Making a photographic print using silver halides

The light sensitive silver halides, silver chloride, silver bromide and silver iodide, are used to make photographic film and photographic paper. In this experiment, students produce photographic paper coated in silver chloride by bringing solutions of silver nitrate and potassium chloride solutions into contact on the paper in the absence of light. They can then obtain a photographic image of an object placed on it when the paper is dried and exposed to strong light.

Depending on the availability of a darkened room, this experiment can be done either as a demonstration or a class experiment. It may even be possible to do it successfully in an open laboratory if the paper can be protected from strong light once the silver nitrate solution has been applied to it, and during the final drying.

Before producing the photographic paper, the formation of the insoluble silver halides as precipitates on mixing silver nitrate solution with solutions of potassium chloride, bromide and iodide in test tubes should be demonstrated or done as part of the class experiment.

On placing the test tubes containing the precipitates in strong light – eg on a window sill – the silver chloride darkens rapidly. The change is much slower with the silver bromide and iodide produced in this way.

When done as a demonstration, these experiments should take about 10 minutes, excluding light exposure time.

Equipment

Apparatus

• Eye protection
• Protective gloves (preferably nitrile gloves)
• A square of white paper, about 10 x 10 cm, or a filter paper of similar size
• Small paint brushes, x2
• Test tubes, x3
• Test tube rack
• Hairdryer (see note 5 below)
• Ultraviolet light source (optional) (see note 6)

Chemicals

• Access to 0.1 M solutions of the following:
  • Potassium chloride, 10 cm³
  • Potassium bromide, 5 cm³
  • Potassium iodide, 5 cm³
  • Silver nitrate, 10 cm³

Health, safety and technical notes

• Read our standard health and safety guidance.
• Wear eye protection throughout.
• Potassium chloride, KCl(aq), potassium bromide, KBr(aq) and potassium iodide, KI(aq), solutions are all LOW HAZARD. Solutions of the sodium salts can be used in place of the potassium salts. See CLEAPSS Hazcard HC047b, plus CLEAPSS Recipe Book RB068 and RB072.
• Silver nitrate solution, AgNO₃(aq), is LOW HAZARD at this concentration but will still stain skin, clothing and some bench materials. The silver nitrate solution should be made up using distilled or deionised water as the chloride content of tap water gives a cloudy solution due to the formation of a small amount of silver chloride. See CLEAPSS Hazcard HC087 and CLEAPSS Recipe Book RB077 for further information and disposal instructions.
• Ensure the hairdryer has had a portable electrical appliance test.
• The UV light source should be a safe UVA, or so-called ‘blacklight’, lamp such as that used for locating spots in chromatography. Shield the lamp so that it cannot be viewed directly.

Procedure

Silver halide precipitates

1. Place about 5 cm³ of potassium chloride, potassium bromide and potassium iodide solution in three separate test tubes.
2. To each solution add about 1 cm³ of silver nitrate solution. A silver halide precipitate forms – ranging in colour from white (silver chloride), to cream (silver bromide) and yellow (silver iodide).
3. Place the test tubes containing the precipitates in strong light – eg on a window sill. After several minutes, depending on the light level, the silver chloride darkens to a dark grey colour as metallic silver forms. The other two silver halides change much more slowly, if at all, under these conditions.

Photographic paper

1. Paint one side of the piece of paper with some of the remaining potassium chloride solution. Dry the paper with a hair dryer.
2. In a darkened room, or shielding the paper from as much bright light as possible, paint the same side of the paper with some of the remaining silver nitrate solution, using a different brush. Dry the paper with a hairdryer.
3. Put your chosen object (preferably something flat, with a sharp outline eg a coin or a key) on the treated side of the paper and place it in bright sunlight or under UV light until the exposed part of the paper darkens. Do not look directly at the UV light.
4. Remove the object and the light source. An image of your object should be visible on the paper.

Teaching notes

The precipitation reactions forming the silver halides are also used as tests for the presence of halide ions in solution. The general equation for these reactions is:

MX(aq) + AgNO₃(aq) → AgX(s) + MNO₃(aq)

where M = K or Na and X = Cl, Br or I.

Or more simply in ionic form:

Ag⁺(aq) + X^–(aq) → AgX(s)

The decomposition of the silver halides in light is a photochemical redox reaction in which an electron is transferred from the halide ion to the silver ion, forming silver atoms and chlorine atoms:

Light

AgX → Ag + Cl

The formation of metallic silver causes the darkening of the areas exposed to light. In photography the exposed paper is then ‘fixed’ to remove the unexposed silver chloride.

Digital photography has of course now largely replaced silver-based film for home photography.