Issues of chemical composition and operating properties of chlorine based inorganic liquid chemical germicides

Issues of chemical composition and operating properties of chlorine based inorganic liquid chemical germicides

Published in VNMT magazine, #4, 2003

V. M. Bakhir, B.I.Leonov, S.A.Panicheva, V.I.Prilutsky, N.Y.Shomovskaya

The Russian Scientific & Research Institute for Medical Engineering (VNIIIMT MZ RF)

More then hundred years chlorine-based inorganic compounds have been used as chemical germicides as well as consistent investigations of influence of hypochlorous acid and hypochlorites on human and environment have been going on.

The first commercial installation for drinking water disinfection by hypochlorite, generated through direct electrolysis of natural fresh water contained chlorides of alkaline and alkaline-earth metals (the Woulf process) was made in 1893 in a suburb of New-York – Brewster [1].

In health care sodium hypochlorite was first used for disinfection and later, in 1915, it was applied as antiseptic agent for infected wounds treatment [2,3]. It is also known fact, that sodium hypochlorite has been used for dental applications since 1920 [4]. In spite of the fact that the first publication about sodium hypochlorite medicinal application in dentistry was in 1989 [5], modern hypochlorite solutions, produced by “Chlorox” (USA) and “Parcan” (France), are widely used in many Russian dental clinics due to their superiority over other disinfectants, such as alcohols, phenols, quarternary ammonium compounds, dichlorisocyanurats, etc [6].

Nowadays chlorine and chlorine-based compounds are extensively used for drinking water disinfection. For example, Moscow spend about 200 thousands tons of liquid chlorine each year. More over, 98% of drinking water in the USA is chlorinated vs. 0,37% ozonation [7]. This can be explained by the fact, that chlorination is the most effective and economical method of drinking water disinfection at fewer disadvantages than ozonation or ultraviolet irradiation [8-10]. Hypochlorite solutions are used for the purpose of disinfection in food industry, pharmaceutical industry, medicine, municipal waste and water treatment, veterinary, farming, at transport and in many other industries in accordance with regulations.

As presented in number of scientific publications, the multicellular organisms, including human organism, form hypochlorous acid and super-active metastable oxy-chlorine- and hydroperoxi- compounds (mixture of metastable oxidants) by special cellular structures to defend organism against pathogens and foreign compounds [11,12]. The use of similar mixture of oxidants as a broad spectrum germicide is the basic principal for development of new liquid chemical germicides – electrochemically activated solutions (ECASOL), known also as anolyte ANK. The introduced solutions are generated from diluted aqueous solution of sodium chloride or potassium chloride by family of STEL devices as well as by variations of that devices for specific application, such as Renofilter, Endosteril [13].

This article’s goal is to evaluate, based on published results of the original research work, the biocidal efficacy of the main chlorine-based liquid chemical germicides such as sodium hypochlorite, chlorinated water and hypochlorous acid in respect to their chemical equilibrium. The following analysis could be useful for development of the disinfection strategy concept for a sustainable environment, which is absent at present time.

Hypochlorous acid dissociates in aqueous media with formation of hypochlorite-anion and hydrogen-ion: HClO ClO- + Н+.

Concentrations of HClO and hypochlorite-anions ClO- are close at neutral pH (fig. 1). A decrease in pH shifts reaction balance towards to HClO, and an increase of pH raises concentration of hypochlorite-anions. A reaction balance under рН < 3.5 is determined by hypochlorous acid and molecular chlorine, dissolved in water. In рН range from 3.5 up to 5.5 all species of free available chlorine (the chlorine is called in such a way, which is educed as gas under reaction with hydrochloric acid) are presented only by hypochlorous acid. Chloramine also relates to active chlorine compounds, which are called as “combined” chlorine as against “free” chlorine, i.e. dissolved chlorine, hypochlorous acid and hypochlorite-anions. We do not discuss chloramines in this article.

Sodium hypochlorite has much less bactericidal activity, than hypochlorous acid. It is illustrated by the diagram [8] at the figure 2.

