The Chemistry of Iron and Thiocyanates
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The following article was originally published in the journal for educators Chemia w Szkole (eng. Chemistry in School) (1/2024):
The natural sciences, including chemistry, are disciplines in which experimentation is essential for gaining deeper insight into the mysteries of the microscopic world. Knowledge in these fields is built not only on theory but also on practice, which reveals the connections between abstract concepts and the real phenomena we encounter in everyday life.
Chemistry is the science that studies the structure, properties, and transformations of matter. Teaching it effectively requires more than memorizing facts or formulas; it calls for a balanced approach that connects theory with experience. While abstract principles are essential, they become truly meaningful only when students can see and touch the processes they describe. Hands-on experiments make chemistry come alive, helping students understand concepts that would otherwise remain distant or purely theoretical. Unfortunately, this crucial element of learning is sometimes overlooked in today’s classrooms.
It is also important to remember that the primary goal of teaching chemistry is not simply to transmit information but to cultivate analytical skills and a critical way of thinking. Chemical experiments therefore serve as an irreplaceable educational tool enabling students to transform theory into practice. Carrying out experiments reinforces knowledge and makes it easier to retain. Sensory impressions in the form of visual cues such as changes in solution color, auditory stimuli such as the bubbling of liquids in flasks, or even the distinctive odors of certain chemicals make abstract concepts more tangible and engaging for learners.
For these reasons even simple experiments that can be set up quickly without complex equipment may hold considerable educational value, particularly when it comes to sparking students’ curiosity. One such experiment is the demonstration of iron “bleeding.”
Experiment
To carry out the experiment we need the following substances:
- potassium thiocyanate KSCN or ammonium thiocyanate NH4SCN [1],
- concentrated hydrochloric acid HCl(aq),
- 3% hydrogen peroxide H2O2 (pharmaceutical-grade solution),
- iron Fe (steel).
Potassium thiocyanate, also known as potassium rhodanide, is an inorganic compound from the thiocyanate group, the potassium salt of thiocyanic acid. At room temperature it is a white crystalline solid.
Although thiocyanates themselves are not highly toxic, they can release hazardous gases when combined with other substances. Concentrated hydrochloric acid HCl is strongly corrosive and releases hydrogen chloride gas, which is both irritating and toxic. Appropriate personal protective equipment is therefore essential.
To begin, prepare a solution by dissolving 0.2 g (0.007 oz) of potassium thiocyanate or the same amount of ammonium thiocyanate in 25 cm3 (≈0.85 fl oz) of distilled water. Add three drops of concentrated hydrochloric acid and several drops of 3% hydrogen peroxide. The resulting solution is completely colorless (Photo.1).
A steel or iron object is also required. A plain, ungalvanized nail works well (Photo.2).
To initiate the reaction, simply place the steel object into the solution (Photo.3). At first no dramatic changes are visible, apart from a slight darkening of the metal surface.
However, after only a few seconds something remarkable takes place: a dark red liquid begins to flow from the surface of the metal, closely resembling blood (Photo.4). With gentle stirring the entire solution takes on a red coloration [2].
The experiment can also be carried out in another, perhaps even more dramatic way. In this variation, the skin (without any cuts, since the chemicals are not harmless) is moistened with the colorless solution. Any contact with a steel tool, for example a specially prepared blunt knife blade, then leaves marks resembling blood. This effect has occasionally been used as a practical special effect in films.
Explanation
The nail used in the experiment is made of steel, whose main component is iron. It is this iron that reacts with dilute hydrochloric acid in the presence of hydrogen peroxide, producing ferric ions Fe3+ in solution.
Because potassium thiocyanate is also present, the thiocyanate ions react with the ferric ions to form a dark red complex strikingly similar to blood. This complex is responsible for the apparent “bleeding” of the metal.
This reaction is extremely sensitive and is widely used in chemical analysis to detect ferric ions Fe3+ [3].
References:
- [1] Hassa R., Mrzigod J., Nowakowski J., Podręczny słownik chemiczny, Wyd. I, Videograf II, Katowice, 2004, p. 384 back
- [2] Lide D. R. (red.), CRC Handbook of Chemistry and Physics, wyd. 88, CRC Press, 2007, pp. 8-16 back
- [3] De Meyer T., Substituent effects on absorption spectra of pH indicators: An experimental and computational study of sulfonphthaleine dyes, Dyes and Pigments, 2014, 102, pp. 241-250 back
All photographs and illustrations were created by the author.
Addendum
The experiment’s result can be seen in the video:
Marek Ples