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Material World
Element · I

Iodine

An element that is essential to human development in microgram quantities and whose deficiency was once a widespread cause of preventable harm.

Iodine is a dark violet-black solid that sublimes readily into a striking purple vapour. It is the heaviest stable halogen, and is extracted from caliche deposits in Chile and from iodine-rich brines associated with natural gas.

Its biological role is disproportionate to the quantities involved. The thyroid gland concentrates iodine to make hormones that regulate metabolism and, critically, brain development. Iodine deficiency during pregnancy and infancy causes irreversible cognitive impairment, and it was historically common in regions far from the sea.

Uses

Adding iodine to table salt is one of the cheapest and most effective public health measures ever implemented, and it has substantially reduced deficiency disorders worldwide.

Iodine's other uses follow from its chemistry and its density to X-rays. Iodinated compounds are the standard contrast agents for angiography and CT imaging. Povidone-iodine remains a widely used topical antiseptic. Silver iodide was a photographic material and is used in cloud seeding, and iodine compounds serve as catalysts and as the electrolyte in some solar cells.

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.

is used as

  • Medical imaging — application · iodinated compounds for angiography and CT

is produced by

  • Brine evaporation — process · from the caliche brines of northern Chile and from Japanese gas-field water, which between them supply nearly all of it

is found in

  • Seawater — material · at sixty parts per billion, which is dilute even by the sea's standards — seaweed concentrates it by a factor of thousands, and that is how it was first isolated and how it reached the caliche brines that supply it now

Sources

  • Material World
    Our own writing
  • Wikidata
    Wikimedia Foundation · Creative Commons CC0 1.0 Universal (public domain dedication)
  • PubChem
    US National Library of Medicine · Mixed — aggregated third-party content, rights retained by depositors
  • NIST — National Institute of Standards and Technology
    United States Department of Commerce · US Government work — public information, credit requested

Questions this page answers

Where it comes from, and what it becomes

Follow Iodine 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

  • Iodine → is produced by (from the caliche brines of northern Chile and from Japanese gas-field water, which between them supply nearly all of it) → Brine evaporation → takes as input (and the pond sequence is run so that the less soluble salts crystallise out first, leaving the sodium chloride to be harvested on its own) → 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 sourced from (what is left when a body of seawater evaporates faster than it is replenished — the salts come out in order of solubility, which is why the sequence is readable) → 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
  • Iodine → is produced by (from the caliche brines of northern Chile and from Japanese gas-field water, which between them supply nearly all of it) → 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
  • Iodine → is produced by (from the caliche brines of northern Chile and from Japanese gas-field water, which between them supply nearly all of it) → Brine evaporation → takes as input (and the pond sequence is run so that the less soluble salts crystallise out first, leaving the sodium chloride to be harvested on its own) → 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
  • Iodine → is produced by (from the caliche brines of northern Chile and from Japanese gas-field water, which between them supply nearly all of it) → Brine evaporation → takes as input (and the pond sequence is run so that the less soluble salts crystallise out first, leaving the sodium chloride to be harvested on its own) → 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
  • Iodine → is produced by (from the caliche brines of northern Chile and from Japanese gas-field water, which between them supply nearly all of it) → Brine evaporation → takes as input (and the pond sequence is run so that the less soluble salts crystallise out first, leaving the sodium chloride to be harvested on its own) → 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 (the bulk of most evaporite sequences, and the reason the rock is called salt) → Halite
  • Iodine → is produced by (from the caliche brines of northern Chile and from Japanese gas-field water, which between them supply nearly all of it) → Brine evaporation → takes as input (and the pond sequence is run so that the less soluble salts crystallise out first, leaving the sodium chloride to be harvested on its own) → 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 (one of the last salts to precipitate, needing near-total desiccation — which is why potash deposits are far rarer than salt ones) → Sylvite

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

Downstream — what it becomes