Eh-pH diagram for SILVER (Ag): Geochemistry

Ag-O-H System, Ag=1e-10, SO4 200 ppm; Acanthite Formation

October 22, 2010 (Coal Geology) Silver (Ag) occurs naturally in its pure, free form (native silver), as an alloy with gold and other metals, and in minerals such as argentite and chlorargyrite. Most silver is produced as a by-product of copper, gold, lead, and zinc refining. The Eh-pH diagram shows the two main phase of the Silver, the metal silver and silver ion (Ag+).

Eh–pH diagram is any of a class of diagrams that designates the fields of stability of mineral or chemical species in terms of the activity of hydrogen ions (pH) and the activity of electrons (Eh). All of the reactions illustrated on Eh–pH diagrams involve either proton transfer (e.g., hydrolysis) or electron transfer (oxidation or reduction) or both.  Lets check out the Eh-pH diagram for SILVER (Ag) in Ag-O-H system.

Ag-O-H System, Ag=1e-10 ppm
Figure 1: Ag-O-H System, Ag=1e-10 ppm

Looking at the Eh-pH diagram you can tell that pH has no control on Silver geochemistry. It is solely controlled by the Eh condition.

Lets see what happens if we introduce some sulfate (200 ppm) in the system.


Acanthite  + 2 O2(aq)  = 2 Ag+  + SO4–

Log K’s:
0 °C:  112.4132        150 °C:   67.2640
25 °C:  102.2965        200 °C:   57.5946
60 °C:   90.2467        250 °C:   49.1181
100 °C:   78.9067        300 °C:   41.3869

Polynomial fit:
log K = 112.4 – .4328 × T + .001199 × T^2 – 2.44e-6 × T^3 + 2.078e-9 × T^4

Equilibrium equation:
log K = 2 × log a[Ag+] + log a[SO4–] – 2 × log a[O2(aq)]

Ag-O-H System, Ag=1e-10, SO4 200 ppm; Acanthite Formation
Figure 2: Ag-O-H System, Ag=1e-10, SO4 200 ppm; Acanthite Formation

Figure 2 has an additional field for the Silver Sulfide Ag2S mineral ACANTHITE.

So, we conclude that the native silver form is the most stable under near earth conditions.

Questions, comments? If you have different geochemical condition to evaluate, contact me at for further help.

Silver  is a precious metal with atomic number 47.

Facts you should know about Silver:
1. Silver has the highest electrical conductivity of any element
2. Silver has the highest thermal conductivity of any metal.
3. Naturally occurring silver is composed of two stable isotopes, 107Ag and 109Ag, with 107Ag being the most abundant (51.839% natural abundance).
4. Silver’s atomic weight is 107.8682(2) g/mol
5. Silver has Twenty-eight radioisotopes

Silver is found in native form, as an alloy with gold, and in ores containing sulfur, arsenic, antimony or chlorine. Ores include argentite (Ag2S), chlorargyrite (AgCl) which includes horn silver , and pyrargyrite (Ag3SbS3). The principal sources of silver are the ores of copper, copper-nickel, lead, and lead-zinc obtained from Peru, Mexico, China, Australia, Chile, Poland and Serbia. Peru and Mexico have been mining silver since 1546 and are still major world producers.

Top silver-producing mines are Proaño / Fresnillo (Mexico), Cannington (Queensland, Australia), Dukat (Russia), Uchucchacua (Peru) and Greens Creek mine (Alaska).

Silver is used to make ornaments, jewelry, high-value tableware, utensils

Common Chemistry for Silver (For more visit Silver Wiki)
Silver metal dissolves readily in nitric acid (HNO3) to produce silver nitrate (AgNO3), a transparent crystalline solid that is photosensitive and readily soluble in water.

Silver nitrate is used as the starting point for the synthesis of many other silver compounds, as an antiseptic, and as a yellow stain for glass in stained glass.

Silver metal does not react with sulfuric acid, which is used in jewelry-making to clean and remove copper oxide firescale from silver articles after silver soldering or annealing

However, silver reacts readily with sulfur or hydrogen sulfide H2S to produce silver sulfide, a dark-colored compound familiar as the tarnish on silver coins and other objects. Silver sulfide also forms silver whiskers when silver electrical contacts are used in an atmosphere rich in hydrogen sulfide.

4 Ag + O2 + 2 H2S ? 2 Ag2S + 2 H2O

Silver chloride (AgCl)
is precipitated from solutions of silver nitrate in the presence of chloride ions, and the other silver halides used in the manufacture of photographic emulsions are made in the same way using bromide or iodide salts. Silver chloride is used in glass electrodes for pH testing and potentiometric measurement, and as a transparent cement for glass. Silver iodide has been used in attempts to seed clouds to produce rain.[6] Silver halides are highly insoluble in aqueous solutions and are used in gravimetric analytical methods.

Silver oxide (Ag2O) can be produced when silver nitrate solutions are treated with a base; it is used as a positive electrode (anode) in watch batteries. Silver carbonate (Ag2CO3) is precipitated when silver nitrate is treated with sodium carbonate (Na2CO3).

2 AgNO3 + 2 OH- ? Ag2O + H2O + 2 NO3-
2 AgNO3 + Na2CO3 ? Ag2CO3 + 2 NaNO3

Silver fulminate (AgONC), a powerful, touch-sensitive explosive used in percussion caps, is made by reaction of silver metal with nitric acid in the presence of ethanol (C2H5OH). Another dangerously explosive silver compound is silver azide (AgN3), formed by reaction of silver nitrate with sodium azide (NaN3).

Silver metal is attacked by strong oxidizers such as potassium permanganate (KMnO4) and potassium dichromate (K2Cr2O7), and in the presence of potassium bromide (KBr), these compounds are used in photography to bleach silver images, converting them to silver halides that can either be fixed with thiosulfate or re-developed to intensify the original image. Silver forms cyanide complexes (silver cyanide) that are soluble in water in the presence of an excess of cyanide ions. Silver cyanide solutions are used in electroplating of silver.

Leave a Comment

Your email address will not be published. Required fields are marked *