What are the factors affecting the stability of hypochlorous acid water?

The “GB38850-2020 List of Disinfectant Raw Materials and Prohibited Substances” issued on April 9, 2020 pointed out that acidic oxidizing potential water and hypochlorous acid (including slightly acidic electrolyzed water) are “disinfectant raw materials (effective) ingredients”.

At present, hypochlorous acid is classified as a chlorine-containing disinfectant. In addition to pH and ORP stability, the stability of its available chlorine is also a difficult problem to overcome. “Sterilization Technical Specifications 2002 Edition 2.2.3 Stability Determination of Disinfection Products” It is pointed out that the results of accelerated test method are evaluated as non-compliance with the decrease rate of effective ingredients exceeding 10%. If the sample stored at 37°C for 3 months, its sterilization effective ingredient content decline rate is ≤10%, the storage validity period can be set as 2 years; if stored at 54°C for 14 days, the sterilization effective ingredient drop rate ≤10%, the storage validity period Can be set as 1 year.

Hypochlorous acid is usually made on-the-spot. With the prolonged storage time, the pH, ORP and available chlorine of the product decrease, and the performance decline causes its sterilization effect to gradually disappear, which cannot meet the national standard for chlorine-containing disinfectants.

Hypochlorous acid decomposition reaction formula: 2HClO = light/heat = 2HCl+O₂↑

01 The impact of raw water used in production

Studies have found that organic matter (such as protein, carbohydrates, especially substances containing NH2- or CHO- groups) and inorganic ions (such as NO2-, SO32-, PO32-, Fe2+, Cu2+) in raw water for production The stability of chloric acid has an impact.

The tap water contains a large number of impurities, which are classified according to particles from large to small, including: suspended matter, colloidal matter, dissolved matter (ions and molecules), organic matter and water molecules themselves. Therefore, it is recommended that the raw water used in the production of hypochlorous acid must be purified to ensure that the conductivity is ≤5.1μs/cm (25°C).

02 The impact of production equipment

It is generally believed that the effective components of the hypochlorous acid water produced by electrolysis include available chlorine and active oxygen. Among them, the available chlorine includes Cl2, HCIO, CIO-, etc., while the active oxygen is H2O2, O3, and ·OH. While the oxygen and chlorine evolution reactions occur at the anode, other complex electrochemical reactions also occur to generate nascent oxygen. It has extremely strong activity and can oxidize O2 and H2O to generate strong oxidizing O3 and H2O2.

However, the current method for detecting the concentration of effective chlorine in accordance with the “Disinfection Technical Specification 2002 Edition” cannot separate effective chlorine from active oxygen, and mistakenly regards active oxygen as effective chlorine, but active oxygen is extremely unstable and easier to decompose and disappear. In addition, the difference in electrode materials will lead to differences in the production of active oxygen. For example, the platinum electrode has a higher oxygen evolution electrode, which is easier to promote the generation of active oxygen.

The purity of the electrolyte [hydrochloric acid or (and) sodium chloride] used in the electrolysis method and the purity of the raw materials used in the acidified hypochlorite ion method to produce hypochlorous acid will affect the stability of the hypochlorous acid produced. Improve the purity of raw materials for preparing hypochlorous acid water as much as possible.

03 The impact of the production environment

In the process of industrial production and packaging of hypochlorous acid, the material of equipment that is in contact with hypochlorous acid, such as pipelines, storage containers, filling equipment, and packaging materials for hypochlorous acid water, will also affect the stability of hypochlorous acid. Therefore, food-grade stainless steel, PVC or PPR should be chosen as far as possible for conveying pipes, and food-grade stainless steel should be chosen for filling equipment.

In addition, the container for hypochlorous acid water should also be cleaned regularly to avoid the deposition of impurities and the entry of organic pollutants.

Thirdly, the hands of personnel also contain a lot of grease, and the hypochlorous acid aqueous solution should try to avoid direct contact with the hands of personnel during the production process.

04 Influence of storage conditions

Storage conditions are also very important factors that affect the stability of hypochlorous acid, such as temperature, ultraviolet light, packaging capacity, and container tightness.

Temperature has a huge influence on the stability of hypochlorous acid. When the temperature exceeds 40℃, the decomposition of hypochlorous acid is accelerated, so low temperature is helpful for the preservation of hypochlorous acid water;

Ultraviolet rays will accelerate the dissipation of available chlorine, so transparent bottles should be avoided for containers containing hypochlorous acid;

The packaging capacity also has a certain impact on the stability of hypochlorous acid, especially when the accelerated test is performed at 54°C, the smaller the capacity, the faster the effective chlorine concentration will decrease;

Avoid contact with air to ensure the reliability of the seal.

To sum up, the stability of hypochlorous acid is affected by a series of complex factors. It is necessary to ensure the various processes of hypochlorous acid production in order to finally ensure the stability of the product.

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