Solvay Process

Work in a fume hood or outdoors. Ammonia vapour is sharp and irritating.

1. Dissolve sodium chloride in water until no more will dissolve — a saturated brine, roughly 36 g per 100 ml at room temperature.
2. Cool the brine in an ice bath.
3. Add ammonia solution and keep it cold.

Carbon dioxide comes from limestone — in industry by burning it, on the bench by acid.

1. Generate CO₂ by adding acid to calcium carbonate in a stoppered flask, or use a cylinder.
2. Lead the gas through tubing to the bottom of the cold ammoniacal brine.
3. Bubble steadily for 20-30 minutes, keeping the beaker in ice.

NaCl + NH₃ + CO₂ + H₂O → NaHCO₃↓ + NH₄Cl

Collect the solid and wash it.

1. Filter the cold slurry through filter paper in a funnel.
2. Wash the crystals with a little ice-cold water to remove ammonium chloride.
3. Press between filter papers and let dry.

Heating drives the bicarbonate to the carbonate — the actual product.

1. Heat the dried solid gently in an evaporating dish or crucible.
2. Hold it hot until it stops losing mass.

2 NaHCO₃ → Na₂CO₃ + H₂O + CO₂↑

Confirm you made carbonate and not something else.

1. Dissolve a little in water and test with indicator — sodium carbonate is distinctly alkaline, more so than the bicarbonate you started from.
2. Add acid: it fizzes, releasing CO₂.

Ernest Solvay was 23 and working at his uncle's gas works when he began experimenting with ammonia and brine. He was not the first to think of it — the reaction had been described in Britain decades earlier, and several attempts to industrialise it had failed. Solvay's contribution was engineering rather than chemistry: he designed a tall carbonating tower that brought gas and liquid into efficient contact, and he made the ammonia recovery work, which is what everyone before him had failed at.

Recovery is the whole economics. Ammonia is expensive; salt and limestone are nearly free. Treating the ammonium chloride with lime — itself made by burning the same limestone that supplies the CO₂ — regenerates the ammonia: 2 NH₄Cl + Ca(OH)₂ → CaCl₂ + 2 NH₃ + 2 H₂O. Run in a loop, the process consumes only brine and limestone.

Against Leblanc it was decisive: less fuel, no sulfur, no hydrogen chloride pollution, continuous rather than batch. Leblanc production in Britain collapsed over the following decades. Solvay became extremely rich and funded the Solvay Conferences, the physics meetings where quantum mechanics was argued out in the 1920s.

The honest weakness: calcium chloride. Every tonne of soda ash leaves roughly a tonne of it, and there is nothing like enough demand for it. Most is discharged, and where plants discharge to rivers or shallow seas the local salinity damage is real and well documented. A process can be a great improvement on its predecessor and still leave a serious waste problem.