Silver

Typical wastewaters

• electroplating and metal finishing (silver plating) Ag⁺ in rinse water, held in solution as cyanide complexes until the cyanide is destroyed; precipitated as AgCl, Ag₂S or into the hydroxide sludge US limit 0.43 mg/L daily, 0.24 mg/L monthly
• photographic processing (fixer) silver thiosulfate complexes, which keep silver dissolved past chloride precipitation declining since digital photography (WHO); no fixer concentration read
• municipal sewage (nanoparticles from antimicrobial textiles and consumer products) silver nanoparticles, aggregated and coated, releasing Ag⁺; converted to Ag₂S in sludge and sediment coated nanoparticles are expected to be removed by conventional treatment
• textile and leather wet processing (antimicrobial silver finishes) total silver as a ZDHC Table 2 metal (0.1 mg/L foundational); sludge threshold 50 mg/kg dry weight

1 · Identity

Symbol, number

Ag, 47

Oxidation states in water

+1 only: Ag⁺ from silver nitrate and from dissolving nanoparticles; AgCl, slightly soluble, and its chloride complexes in chloride rich water; Ag₂S, the insoluble thermodynamic sink (WHO). 0 as the metal, as silver nanoparticles of 1 to 100 nm that release Ag⁺ at their surface, and as the silver coating of filter media.

Note

The element entry gives the nitrate, the halides and the sulfide tarnish. In water everything turns on chloride and sulfide: they decide whether silver is a dissolved biocide, a colloid of AgCl, or a sulfide particle that does nothing.

2 · Occurrence in water

Natural sources

Silver occurs mainly as its very insoluble and immobile sulfide, oxides and some salts (WHO fact sheet); in soil as the chloride or sulfide, mobile only if the sulfide oxidises to sulfate. In river water silver is dissolved by complexation with chloride and humic matter (WHO background document).

Anthropogenic sources

Point of use filters with silver spiked activated carbon or silver coated ceramics; copper-silver ionisation in building water systems; silver nanoparticles washed out of antimicrobial textiles and consumer products into sewage; silver plating and metal finishing (US limit 0.43 mg/L daily); photographic fixer (declining since digital photography, WHO); silver and base metal mining. Nanoparticles released via wastewater or industrial discharge can be mobile and enter ground and drinking water supplies (WHO).

3 · Speciation

Ag⁺ is a soft cation that binds sulfide, chloride and organic sulfur ligands. In low chloride fresh water it stays ionic, complexed by humic matter; as chloride rises it precipitates AgCl and then redissolves as AgCl₂⁻ and higher chloro complexes, so silver in seawater is a dissolved chloride complex. Any sulfide converts it to Ag₂S, which is the final sink for both ions and nanoparticles and is biologically inert. Silver nanoparticles aggregate because of their surface energy (WHO) and release Ag⁺ by surface oxidation; their coating decides whether conventional treatment catches them. Ionic release from nanoparticles makes it hard to say whether silver found in water came from ions or particles (WHO fact sheet).

Solubility

AgCl is slightly soluble; Ag₂S is among the least soluble sulfides; silver nitrate is freely soluble. EPA 200.8 notes silver is only slightly soluble in the presence of chloride unless there is enough chloride to form the soluble chloride complex, which is an analytical and a treatment fact at once. No solubility products quoted; the sources read print none.

Hydrolysis

Negligible at natural pH; Ag₂O precipitates only in strong alkali.

Complexation

Chloride, humic matter (WHO), thiosulfate (the basis of photographic fixer), ammonia and cyanide in plating baths; all keep silver dissolved past the chloride precipitation.

Precipitates

AgCl, Ag₂S, metallic silver from reduction, silver in hydroxide sludge of metal finishing plants.

4 · Role in treatment

5 · Removal and control

6 · Analytics

Sampling pitfalls

Silver precipitates as AgCl and adsorbs on container walls unless the sample is digested or carries enough chloride to form the chloride complex (EPA 200.8 section 1.7); the method requires hydrochloric acid to keep silver and antimony stable and recommends digestion before determination. Dissolved silver by filtration therefore understates silver in chloride bearing water. Ionic silver released from nanoparticles cannot be told from ionic silver present as such without particle methods (WHO).

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

Argyria, the blue grey discolouration of skin and organs by deposited Ag₂S, is the only established effect of chronic oral silver; the lowest human LOAEL is about 0.6 mg/kg per day of colloidal silver for 16 months, and 10 g of silver over a lifetime is taken as a NOAEL (WHO). Animal data on nanoparticles at high doses hint at effects of unclear relevance to the low µg/L found in drinking water (WHO background document).

Bioaccumulation

Silver deposits in skin, kidney and liver as insoluble sulfide after ingestion of ions or nanoparticles (WHO); aquatic bioaccumulation is not addressed in the sources read.

Ecotoxicity

Silver ion is among the most toxic metals to aquatic life: US EPA aquatic life criteria 3.2 µg/L freshwater acute and 1.9 µg/L saltwater acute (1980), no chronic criteria; the biocidal use and the ecotoxicity are the same property, and sulfide and chloride complexation blunt both.

