Production of Sodium Hypochlorite

NaOCl is essentially produced by two methods: chemical and electrochemical.

Chemical Method

Cl2 is made to react with a solution of NaOH. In this way, NaOCl, NaCl and water are produced according to the reaction:

This method produces solutions of high concentrations, but their purity and stability do not satisfy the quality characteristics that are necessary for their use in the food and medical sectors.

Electrochemical Method

We start with a solution obtained by dissolving NaCl, until obtaining concentrated brine. The solution is then electrolyzed in an 'undivided' cell, forming an alkaline solution of NaOCl. At the same time, gaseous hydrogen is formed. For use in food and medical disinfection, electrolytic hypochlorite is preferred, inasmuch as it is appreciably purer and more stable than the other chemical. Moreover, equipping the cell with DSA-type electrodes assures the minimal presence of chlorates and solution-destabilizing impurities like:

• suspended solids

• metallic ions

• graphite particles.

Fig.1. Electrochemical cell for NaCl electrolysis and production of chlorine.

When a salt (NaCl) solution is formed in water, sodium ions (Na+) and chlorine ions (Cl-) are produced according to the reaction:

In the electrochemical cell, a potential difference is applied between the anode (+) and cathode (-), and the following reactions are generated at the electrodes:

At the anode, two oxidation reactions (loss of electrons) are possible:

At the cathode, there is a reduction reaction (gain of electrons):

The formation of undesired chlorates in the electrolytic cell is possible, either by chemical path in the 'bulk' of the solution, or by electrochemical path at both the anode and the cathode (fig. 1).

Anode: 2ClO- + 3H2O ^ 2ClO3- + 4Cl- + 6H+ + 1.5O2 + 6e- Ev = 0.46 V

Electrolysis of an aqueous solution of NaCl produces a mixture of hydrogen and Cl2 and an aqueous solution of sodium hydroxide (NaOH).

2NaCl(aq) + 2H2O(l) ^ 2Na+(aq) + 2OH-(aq) + H2(g) + Cl2(g)

The dotted line visible in the electrolytic cell portrayed in the above figure represents a diaphragm or a membrane that prevents the Cl2 product at the cell

anode from coming into contact with the soda-rich solution (NaOH) that accumulates in the cathode compartment. When this separation is removed from the cell, the electrolysis products of the aqueous solution of NaCl react to form NaOCl, which is the first step for producing bleaching solutions based on NaOCl, which by hydrolysis will produce HClO:

The conversion of the salt can be optimized by running the electrolysis system with a solution of an optimal level of concentration. With the patented system employed in producing concentrated Alcavis, an optimal quantity of the product is obtained by using DSA-type electrodes with electrocatalytic coatings of oxides of precious metals and with a concentration of the brine of 180 g/l of NaOCl. For food or medical quality (i.e., greatest purity) of the electrolytic NaOCl solution, it is necessary to optimize the electrolytic solution, the electrodes and the cell, both as to surface and to inter-electrode distance.

A recent study1 has analyzed the optimal conditions to produce NaOCl elec-trolytically. The electrolytic solutions must be prepared by dissolving pure NaCl (reagent grade) in distilled water. As the anode, old production cells employed graphite; to obtain solutions for medical use, the anodes in more recent cells are constructed of titanium coated with precious metals or with layers of oxides of precious metals (Pt, Ir, Ru, Os, etc.). In fact, graphite electrodes release micro-and nano-particles of graphitic carbon that are very difficult to eliminate even with sophisticated filtration methods. These impurities represent one of the main causes of instability of NaOCl, which decomposes starting from the surface of these particles, which function as catalysts for the decomposition reactions.

With DSA-type anodes (e.g., Ti/RuO2), the solution proves particularly pure and free of particles in suspension and is stable for very long periods.

The common electrodes used in electrochemical cells for the production of NaOCl are:

Anode: graphite (not advised), titanium with a platinum coating (Ti/Pt) (good), titanium with a coating of ruthenium oxide (Ti/RuO2) (excellent).

Cathode: graphite (not advised), stainless steel or nickel (risky2), Ti (good3), titanium coated with iridium oxide (very good).

Let us now summarize the characteristics of an electrolysis cell for production of NaOCl, analyzing some graphs obtained from the previously mentioned study, where:

2The release of Fe++ and Ni++ ions causes the solution to be unstable, with considerable rapidity of decomposition.

3Titanium is permeable to hydrogen, for which reason it is very valid electrochemically, but there can be drawbacks of dimensional stability of the electrodes.

Fig. 2. Effect of Sa/Sc ratio; Ti/TiRuO2 anode; Ti cathode; current density, 10 A/dm2; NaCl concentration, 2 m; temperature, 20°C; electrolysis time, 1 h.

Sa = Anode surface

Sc = Cathode surface

AC = Active chlorine in solution

Influence of Cell Parameters

Sa/Sc Effect

Figure 2 shows the effects of the ratio of the surface areas of the electrodes on the production of active chlorine. First of all, it is noted that, with increasing Sa, AC increases in an almost linear manner, up to an optimal value. The maximum value of AC is found at an optimal value of the Sa/Sc ratio of 1.33, after which AC decreases. This is in agreement with what is described in the literature, which indicates how an Sa > Sc favors the production of AC. In fact, a large Sa promotes the transformation of chloride ions into hypochlorite ions, while a small Sc works against cathodic reduction of the same hypochlorite ion.

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