Indium

Typical wastewaters

• flat panel display and semiconductor manufacture (indium tin oxide, indium phosphide) hydrolysed In(III), In(OH)₃^0 and In(OH)₄⁻, particle bound; the industrial signal is 2 to 300 pmol/L in Japanese coastal water the review quotes the receiving coastal water, not the effluent
• hospital effluent and sewage (radiopharmaceuticals) indium-111 as the In(DTPA)²⁻ chelate, which keeps it in solution the review's interpretation of evidence, not a measurement

1 · Identity

Symbol, number

In, 49

Oxidation states in water

+3 only; hydrolysed to In(OH)₃^0 and In(OH)₄⁻ (the review's thermodynamic statement for Ga and In); anthropogenic In(DTPA)²⁻ from medical use

Note

The element entry covers the metal, In₂O₃ and ITO. In water indium is a strongly hydrolysed trivalent cation with even lower solubility than gallium.

2 · Occurrence in water

Natural sources

Trace substitution in sphalerite and other sulfides (element entry); weathering releases very little and hydrolysis keeps it on particles.

Anthropogenic sources

Indium tin oxide for flat panel displays (more than half of use), indium phosphide and semiconductor manufacture, ITO sputtering scrap recycling; medical indium-111 DTPA; Japanese coastal water carries the industrial signal.

3 · Speciation

Dissolved indium is the neutral trihydroxide with some In(OH)₄⁻; the free In³⁺ ion exists only in acid solution. Anthropogenic indium from medical imaging travels as the In(DTPA)²⁻ chelate, which keeps it in solution.

Solubility

Very low, controlled by In(OH)₃; no solubility product read.

Hydrolysis

Complete in neutral water (qualitative).

Complexation

Strong aminopolycarboxylate (DTPA) chelation from medical use; no natural ligand data read.

Precipitates

In(OH)₃ on neutralisation of acid indium solutions.

4 · Role in treatment

Not relevant or not given for this element.

5 · Removal and control

Not relevant or not given for this element.

6 · Analytics

Sampling pitfalls

Sub-picomolar seawater levels demand trace-metal clean sampling; indium contamination from tin solder and ITO coated laboratory glass is a plausible blank (writer's caution, not sourced).

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

No human health guideline for stable indium in water anywhere read; no ingestion toxicology was read this session.

Bioaccumulation

No published concentration data for marine organisms other than one reference material value (review). Medaka accumulated In dose dependently with sublethal growth and behaviour effects.

Ecotoxicity

96 h LC₅₀ for the freshwater shrimp Macrobrachium nipponense 6.89 mg/L In(III), the least toxic of the five semiconductor metals tested (Ga 2.77, Sb 1.96, Cd 0.054, Cu 0.031 mg/L). In acidic paddy soil above 4 mmol/kg In, medaka died mainly from released aluminium.

Flags

• The In(DTPA)²⁻ statement is the review's interpretation of evidence, not a measurement quoted here.
• Japanese coastal average 25 plus or minus 55 pmol/L has a standard deviation larger than the mean; treat as a range.

Gaps

• No river, groundwater, municipal or industrial wastewater concentration was read.
• No hydrolysis constants or solubility product were read.
• No removal process for indium from water was read; ITO etching effluent treatment was not found in a readable source.
• No GCC discharge standard was read.

Sources

Identity

Name and symbol

Indium, In

Atomic number

49 protons

Position

group 13 · period 5 · p-block · post-transition metal

CAS number

7440-74-6

Atomic structure

Atomic mass

114.818 u

Electron configuration

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p¹
[Kr] 5s²⁴d¹⁰⁵p¹

Electrons per shell

2, 8, 18, 18, 3

Valence electrons

3 outer shell

Physical properties

State at room temperature

Solid

Melting point

429.75 K (156.6 °C)

Boiling point

2,345 K (2,071.85 °C)

Density

7.31 g/cm3

Appearance

silvery lustrous gray

Thermal conductivity

81.8 W/(m·K)

Electrical resistivity

83.7 nΩ·m at 20 °C

Electrical conductivity

11.95 MS/m

Crystal structure

body-centered-tetragonal

Molar heat capacity

26.74 J/(mol·K)

Chemical properties

Oxidation states

+3

Electronegativity

1.78 (Pauling Scale)

Ionisation energy

5.786 eV
1st 558.3, 2nd 1,820.7, 3rd 2,704 kJ/mol

Electron affinity

0.3 eV

Atomic radius

empirical 142, covalent 142, van der Waals 193 pm

Ionic radius

In³⁺ 80 pm

Reactivity

A very soft group 13 metal, less reactive than gallium: stable in air and water at room temperature, dissolved by mineral acids but, unlike gallium and aluminium, not by aqueous alkali; nearly all its chemistry is indium(III).

