SILVER NITRATE (AGNO3): STRUCTURE, PROPERTIES, USES, TOXICITY - CHEMISTRY - 2025

The silver nitrate is an inorganic salt having the chemical formula AgNO ₃. Of all the silver salts, it is the most economical and the one that has a relative stability against sunlight, so it tends less to decompose. It is the soluble and preferred source of silver in any teaching or research laboratory.

In teaching, aqueous solutions of silver nitrate are used to teach silver chloride precipitation reactions. Likewise, these solutions are put in contact with metallic copper so that a redox reaction takes place, in which metallic silver precipitates in the middle of the solution formed of copper nitrate, Cu (NO ₃) ₂.

Silver nitrate sample container. Source: W. Oelen / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)

The top image shows a bottle with silver nitrate. It can be kept exposed to light without an early darkening of its crystals, due to the appearance of silver oxide.

As a result of alchemical customs, and the antibacterial properties of metallic silver, silver nitrate has been used to disinfect and cauterize wounds. However, for this purpose very dilute aqueous solutions are used, or their solid mixed with potassium nitrate applied through the tip of some wooden rods.

Structure of silver nitrate

Ions that make up silver nitrate crystals. Source: CCoil / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)

The image above shows the Ag ⁺ and NO ₃ ^- ions of silver nitrate, which are represented by a model of spheres and bars. The formula AgNO ₃ indicates the stoichiometric proportion of this salt: for each Ag ⁺ cation there is a NO ₃ anion ^- interacting with it electrostatically.

The anion NO ₃ ^- (with red and bluish spheres) has a trigonal plane geometry, with the negative charge delocalizing between its three oxygen atoms. Therefore, the electrostatic interactions between both ions take place specifically between the Ag ⁺ cation and an oxygen atom of the NO ₃ ^- anion (Ag ⁺ -ONO ₂ ^-).

In this way, each Ag ⁺ ends up coordinating or surrounding itself with three adjacent NO ₃ ^- in the same plane or crystallographic layer. The grouping of these planes ends up defining a crystal whose structure is orthorhombic.

Preparation

Silver nitrate is prepared by etching a piece of burnished metallic silver with nitric acid, either diluted cold, or concentrated hot:

3 Ag + 4 HNO ₃ (diluted) → 3 AgNO ₃ + 2 H ₂ O + NO

Ag + 2 HNO ₃ (concentrated) → AgNO ₃ + H ₂ O + NO ₂

Note the formation of the gases NO and NO ₂, which are toxic, and force this reaction not to take place outside of an extractor hood.

Physical and chemical properties

Physical appearance

Colorless crystalline solid, odorless, but with a very bitter taste.

Molar mass

169.872 g / mol

Melting point

209.7 ºC

Boiling point

440 ° C. However, at this temperature it undergoes thermal decomposition, in which metallic silver is produced:

2 AgNO ₃ (l) → 2 Ag (s) + O ₂ (g) + 2 NO ₂ (g)

There are therefore no AgNO ₃ vapors, at least not under ground conditions.

Solubility

AgNO ₃ is an incredibly soluble salt in water, having a solubility of 256 g / 100 mL at 25 ºC. It is also soluble in other polar solvents such as ammonia, acetic acid, acetone, ether, and glycerol.

Density

4.35 g / cm ³ at 24 ºC (room temperature)

3.97 g / cm ³ at 210 ° C (just at the melting point)

Stability

AgNO ₃ is a stable substance as long as it is properly stored. It will not ignite at any temperature, although it can decompose releasing toxic fumes of nitrogen oxides.

On the other hand, although silver nitrate is not flammable, it is a powerful oxidizing agent that when in contact with organic matter and a heat source is capable of triggering an exothermic and explosive reaction.

In addition to this, this salt should not be exposed to sunlight for too long, as its crystals darken due to the formation of silver oxide.

Silver nitrate uses

Precipitating and analytical agent

In the previous section, mention was made of the incredible solubility of AgNO ₃ in water. This implies that the Ag ⁺ ions will dissolve without any problem and will be available to interact with any ion in the aqueous medium, such as the halide anions (X = F ^-, Cl ^-, Br ^- and I ^-).

Silver as Ag ⁺, and after the addition of dilute HNO ₃, precipitates the fluorides, chlorides, bromides and iodides present, which consist of whitish or yellowish solids:

Ag ⁺ (aq) + X ^- (aq) → AgX (s)

This technique is very recurrent for obtaining halides, and is also used in numerous quantitative analytical methods.

Tollens reagent

AgNO ₃ also plays an analytical role in organic chemistry, as it is the main reagent, along with ammonia, for the preparation of the Tollens reagent. This reagent is used in qualitative tests to determine the presence of aldehydes and ketones in a test sample.

Synthesis

AgNO ₃ is an excellent source of soluble silver ions. This, in addition to its relative low cost, makes it a requested reagent for countless organic and inorganic syntheses.

Whatever the reaction, if you need Ag ⁺ ions, then chemists are quite likely to turn to AgNO ₃.

Medicinal

AgNO ₃ became popular in medicine before the advent of modern antibiotics. Today, however, it is still used for specific cases, as it has cauterizing and antibacterial properties.

It is usually mixed with KNO ₃ on the tip of some wooden sticks, so it is reserved exclusively for topical uses. In this sense, it has served to treat warts, wounds, infected nails, mouth ulcers and nosebleeds. The AgNO ₃ -KNO ₃ mixture cauterizes the skin, destroying damaged tissue and bacteria.

The bactericidal action of AgNO ₃ has also been used in water purification.

Toxicity and side effects

Silver nitrate can cause burns that are visible by its purple or dark spots. Source: Jane of baden at English Wikipedia / Public domain

Although silver nitrate is a stable salt and does not represent too many risks, it is a highly caustic solid, the ingestion of which can cause severe gastrointestinal damage.

That is why its handling with gloves is recommended. It can burn the skin, and in some cases, darken it to purple, a condition or disease known as argyria.

References

1. Shiver & Atkins. (2008). Inorganic chemistry. (Fourth edition). Mc Graw Hill.
2. Wikipedia. (2020). Silver nitrate. Recovered from: en.wikipedia.org
3. National Center for Biotechnology Information. (2020). Silver nitrate. PubChem Database., CID = 24470. Recovered from: pubchem.ncbi.nlm.nih.gov
4. Elsevier BV (2020). Silver Nitrate. ScienceDirect. Recovered from: sciencedirect.com
5. University of Iowa. (2020). Silver nitrate use and toxicity. Recovered from: medicine.uiowa.edu
6. PF Lindley & P. ​​Woodward. (1966). An X-ray investigation of silver nitrate: a unique metal nitrate structure. Journal of the Chemical Society A: Inorganic, Physical, Theoretical.
7. Lucy Bell Young. (2020). What are the Medical Uses of Silver Nitrate. ReAgent Chemicals. Recovered from: chemicals.co.uk