Materials Included In Kit

Additional Materials Required

Safety Precautions

The combined solution will dye the skin a dark blue. The solution (Prussian-blue in color) is non-toxic, and its color will fade with time. Dangerous hydrogen cyanide gas may result when potassium ferricyanide is heated or placed in contact with strong acid. Ferric nitrate is a strong oxidizer and a skin and tissue irritant. Oxalic acid is moderately toxic if ingested and is an eye and skin irritant. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Please consult current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures governing the disposal of laboratory wastes. Please dispose of Solution 1 (ferric nitrate and oxalic acid) according to Flinn Suggested Disposal Method #26b. Provide a disposal container for the Solution 2 waste. Dispose of potassium ferricyanide solution according to Flinn Suggested Disposal Method #14.

Teacher Tips

• Enough chemicals and weighing dishes are provided in this kit for 30 students working in pairs, or for 15 groups of students. Enough filter paper (100 sheets) is provided for each group to make at least six “photographs” or images. This laboratory activity can reasonably be completed in one 50-minute class period.
• Perform this lab procedure on a day when the students are able to develop their images outside. A sunny day will provide for the fastest developing of the images; however, an overcast day will set the stage for additional discussion of ultraviolet radiation. Consider having students develop one print in a sunny area and one in a cloudy or shaded area. Compare the results. Have students try translucent objects.
• If your situation prohibits students from going outside, try developing the paper using an artificial ultraviolet “black” light. Or try developing the paper in front of a sunny window; however, some windows may block or filter some UV rays.
• When actual blueprints are made, instead of placing objects directly on the paper, a sketch is drawn on colorless plastic and then placed over the blueprint paper before being exposed to UV light. The drawing forms white lines on the paper, which prevent the transmission of UV light and therefore does not expose the photographic chemical in those areas.
• Consider purchasing Sun Print Paper from Flinn Scientific (Catalog No. FB1554), which is a package containing 30 sheets of sun-sensitive paper. After students make their own UV-sensitive paper in the laboratory, they can compare their handmade paper to the Sun Print Paper.

Further Extensions

• Test various sunscreens and determine the effectiveness in blocking UV rays. (Try Flinn’s FB1561 Sun Protection Factor Kit.)
• Use black-and-white negatives to produce an image on the blueprint paper.

Alternate Procedure (for Metallic Objects)

• An alternative procedure involves allowing the paper to dry completely (in the dark or very dimly lit space) prior to exposure to light. This is less messy, but treating the filter paper would need to be done on Day 1 while the exposure would wait until Day 2. If this is done, a metal object (e.g., key, coin) can be used to make the pattern because if the paper is dry, no redox reaction will occur with the iron(III) ion as it might when the paper is moist.

Answers to Questions

1. Why is it best to store the ferric oxalate and other photographic chemicals in brown or dark-colored bottles?
   The brown bottles help filter out UV light, extending the life of the chemical.
2. Why does the lab work on overcast days as well as on clear days?
   Ultraviolet radiation can and does penetrate the clouds.
3. Explain why slow deterioration of poorly-rinsed blueprints or photographs may result.
   Unreacted chemicals left on the paper will react with the light, ruining the image on the print.
4. Where is the best place for storing photographs? What would you predict sunlight would to to photographs? Why?
   Photographs are best stored in a dark area out of the sunlight. Sunlight would gradually lighten the image causing it to fade.
5. Why doesn’t incandescent light (classroom lighting) develop the prints?
   Higher energy ultraviolet rays (which are not present in incandescent bulbs) are needed to develop the print.
6. Explain why the object must be non-metallic.
   If the object to be developed was metallic, an oxidation–reduction reaction would occur with the iron(III) ions on the moist paper.
7. (Optional) Compare the reduction potentials for silver and iron. Which is more likely to become insoluble? What other metals might also work well for photography?
   Silver is more easily reduced to the solid than iron, either +2 or +3 ionic state (E^o_red = 0.7996 V for silver versus –0.447 V and –0.037 V, for iron(II) and iron(III), respectively). Gold has a higher reduction potential than silver (E^o_red = 1.498 V), but its high cost keeps it from being used as an alternative to silver in photographic processes.

References

Orna, M. V.; Schreck, J. O.; Heikkinen, H., eds. SourceBook, Vol. 3 (PHOT), ChemSource, Inc: NY, 1994; pp 12–14.

Introduction

Make your own ultraviolet-sensitive paper and develop images of any object you want! Reproduce the ultraviolet-activated chemical reaction used in blueprinting and mimic certain aspects of the photographic process with this blueprinting paper lab. Images produced appear in just minutes and are permanent.

Concepts

• Light-activated reactions
• Photochemistry
• Ultraviolet (UV) light
• Blueprinting

Background

One preparation of UV-sensitive paper (sometimes called blueprint paper) involves the combination of two iron salts—ferric oxalate, Fe₂(C₂O₄)₃, and potassium ferricyanide, K₃Fe(CN)₆. Ferric oxalate can be produced by combining ferric nitrate and oxalic acid as shown in Equation 1.

Exposing porous paper that has been dipped in ferric oxalate and potassium ferricyanide solutions to UV light results in a breakdown of the ferric oxalate to produce the ferrous [iron(II)] ion. It is the iron(II) that combines with the hexacyanoferrate(III) ion from the potassium ferricyanide, resulting in the characteristic dark blue color of the exposed part of the paper. The dark blue color is due to a ferrous–ferric [iron(II)–iron(III)] ion, Fe[Fe(CN)₆]^–₃ (see Equation 3). In the areas where the paper is covered by an opaque object (and protected from UV light), this reaction does not occur and the paper remains its original color.