with water

Does not react with water.

with oxygen, air

Does not react with oxygen at room temperature; burns when heated in air to indium(III) oxide: $\ce{4 In + 3 O2 -> 2 In2O3}$.

with acids

Dissolves in mineral acids to indium(III) salts and hydrogen: $\ce{2 In + 6 HCl -> 2 InCl3 + 3 H2}$; insoluble in aqueous alkali.

with halogens

Oxidised by the halogens to the trihalides: $\ce{2 In + 3 Cl2 -> 2 InCl3}$; InF3 is polymeric, InCl3 and InBr3 colourless, InI3 yellow.

Typical compounds

• In₂O₃ indium(III) oxide with tin oxide gives ITO, the transparent conductor
• InCl₃ indium trichloride Lewis acid, common indium(III) salt
• InP indium phosphide III-V semiconductor for high-speed electronics
• InSb indium antimonide infrared detector semiconductor
• InN indium nitride with gallium nitride in LEDs
• In₂S₃ indium(III) sulfide chalcogenide from zinc-ore processing

Occurrence, production and use

Crustal abundance

2.5×10-1 milligrams per kilogram

Oceanic abundance

2×10-2 milligrams per liter

Occurrence and sources

Indium is most frequently associated with zinc materials, and it is from these that most commercial indium is now obtained; however, it is also found in iron, lead, and copper ores.

• trace substitution in sphalerite (ZnS), under 1 to 100 ppm indium zinc deposits worldwide; recovered at zinc refineries in China, Korea, Japan, Canada, Belgium, France
• traces in chalcopyrite and stannite copper and tin sulfide ores; not economic to recover
• crustal and oceanic abundance about 0.052 ppm crust (BGS via RSC); 0.25 mg/kg crust and 0.02 mg/L seawater (PubChem)

Extraction, production

By-product of zinc refining

indium is concentrated in zinc smelter residues and refined to 99.99 percent metal; neither source states the chemistry

Recycling of indium tin oxide scrap

the main secondary route, in Japan and the Republic of Korea; quantity not reported

Uses

Indium is used to coat the bearings of high speed motors since it allows for the even distribution of lubricating oil. Indium is used to dope germanium to make transistors. It is also used to make other electrical components such as rectifiers, thermistors and photoconductors. Indium can be used to make mirrors that are as reflective as silver mirrors but do not tarnish as quickly. Indium is also used to make low melting alloys. An alloy of 24% indium and 76% gallium is a liquid at room temperature.

It has found application in making low-melting allows; an allow of 24% indium - 76% gallium is liquid at room temperature. It is used in making bearing alloys, germanium transistors, rectifiers, thermistors, and photoconductors. It can be plated onto metal and evaporated onto glass, forming a mirror as good as that made with silver but with more resistance to atmospheric corrosion.

• Electronics and displays: indium tin oxide transparent conductive coatings on liquid crystal and other flat-panel displays and touch screens; most global consumption; indium phosphide substrates, lasers and photodetectors for 5G fibre optics and data-centre links; indium nitride and antimonide semiconductors; indium-based solders and alloys, ITO coating of data-centre cables ITO accounts for most global indium consumption (usgs-mcs2025-indium); no percentage given
• Energy: transparent electrodes in solar panels; indium in LEDs replacing mercury fluorescent lamps
• Nuclear and safety alloys: cadmium-silver-indium control-rod alloys (hafnium can substitute); low-melting alloys for fire sprinklers; gallium-indium-tin replacing mercury in thermometers
• Mining: indium content of zinc ores; a zinc-copper-silver-indium project in Utah was permitted in 2024

Safety, toxicity

There is evidence that indium has a low order of toxicity; however, care should be taken until further information is available.

GHS classification, signal word Danger

• H302 Harmful if swallowed Acute toxicity, oral
• H312 Harmful in contact with skin Acute toxicity, dermal
• H315 Causes skin irritation Skin corrosion/irritation
• H319 Causes serious eye irritation Serious eye damage/eye irritation
• H332 Harmful if inhaled Acute toxicity, inhalation
• 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
• H303 May be harmful if swallowed Acute toxicity, oral

Discovery and name

Discovered by

Ferdinand Reich and Hieronymous Theodor Richter

Discovered

1863

First isolated

Hieronymous Theodor Richter

Named by

not in sources

Origin of the name

for the indigo blue line in its spectrum

Indium is available in ultra pure form. Indium is a very soft, silvery-white metal with a brilliant luster. The pure metal gives a high-pitched "cry" when bent. It wets glass, as does gallium.