For example, if anthrax spores B. anthracis were killed by aqueous solution of active chlorine compounds at 50 mg/l and pH=8,6 during 40 minutes, it took 20 minutes for the solution at 5 mg/l and pH=7,2 to produce the same results. Comparing presented data by using Fig.1 one can see, that in the first type of biocidal solution consists of hypochlorite-ions in more than with 90 %, whereas in the second case hypochlorite-ions take a share of less than a half among oxidants, and content of hypochlorous acid rises following the pH change. Oxygen-chlorine compounds have the highest bactericidal activity in the range of pH from 7,0 to 7,6, where concentrations of hypochlorite-ions and hypochlorous acid are close to each other. It can be explained by the fact that mentioned chemicals are conjugate acid and base (HClO + H2O ® H3O+ + ClO- ; ClO- + H2O ® HClO + OH- ), and they form under the above mentioned range of pH values a metastable system, which can generate further active components which possess higher than hypochlorous acid biocidal efficiency: 1О2 – singlet molecular oxygen; ClO· – hypochlorite-radical; Cl· – chlorine-radical (atomic chlorine); O· – atomic oxygen; ОН· – hydroxyl-radical. H+ and OH- are the catalysts of reactions with participation of oxi-chlorine ions and radicals. Concentrations of H+ and OH- ions are close in water under pH close to neutral [14].

A special role of oxy-chlorine oxidants in a biological shielding of human organism and other warm-blooded organisms can be stressed by the fact, that diluted (less than 0,1 %) solutions of sodium hypochlorite and hypochlorous acid considerable increase their bactericidal activity at temperature of 36 - 37 ° С. Biocidal activity and oxidation ability of organic compounds by diluted solutions under this temperature is comparable to 3 – 5 % solutions, at the same time diluted solutions have minimal toxicity and irritant action. De-toxication of human’s organism can be done by intravenous injection of sodium hypochlorite with concentration 300 – 600 mg/l, generated from 0,9 % sodium chloride aqueous solution by electrolysis in, for example, non-diaphragm EDO-4 device. It is now confirmed safety and high efficacy of sodium hypochlorite application for this purpose [15]. The main advantage of this method over the present ones, beside simplicity and low cost in hypochlorite solutions generation, is in almost total absence of any know side-effects to the patient as well as in lack of microorganisms’ resistance development. It should be mentioned that hypochlorite solution changes its pH value (chemical composition) when it is injected into blood vessels. At pH 7,2 – 7,3 (pH value of blood), sodium hypochlorite solution converts into a metastable system consisted of HClO and NaClO (fig.1), that posses the highest germicidal activity.

There for, to prevent the microbial resistance development against applied liquid chemical germicides, the biocidal agents should be applied in metastable conditions.

A metastable solution cannot be stored for a long time. Sodium hypochlorite solution is more stable than hypochlorous acid. For example, 0,5 % sodium hypochlorite solution (pH = 11,0) decomposed in a half at room temperature during 5 years, while sodium hypochlorite at pH = 9,0 decomposes in a half during 45 days. The same solution loses a half of active chlorine during 8 days at pH = 7,0. At the first glance, the simple way of regulating pH of solution before its application in order to decrease concentration of active ingredients and increase of its biocidal activity, gives some difficulties in practical realization. pH reduction of hypochlorite solutions at more than 0,1 %, is dangerous due to chlorine off-gassing when mixing with acid (fig. 1). Besides, most probably, the local loss of active chlorine may occur (in a small volume) during the moment of acidification. In any case, such approach requires special equipment use, qualified maintenance personnel, that will increase a cost for the end product.

Nevertheless, unique ability of hypochlorous acid to form metastable mixture of oxidants with broad spectrum biocidal activity is widely used in many disinfectants inorganic and organic nature, for instance, based on cyanuric acid (Aquatabs, Deochlor, Chlorsept, Presept, Javelion, Chlor-Clean, Sanival etc.), which helps to get higher biocidal activity at 10 times decreased concentration of active chlorine in working solutions of disinfectants in comparison to sodium hypochlorite.

As an example, let’s review the mechanism of action of “Presept”, produced by “Johnson and Johnson”. It is claimed that the basic active ingredient is sodium dichlorisocyanurate. In fact, active ingredient is hypochlorous acid, that is formed by interaction between sodium dichlorisocyanurate and water at pH = 6,2, which is maintained by adipic acid included in Presept’s composition. Almost all agents of this type presented on the market, have similar mechanism of action. However, their application is unsafe for the human and warm-blooded animals since such disinfectants contain chlorine-organic compound, in particular, sodium dichlorisocyanurate, which does not disappear after drying up, but accumulates in environment and organisms, in contrast to inorganic oxi-chlorine compounds. These compounds, its derivatives and degradation products are dangerous, as they have no smell. At the same time, their maximum permissible concentration (MPC) is the same as for chlorine or ozone. Usually, the side-effect of such chemicals as well as other xenobiotics, develops over time when hard to originate diseases of organism occurred.

