Tin

Typical wastewaters

• electroplating and metal finishing (tin plating and tinplate lines) Sn²⁺ and its chloride complexes from acid baths, stannate Sn(OH)₆²⁻ from alkaline baths; hydrous tin oxide precipitates on neutralisation the US metal finishing category covers tin plating but sets no tin limit
• textile and leather finishing and coating (organotin biocides and stabilisers) mono, di and tributyl, methyl, octyl and phenyl tins, sorbed to solids; 0.01 µg/L reporting limit per compound MRSL substances, so detection above the reporting limit is a failure
• harbours, ship hull maintenance and dredge spoil (legacy antifouling paint) tributyltin cation (C₄H₉)₃Sn⁺ with its chloride and hydroxide, sorbed to sediment and remobilised by dredging; dibutyl and monobutyl tin as degradation products application banned since 17 September 2008 under the AFS Convention

1 · Identity

Symbol, number

Sn, 50

Oxidation states in water

+4 as hydrous SnO₂ and stannic hydroxide, insoluble at any natural pH, and as stannate Sn(OH)₆²⁻ only in strong alkali; +2 as Sn²⁺, a reductant that hydrolyses readily and oxidises to Sn(IV) in air, used as tin(II) chloride; and the organotins, above all the tributyltin cation (C₄H₉)₃Sn⁺, a lipophilic biocide that behaves like an organic pollutant, not like a metal.

Note

The element entry gives the allotropes, cassiterite, tinplate and the halides, and its reactivity text notes tin resists distilled water, sea water and soft tap water under its oxide film. The water chapter splits at the carbon tin bond: inorganic tin is a non problem, organotin a priority hazard.

2 · Occurrence in water

Natural sources

Weathering of cassiterite is slow and the product is insoluble SnO₂; natural water carries inorganic tin at trace levels. WHO: levels in drinking water above 1 to 2 µg/L are exceptional.

Anthropogenic sources

Tinplate for cans (food is the main exposure); tin solder in domestic plumbing, increasing, and tin proposed as a corrosion inhibitor (WHO); tin plating and tinplate lines (metal finishing); organotin antifouling paints on ships, banned from application since the AFS Convention entered into force on 17 September 2008 but persistent in harbour sediment; organotin PVC stabilisers and organotin biocides in textiles and leather (ZDHC Table 1M); the ledger's chemical and textile chapters carry the plant figures.

3 · Speciation

Inorganic tin(IV) is hydrolysed completely to hydrous tin dioxide at every natural pH, so it is particulate and immobile, which is why drinking water carries so little and why tin resists water under its oxide film. Tin(II) is a reductant: in air it oxidises to Sn(IV) within hours and hydrolyses on the way, so tin(II) chloride solutions are kept acid and fresh. The organotins are different in kind: the tributyltin cation is stable to hydrolysis, lipophilic, adsorbed to suspended solids and sediment, and degrades slowly by stepwise dealkylation to dibutyl, monobutyl and inorganic tin; it is toxic to molluscs at nanograms per litre, hence an EQS three orders of magnitude below any metal's.

Solubility

SnO₂ is insoluble; tin(II) salts are soluble but unstable; organotins are sparingly soluble organics that partition to solids. No solubility products quoted; the sources read print none.

Hydrolysis

Sn(IV) complete; Sn(II) readily, to Sn(OH)₂ and basic salts unless kept acid (Stumm and Morgan chapter 6, from the chapter, not re-read).

Complexation

Chloride for Sn(II); fluoride and hydroxide for Sn(IV) in plating; the organotins carry their ligands covalently.

Precipitates

SnO₂ and hydrous tin oxide; Sn(OH)₂ and basic tin(II) salts; SnS and SnS₂ in sulfidic sediment; organotins sorbed on sediment rather than precipitated.

4 · Role in treatment

5 · Removal and control

6 · Analytics

Sampling pitfalls

Inorganic tin needs digestion; tin(II) needs acid and prompt analysis or it oxidises. Organotins sorb to glass and solids and photodegrade: sample in amber glass, keep cold, extract quickly, and report whole water because the sediment fraction carries most of the tributyltin.

7 · Regulatory limits

Limits change, and many are set locally. Treat these as the published values to start from, not as your compliance target: check the standard in force at your site and the numbers written into your own permit.

8 · Health and environmental effects

Toxicity

Inorganic tin is poorly absorbed, does not accumulate and is rapidly excreted; the only human effect is acute gastric irritation from canned beverages above 150 mg/kg or foods above 250 mg/kg; rat NOAEL 20 mg/kg per day in a long term feeding study, no carcinogenicity or teratogenicity (WHO fact sheet). Organotin toxicity is the reason for the chapter and is not covered by the WHO tin fact sheet.

