid, the method may lead to improved stability. In various embodiments, the acidified solution comprises active chlorine, and the decrease in the concentration of the active chlorine 4 weeks after preparing the acidified solution may be no greater than 10%, preferably no greater than 5%. The decrease in active chlorine may be measured following storage in darkness, at a temperature of about 5 degrees Celsius.

The aqueous solution may comprise an aqueous solution of a hypochlorite salt. Suitably, the hypochlorite salt may comprise sodium hypochlorite and/or calcium hypochlorite.

As will be apparent to those skilled in the art, the concentration of hypochlorite in the aqueous solution may be chosen based on a desired active chlorine level in the acidified solution.

The acidified solution may advantageously comprise in the range of from 100 to 8000 ppm active chlorine, for example in the range of from 200 to 5000 ppm. In various embodiments, the acidified solution comprises at least 1000 ppm active chlorine.

The acidifier may suitably comprise a chloride-free acid. The acidifier may optionally comprise or consist of at least one of acetic acid, formic acid, hydrofluoric acid, nitric acid, perchloric acid, phosphoric acid, and sulfuric acid. Advantageously, the acidifier may advantageously comprise or consist of at least one of nitric acid, sulphuric acid and phosphoric acid. In various embodiments, the acidifier comprises or consists of phosphoric acid.

From a further aspect, the invention provides an acidified solution comprising hypochlorous acid, obtainable by the method.

From still another aspect, the invention provides an apparatus for producing an acidified solution comprising hypochlorous acid, the apparatus comprising : a vessel for holding aqueous solution comprising hypochlorous acid and/or hypochlorite anions in use; and an acidification system for adding acidifier to the vessel, wherein the apparatus is arranged to counteract localised regions of low pH in the vessel in use on addition of acidifier by the acidification system.

The acidification system may comprise a controller for controlling a rate at which acidifier is added to the reaction vessel. In various embodiments, the acidification system comprises a flow controller for controlling a flow rate of liquid acidifier added into the reaction vessel.

Suitably, the acidification system may comprise a supply of diluted acidifier and a conduit for adding the diluted acidifier to the vessel. The diluted acid may, for example, be as hereinabove defined. Optionally, the acidifier may comprise acid in aqueous solution at a concentration of at most 16M, e.g. as hereinabove defined.

The acidification system may optionally comprise one or more dispensers for adding acidifier to a plurality of distinct regions of the vessel.

Suitably, the vessel may comprise mixing means, for example a stirrer or paddle, capable of contacting acidifier within 10 seconds, optionally within 5 seconds of addition to the vessel from the acidification system in use.

Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of the words, for example "comprising" and "comprises", mean "including but not limited to", and do not exclude other components, integers or steps. Moreover the singular encompasses the plural unless the context otherwise requires: in particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

Preferred features of each aspect of the invention may be as described in connection with any of the other aspects. Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and/or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and/or features of any embodiment can be combined in any way and/or combination, unless such features are incompatible.

BRIEF DESCRIPTION OF THE DRAWING

In order that the invention may be more readily understood, reference will now be made, by way of example, to the accompanying Figure 1, which is a schematic view of an apparatus for producing acidified hypochlorous acid solutions in accordance with an embodiment of the invention.

DETAILED DESCRIPTION

Referring to Figure 1, an apparatus 100 for producing acidified hypochlorous acid solutions comprises a reaction vessel 110 for holding an aqueous solution of hypochlorous acid and/or hypochlorite anions; a supply of acidifier 120 and a conduit 130 for delivering acidifier from the supply 120 to the reaction vessel 110.

In use, an aqueous solution of a hypochlorite salt, for example sodium or calcium hypochlorite, may be prepared in the reaction vessel 110, followed by the addition of acidifier from the supply 120.

To enhance the stability of the acidified solutions, the apparatus comprises features to counteract localised regions of low pH during acidification of aqueous solution in the reaction vessel.