Coming back to less ecologically dangerous yet more concentrated and stable solutions of inorganic oxy-chlorine compounds, it is worse to note, that differentiation solutions of sodium hypochlorite is lately getting by the method or device used for its synthesis. Many textbooks for epidemiologists and data directories for disinfections [16,17] present different “types” of sodium hypochlorite solution, which are differentiated by the types of devices or methods for obtaining and by application. It is pretty common situation, when the certain “type” of hypochlorite solution requires different concentrations of active ingredient, such as sodium hypochlorite, for particular application to be in compliance with Recommendations approved by the RF State Committee of Sanitary and Epidemic Control.

These statements may confuse consumer regarding the special electrochemical systems, which produce hypochlorite solutions, or some differences in technologies of industrial producers of hypochlorite solutions. Such a notions, obviously, are baseless.

In fact, such a “secret” of property’s difference among hypochlorite solutions is caused by its chemical composition, determined not only by concentration of active chlorine, but also by content of sodium chloride and above all by alkalinity, i.e. sodium hydroxide concentration.

Solution of sodium hypochlorite is produced by chlorination of sodium hydroxide in industry. For example, in compliance with all-Union State Standard 11086-64, 0.162 tons of chlorine and 0,19 tons of sodium hydroxide is required for production of 1ton sodium hypochlorite solution (185 g/l of active chlorine).

Usually, solution of sodium hypochlorite, produced by industrial method, can contain from 70 – 100 to 185 – 200 g/l of active chlorine, from 10 to 90 g/l of sodium hydroxide and about 130 – 180 g/l of sodium chloride. Presence of chlorates in hypochlorite solution influence its disinfection ability less than presence of sodium hydroxide, therefore we do not consider its effect on hypochlorite solution in this article.

Hypochlorite solution, produced by on-site devices of different types by means of non-diaphragm electrolyses of 2 – 5 % solution of sodium chloride, has considerably lower concentration of active chlorine and much higher specific content of sodium chloride as compared to hypochlorite solutions produced by industrial method. It is stipulated by capabilities of each method. Let’s consider the technical data obtained from All-Russian Research Institute “Synthesis” which developed the on-site generator for hypochlorite solution as typical example of technology for sodium hypochlorite on-site generation.

Concentration of sodium hypochlorite in resulting solution is from 6 to 8 g/l; initial concentration of sodium chloride is from 35 to 45 g/l; energy consumption is from 5,5 to 7,7 watt-hour/kg; sodium chloride consumption is from 5,0 to 6,0 kg/kg; service life of anode coating - not less than 8000 hours; life time of device, based on timely replacement of anode coating, is no less than 5 years. Specific consumptions are related to 1 kg of resulting product. Warranty for the device is 1 year at one-shift operation. Warranty period depends on conditions of operation.

It is essential, that producer does not indicate sodium hydroxide concentration in hypochlorite solution or in device manual. However, concentration of sodium hypochlorite, produced by different devices of such a type (“Saner”, “ELMA” etc.), can be from 0.5 to 3 g/l and higher depends not only on device design and its operating mode, but also on chemical composition of sodium chloride and water, used for preparation of brine. It is clear, that water dilution of different hypochlorite solutions, produced by an industrial method or by on-site non-diaphragm electrolyzers, will give solutions with different pH, mineralization and, therefore, with very differing from each other biocidal activity (fig. 1, 2). Influence of sodium chloride on biocidal activity of hypochlorite solution reveals the fact, that its coagulating effect on protein protects microorganisms from contact with bactericidal agent (hypertonic solution). Thus, increase of sodium chloride concentration results in decrease of biocidal activity of solution. Biocidal activity of hypotonic solutions of hypochlorite is higher than isotonic or hypertonic solutions due to intensification of osmotic transfer of solution components through biological membrane to the internal environment of microorganisms. Possessing an intermediate biocidal activity among above mentioned solutions, isotonic hypochlorite solutions are also more stable due to their interaction with biological objects is minimal due to electrolytic homogeneity. The same is true for any liquid chemical germicides, in other words one should always take into consideration the concentration of dissolved compounds (osmotic properties) in an attempt to increase of their biocidal activity.

Differences in biocidal activity of hypochlorite solutions, contained sodium hydroxide, are inessential when used for drinking water disinfection, due to high buffer water capacity and high dilution rate. On the contrary, the differences are quite visible if applied concentration of sodium hypochlorite exceeds 0,1 %.

So, comparison of hypochlorite solutions should be done based not only the concentration of active chlorine, but also by alkalinity (pH value) and total amount of all dissolved compounds.

