Making a photographic print

Read our standard health & safety guidance

Lesson organisation

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 – e.g. 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.

Apparatus and chemicals

Eye protection

For one demonstration or one pair of students:

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, 2
Test-tubes, 3
Test-tube rack

Access to:

Hairdryer (see note 1)
Ultraviolet light source (Optional) (see note 2)

Potassium (or sodium) chloride solution, 0.1 mol dm^-3 (Low hazard), 10 cm³
Potassium (or sodium) bromide solution, 0.1 mol dm^-3 (Low hazard), 5 cm³
Potassium (or sodium) iodide solution, 0.1 mol dm^-3 (Low hazard), 5 cm³
Silver nitrate solution, 0.1 mol dm^-3 (Low hazard but stains fingers and clothing at this concentration, Danger to the environment), 10 cm³ (see note 3)

Technical notes

Potassium chloride solution (Low hazard) Refer to CLEAPSS Hazcard 47B and Recipe card 51
Sodium chloride solution (Low hazard) Refer to CLEAPSS Hazcard 47B and Recipe card 63
Potassium bromide solution (Low hazard) Refer to CLEAPSS Hazcard 47B
Sodium bromide solution (Low hazard) Refer to CLEAPSS Hazcard 47B
Potassium iodide solution (Low hazard) Refer to CLEAPSS Hazcard 47B and Recipe card 55
Sodium iodide (Low hazard) Refer to CLEAPSS Hazcard 47B
Silver nitrate solution (Low hazard at concentration used, Dangerous for the environment) Refer to CLEAPSS Hazcard 87 and Recipe card 58

1 Ensure hairdryer has had a portable electrical appliance test.

2 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 can not be viewed directly.

3 The silver nitrate solution should be made up using distilled or deionised water. The chloride content of tap water gives a cloudy solution due to the formation of a small amount of silver chloride.

Procedure

HEALTH & SAFETY: Wear eye protection

Silver halide precipitates

a Place about 5 cm³ of potassium chloride, potassium bromide and potassium iodide
solution in three separate test-tubes.

b 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).

c Place the test-tubes containing the precipitates in strong light – e.g. 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

a Paint one side of the piece of paper with some of the remaining potassium chloride solution. Dry the paper with a hair dryer.

b 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.

c 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.

d 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.

Health & Safety checked, February 2008

Web links

This website contains still pictures of an alternative method of producing photosensitive paper.

www.jce.divched.org/JCESoft/CCA/CCA3/MAIN/PHOTOAG/PAGE1.HTM

(Website accessed August 2007)