Effect of Electrolysis Times

Figure 9 indicates that the value of AC concentration increases in every case up to a maximum value and then decreases for prolonged periods. Indeed, for prolonged electrolysis times, the current flow drops, there is a strong development of oxygen, and transformation of Cl~ ions into ClO~ decreases, favor-

Fig. 8. Effect of current density as a function of the nature of the anode. Ti cathode; NaCl concentration, 2 m; temperature, 20°C; electrolysis time, 1 h.
45 40 35 30 25 20 15 10 5 0

Carbon -■- Ti/Pt

/

/

j

1

Electrolysis time (min)

50 100 150

Electrolysis time (min)

Fig. 9. Effect of electrolysis time as a function of the nature of the anode. Ti cathode; Sa/Sc = 1.33 cm; current density, 15 A/dm2; NaCl concentration, 2 m; temperature, 20°C.

ing the formation of chlorate. The behavioral superiority of Ti/RuO2 anodes in comparison to other anodes is undeniable, in as much as it is possible to employ higher currents with less production of ClO3. These results confirm that the best performances, in terms of producing active chlorine and efficiency of current, were given by the Ti/Pt and Ti/RuO2 anodes. So, the criteria of choice of anode must take into account an important factor that influences the yield of the cell, i.e., the capacity to operate with long electrolysis times. While temperature does not have a very appreciable effect, current density, electrolyte concentration and electrolysis time are the parameters that most influence the parameters of the electrochlorination reaction.

After the previous observations, some consequential conclusions can be drawn. The production of NaOCl is strictly correlated with the nature of the anode. The best result is obtained with anodes of titanium coated with ruthenium oxide; however, the operative parameters tied to NaCl concentration, current density and electrolysis time have a significant influence.

Combining all the factors in an optimal manner, it is possible to obtain, by electrochemical path in undivided cells, active chlorine concentration values up to 66 g/l with Ti/RuO2 and 60 g/l with Ti/Pt. However, it is important to note the negative effect of current density on graphite anodes. In addition, these anodes have weak resistance to oxidation, with effects of crumbling and release of microparticles of graphite into solution. Therefore, use of these materials for producing NaOCl is not advisable, especially for food and medical use.

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