Anolyte and catholyte: the two outputs of a diaphragm cell
A diaphragm electrolysis cell separates the anode and cathode chambers, so it produces two distinct solutions from salt, water and electricity. The anolyte, formed at the anode, is a hypochlorous acid-dominant oxidant used for disinfection. The catholyte, formed at the cathode, is a sodium hydroxide (caustic) solution that can be used on site for alkaline cleaning, CIP or pH correction. Both are produced where they are needed, with no hazardous chemical transport.
Electrolysis of a dilute salt solution always produces something at both electrodes. In a single-chamber cell those products mix into one solution. A diaphragm cell keeps them apart — and in doing so turns one process into two useful outputs.
One cell, two chambers
In a diaphragm cell the anode and cathode sit in separate chambers, divided by a diaphragm that lets ions pass but keeps the two solutions from mixing. Brine is fed to the cell, electricity is applied, and two streams leave it: the anolyte from the anode side and the catholyte from the cathode side.
At the anode: the disinfecting solution
Chloride ions are oxidised at the anode and react with water to form hypochlorous acid (HOCl). This anolyte is the main product: a hypochlorous acid-dominant oxidant used for water-line, surface and process disinfection.
Why the separation matters here comes down to pH. How much of the chlorine is in the active hypochlorous acid form is governed by an equilibrium with a dissociation constant of pKa ≈ 7.5;1 the higher the pH, the more chlorine shifts to the weaker hypochlorite ion. Because the hydroxide formed at the cathode stays on the other side of the diaphragm, the anolyte remains in the range where hypochlorous acid dominates.
At the cathode: sodium hydroxide
At the cathode, water is reduced to hydrogen gas and hydroxide ions. Sodium ions crossing from the anode side combine with that hydroxide to form sodium hydroxide (NaOH) — caustic. The hydrogen gas is vented as part of the system design.
The catholyte is not a by-product to be disposed of. Caustic is a chemical many facilities already buy, transport and store.
Where the catholyte is used
- Alkaline cleaning and CIP: caustic removes fat, protein and organic soil. In a typical cleaning cycle the alkaline step comes first and disinfection follows, so a single cell supplies both steps.
- pH correction: where a process circuit calls for raising pH, the catholyte can be used for the adjustment.
- Pre-cleaning before disinfection: on surfaces and equipment carrying organic soil, an alkaline wash means the hypochlorous acid that follows acts on a clean surface rather than being consumed by the soil.
Whether and where the catholyte is used is decided per site, according to the facility’s cleaning and process needs.
Why this matters for the operator
Producing both solutions on site removes two hazardous chemicals from the supply chain rather than one. The literature on electrolyzed water in industry records that the method’s only chemical input is salt, that generation can take place at the point of use, and that it offers cost and environmental advantages over conventional systems.2
The AquaSky system is built on this principle: a diaphragm cell producing a hypochlorous acid-dominant anolyte for disinfection and a caustic catholyte for on-site use. Skytech Water sizes it to the facility, then designs, installs, commissions and services it on site.