Industrial Production of Sodium Hydroxide: Processes & Equations

Instructor: Laura Foist

Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

In this lesson we will learn how sodium hydroxide is produced using the Castner-Kellner, Nelson Diaphragm, and membrane cells. We will also look at the chemical equations.

Industrial Production

What do paper, soap, and (some) clothing have in common? We need to use sodium hydroxide (NaOH) to make them! As of 2004, we produce 60 million metric tons per year worldwide! As you can guess, industrial processes for sodium hydroxide have therefore become very efficient.

Let's take a look at the first method used to create it, look at the corresponding equations, and explore some of the newer methods as well.

Castner-Kellner Cell

In the mid 1800's the foundation for the Castner-Kellner process was developed. This cell had a positive electrode and a negative electrode inside a tank. The reactions proceeded as follows:

  1. A sodium chloride brine (salt water) is put into the tank.
  2. The positive electrode (the anode) attracts the chloride away from the sodium.
  3. The chloride builds up and combines to form chlorine gas (an excellent disinfectant). But we now have free sodium ions roaming around.
  4. The water is broken apart with the negative electrode (the cathode) into hydrogen ions (which form into hydrogen gas, which also has many practical applications) and hydroxide (OH) ions.
  5. Then hydroxide and sodium are combined, forming sodium hydroxide.

At first this method wasn't very effective because sodium (formed at the cathode) kept reacting with chlorine at the anode or with other constituents. But by the late 1800's this problem was fixed by using mercury as the cathode, which amalgamated with sodium.

Castner-Kellner cell. The right and left sections have a sodium chloride solution, with a graphite anode (A), and a mercury cathode (M). The middle section has a sodium hydroxide solution, the same mercury anode (M), and an iron cathode (D).
Castner-Kellner Cell

The name Castner-Kellner came about because two men, Hamilton Castner in the US, and Karl Kellner of Austria, developed the mercury method almost simultaneously. Instead of fighting over rights they decided to work together and put both their names on it.

Sodium hydroxide forms a white solid
NaOH picture

How To Break Apart Water

In reality the reactions happening are a lot more complicated. In an electric environment sodium chloride will easily break apart, as its ionic bonds separate easily. But breaking the covalent bonds of water is a lot more difficult.

In water, an oxygen atom is bonded to two hydrogen atoms. The oxygen has two lone pairs (4 electrons), so there are a total of 8 electrons in the entire molecule.

Note the eight electrons in a water molecule.
water molecule

To break it apart we need to use 2 molecules of water to form 1 molecule of hydrogen gas (H2) and 2 molecules of hydroxide (2OH).

The hydrogen gas brings 2 electrons and the hydroxide brings 16 electrons (8 from each), so we end up with a total of 18 electrons. How do we start with 16 electrons and end up with 18?

This does not just magically occur. Let's look back at the production of chlorine gas. When the chlorine and sodium separate, chlorine comes away with an extra electron (a total of 8 valence electrons) since it took one away from the sodium. Combining two chlorine atoms equals a total of 16 electrons when we only need 14. The extra 2 electrons are pulled off and used to break apart water.

Therefore, the anode facilitates as an oxidizer for the chlorine (it loses electrons) and the cathode facilitates as a reducer for the water (it gains electrons). But why doesn't the sodium simply go over to the cathode and become reduced? It has a positive charge, you would think it would want to be reduced. But sodium is actually very stable with a positive charge so it doesn't undergo reduction.

Sodium Hydroxide Production Equation

Now that we know how this reaction occurs let's look at the equations a little closer:

Full equation

But really this occurs as a series of steps. First the 2 sodium chlorides break apart into ions:

Sodium chloride equation

The sodium is quite stable as a positive ion so it does not further reduce or oxidize. But the chlorine gas is further oxidized at the anode:

Chlorine equation

Using these electrons the water can be reduced at the cathode:

Water equation

Now the sodium and hydroxide ion can combine to form sodium hydroxide:

Combination equation

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