Firstly, to permit addition of the acidifier in diluted form, the supply of acidifier 120 comprises an aqueous, diluted acid, for example diluted phosphoric acid.

Secondly, to permit addition of the acidifier over an extended period, the conduit 130 comprises a flow controller in the form of a metered valve 140, for restricting delivery of the acidifier via the conduit 130 to the reaction vessel 110.

Thirdly, to permit dispersed addition of the acidifier to a plurality of regions of the aqueous solution, the conduit leads to two of spaced apart inlets 160 into the reaction vessel 110. To further disperse the acidifier each inlet is defined by a spray nozzle 160 such that the acidifier is spread as it exits the inlets and enters the reaction vessel 110.

Fourthly, the reaction vessel comprises a stirrer to provide for mixing of the acidifier and aqueous solution on addition of the acidifier.

Of course, modifications can readily be made to the apparatus 100 without departing from the scope of the invention. For example, it may not be necessary to include all four features to counteract localised regions of low pH during acidification. A single one of the features, or a combination of any two or three of the features may be sufficient in various embodiments. EXAMPLES

In the following Examples, aqueous solutions ("batches") comprising hypochlorous acid and/or hypochlorite anions were prepared under ambient conditions in a reaction vessel and then acidified according to the following general method :

Dissolution of sodium or calcium hypochlorite in water

Acidification by addition of phosphoric acid to form hypochlorous acid

The sodium hypochlorite used in the examples met the standard of BS EN 901 :2013 for

'Chemicals used for treatment of water intended for human consumption.' The calcium

hypochlorite used in the Examples meet the standard of BS EN 900:2007 for 'Chemicals used for treatment of water intended for human consumption - calcium hypochlorite'. The phosphoric acid was of European Pharmacopeia quality standard (EP grade) 90% unless stated otherwise.

Acidification of the aqueous solutions was studied as set out in each of the Examples. Phosphoric acid was added to solutions at different rates and pH and chloride levels were measured. The active chlorine levels were also monitored over time. For each example, the pH of the batches being compared was ± 0.1. All variables between the batches are explicitly indicated in the Examples.

Acidified solutions subjected to stability studies were stored in white polyethylene bottles under ambient conditions, e.g. about 20 degrees Celsius.

Active chlorine levels were measured by a thiosulphate / KI /starch titration whereby the active chlorine first reacts with acidified potassium iodide producing iodine which is titrated with thiosulphate using starch as an end point indicator. This test method complies with the method recommended in BSI - EN 901 :2013.

Chloride levels were determined by titration with silver nitrate using dichlorofluoroscein as an adsorption indicator. The chloride levels were corrected for the possible interference from hypochlorite by first reducing hypochlorite to chloride using hydrogen peroxide under basic conditions and correcting for the contribution to the total chloride.

The acidified batches contained active chlorine content predominantly in the form of hypochlorous acid. This was deduced from a spectroscopic analysis of a representative batch, which was found to contain approx. 91% of the active chlorine in the form of hypochlorous acid one week after preparation. Example 1 Manufacture of a solution of approximately 250DDm active chlorine from sodium hypochlorite; pH 5.8

In this study, the same total amount of phosphoric acid was added to two separate batches as follows: i. phosphoric acid added straight into reaction vessel

ii. phosphoric acid added in 4 parts with stirring prior to next addition

Chloride levels were measured in each batch after acid addition. The chloride levels in batch ii were 91% of those in batch i.

Example 2 Manufacture of a solution of approximately 25Qppm active chlorine from sodium hypochlorite; pH 5.3

In this study, the same amount of phosphoric acid was added at different rates in three separate batches, as follows: iii. phosphoric acid added without delay

iv. phosphoric acid added at 1.54g/min

v. phosphoric acid added at 0.77g/min

Chloride levels were measured in each batch before and immediately after acid addition. Increases in chloride ion concentration were compared and the results are shown in Table 1.

Table 1 - Immediately after acid addition

Active chlorine levels were measured in each batch immediately after acid addition and after a period of four weeks and a decrease calculated. The results are shown in Table 2.

