Chemistry of Photography-2.ppt

8/11/2019 Chemistry of Photography-2.ppt

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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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Chemistry of Photography-2.ppt

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