- Structure of silver bromide
- Crystal defects
- Synthesis
- Properties
- Appearance
- Molecular mass
- Density
- Melting point
- Boiling point
- Water solubility
- Refractive index
- Heat capacity
- Sensitivity to light
- Applications
- References
The silver bromide is an inorganic salt having the chemical formula AgBr. Its solid is composed of Ag + cations and Br - anions in a 1: 1 ratio, attracted by electrostatic forces or by ionic bonds. It can be seen as if metallic silver had given up one of its valence electrons to molecular bromine.
Its nature resembles its "siblings" silver chloride and iodide. All three salts are insoluble in water, have similar colors, and are also sensitive to light; that is, they undergo photochemical reactions. This property has been used in obtaining photographs, as a result of the reduction of Ag + ions to metallic silver.
Silver bromide ions. Source: Claudio Pistilli
The image above shows an Ag + Br - ion pair, in which the white and brown spheres correspond to the Ag + and Br - ions, respectively. Here they represent the ionic bond as Ag-Br, but it is necessary to indicate that there is no such covalent bond between both ions.
It may seem contradictory that silver is the one that contributes the black color to photographs without color. This is because the AgBr reacts with light, generating a latent image; which, then, is intensified by increasing the reduction of silver.
Structure of silver bromide
Crystal structure of silver bromide. Source: Benjah-bmm27 via Wikipedia.
Above is the lattice or crystal structure of silver bromide. A more faithful representation of the difference in size between the ionic radii of Ag + and Br - is shown here. The Br - anions, more voluminous, leave interstices where the Ag + cations are located, which is surrounded by six Br - (and vice versa).
This structure is characteristic of a cubic crystalline system, specifically of the rock salt type; the same, for example, as for sodium chloride, NaCl. In fact, the image facilitates this by providing a perfect cubic boundary.
At first glance it can be seen that there is some difference in size between the ions. This, and perhaps the electronic characteristics of Ag + (and the possible effect of some impurities), leads to the AgBr crystals showing defects; that is, places where the ordering sequence of ions in space is "broken".
Crystal defects
These defects consist of voids left by absent or displaced ions. For example, among six Br - anions there should normally be the Ag + cation; but instead, there may be a gap because the silver has moved into another gap (Frenkel defect).
Although they affect the crystal lattice, they favor the reactions of silver with light; and the larger the crystals or their cluster (size of grains), the greater the number of defects, and therefore, it will be more sensitive to light. Also, impurities influence the structure and this property, especially those that can be reduced with electrons.
As a consequence of the latter, large AgBr crystals require less exposure to light to reduce them; that is, they are more desirable for photographic purposes.
Synthesis
In the laboratory, silver bromide can be synthesized by mixing an aqueous solution of silver nitrate, AgNO 3, with the sodium bromide salt, NaBr. The first salt contributes the silver, and the second the bromide. What follows is a double displacement or metathesis reaction that can be represented by the chemical equation below:
AgNO 3 (aq) + NaBr (s) => NaNO 3 (aq) + AgBr (s)
Note that the sodium nitrate salt, NaNO 3, is soluble in water, while AgBr precipitates as a solid with a faint yellow color. Subsequently the solid is washed and subjected to vacuum drying. In addition to NaBr, KBr could also be used as a source of bromide anions.
On the other hand, AgBr can naturally be obtained through its bromirite mineral and its due purification processes.
Properties
Appearance
A whitish yellow clay-like solid.
Molecular mass
187.77 g / mol.
Density
6.473 g / mL.
Melting point
432 ° C.
Boiling point
1502 ° C.
Water solubility
0.140 g / mL at 20 ° C.
Refractive index
2,253.
Heat capacity
270 J / Kg · K.
Sensitivity to light
It was said in the previous section that there are defects in AgBr crystals that promote the sensitivity of this salt to light, since they trap the electrons formed; and thus, in theory, they are prevented from reacting with other species in the environment, such as oxygen in the air.
The electron is released from the reaction of Br - with a photon:
Br - + hv => 1 / 2Br 2 + e -
Note that Br 2 is produced, which will stain the solid red if not removed. The released electrons reduce the Ag + cations, in their interstices, to metallic silver (sometimes represented as Ag 0):
Ag + + e - => Ag
Having then the net equation:
AgBr => Ag + 1 / 2Br 2
When the "first layers" of metallic silver form on the surface, it is said that there is a latent image, still invisible to the human eye. This image becomes millions of times more visible if another chemical species (such as hydroquinone and phenidone, in the development process) increases the reduction of the AgBr crystals to metallic silver.
Applications
Black and white photograph of pocket watch. Source: Pexels.
Silver bromide is the most widely used of all its halides in the field of photographic film development. AgBr is applied on said films, made with cellulose acetate, suspended in a gelatin (photographic emulsion), and in the presence of 4- (methylamino) phenol sulfate (Metol) or phenidone, and hydroquinone.
With all these reagents, the latent image can be brought to life; finish and accelerate the transformation of ionic to metallic silver. But, if you do not proceed with certain care and experience, all the silver on the surface will oxidize, ending the contrast between the black and white colors.
That is why the stop, fixation and wash dipping steps of photographic film are vital.
There are artists who play with these processes in such a way that they create shades of gray, which enrich the beauty of the image and their own legacy; And they do all this, sometimes perhaps without suspecting it, thanks to chemical reactions, whose theoretical basis can become a bit complex, and to a light-sensitive AgBr that marks a starting point.
References
- Wikipedia. (2019). Silver bromide. Recovered from: en.wikipedia.org
- Michael W. Davidson. (2015, November 13). Polarized Light Digital Image Gallery: Silver Bromide. Olympus. Recovered from: micro.magnet.fsu.edu
- Crystran Ltd. (2012). Silver bromide (AgBr). Recovered from: crystran.co.uk
- Lothar Duenkel, Juergen Eichler, Gerhard Ackermann, and Claudia Schneeweiss. (June 29, 2004). Self-made silver-bromide-based emulsions for users in holography: manufacturing, processing, and application, Proc. SPIE 5290, Practical Holography XVIII: Materials and Applications; doi: 10.1117 / 12.525035;
- Alan G. Shape. (1993). Inorganic chemistry. (Second edition.). Editorial Reverté.
- Carlos Güido and Ma Eugenia Bautista. (2018). Introduction to photographic chemistry. Recovered from: fotografia.ceduc.com.mx
- García D. Bello. (January 9, 2014). Chemistry, photography and Chema Madoz. Recovered from: dimethylsulfuro.es