Table 2 - After 4 weeks

Example 3 Manufacture of a solution of approximately IQOQppm active chlorine from sodium hypochlorite; pH 5.2

In this study, the same amount of phosphoric acid was added at different rates in three separate batches as follows: vi. phosphoric acid added without delay

vii. phosphoric acid added at 6.34g/min

viii. phosphoric acid added at 3.17g/min

The pH in the reaction vessel was measured during acid addition and the lowest pH was recorded. The results are shown in Table 3.

Table 3 - During acid addition

Active chlorine levels were measured in each batch immediately after acid addition and after a period of four weeks and a decrease calculated. The results are shown in Table 4.

Table 4 - After 4 weeks

Example 4 Manufacture of a solution of approximately IQOOoom active chlorine from calcium hypochlorite; pH 5.3

In this study the same amount of phosphoric acid (food grade 81%) was added at different rates in three separate batches, as follows: ix. phosphoric acid added without delay

x. phosphoric acid added at 5.94g/min

xi. phosphoric acid added at 2.97g/min

Chloride levels were measured in each batch before and immediately after acid addition. Increases in chloride ion concentration were compared and the results are shown in Table 5. Table 5 - Immediately after acid addition

Active chlorine levels were measured in each batch immediately after acid addition and after a period of four weeks and a decrease calculated. The results are shown in Table 6.

Table 6 - After 4 weeks

Experiment 5 Manufacture of a solution of approximately 4000RRITI active chlorine from calcium hypochlorite; pH 4.7

In this study the same amount of phosphoric acid (food grade 81%) was added at different rates in three separate batches, as follows: xii. phosphoric acid added without delay

xiii. phosphoric acid added at 23.94g/min

xiv. phosphoric acid added at 11.97g/min

Active chlorine levels were measured in each batch immediately after acid addition and after a period of four weeks and a decrease calculated. The results are shown in Table 7.

Table 7 - After 4 weeks

Example 6 Manufacture of a solution of approximately lOQppm active chlorine from sodium hypochlorite: oH 5.3

In this study the same amount of phosphoric acid was added at different rates in three separate batches as follows: xv. phosphoric acid added without delay

xvi. phosphoric acid added at 0.62g/min

xvii. phosphoric acid added at 0.31g/min

Active chlorine levels were measured in each batch immediately after acid addition and after a period of four weeks and a decrease calculated. The results are shown in Table 8.

Table 8 - After 4 weeks

Example 7 Manufacture of a solution of approximately 2000RRITI active chlorine from sodium hypochlorite; pH 3.9

In this study the same amount of phosphoric acid was added at different rates in three separate batches as follows: xviii. phosphoric acid added without delay

xix. phosphoric acid added at 1.54g/min

xx. phosphoric acid added at 0.77g/min

Active chlorine levels were measured in each batch immediately after acid addition and after a period of four weeks and a decrease calculated. The results are shown in Table 9.

Table 9 - After 4 weeks

Example 8 Manufacture of a solution of approximately 200Qppm active chlorine from calcium hypochlorite: oH 5.2

In this study the same amount of phosphoric acid (food grade 81%) was added at different rates in three separate batches, as follows: xxi. phosphoric acid added without delay xxii. phosphoric acid added at 1.49g/min

xxiii. phosphoric acid added at 0.75g/min

Active chlorine levels were measured in each batch immediately after acid addition and after a period of four weeks and a decrease calculated. The results are shown in Table 10.

Table 10 - After 4 weeks

Example 9 Manufacture of a solution of approximately 5000RRITI active chlorine from calcium hypochlorite; pH 4.4

In this study the same amount of phosphoric acid (food grade 81%) was added at different rates in three separate batches, as follows: xxiv. phosphoric acid added without delay

xxv. phosphoric acid added at 3.74g/min

xxvi. phosphoric acid added at 1.87g/min

Active chlorine levels were measured in each batch immediately after acid addition and after a period of four weeks and a decrease calculated. The results are shown in Table 11.

