Choosing Conditions for Chlorine Production from Seawater
A remote aquaculture facility needs to generate a disinfectant (active chlorine as HOCl/OCl−) from seawater by electrolysis. The seawater has [Cl−] = 0.50 M, pH 8.1, and is at 20°C. The facility has a graphite anode and a stainless steel cathode. The engineer observes that at the existing conditions the current efficiency for active chlorine is only 12%, with most charge going to oxygen evolution. The relevant standard potentials are E°(Cl2/Cl−) = +1.36 V and E°(O2/H2O) = +1.23 V (vs SHE at pH 0). The Tafel slope for oxygen evolution on graphite is 0.12 V per decade, and for chlorine evolution is 0.08 V per decade. At the operating current density of 20 mA cm−2, the oxygen overpotential is 0.55 V and the chlorine overpotential is 0.15 V. The facility cannot change the anode material, but can adjust pH, chloride concentration (by adding salt), and temperature (within 10–30°C). The target is to raise current efficiency for active chlorine above 70%. Assume activity coefficients are unity and that chlorine hydrolyses rapidly to HOCl/OCl− at pH > 7.
1. Calculate the reversible potentials and the actual anode potentials for oxygen and chlorine evolution at pH 8.1 and [Cl−] = 0.50 M. Show which reaction is thermodynamically and kinetically favoured under the current conditions.
Read the worked answer
For O2 evolution: O2 + 4H+ + 4e− ⇌ 2H2O. At pH 8.1: E(O2) = 1.23 − 0.0592 × pH = 1.23 − 0.0592 × 8.1 = 1.23 − 0.480 = 0.750 V vs SHE. With an overpotential of 0.55 V, the actual anode potential needed for oxygen evolution at 20 mA cm−2 is 0.750 + 0.55 = 1.300 V.
For chlorine evolution: Cl2 + 2e− ⇌ 2Cl−. Using [Cl−] = 0.50 M and P(Cl2) = 1 bar: E(Cl2) = 1.36 − (0.0592/2) log10([Cl−]^2) = 1.36 − 0.0592 log10(0.50) = 1.36 + 0.0178 = 1.378 V. With an overpotential of 0.15 V, the actual anode potential needed for chlorine evolution is 1.378 + 0.15 = 1.528 V.
Oxygen evolution requires a lower anode potential (1.300 V) than chlorine evolution (1.528 V), so oxygen is favoured both thermodynamically and kinetically. This explains the low 12% current efficiency for active chlorine.
2. The engineer proposes acidifying the seawater to pH 2.0. Calculate the new reversible and actual anode potentials for oxygen and chlorine evolution, assuming overpotentials are unchanged. Explain qualitatively how the current efficiency for chlorine would change.
Read the worked answer
At pH 2.0: E(O2) = 1.23 − 0.0592 × 2.0 = 1.23 − 0.118 = 1.112 V. Actual oxygen potential = 1.112 + 0.55 = 1.662 V.
The chlorine half-reaction does not involve H+, so its reversible potential remains 1.378 V and its actual potential remains 1.378 + 0.15 = 1.528 V, assuming [Cl−] is unchanged.
Now chlorine evolution requires the lower potential (1.528 V vs 1.662 V for oxygen) by 0.134 V. The thermodynamic preference has reversed, so chlorine evolution should become the dominant anode reaction and the current efficiency for chlorine should rise substantially. However, at pH 2 the product is Cl2 gas rather than HOCl/OCl−, because chlorine hydrolysis is suppressed at low pH. The proposal therefore increases chlorine production but changes the chemical form of the disinfectant.
3. The facility wants active chlorine (HOCl/OCl−) for disinfection, not Cl2 gas. Evaluate the engineer’s pH 2 proposal in terms of product suitability, safety, and practical constraints. Recommend a better strategy or conclude that the anode must be changed.
Read the worked answer
The pH 2 proposal would improve chlorine current efficiency, but it is a poor engineering choice for this facility. At pH 2, chlorine exists mainly as dissolved Cl2 gas, not as HOCl/OCl−. Cl2 is a toxic, volatile gas with lower disinfection efficiency in water and a strong odour; it also poses a significant inhalation hazard in an aquaculture setting. Acidifying 0.50 M chloride seawater to pH 2 requires a large acid dose, causes severe corrosion of the stainless steel cathode and plumbing, and is difficult to control safely.
Adding more chloride is not a solution either: increasing [Cl−] from 0.50 M to 5.0 M changes E(Cl2) by only −0.0592 log10(5.0/0.50) = −0.059 V, which is much smaller than the 0.134 V disadvantage at pH 8.1. Temperature adjustment within 10–30°C cannot reverse the overpotential difference either.
The honest conclusion is that the graphite anode is the limiting factor. A dimensionally stable anode (DSA, e.g., IrO2/Ta2O5 on titanium) has a much higher oxygen overpotential and would favour chlorine evolution at near-neutral pH, producing HOCl/OCl− directly. If the anode cannot be changed, the facility should accept low chlorine efficiency and use a separate disinfectant, rather than acidifying seawater to pH 2.