PROCESSING OF LATENT IMAGE

1. PROCESSING OF LATENT IMAGE by: Prof. Susan A. Olavidez Last Revised: June 2004

2. THE CONCEPT OF pH • pH is a quantitative method of measuring the degree of acidity or alkalinity of a solution. • Pure water is chosen as the basis of the pH scale. H2O is slightly ionized into H+ ions and OH- ions. • H+ ion content is the acid portion of a substance in solution. OH- ion content is the alkaline portion. • Water is neither + nor -, therefore these ions are present in equal concentrations. • pH has a value of 0-14. With 7 being neutral, value below 7 are acid and values above 7 are alkali.

3. THE CONCEPT OF pH • Developers • Most developers for automatic processing have a pH range of 9.6-10.6. • To work to its optimum efficiency pH should be maintained within +0.2 of the manufacturer’s recommended value. • Fixers • Fixers for automatic processing have a pH range of 4.2-4.9. • It should also be maintained at recommended value.

4. DEVELOPER • Developer chemicals has 2 functions: • Conversion- must be able to convert exposed crystal to black metallic silver. • Selectivity - must differentiate between exposed silver halide from unexposed silver halide, changing only the exposed crystal to metallic silver.

5. Developer Constituents • DEVELOPING AGENT • Agent • phenidone • hydroquinone • Other Agent Used • methol (instead of hydroquinone) • Function • Converts exposed silver halide crystal to black metallic silver.

6. Developer Constituents • PRESERVATIVE • Agent • potassium metabisulphite • Other Agent Used • sodium sulphite • Function • reduce aerial oxidation to minimum developing agents + oxygen = oxidized dev. agents + OH- ions

7. Methods of Reducing Aerial Oxidation • High preservative level in the developer. • Floating lid in the developer replenisher tank. • Closely fitting rollers. • Deep narrow tanks.

8. Developer Constituents • ACCELERATOR (activator) • Agent • sodium hydroxide • potassium hydroxide • Other Agent Used • sodium carbonate • Function • Controls developer activity by ensuring correct pH value

9. Developer Constituents • RESTRAINER (anti-foggant) • Agent • potassium bromide (inorganic) • benzotriazole (organic) • Function • Improves selectivity of developer, ensuring low fog and high image contrast. • Its action is to increase the effective bromine barrier that exist around silver bromide crystal.

10. Developer Constituents • BUFFER • Buffer is a solution in which the hydrogen concentration is practically unchanged by dilution, and which resists change in pH on the addition of acid or alkali. • Agent • boric acid + sodium hydroxide • Function • Buffer helps maintain pH and therefore activity by absorbing harmful by-products of development action (products of exhaustion). Development process releases bromine ions from silver bromide complexes and hydrogen ions from developing agent bromine ions + hydrogen ions = hydrobromic acid

11. Developer Constituents • SEQUESTERING AGENT • Agent • EDTA Sodium salt (Sodium salt of ethylene diamine tetra-acetic acid) • Other Agent • Calgon (sodium hexametaphosphate) • Function • Soften hard water supplies, thus preventing precipitation of the calcium and magnesium salts onto the surface of the film.

12. Developer Constituents • SOLVENT • Agent • water • Function • Acts as a solvent for all chemicals and by-products of developer action.

13. Developer Constituents • OTHER ADDITIONS • Hardening Agent • glutaraldehyde • Reduces emulsion swelling and softening. • Wetting Agent • detergent-based derivatives • Stimulate uniform development be reducing the surface tension between the developing solution and film emulsion. • Anti-frothant (anti-foaming) • Reduce foaming mainly dur to the presence of wetting agent. • Fungicide • Prevents the growth of fungi.

14. Developer Constituents • STARTER SOLUTION • Added to machine tank developer, when starting the processor from dry. • Agent • potassium bromide + acetic acid • Function • Depresses the pH of the developer, therefore reducing the activity of the solution.

15. Developer Replenisher • REPLENISHER replaces the exhausted developer and maintains the concentration of other active components at the correct level. • Average replenishment is 40-60 ml for every 35 cm film.

16. Developer Replenisher • FACTORS AFFECTING DEVELOPER REPLENISHMENT • area of film processed • average density of the film • silver content of the film • maximum density required • thickness of emulsion • single or duplitized coating • amount of aerial oxidation

17. FIXER • Fixer chemicals has 2 functions: • Conversion - must be able to convert unexposed, undeveloped silver halide crystals to soluble silver complexes. • Selectivity - must have no effect on either the metallic silver in the developed image or the gelatin in which it is suspended.

18. Fixer Constituents • FIXING AGENT • Agent • ammonium thiosulphate (10-13 sec. Fixing time) • sodium thiosulphate(8-10 minutes fixing time) • Function • Converts unexposed, undeveloped silver bromide into water soluble silver complexes. • This renders the image permanent. • Also produces the ‘transparent appearance’ of the film as the areas of the film that receive no exposure.

19. Fixer Constituents • ACID • Agent • Acetic acid (with aluminum chloride as hardener) • Sulphuric acid (with aluminum sulphate as hardener) • Function • Stop development by neutralizing the alkali developer. • Provide the correct pH level for the hardening agent.

20. Fixer Constituents • BUFFER • Agent • Acetic acid + preservative • Function • Maintain the pH of the fixing solution within fine tolerances.

21. Fixer Constituents • PRESERVATIVE • Agent • sodium sulphite • potassium sulphite • Function • Prevent or reduce to minimum the breakdown of fixing agent into sulphur particles. Thiosulphate + acid = sulphurization + sulphite ions = soluble complex

22. Fixer Constituents • HARDENER • Agent • aluminum chloride • aluminum sulphate • Function • Control the swelling and softening of the emulsion. • Aluminum ions causes cross-linking of gelatin polymer within the emulsion.

23. Fixer Constituents • SOLVENT • Agent • water • Function • Acts as a solvent for fixer chemicals and by-products of fixation.

24. Fixer Replenisher • Average replenishment is 80-100 ml for every 35 cm film. • FACTORS AFFECTING FIXER REPLENISHMENT • Area of film processed • average density of the film • silver content of the film • thickness of emulsion • single or duplitized coating • type of fixing agent • silver level required • pH level required

25. WASHING • Remove from the film emulsion, fixer that has not been used in the conversion of unexposed, undeveloped silver bromide into soluble complexes. • Remove the remainder of the soluble silver complex salts that have not been removed by the water present in the fixing solution.

26. ARCHIVAL PERMANENCE • ARCHIVAL PERMANENCE is defined as the length of time a film will store without significant deterioration in its image quality. • SALTS RESPONSIBLE FOR IMAGE DEGRADATION • Silver complex salts that remain in the film due to inadequate fixing. • Soluble complex salts that has not been remove from the surface of the film. • Remaining thiosulphate in the emulsion.

27. SILVER RECOVERY • Metal ion exchange • Silver salt solution is brought into contact with a base metal, the base metal is replaced by the silver and the base metal ions are released into solution. • Electrolytic method • uses two electrodes. Silver ions are attracted to cathode where their charge is neutralized and they plate out as silver metal. • Precipitation • Sodium sulphide (20%) is added to waste fixer, a silver sulphide sludge is produced at the bottom of the container. This sludge requires collection, drying, and refining.

28. THANK YOU