Bioaccumulation

Inorganic tin does not accumulate in tissues (WHO). Tributyltin bioaccumulates in molluscs and fish and biomagnifies; the EQS regime treats it as a priority hazardous substance for that reason.

Ecotoxicity

Tributyltin causes imposex (sex change) in whelks and shell deformation in oysters at nanograms per litre (IMO). US EPA aquatic life criteria for tributyltin (2004): freshwater 0.46 µg/L acute and 0.072 µg/L chronic, saltwater 0.42 and 0.0074 µg/L; EU annual average EQS 0.0002 µg/L.

Flags

• The hydrolysis equations are cited to Stumm and Morgan chapter 6 from memory of the text, not re-read this session.
• The mercury reduction equation is the electron balance; the book's mercury chapter names stannous chloride as a reductant from the EPA capsule report without printing it.
• No natural water, seawater, wastewater or tributyltin concentration was read; only the WHO drinking water statement and the limits.
• Organotin degradation and sorption behaviour is textbook environmental chemistry, not from a source read; no figure is given.
• EU law was read on legislation.gov.uk mirrors because eur-lex did not respond; eur-lex urls kept for consistency.
• Abu Dhabi: sewer 10 mg/L, no marine row; other GCC states not read.
• Standard Methods and ISO method numbers other than those in the sources read (EPA 200.8, ISO 17294-2 and the methods the WHO documents cite) are quoted from memory and were not confirmed this session. ISO 17294-2 is cited for tin from the standard's title list, not from the text.

Gaps

• No concentration of tin or tributyltin in natural water, seawater, sediment, sewage or plating effluent was read.
• The WHO background document on inorganic tin (2004) and any organotin risk assessment were not read.
• Tin plating bath chemistry and rinse treatment are not described in any source read; the ledger's chemical chapter may hold them.
• No detection limit for inorganic tin by ICP was read.
• EU law was read on legislation.gov.uk mirrors; other GCC discharge standards were not read.
• The stepwise dealkylation of tributyltin to dibutyl, monobutyl and inorganic tin is described by the sources as a pathway; none prints a stoichiometry or a rate, so no degradation equation is written.

Sources

Identity

Name and symbol

Tin, Sn

Atomic number

50 protons

Position

group 14 · period 5 · p-block · post-transition metal

CAS number

7440-31-5

Atomic structure

Atomic mass

118.71 u

Electron configuration

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p²
[Kr] 5s²⁴d¹⁰⁵p²

Electrons per shell

2, 8, 18, 18, 4

Valence electrons

4 outer shell

Physical properties

State at room temperature

Solid

Melting point

505.08 K (231.93 °C)

Boiling point

2,875 K (2,601.85 °C)

Density

7.287 g/cm3

Appearance

silvery-white (beta, β) or gray (alpha, α)

Thermal conductivity

66.8 W/(m·K)

Electrical resistivity

115 nΩ·m at 0 °C

Electrical conductivity

8.7 MS/m

Crystal structure

body-centered tetragonal

Molar heat capacity

27.112 J/(mol·K)

Chemical properties

Oxidation states

+4, +2

Electronegativity

1.96 (Pauling Scale)

Ionisation energy

7.344 eV
1st 708.6, 2nd 1,411.8, 3rd 2,943 kJ/mol

Electron affinity

1.2 eV

Atomic radius

empirical 139, covalent 139, van der Waals 217 pm

Ionic radius

Sn⁴⁺ 69 pm

Reactivity

A group 14 metal with two allotropes (grey below 13.2 C, white above) that resists water and air at room temperature under a thin oxide film, which is why it coats steel cans; it is attacked by strong acids, alkalis and acid salts, and shows both +2 and +4 states.

with water

Resists distilled water, sea water and soft tap water; dissolved oxygen accelerates any attack.

with oxygen, air

Stable in air at room temperature; heated in air it forms tin(IV) oxide, feebly acidic and giving stannates with basic oxides: $\ce{Sn + O2 -> SnO2}$.

with acids

Attacked by strong acids, alkalis and acid salts; hydrochloric acid gives tin(II) chloride and hydrogen: $\ce{Sn + 2 HCl -> SnCl2 + H2}$, and hot alkali gives stannates.

with halogens

Burns in chlorine to the volatile tin(IV) chloride: $\ce{Sn + 2 Cl2 -> SnCl4}$; the tin(II) halides such as SnF2 and SnCl2 are made from the metal and the hydrogen halide.

