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1.2. SELECTIVITY 9 reactions) is the main source. On the other hand, for industrial chlorate production keeping the pH low to control the rate of oxygen formation is not possible as an optimal pH value between pH 6-7 exists for the homogeneous chlorate formation reaction 1.5[30, 55–58]. Furthermore, the chlorine evolution reaction is concentration dependent. Increas- ing the chloride concentration is one of the main ways (together with pH control, and as will be discussed, current density increase) that the selectivity for the de- sired products can be maximized[31, 35, 57–65]. This is due partly to improved thermodynamics, but perhaps mainly due to increased mass transfer rates of chlo- ride to the surface as well as competitive adsorption of chlorides on the electrode surface[62, 66]. For modern DSA, the Tafel slope is about 40 mV/decade for chlorine evolution[67], while the Tafel slope for OER has been found to be about 60 mV/dec in acidic so- lutions and ca 120 mV/dec in neutral solutions[68]. With increasing current den- sity, the overpotential for OER should thus increase more rapidly than the over- potential for ClER. A high current density should result in a high selectivity for chlorine evolution, as is indeed noted in practice[35, 43, 55, 57, 61, 62, 66, 69–72]. However, the picture seems to be more complicated in chlorate production, since it has been found that the optimal chlorate efficiency is found close to the so-called critical potential, close to E = 1.4 V vs SHE[73]. The effect of temperature is complex. Increasing the cell temperature increases the rate of both main and parasitic reactions, and it has been connected to increased oxygen selectivity under chlorate conditions[57, 69, 71, 72]. When it comes to concentrations of intermediates, increased hypochlorite con- centrations have been found to yield increased oxygen selectivity[57, 60, 64, 69, 71, 74–77], likely due to decomposition of hypochlorite, while increasing con- centrations of chlorate have been found to yield decreased oxygen selectivity[10, 57, 58, 69]. Both of these species should be avoided in chlor-alkali production, and in chlorate production the hypochlorite concentration is kept relatively low (at 1g/dm3 to 5g/dm3[5, 9, 10]) while the chlorate concentration is kept high (450 g/dm3 to 650 g/dm3[5, 9, 10]). However, the concentration of chlorate is kept high primarily to allow crystallization of NaClO3. 1.2.2 Electrolyte contamination The effects of electrolyte contamination, e.g. by metals, is less well-studied. Fur- thermore, most contaminants have been studied either in pure electrolytes (e.g. including only hypochlorite species and chloride) or otherwise under conditions (e.g. temperature and concentrations) that depart from those used industrially. The main effects that have been studied revolve around competitive adsorption on the electrode, with phosphates[58, 78, 79], sulphates[79, 80] and nitrates[78] allPDF Image | Studies of Electrode Processes in Industrial Electrosynthesis
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