Chemistry of Film Photography?
I'm doing a project for Chemistry on photography. I understand that silver halide crystals in the emulsion layer of the film are sensitive to light, but I don't understand what happens after that? What happens to the grains after the photons hit it? Also, what does this formula mean:
Ag+Br- (crystal) + hv (radiation) --> Ag+ + Br+ e-
This is not the forum for your question. I suggest you ask a moderator to move it so it can get the proper attention.
Your equation is incomplete and only shows part of the reaction. See Mees and James or Haist. These books can be found at most libraries. They show the whole thing.
Ag+ Br- + hv --> Ag+ + Br + e- Please forgive my notation because my keyboard does not let me type superscripts. Your formula is simple to understand although it might be subject to some argument. Describing it in the most basic terms then:
Ag+ of course represents a silver atom which in a silver halide crystal has a charge of plus one. Therefore the notation shows a single plus sign to indicate that it is a positive ion, also called a cation. Since it is positively charged it is attracted to negatively charged ions such as Br-; this is because opposites attract. Br represents the element Bromine which is a brown gas that can form a crystal with metals. Bromine belongs in the column of the periodic table called halides which includes another gas, Chlorine, and a liquid with a high vapor pressure, Iodine. These halides can all form crystals with silver, Ag. Halides will typically absorb only one electron and therefore they often are written with a single negative sign, as in your formula.
The most common silver halide is AgBr, although films will contain lesser amounts of the other halide crystals, AgCl and AgI. All silver halides are light sensitive and no silver halide crystals are soluble in water. To make a silver halide crystal simply mix an aqueous solution of silver ions, (typically a silver nitrate solution) with a aqueous solution of halide ions, (typically a potassium bromide solution), and the silver halide crystals will precipitate out since they are not soluble in water. This must be done in the dark since the silver halide crystals are light sensitive; and generally they add gelatin to the solution as a kind of glue to bind the whole emulsion together.
The crystals can be formed large or small and with other chemicals to make them more sensitive, but basically they are nothing more than little grains of salt which are basically white in color. The term hv in your formula is the universal expression for light energy. It is used in formulas for photography because light energy is a factor which effects a chemical change in the silver halide crystal.
To answer your question: when a photon of light energy, hv, hits a silver halide molecule the photon knocks an electron, e-, into the silver cation, Ag+, changing the silver cation into pure silver metal, Ag superscript 0. Silver metal is gray in color. So if you place a piece of unexposed film in sunlight for about a minute you will notice that it begins to turn dark gray. The silver cations are being transformed into pure silver metal. If the electrons have come from Br- then the Br- (bromide) will be transformed to Br superscript 0 (bromine) which is a gas and will float off. Bromine is diatomic so it takes two Br- anions to form one gas molecule. The formula you show has only part of the reaction: the electron, e-, has been knocked off the Br- but it has not yet been attracted to Ag+ to form pure Ag metal.
Silver halide crystals are unique among all substances in that if only a few cations have been changed into pure metal, the ENTIRE crystal consisting of millions of atoms can be developed into pure silver. This effect is known as amplification and is the reason why silver halide photography allows exposures of less than a second.
If you have any lasting interest in the chemistry of photography you will naturally be drawn to consult Mees and James or Haist, which are classics in the field. IMHO PE is one of the great experts in the subject.
How do you know which forum is which? The wrong forum. Hmm. All I see is "New Posts". And everything there is from all the forums lumped together. Maybe that's why I was kicked off once. My posts must have been showing up all over the place, out of place, and making me look like a nut.
As far as the Original Poster, I have learned one thing well enough, and that is that PE just wrote the best answer you'll ever found.
An easy to read book is "The Chemistry of Photography" by Roger Bunting.
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Well, the process is more complicated and somewhat different than that explained above, so I still suggest a textbook. Haist "Modern Photographic Processes" is 2 volumes of excellent explanation from the POV of a Kodak researcher of many years and a chemist.
Just to elaborate, it normally takes 3 photons and 3 electrons to form a stable latent image that is developable. Recently, Kodak has described 2 electron sensitization.
Also, only Cl, Br and I are used. The two other halides, Fluorine and Astatine are unsuitable due to solubility and radioactivity respectively.
So, there is more to the story, and more than can be posted here unless one wants to write a book. So, pick up a good textbook.
Assuming you are working from a computer, rather than something like an internet enabled telephone ...
Originally Posted by Tom1956
When you look at the top left of your screen, under the APUG banner, you will see a toolbar with a bunch of links on it. The left hand one says Forum. Click on it and you will see a list of forums. The "Feedback and Discussion" forum is actually just for dealing with issues concerning how the website works.
“Photography is a complex and fluid medium, and its many factors are not applied in simple sequence. Rather, the process may be likened to the art of the juggler in keeping many balls in the air at one time!”
Ansel Adams, from the introduction to The Negative - The New Ansel Adams Photography Series / Book 2
Originally Posted by tjc45
here is a website that might help you understand what goes on.
good luck !