Table 11 - After 4 weeks

Example 10 - Manufacture of a solution of approximately 25Qppm active chlorine from sodium hypochlorite; pH 5.9

In this study the same amount of phosphoric acid was added directly into the sodium hypochlorite solution with different levels of mixing in three separate batches, as follows: xxvii. no mixing during addition of phosphoric acid added; mixing after addition only xxviii. slower mixing during and after addition of phosphoric acid

xxix. "normal" mixing during and after addition of phosphoric acid Chloride levels were measured in each batch before and immediately after acid addition. Increases in chloride ion concentration were compared and the results are shown in Table 12.

Table 12 - Immediately after acid addition

Active chlorine levels were measured in each batch immediately after acid addition and after a period of four weeks and a decrease calculated. The results are shown in Table 13.

Table 13 - After 4 weeks

Example 11 - Manufacture of a solution of approximately 2000rriti active chlorine from calcium hypochlorite: PH 5.4

In this study the same amount of phosphoric acid (food grade 81%) was added directly into the sodium hypochlorite solution with different levels of mixing in two separate batches, as follows: xxx. no mixing during addition of phosphoric acid added; mixing after addition only xxxi. "normal" mixing during and after addition of phosphoric acid

Chloride levels were measured in each batch before and immediately after acid addition. Increases in chloride ion concentration were compared and the results are shown in Table 14.

Table 14 - Immediately after acid addition

Active chlorine levels were measured in each batch immediately after acid addition and after a period of four weeks and a decrease calculated. The results are shown in Table 15. Table 15 - After 4 weeks

Example 12 - Manufacture of a solution of approximately 250PDm active chlorine from sodium hypochlorite: PH 5.5

In this study the same number of moles of phosphoric acid was added to two separate batches at different concentrations, as follows: xxxii. 90% phosphoric acid added

xxxiii. 18% phosphoric acid added

Active chlorine levels were measured in each batch immediately after acid addition and after a period of four weeks and a decrease calculated. The results are shown in Table 16.

Table 16 - After 4 weeks

Example 13 - Manufacture of a solution of approximately 200Qppm active chlorine from sodium hypochlorite: oH 5.2

In this study the same number of moles of phosphoric acid was added to two separate batches at different concentrations, as follows: xxxiv. 90% phosphoric acid added

xxxv. 18% phosphoric acid added

Active chlorine levels were measured in each batch immediately after acid addition and after a period of four weeks and a decrease calculated. The results are shown in Table 17.

Table 17 - After 4 weeks

Example 14 - Manufacture of a solution of approximately 250DDm active chlorine from sodium hypochlorite; pH 5.8

In this study the same amount of phosphoric acid was added directly into the sodium hypochlorite solution in two separate batches with different methods of addition, as follows: xxxvi. gradual addition of acid with mixing

xxxvii. direct addition of acid with no mixing

Active chlorine levels were measured in each batch immediately after acid addition and after a period of four weeks and a decrease calculated. The results are shown in Table 18.

Table 18 - After 4 weeks

Claims

1. A method of producing an acidified solution comprising hypochlorous acid, the method comprising adding a substantially chloride-free acidifier to an aqueous solution comprising hypochlorous acid and/or hypochlorite anions to form said acidified solution, wherein the method comprises counteracting localised regions of low pH by one or more of:

(i) addition of the acidifier over an extended time period;

(ii) addition of the acidifier in diluted form;

(iii) dispersed addition of the acidifier to a plurality of regions of the aqueous solution; and

(iv) substantially immediate mixing of the acidifier and aqueous solution on addition of the acidifier.

2. The method of claim 1, wherein (i) the acidifier is added over an extended time period.

3. The method of claim 2, wherein the extended time period is at least 30 seconds, optionally at least 1 minute, and/or at least 1 second per litre of aqueous solution.

4. The method of claim 2 or claim 3, wherein the method comprises restricting or controlling a rate of addition of the acidifier during the extended time period.

