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Material World
Compound · NaOH

Sodium hydroxide

Caustic soda — the strong base that underlies paper, soap, aluminium refining and much of chemical manufacturing.

Sodium hydroxide is the workhorse strong base of industry, and its production is inseparable from chlorine's. Electrolysing brine splits sodium chloride into chlorine at one electrode and sodium hydroxide with hydrogen at the other, so the two are made in fixed proportion whether or not demand for them matches — a structural quirk that has shaped the chemical industry's economics for a century.

It is aggressively hygroscopic, drawing water from the air until solid pellets deliquesce into a puddle, and it attacks skin and eyes severely. Dissolving it releases substantial heat.

Uses

Pulp and paper is the largest consumer, where it dissolves lignin to free the cellulose fibres. Aluminium production depends on it too: the Bayer process uses hot caustic soda to dissolve alumina out of bauxite and leave the iron-rich residue behind.

It saponifies fats into soap, which is its oldest use and still a large one. It is also used to make detergents, textiles, and a wide range of chemicals; to neutralise acidic waste streams; and, in dilute form, in drain cleaners and oven cleaners.

Medium confidence Weak evidence

How we know: checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source.

How this connects

Where a connection has been confirmed by an outside reference, that reference is named beside it.

contains

is sourced from

  • Halite — mineral · electrolysis of brine, which produces chlorine in fixed proportion

is used as

is an input to

  • Bayer process — process · hot concentrated caustic soda dissolves the aluminium hydroxides
  • Kraft pulping — process · with sodium sulfide, breaks the lignin into fragments soluble in the alkaline liquor

is produced by

is used in

  • Chemical manufacture — industry · and it is made as the inseparable co-product of chlorine, which ties the markets for two unrelated chemicals together permanently

Sources

  • Material World
    Our own writing
  • PubChem
    US National Library of Medicine · Mixed — aggregated third-party content, rights retained by depositors

Questions this page answers

Where it comes from, and what it becomes

Follow Sodium hydroxide back to what it starts as, and forward into what it becomes. Each step is a documented one — a real route material takes, not a chain of inference.

Upstream — what it comes from

  • Sodium hydroxide → is produced by (at the cathode, in fixed proportion to the chlorine whether demand agrees or not) → Chlor-alkali electrolysis → takes as input (the largest single use of salt, and the process the whole chlorine and caustic soda industry rests on) → Salt → is produced by (in the solar route — the same process, read from the other end) → Brine evaporation → takes as input (the feedstock for solar salt and, in the salars, for the far stronger brines that lithium comes from) → Seawater → is composed of (about 96.5 per cent by mass — seawater is 3.5 per cent dissolved solids and the rest of it is this) → Water
  • Sodium hydroxide → is sourced from (electrolysis of brine, which produces chlorine in fixed proportion) → Halite
  • Sodium hydroxide → is produced by (at the cathode, in fixed proportion to the chlorine whether demand agrees or not) → Chlor-alkali electrolysis → takes as input (as the brine the cell electrolyses, and as the source of the hydrogen that comes off the cathode) → Water → is sourced from (by desalination, where energy is cheap and coastline available — the only route that adds fresh water rather than moving it) → Seawater
  • Sodium hydroxide → is produced by (at the cathode, in fixed proportion to the chlorine whether demand agrees or not) → Chlor-alkali electrolysis → takes as input (as brine; the salt is the feedstock for both products at once) → Halite
  • Sodium hydroxide → is produced by (at the cathode, in fixed proportion to the chlorine whether demand agrees or not) → Chlor-alkali electrolysis → takes as input (the largest single use of salt, and the process the whole chlorine and caustic soda industry rests on) → Salt → is sourced from (by solar evaporation, which needs a dry sunny coast and is the cheapest route there is) → Seawater → is composed of (about 96.5 per cent by mass — seawater is 3.5 per cent dissolved solids and the rest of it is this) → Water
  • Sodium hydroxide → is produced by (at the cathode, in fixed proportion to the chlorine whether demand agrees or not) → Chlor-alkali electrolysis → takes as input (the largest single use of salt, and the process the whole chlorine and caustic soda industry rests on) → Salt → is sourced from (mined as rock salt from bedded deposits, or dissolved in place and pumped up as brine — which is what the chemical industry wants anyway) → Evaporite → is composed of (precipitated early, when the water has reduced to roughly a fifth of its volume) → Gypsum

These are the most distinct paths back. Sodium hydroxide can be traced through others besides.

Downstream — what it becomes

  • Sodium hydroxide → is an input to (with sodium sulfide, breaks the lignin into fragments soluble in the alkaline liquor) → Kraft pulping → produces (as tall oil rosin from the black liquor — a by-product of making paper, and now a larger source than tapping) → Resin → is used in (as the flux in rosin-cored solder wire: it strips the oxide off copper at soldering temperature so the solder can wet the metal, and the smell of soldering is the smell of pine) → Electronics manufacture → is associated with (and the supply chain it produced is the deepest and most concentrated in the world economy) → The semiconductor era complete chain
  • Sodium hydroxide → is an input to (hot concentrated caustic soda dissolves the aluminium hydroxides) → Bayer process → produces (crystallised from the liquor and calcined, ready for the smelter) → Aluminium oxide → is an input to (dissolved in molten cryolite and electrolysed) → Hall–Héroult process → produces (collects at the cathode while the carbon anodes are consumed) → Aluminium → is used as → Electrical conduction
  • Sodium hydroxide → is used as (in the Bayer process and in metal cleaning before coating) → Corrosion protection
  • Sodium hydroxide → is used in (and it is made as the inseparable co-product of chlorine, which ties the markets for two unrelated chemicals together permanently) → Chemical manufacture
  • Sodium hydroxide → is an input to (with sodium sulfide, breaks the lignin into fragments soluble in the alkaline liquor) → Kraft pulping → produces (via pulp — most chemical pulp and therefore most quality paper) → Paper → is used as (cartons and cases, and the material most plastic packaging is now being asked to justify itself against) → Packaging
  • Sodium hydroxide → is an input to (with sodium sulfide, breaks the lignin into fragments soluble in the alkaline liquor) → Kraft pulping → produces (as tall oil rosin from the black liquor — a by-product of making paper, and now a larger source than tapping) → Resin → is a source for (the same material, forty million years later: the volatiles gone and the remainder cross-linked into a network that no longer dissolves) → Amber → is used as (worked since the Upper Palaeolithic, and traded the length of Europe before anyone smelted a metal) → Jewellery and adornment

These are the most distinct paths onward. Sodium hydroxide ends up in others besides.