Solutions of ferric oxalate, Fe2(C₂O₄)₃, contain several complex ions of Fe³⁺. One of these ions is [Fe(C₂O₄)(H₂O)₄]⁺, which can be simplified to Fe(C₂O₄)⁺. When irradiated with UV light, this ion can transfer an electron from oxalate to iron, resulting in the iron(II) ion and a radical oxalate anion:

The (C₂O₄)^–• is a very unstable radical anion and decomposes into CO₂ and the CO₂^–• radical anion. The CO₂^–• dimerizes to produce (C₂O₄)^2–, the oxalate anion. The oxalate anion is stable.

The resulting Fe^₂₊ reacts with hexacyanoferrate(III) ion to produce the deep Prussian-blue (Turnbull’s blue) color:

It is this ion that forms an insoluble salt with iron(III) in solution. The overall reaction is

This process is similar to the photographic process. Instead of iron salts, black and white photography uses silver salts to coat cellulose acetate paper. Silver is easily and quickly reduced from Ag⁺ to Ag(s) when exposed to light. In areas of black, all the silver has been reduced and becomes insoluble in the cellulose. Areas of white are just the opposite. Where there are shades of gray, not all of the silver receives enough light to be activated, resulting in only some of the silver becoming insoluble. As with blueprinting paper, areas that were blocked from the light source result in the lightest areas, whereas those most exposed result in the darkest. The fixing and washing processes remove the excess reactants, preventing further darkening of the photograph.

Experiment Overview

Today’s experiment involves two parts. First, you will prepare the ultraviolet-sensitive paper by combining two iron salt solutions, as described. Then you will develop various images on the treated paper by exposure to ultraviolet radiation. Be creative and experiment with a variety of different objects—opaque and translucent. Try exposing them in a variety of areas outside, of differing degrees of brightness, to determine the extent of ultraviolet radiation present.

Materials

Prelab Questions

Collect various non-metallic objects to place on the paper. Opaque and translucent objects work well.

(Note: If you wish to use metallic objects for developing the paper, consult your teacher for an alternative procedure.)

Safety Precautions

The combined solution will dye the skin a dark blue. The solution (Prussian-blue in color) is non-toxic, and its color will fade with time. Dangerous hydrogen cyanide gas may result when potassium ferricyanide is heated or placed in contact with strong acid. Ferric nitrate is a strong oxidizer and a skin and tissue irritant. Oxalic acid is moderately toxic if ingested and an eye and skin irritant. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Please review current Safety Data Sheets for additional safety, handling and disposal information. Wash hands thoroughly with soap and water before leaving the laboratory.

Procedure

Part 1. Preparing UV-Sensitive Paper

1. Measure 10 mL of 0.10 M ferric nitrate solution and pour it into a weighing dish.
2. Measure 10 mL of 0.15 M oxalic acid solution. Combine it with the ferric nitrate in the weighing dish and mix well with a stirring rod. This solution (which is now ferric oxalate) will be Solution 1. Set the dish containing Solution 1 aside for use in step 4.
3. Measure 10 mL of 0.10 M potassium ferricyanide solution and pour it into a separate weighing dish. This will be Solution 2.
4. Use a pair of tongs or forceps to soak a piece of filter paper in Solution 1. As soon as it is completely saturated, remove the filter paper and blot it on a paper towel.
5. Take the same piece of filter paper and dip it in Solution 2, turning it over to wet both sides. Remove it and blot it on a paper towel.
6. Place the treated filter paper on a piece of cardboard. Cover it with a second piece of cardboard.

Part 2. Developing Images on the Treated Paper

7. Locate both a sunny area and a cloudy or shaded area outside.
8. Bring a non-metallic object (or objects) and the treated filter paper into the shaded area. Remove the top piece of cardboard, and quickly place the object(s) on the paper. (Note: Doing this in the shade helps to block the paper from direct sunlight.)
9. With the object setting on the treated paper on the cardboard, move to a sunny area. Set the paper in the sunlight, exposing it to bright, direct sunlight until the paper turns a dark blue color. This step will only take about five minutes, depending on the brightness of the sun.
10. After the paper is developed, remove the object and cover the paper with the top cardboard piece. Bring it indoors (out of the sunlight). Holding the paper with tongs, rinse it under cold water for a few minutes to remove the yellow color of excess iron(III) ions. Rinse until the solution turns clear and until the paper turns a lighter blue.
11. Repeat steps 4–10, trying a variety of different objects.
12. Solution 1 may be rinsed down the drain with plenty of water. Return Solution 2 to your instructor or place it in the waste container provided for potassium ferricyanide waste solutions.

Post-Lab Questions

Answer the following questions on a separate sheet of paper.

1. Why is it best to store the ferric oxalate and other photographic chemicals in brown or dark-colored bottles?
2. Why does the lab work on overcast days as well as on clear days?
3. Explain why slow deterioration of poorly-rinsed blueprints or photographs may result.
4. Where is the best place for storing photographs? What would you predict sunlight would do to photographs? Why?
5. Why doesn’t incandescent light (classroom lighting) develop the prints?
6. Explain why the object must be non-metallic.
7. (Optional) Compare the reduction potentials for silver and iron. Which is more likely to become insoluble? What other metals might also work well for photography?