Post on 02-Jun-2018
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Chemistry of Photography
1) The light sensitive emulsion
2) The latent image
3) Developing the image
4) Fixing the image
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The emulsion AgNO3+ KBr = AgBr + KNO3 in gelatin
AgBr precipitates (WHY??) and remain in the gelatin to form minute grains.
AgBr is light sensitive, forming a latent image that can be developed
But how?
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The sensitivity of the grains are proportional to their sizes. If all the grains
were the same size, there would be no shades of grey at all! Typical
densities of grains are about 5 x 108grains per cm2. If you consider a grain
to be equivalent to a pixel, you see that photographic film (taken by itself)
it quite a bit more capable of resolving detail than our current digital
cameras.
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The latent image
For many years, it was thought that 2AgBr + light = Ag2Br + Br
(the sub-haloid hypothesis). But there was never evidence of a chemical
change. Less than 5 silver atoms are involved at any site!!
X-ray spectroscopy finally showed that silver is liberated
Br - + lightBr + e
The electron then migrates to a shallow trap (called a sensitivity site).
Ag + + e -Ag
Species produced include: Ag2+, Ag2
o, Ag3+, Ag3
o, Ag4+, Ag4
o
Why doesnt it go the other way? i.e. why is it stable?
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The Mechanism of Development
The photographic process depends upon the fact that the reaction:
Ag ++ e Ag(i.e. the reduction of silver ion to metallic silver by a developing
solution), proceeds much more easily for an exposed silver halide
grain than for an unexposed grain. The gain can be ~109.
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Development- Continued
The reduction potential of the developer must be such that it will develop those
exposed silver halide grains, but not large enough to develop them all. (Afogging developer)
What actually happens?
C6H4(OH)2 + Na2SO3 + 2AgBr +NaOHC6H3(OH)2SO3Na +2NaBr+H2O +2Ag
Hydroquinone sodium sulphite silver bromide sodium hydroxide hydroquinone sulphonate sodium bromide water SILVER!
| |
stabilizer ya gotta do something for the bromine! (plus it adjusts the pH)
Chemical velocity: T= 1oC vchem= 10%.
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Hurter and DriffieldD vs. log E
Reflectivity amount of light reflected / amount of light incident = R
Transmissivity 1 R = T
T = (1amount reflected) / amount incident
= (amount incidentamount reflected) / amount incident
= amount transmitted / amount incident
Opacity 1/ T
Density log Opacity = D
Exposure = amount of energy incident on each square centimeter of film
Question: Why do we use logarithms?
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Fixing the image
The biggest problem after the invention of photography in the 1830s
was the lack of permanency. You have to get rid of that remaining
bromide, or eventually the photograph will go black. There are no true
solventsof AgBr. When sugar is dissolved in water, and then
evaporated, the sugar is recovered. This never happens with AgBr.
The residue left behind is always a transformed salt. So what we
need to do is make sure the transformed salt is soluble, so it can bewashed away.
AgBr + Na2S2O3 = AgNaS2O3+ NaBr (only slightly soluble)
But if we have a more liberal solution of sodium thiosulphate:
2AgBr + 3 Na2S2O3= Ag2Na4(S2O3)3+ 2 NaBr (bingo!)
Does anything else work? KCN. We wont go there.
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