Flags

• The WHO 0.1 mg/L is a provisional reference value that WHO says must not be used for regulations; it is listed separately from the no guideline row.
• The French 8 to 49 µg/L mean drinking water estimate is quoted as WHO printed it; it sits far above the US and Canadian surveys.
• The 2021 WHO background document read is the December 2020 public review draft (iris.who.int returned 403 for the final WHO/HEP/ECH/WSH/2021.7).
• The sulfide precipitation equation is written here; WHO names Ag₂S as the sink without printing it.
• The metal finishing removal train is inferred from the limit and the CWW BREF's generic precipitation chapter; no silver specific effluent treatment source was read.
• Abu Dhabi values cover two media (marine 0.005 mg/L, sewer 5 mg/L); 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.

Gaps

• No seawater silver concentration and no photographic or plating effluent concentration was read; silver recovery from fixer is not covered.
• No solubility products for AgCl and Ag₂S and no chloride complex constants are quoted.
• Copper-silver ionisation silver doses in µg/L are given by WHO only as 'low µg/L'; no design figure was read.
• EU law was read on legislation.gov.uk mirrors; other GCC discharge standards were not read.
• The nanoparticle oxidative dissolution is written here from the surface oxidation the WHO background document describes; no rate or solubility figure was read.
• No stability constants were read for the thiosulfate or cyanide complexes; both equations are written from the complexes the sources name.

Sources

Identity

Name and symbol

Silver, Ag

Atomic number

47 protons

Position

group 11 · period 5 · d-block · transition metal

CAS number

7440-22-4

Atomic structure

Atomic mass

107.8682 u

Electron configuration

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s¹ 4d¹⁰
[Kr] 5s¹⁴d¹⁰

Electrons per shell

2, 8, 18, 18, 1

Valence electrons

11 ns and (n-1)d

Physical properties

State at room temperature

Solid

Melting point

1,234.93 K (961.78 °C)

Boiling point

2,435 K (2,161.85 °C)

Density

10.501 g/cm3

Appearance

lustrous white metal

Thermal conductivity

429 W/(m·K)

Electrical resistivity

15.87 nΩ·m at 20 °C

Electrical conductivity

63.01 MS/m

Crystal structure

face-centered cubic

Molar heat capacity

25.35 J/(mol·K)

Chemical properties

Oxidation states

+1

Electronegativity

1.93 (Pauling Scale)

Ionisation energy

7.576 eV
1st 731, 2nd 2,070, 3rd 3,361 kJ/mol

Electron affinity

1.302 eV

Atomic radius

empirical 145, covalent 145, van der Waals 172 pm

Ionic radius

Ag⁺ 115; Ag²⁺ 94; Ag³⁺ 75 pm

Reactivity

A noble coinage metal that is stable in pure air and water even at red heat, but is tarnished by sulfur compounds and dissolved by oxidising acids; its chemistry is almost entirely that of Ag+.

with water

Does not react with water.

with oxygen, air

Does not react with oxygen even at red heat; tarnishes to black silver sulfide in air containing hydrogen sulfide or sulfur compounds, and with ozone: $\ce{4 Ag + 2 H2S + O2 -> 2 Ag2S + 2 H2O}$.

with acids

Resists hydrochloric and other non-oxidising acids; dissolves readily in dilute or concentrated nitric acid and in hot concentrated sulfuric acid: $\ce{3 Ag + 4 HNO3 -> 3 AgNO3 + NO + 2 H2O}$.

with halogens

Combines with the halogens to the silver halides AgF, AgCl, AgBr and $\ce{AgI: 2 Ag + Cl2 -> 2 AgCl}$.

Typical compounds

• AgNO₃ silver nitrate the workhorse silver salt, precursor to the rest
• AgCl silver chloride horn silver; white, light-sensitive
• AgBr silver bromide photographic film emulsion
• Ag₂S silver sulfide argentite ore and black tarnish
• Ag₂O silver oxide easily reduced; unstable above 160 C
• AgF silver fluoride the one water-soluble silver halide

Occurrence, production and use

Crustal abundance

7.5×10-2 milligrams per kilogram

Oceanic abundance

4×10-5 milligrams per liter

Occurrence and sources

Silver occurs natively and in ores such as argentite (Ag2S) and horn silver (AgCl); lead, lead-zinc, copper, gold, and copper-nickel ores are principal sources. Mexico, Canada, Peru, and the U.S. are the principal silver producers in the western hemisphere.