Influence of NaOH on biocidal activity of hypochlorite solutions is as higher as further pH value is from the neutral value.

We can come to a conclusion, that solutions of hypochlorite with known chemical composition, irrespectively to a method of their obtaining, should be used in all permitted fields of application of hypochlorite solutions in accordance with existent methodic recommendations. No additional certification for their application is needed. However, hypochlorite solutions, which are used, for example, for medical applications, must not contain harmful impurities leaching from the method/device. For example, one must not use chlorine and alkali produced by electrolyses utilizing the mercury cathode or sacrificial electrodes, or salt, contained dangerous admixtures.

Solutions, which possess the highest biocidal activity among all known liquid chemical germicides at the low or no toxicity for warm-blooded organisms, are electrochemically activated solutions, in particular, neutral anolyte ANK produced by STEL devices. STEL devises are designed for generation of biocidal solutions through the electrochemical conversion of sodium chloride solution and are different from any known on-site hypochlorite generators. The main advantage of such devices is in ability to generate biocidal solutions with targeted parameters in pH range from 3 to 9 and particularly with neutral pH. Fairly often these solutions are mixed with hypochlorite solutions. It can be explained by insufficient information and people’s explicable wishes to simplify by attribution to the well-known hypochlorite solutions the classification of activated solutions produced by new technology – technology of electrochemical activation.

Anolyte ANK is approved in Russia as a sterilizing agent at the concentration of oxidants of 0,005 – 0,05 % versus 0,5 – 5,0 % sodium hypochlorite solutions which can be used for disinfection purposes only.

The main principle for activated solutions generation is in unipolar joint electro-physical and electrochemical effect on treated media. During this process which is accompanied by releasing and accepting electrons, the directional change of physical-chemical, structural-energetic and catalytic properties of the treated media is taking place.

Comparison of bactericidal efficiency of anolyte ANK (lower dotted line, pH=7,0), hypochlorous acid (pH=6,5), sodium hypochlorite (pH=9,0) and chloramine (pH=9,0) is presented on the figure 3. This diagram (without data concerning anolyte ANK) is published in [8]. Positions of each line were experimentally confirmed in similar conditions (temperature, concentration of active ingredients and microbial isolates). Then the anolyte ANK was added in accordance with the obtained results. The research work was conducted by GUP “Gydrobios” in VNIIIMT MZ RF in 1999.

Active ingredients of anolyte ANK are mixture of oxy-chlorine compounds (HClO – hypochlorous acid; ClO- – hypochlorite ion; ClO· – hypochlorite-radical; ClO2 – chlorine dioxide) and peroxide compounds (НO· – hydroxyl radical; НО – peroxide anion; 1О2 – singlet oxygen; О – superoxyde-anion; O3 – ozon; O· – atomic oxygen).

Such a combination of active ingredients prevents the development of microbial resistance to biocide effect of anolyte ANK, yet low total concentration of active ingredients (oxygen and chlorine compounds) provides its safety for human and envirenment under over the long time.

Anolyte ANK is a universal germicidal agent that can be applied for all levels disinfection and sanitation, pre-sterilizing treatment, sterilization of hard surfaces as well as for antiseptic applications.

A total concentration of free oxygen and chlorine compounds in anolyte ANK (total oxidant content) is within 100 to 500 mg/l, that is dozens times less than in most solutions of currently used disinfectants. Anolyte ANK does not cause coagulation of protein, which protects microorganisms and, due to its loosened structure, easily penetrate into pinholes of living and lifeless matter. Anolyte ANK does not leave any synthetic resedue and, due to low concentration of active ingredients, it does not require wiping/rinsing off from treated surfaces after use.

Anolyte ANK is generated by STEL-devices from dilute solution of sodium chloride in tap water. Total mineralization of initial solution for anolyte ANK generation is within 1.5 to 5.0 g/l. That low concentrations prevent from accumulation of anolyte’s active ingredients in porous materials after it’s used and dried up. ANK has a low corrosion activity and does not damage surfaces of equipment and devices at mineralization of less than 2.5 g/l.

Production capacity of STEL devices is from 10 to 1000 liters of anolyte ANK per hour.

Anolyte is produced in the automatic mode re-filling plastic tanks.

In conclusion, the most efficient biocidal solutions are low or non-toxic, metastable, low-mineralized, chlorine-oxygen based (electrochemically activated solutions). There is no alternative to these solutions while the life on the Earth is presented by various forms of proteine substances in an aqueous electrolyte solutions of chlorine ions, sodium ions and some others.

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