Processing of latent image
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PROCESSING OF LATENT IMAGE. by: Prof. Susan A. Olavidez. Last Revised: June 2004. THE CONCEPT OF pH. pH is a quantitative method of measuring the degree of acidity or alkalinity of a solution.

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Processing of latent image

PROCESSING OF LATENT IMAGE

by:

Prof. Susan A. Olavidez

Last Revised: June 2004


The concept of ph
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.


The concept of ph1
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.


Developer
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.


Developer constituents
Developer Constituents

  • DEVELOPING AGENT

    • Agent

      • phenidone

      • hydroquinone

    • Other Agent Used

      • methol (instead of hydroquinone)

    • Function

      • Converts exposed silver halide crystal to black metallic silver.


Developer constituents1
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


Methods of reducing aerial oxidation
Methods of Reducing Aerial Oxidation

  • High preservative level in the developer.

  • Floating lid in the developer replenisher tank.

  • Closely fitting rollers.

  • Deep narrow tanks.


Developer Constituents

  • ACCELERATOR (activator)

    • Agent

      • sodium hydroxide

      • potassium hydroxide

    • Other Agent Used

      • sodium carbonate

    • Function

      • Controls developer activity by ensuring correct pH value


Developer constituents2
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.


Developer constituents3
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


Developer constituents4
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.


Developer constituents5
Developer Constituents

  • SOLVENT

    • Agent

      • water

    • Function

      • Acts as a solvent for all chemicals and by-products of developer action.


Developer constituents6
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.


Developer constituents7
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.


Developer replenisher
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.


Developer replenisher1
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


Fixer
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.


Fixer constituents
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.


Fixer constituents1
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.


Fixer constituents2
Fixer Constituents

  • BUFFER

    • Agent

      • Acetic acid + preservative

    • Function

      • Maintain the pH of the fixing solution within fine tolerances.


Fixer constituents3
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


Fixer constituents4
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.


Fixer constituents5
Fixer Constituents

  • SOLVENT

    • Agent

      • water

    • Function

      • Acts as a solvent for fixer chemicals and by-products of fixation.


Fixer replenisher
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


Washing
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.


Archival permanence
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.


Silver recovery
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.



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