Electrolysing brine makes chlorine gas at the anode (positive electrode) and hydrogen gas at the cathode (negative electrode), with sodium hydroxide building up around the cathode. A cell carrying 10 amps makes about 13 g of chlorine and 4.5 litres of each gas per hour; run for 10 hours in a 30 cubic-metre shed, that chlorine alone reaches roughly 1,500 ppm, a concentration that is immediately fatal. Keep the gases apart, keep the cell outdoors or in strong ventilation, and absorb the chlorine in alkali.
Part of Industrial Chemistry
Building a brine electrolysis cell: electrodes, divider, power, gas handling and the arithmetic of how much gas it makes.
Which Gas Comes Off Which Electrode
Whenever the electrolyte contains chloride, the cell makes two gases at once. The corpus states the electrode reactions in Chlor-Alkali Process:
- Anode (+): chloride ions are oxidised to chlorine gas, 2Cl⁻ → Cl2 + 2e⁻. Chlorine is yellow-green, heavier than air and extremely toxic.
- Cathode (−): water is reduced to hydrogen gas and hydroxide, 2H2O + 2e⁻ → H2 + 2OH⁻. Hydrogen is flammable and lighter than air.
If the brine is dilute or the anode wrong, some oxygen is made instead of chlorine at the anode. The general rules for electrolysis are in Electrolysis Fundamentals, electrode choice in Electrode Selection, and cell layout in Cell Design.
Why Chlorine And Hydrogen Must Be Kept Apart
A mixture of hydrogen and chlorine can explode when lit or even in strong light, a general chemistry fact. The cell must keep the two gas streams separate from the moment they form, collect them in separate paths, and vent each to a safe place. The hydrogen article’s flammable range is 4-75% in air (Gas Hazards).
If there is no divider, chlorine and sodium hydroxide react to form sodium hypochlorite (bleach). That is useful only if bleach is what you want. Acidifying a hypochlorite solution releases chlorine again, so bleach is never mixed with acid; see Chlorination.
What You Need
- Saturated brine, about 26% salt by weight per the carrier.
- Anode: graphite or platinum, which resist chlorine; cathode: steel or nickel, which resist sodium hydroxide (carrier).
- A divider between the compartments (porous ceramic pot or other membrane that passes ions and not gas) per the carrier.
- DC power: batteries, generator or solar. The carrier says 6-12 V and as much current as the source allows. The corpus’s other article gives 3.5-5 V per cell; the two disagree and neither is altered here. Use the lower voltage that drives the cell, since a lower voltage means less heat and fewer side reactions, and measure rather than assume.
- Separate gas collection and venting for each electrode, with an alkaline scrubber for the chlorine (a tank of sodium hydroxide solution the chlorine is bubbled through; see Waste Treatment).
- An outdoor site, upwind of everything, with a way to switch the power off from a distance.
Method: Order Of Work (No Gas Is Tested With A Flame)
Step 1: Site and ventilate. Put the cell outdoors, or in an open-sided shelter. No sleeping, eating or work area downwind.
Step 2: Check the divider and gas paths with the power off. Check joints with soap solution, never a flame.
Step 3: Connect the scrubber to the chlorine side and an outdoor vent to the hydrogen side.
Step 4: Apply power and check polarity. The electrode that evolves a yellow-green gas with a sharp smell is the anode.
Step 5: Watch the cell; stop at the first gas smell outside the scrubber. Switch the power off from a distance, then ventilate before approaching.
Worked Example: How Much Gas A 10 A Cell Makes
Faraday’s constant is 96,485 coulombs per mole of electrons, which is 26.8 ampere-hours (a recalled constant). Assume 100% current efficiency, which overstates the real chlorine yield.
- Chlorine needs 2 mol of electrons per mole of Cl2: 2 × 26.8 = 53.6 Ah per mole (71 g).
- At 10 A for 1 hour (10 Ah): 10 ÷ 53.6 = 0.187 mol Cl2 = 13.2 g, about 13 g of chlorine per hour.
- Volume at about 24 L per mole: 0.187 × 24 = 4.5 litres per hour. Hydrogen is also 2 electrons per mole, so 0.187 mol = 4.5 litres, 0.37 g.
- Over 10 hours: 45 litres of chlorine and 45 litres of hydrogen.
If it all escaped into a closed shed of 30 cubic metres (30,000 litres): chlorine = 45 ÷ 30,000 = 0.15% = 1,500 ppm. The gas-hazards article gives 50 ppm as lethal within 30-60 minutes and 1,000 ppm as immediately fatal; 1,500 ppm is above it. The hydrogen, 0.15%, is well under the 4% flammable limit, but it rises and collects against the roof, where an unventilated pocket can be much more concentrated. Chlorine at 71 g per mole is 71 ÷ 29 = 2.4 times heavier than air (29 g per mole) and sinks; hydrogen at 2 g per mole is 2 ÷ 29 = 0.07 times as heavy and rises.
Common Mistakes
- Running the cell in a shed, cellar or garage. This is how people have died.
- Collecting both gases in one container.
- Using a steel anode, which dissolves.
- Mixing the cell liquor with acid or with ammonia (see Safety).
- Leaving the cell running unattended.
Safety
Chlorine at the anode and hydrogen at the cathode; one is poison, the other explosive
Chlorine (anode, +) is heavy, sinks into pits and low rooms, and is fatal at a fraction of one per cent in air; hydrogen (cathode, −) is lighter than air, burns almost invisibly and ignites in a very wide range. Run the cell outdoors with strong natural ventilation, keep the two gas paths separate, absorb chlorine in alkali, switch off from a distance, and never use a flame to test for either gas. Do not add acid or ammonia to bleach-bearing liquor. See Toxic Gases and Gas Hazards.
What’s Next
- Learn next: Toxic Gases — chlorine and the other gases in this section
- Combine with: Waste Treatment — absorbing chlorine and neutralising liquor