• by-product silver in lead-zinc, copper and gold ores in descending order of silver output; polymetallic deposits hold more than two thirds of world resources
• native silver, argentite (Ag2S), chlorargyrite (AgCl) primary silver mines (four in the United States in 2024); Mexico is the leading producer
• crustal and oceanic abundance about 0.055 ppm crust (BGS via RSC); 0.075 mg/kg crust and 0.00004 mg/L seawater (PubChem)

Extraction, production

Recovery from base-metal and gold ores and during electrolytic copper refining

silver is recovered from the ore or from the anode slimes of electrolytic copper refining; 24 US refiners produced 2,400 t of commercial-grade silver in 2024 from ores, concentrates and scrap; about 1,200 t (19 percent of US apparent consumption) came from new and old scrap; no leaching or refining reaction is stated by the sources

Cupellation (historical)

molten metal heated in a shallow cup under an air blast oxidises lead and copper and leaves silver; the Chaldean process, no equation stated

Uses

Silver and silver compounds have many uses. Pure silver is the best conductor of heat and electricity of all known metals, so it is sometimes used in making solder, electrical contacts and printed circuit boards. Silver is also the best reflector of visible light known, but silver mirrors must be given a protective coating to prevent them from tarnishing. Silver has also been used to create coins, although today other metals are typically used in its place. Sterling silver, an alloy containing 92.5% silver, is used to make silverware, jewelry and other decorative items. High capacity batteries can be made with silver and zinc and silver and cadmium. Silver nitrate (AgNO3) is light sensitive and is used to make photographic films and papers. Silver iodide (AgI) is used to seed clouds to produce rain.

Sterling silver is used for jewelry, silverware, etc. where appearance is paramount. This alloy contains 92.5% silver, the remainder being copper or some other metal. Silver is of the utmost importance in photography, about 30% of the U.S. industrial consumption going into this application. It is used for dental alloys. Silver is used in making solder and brazing alloys, electrical contacts, and high capacity silver-zinc and silver-cadmium batteries. Silver paints are used for making printed circuits. It is used in mirror production and may be deposited on glass or metals by chemical deposition, electrode position, or by evaporation. When freshly deposited, it is the best reflector of visible light known, but is rapidly tarnished and loses much of its reflectance. It is a poor reflector of ultraviolet. Silver fulminate, a powerful explosive, is sometimes formed during the silvering process. Silver iodide is used in seeding clouds to produce rain. Silver chloride has interesting optical properties as it can be made transparent; it also is a cement for glass. Silver nitrate, or lunar caustic, the most important silver compound, is used extensively in photography. Silver for centuries has been used traditionally for coinage by many countries of the world. In recent times, however, consumption of silver has greatly exceeded the output.

• Electronics and photovoltaics: contacts, printed circuits and silver paints; photovoltaic cell metallisation; batteries, bearings, brazing and solder alloys US 2024: electrical and electronics 29 percent, photovoltaics 12 percent, brazing and solder 4 percent (usgs-mcs2025-silver)
• Investment, coinage, jewellery and silverware: bars, coins and medals; sterling silver (92.5 percent) jewellery and tableware; mirrors US 2024: physical investment 30 percent, coins and medals 12 percent, jewellery and silverware 6 percent (usgs-mcs2025-silver)
• Chemicals: silver catalyst for ethylene oxide; spent catalysts reprocessed for their silver; silver nitrate and silver halides for photography and photochromic glass
• Food and beverage (biocides): silver, silver chloride, silver nitrate and silver-zeolite biocidal actives in cleaning and disinfection
• Textiles (antimicrobial finishing): silver nanoparticles in clothing to stop odour bacteria; silver threads in touchscreen gloves
• Medicine and water treatment: antimicrobial bandages and pharmaceuticals; dental amalgam (silver and mercury as biocides); water purification, wood treatment
• Mining: primary silver mines and by-product recovery from lead-zinc, copper and gold operations

Safety, toxicity

While silver itself is not considered to be toxic, most of its salts are poisonous. Exposure to silver (metal and soluble compounds, as Ag) in air should not exceed 0.01 mg/m3, (8-hour time-weighted average - 40 hour week). Silver compounds can be absorbed in the circulatory system and reduced silver deposited in the various tissues of the body. A condition, known as argyria, results with a grayish pigmentation of the skin and mucous membranes. Silver has germicidal effects and kills many lower organisms effectively without harm to higher animals.

GHS classification, signal word Danger

• H361f Suspected of damaging fertility Reproductive toxicity
• H373 May causes damage to organs through prolonged or repeated exposure Specific target organ toxicity, repeated exposure
• H400 Very toxic to aquatic life Hazardous to the aquatic environment, acute hazard
• H410 Very toxic to aquatic life with long lasting effects Hazardous to the aquatic environment, long-term hazard
• H360 May damage fertility or the unborn child Reproductive toxicity
• H317 May cause an allergic skin reaction Sensitization, Skin
• H320 Causes eye irritation Serious eye damage/eye irritation
• H370 Causes damage to organs Specific target organ toxicity, single exposure
• 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

before 5000 BC

First isolated

not in sources

Named by

not in sources

Origin of the name

a non-Indo-European wanderwort

Pure silver has a brilliant white metallic luster. It is a little harder than gold and is very ductile and malleable, being exceeded only by gold and perhaps palladium. Pure silver has the highest electrical and thermal conductivity of all metals, and possesses the lowest contact resistance. It is stable in pure air and water, but tarnishes when exposed to ozone, hydrogen sulfide, or air containing sulfur. The alloys of silver are important.