The Chemistry of The Chemistry of PhotographyPhotography

Peter JantzPeter Jantz

Chemistry 496Chemistry 496

February 27, 2004February 27, 2004

1824, Joseph-Nicephore Niepce

• First Permanent Image

• “Heliogravure”• Asphalt on a Pewter

Plate • Exposure took 5 days

1839, Louis Daguerre

• “Daguerreotypes” • Sensitized Silver-

plated Copper Sheet• Mercury-vapor

developer

1841, William Henry Fox Talbot

• “Calotypes”

• Salt Solution with Silver nitrate

• Quicker Development/Lower Image Resolution

•1881, George Eastman•Gelatin Dispersion of Silver-HalideHalide

•1887, Hannibal Goodwin •Plastic-backed “modern film”

AgI >> AgBr >> AgClAgI >> AgBr >> AgCl

• Silver halides form imperfect crystals in the DispersionSilver halides form imperfect crystals in the Dispersion

• A photon of light may hit the crystal, exciting the A photon of light may hit the crystal, exciting the electron on the negatively charged halideelectron on the negatively charged halide

• The electron is transferred into the conduction band of The electron is transferred into the conduction band of the silver halide microcrystal at the sensitivity speckthe silver halide microcrystal at the sensitivity speck

0

)(

AgeAg

eBrAghcrystalBrAg

Chemical engineer’s resource page Chemical engineer’s resource page www.cheresources.com/photochemwww.cheresources.com/photochem

• The emulsion layer is ~12% silver halide

• Grain size affects contrast and light sensitivity 0.1 ~ 3 microns

• Anti-halation backing prevents reflections

• More photons = more silver

• A reducing agent (Hydroquinone)

•Sodium hydroxide and sodium carbonate added as accelerators

• Potassium bromide added as restrainer

•Reaction must be timed

•Developers work at high pH

•A weak acid (acetic acid) is used to halt the reaction

• Removes undeveloped silver halide crystals Removes undeveloped silver halide crystals

• Sodium thiosulfate makes water soluble Sodium thiosulfate makes water soluble silver thiosulfatesilver thiosulfate

•Photo paper is rinsed and driedPhoto paper is rinsed and dried

• Silver halides only sensitive up to 525 nm

• Coupling to dyes expands sensitivity range

• Lower resolution than Black and White film

• Latent images are black and white

• paraphenylendiamine

• Silver halide crystals are reduced by developer

• Oxidized developer reacts with the dye couplers, activating the dye color

• Printing paper has dye-coupled layers

[1] [1] Ian R. Gould et al. 2000. Ian R. Gould et al. 2000. Two-Electron Sensitization: A New Concept for Two-Electron Sensitization: A New Concept for Silver Halide Photography.Silver Halide Photography. J. Am. Chem. Soc., 122 (48), pp 11934 -11943. J. Am. Chem. Soc., 122 (48), pp 11934 -11943.

[2] Tao Deng et al. 2000. [2] Tao Deng et al. 2000. Fabrication of Metallic Microstructures Using Exposed, Fabrication of Metallic Microstructures Using Exposed, Developed Silver Halide-Based Photographic Film. Developed Silver Halide-Based Photographic Film. Anal. Chem., 72 (4), pp Anal. Chem., 72 (4), pp 645 -651.645 -651.

[3] Current, Ira. [3] Current, Ira. Photographic Color Printing: Theory and Technique. Photographic Color Printing: Theory and Technique. Copyright Copyright 1987, Butterworth Publishers.1987, Butterworth Publishers.

[4] C. Swedlund. [4] C. Swedlund. Photography.Photography. Copyright 1981. Holt, Rinehart, and Winston. Copyright 1981. Holt, Rinehart, and Winston.

[5] The Chemical Engineer’s Resource Page. [5] The Chemical Engineer’s Resource Page. http://www.cheresources.com/photochem.shtml retrieved 2-19-2004. retrieved 2-19-2004.

[6] T. Naugler. University of Nova Scotia [6] T. Naugler. University of Nova Scotia http://www.nsac.ns.ca/envsci/staff/jhoyle/students/tnaugler2/Index.html#bib retrieved 2-19-2004retrieved 2-19-2004