5. The method of any one of claims 2 to 4, wherein the method comprises adding the acidifier at a substantially constant rate during the extended time period.

6. The method of any one of claims 2 to 5, comprising adding the acidifier in a plurality of distinct, consecutive amounts.

7. The method of any preceding claim wherein (i) the acidifier is added in diluted form.

8. The method of claim 7, wherein the acidifier comprises acid in aqueous solution at a concentration of at most 16M, optionally at most 10M.

9. The method of any preceding claim comprising (iii) dispersed addition of the acidifier to a plurality of regions of the aqueous solution.

10. The method of claim 9, comprising adding the acidifier to at least two distinct regions of the aqueous solution.

11. The method of claim 9 or claim 10, wherein the acidifier is spread from a dispenser onto at least two distinct regions of the aqueous solution.

12. The method of any one of claims 9 to 11, wherein the acidifier is added via a plurality of dispensers.

13. The method of any preceding claim comprising (iv) substantially immediate mixing of the acidifier and aqueous solution on addition of the acidifier.

14. The method of claim 13 comprising contacting the acidifier with a mixing means within 10 seconds of addition, optionally within 5 seconds of addition and/or substantially homogenising the acidifier with the aqueous solution within 60 seconds of addition, optionally within 10 seconds of addition.

15. The method of claim 13 or claim 14 comprising agitating the aqueous solution prior to addition of the acidifier, such that the aqueous solution is in an agitated state on addition of the acidifier.

16. The method of any preceding claim, wherein the aqueous solution comprises chloride ions, and wherein the increase in the concentration of chloride ions in the acidified solution is no greater than 10%, preferably no greater than 5% as compared to the concentration of chloride ions in the aqueous solution.

17. The method of any preceding claim, wherein the acidified solution comprises active chlorine, and wherein the decrease in the concentration of the active chlorine 4 weeks after preparing the acidified solution is no greater than 10%, preferably no greater than 5%.

18. The method of any preceding claim, wherein the aqueous solution comprises an aqueous solution of a hypochlorite salt.

19. The method of claim 18, wherein the hypochlorite salt comprises sodium hypochlorite and/or calcium hypochlorite.

20. The method of any preceding claim, wherein the acidifier comprises at least one of acetic acid, formic acid, hydrofluoric acid, nitric acid, perchloric acid, phosphoric acid, and sulfuric acid.

21. The method of any preceding claim, wherein the acidifier comprises at least one of nitric acid, sulphuric acid and phosphoric acid.

22. The method of any preceding claim, wherein the acidifier comprises phosphoric acid.

23. An acidified solution comprising hypochlorous acid, obtainable by a method according to any preceding claim.

24. An apparatus for producing an acidified solution comprising hypochlorous acid, the apparatus comprising : a vessel for holding aqueous solution comprising hypochlorous acid and/or hypochlorite anions in use; and an acidification system for adding acidifier to the vessel, wherein the apparatus is arranged to counteract localised regions of low pH in the vessel in use on addition of acidifier by the acidification system.

25. The apparatus of claim 24, wherein the acidification system comprises a controller for controlling a rate at which acidifier is added to the reaction vessel.

26. The apparatus of claim 24, wherein the acidification system comprises a flow controller for controlling a flow rate of liquid acidifier added into the reaction vessel.

27. The apparatus of any one of claims 23 to 26, wherein the acidification system comprises a supply of diluted acidifier and a conduit for adding the diluted acidifier to the vessel.

28. The apparatus of claim 27, wherein the acidifier comprises acid in aqueous solution at a concentration of at most 16M.

29. The apparatus of any one of claims 24 to 28, wherein the acidification system comprises one or more dispensers for adding acidifier to a plurality of distinct regions of the vessel.

30. The apparatus of any one of claims 24 to 29, wherein the vessel comprises mixing means capable of contacting added acidifier within 30 seconds, optionally within 10 seconds of addition to the vessel from the acidification system in use.