Typical compounds

• SnO₂ tin(IV) oxide cassiterite, the ore; ceramics and gas sensors
• SnCl₂ tin(II) chloride the most important tin salt; reducing agent, mordant
• SnCl₄ tin(IV) chloride volatile liquid from the metal and chlorine
• SnF₂ tin(II) fluoride polymeric solid; fluoride toothpaste
• SnS₂ tin(IV) sulfide mosaic gold pigment
• (C₄H₉)₃SnH tributyltin hydride organotin; organotins are PVC stabilisers and biocides

Occurrence, production and use

Crustal abundance

2.3 milligrams per kilogram

Oceanic abundance

4×10-6 milligrams per liter

Occurrence and sources

Tin is found chiefly in cassiterite (SnO2). Most of the world's supply comes from Malaya, Bolivia, Indonesia, Zaire, Thailand, and Nigeria. The U.S. produces almost none, although occurrences have been found in Alaska and California. Tin is obtained by reducing the ore with coal in a reverberatory furnace.

• cassiterite (SnO2) the tin belt of China, Thailand, Indonesia and Myanmar; Peru, Bolivia, Brazil, Congo (Kinshasa), Rwanda, Nigeria, Australia
• crustal and oceanic abundance about 1.7 ppm (BGS figure via RSC); 2.3 mg/kg crust and 0.000004 mg/L seawater (PubChem)

Extraction, production

Carbothermic smelting of cassiterite

$\ce{SnO2 (s) + C (s) -> Sn (l) + CO2 (g)}$

RSC states the ore is reduced with coal in a furnace to tin; The added secondary source prints the carbon dioxide form, which is the equation given.

Secondary tin from detinning and scrap

about 18,000 t recycled in the United States in 2024, 10,000 t of it from old scrap at one detinning plant and 31 secondary smelters

Uses

Tin resists corrosion and is used as a protective coating on other metals. Tin cans are probably the most familiar example of this application. A tin can is actually made from steel. A thin layer of tin is applied to the inside and outside of the can to keep the steel from rusting. Once widely used, tin cans have largely been replaced with plastic and aluminum containers.

Tin is used in the Pilkington process to produce window glass. In the Pilkington process, molten glass is poured onto a pool of molten tin. The glass floats on the surface of the tin and cools, forming solid glass with flat, parallel surfaces. Most of the window glass produced today is made this way.

Tin is used to form many useful alloys. Bronze is an alloy of tin and copper. Tin and lead are alloyed to make pewter and solder. An alloy of tin and niobium is used to make superconductive wire. Type metal, fusible metal, bell metal and Babbitt metal are other examples of tin alloys.

Tin salts can be sprayed onto glass to make electrically conductive coatings. These can then be used to make panel lighting and frost-free windshields. Stannous fluoride (SnF2) is used in some types of toothpaste.

• Food and beverage (tinplate cans): tin-coated steel for cans and containers; stannous chloride is also an authorised food additive (E512) in the ledger food graph tinplate 23 percent of US tin use in 2024 (usgs-mcs2025-tin)
• Chemicals: tin(II) chloride reducing agent; tin(IV) oxide for ceramics and gas sensors; zinc stannate flame retardant for plastics; organotin compounds (stabilisers, biocides; antifouling now banned) chemicals 22 percent of US tin use in 2024 (usgs-mcs2025-tin)
• Textiles: tin(II) chloride mordant for dyeing calico and silk (RSC); tin tetrachloride baths for weighting degummed silk (textile BREF 2.6.3.2); organotin compounds as biocides, catalysts and stabilisers in coatings, prints and glitter, restricted under ZDHC MRSL 1L
• Electronics and alloys: solder (lead-free); bronze, phosphor bronze, pewter, babbitt bearing metal, brass; niobium-tin superconducting magnets US 2024: solder 11 percent, alloys 10 percent, babbitt, brass, bronze and tinning 6 percent, bar tin 2 percent (usgs-mcs2025-tin)
• Glass: float glass formed on a bath of molten tin; conductive tin oxide coatings from sprayed tin salts
• Mining: cassiterite mining and smelting; new refineries in Uganda and Congo (Kinshasa) in 2024

Safety, toxicity

The small amount of tin found in canned foods is quite harmless. The agreed limit of tin content in U.S. foods is 300 mg/kg. The trialkyl and triaryl tin compounds are used as biocides and must be handled carefully.

GHS classification, signal word Danger

• H319 Causes serious eye irritation Serious eye damage/eye irritation
• H335 May cause respiratory irritation Specific target organ toxicity, single exposure; Respiratory tract irritation
• H372 Causes damage to organs through prolonged or repeated exposure Specific target organ toxicity, repeated exposure

Discovery and name

Discovered by

not in sources

Discovered

protohistoric, around 35th century BC

First isolated

not in sources

Named by

not in sources

Origin of the name

a Proto-Germanic word

Ordinary tin is composed of nine stable isotopes; 18 unstable isotopes are also known. Ordinary tin is a silver-white metal, is malleable, somewhat ductile, and has a highly crystalline structure. Due to the breaking of these crystals, a "tin cry" is heard when a bar is bent.