3. Fe2+ + K3 4.
[FeIII (CN)6
] → KFeII [FeIII (CN)6
] + 2 K+
The acetonedicarboxylic acid can undergo decarboxylation
two more times, with the Fe3+ ions being reduced. The final products are acetone and carbon dioxide:
5.
If the UV radiation is very strong, white color can form on
the filter paper. There is a transfer of electrons from the ferric (II) citrate to the ferric (III) ion of the potassium ferricyanide (III). This forms a ferric (II) ferricyanide complex, which reacts with the unbound ferric (II) ions to form so-called Prussian white. This process is called the solarization effect.
6. K 3 [Fe III (CN)6] + e- → [FeII (CN)6] 4- + 3 K + 7. Fe 2+ + [FeII (CN) 6 ] 4- → FeII [FeII (CN)6
]2- The Prussian white reacts with atmospheric oxygen to
form Prussian blue. Contrary to the insoluble Prussian blue in chemical equation (3), the complex contains a ferric (II) ion.
8. 4 FeII [FeII (CN)6 ]2- + O2 4 FeIII [FeII(CN)6] - + 4 OH-
+ 2 H2 O →
Part B: Sunscreen by Reflection These three model experiments compare different options for skin protection from UV exposure such as cream, clothing and skin types. Materials: 3 cyanotype filter papers from Part A, 3 glass
sheets, a paintbrush, template of different skin types (see fig- ure 4), a piece of silk fabric, a piece of cotton fabric, 2 clothes pegs, waste container Chemicals: Sun cream, hand cream Time: 15 minutes
Procedure a. Put a cyanotype filter paper between two sheets of glass. Using the paintbrush, apply thin layers of hand cream and sun cream side by side to the top glass sheet. The same quantities should be used of each and the thick- ness of the layers applied should be identical.
b. The pieces of silk and cotton fabric should be placed side by side over a second piece of cyanotype filter paper and fixed with clothes pegs, if necessary.
c. Place the template of different skin types on a third cya- notype filter paper and weigh it down with a glass sheet.
Students prepare photo-sensitive cyanotype filter paper for sunscreen in a model experiment.
4. Expose these three prepared cyanotype filter papers to UV radiation from the sun.
Disposal: Collect the exposed filter papers in a waste
container. Explanation: A comparison between the hand cream and
the sun cream reveals different colors on the cyanotype filter paper. The hand cream provides no protection from UV radia- tion so a photochemically induced reaction occurs, resulting in a blue color. The sun cream, on the other hand, prevents this process and the filter paper remains virtually yellow-green. Min- eral sun creams contain the primary light-screening substance titanium dioxide, in the form of nanoparticles that reflects or absorbs the UV radiation. Silk and cotton also leave different shades of color on the
cyanotype filter paper. Thin fabrics that are not densely inter- woven, such as silk, provide very little protection against UV radiation, although people often like to wear these lightweight fabrics in the summer. The third model experiment demonstrates the protective
effect of different skin types. A color template consisting of four brown tones with varying degrees of transparency provides a basis for this experiment. After the experiment, the differ- ent shades of the color scale are reflected by different degrees of blue color on the filter paper. The color scale is intended to represent the different concentrations of melanin in skin. Melanin is the color pigment that is produced when skin is sun- tanned. The results illustrate the differences between the skin types with regard to their ability to resist UV radiation. The varying formation of Prussian blue symbolizes the exposure of the deeper skin layers to UV radiation.
GREEN TEACHER 109 Page 41
Alexander Engl and Björn Risch
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