Learn how to carry out microscale experiments for greener chemistry teaching – and less washing up.
Last year, I worked with my students on an international Scientixw1 project, using chemistry to investigate metal objects found in UNESCO cultural heritage sites, such as the copper and bronze statues in Heroes’ Square, Budapest, Hungary. We carried out experiments using compounds of iron, copper, nickel, silver, lead, mercury and other heavy metals – materials that are dangerous, toxic and polluting. Although only one or two millilitres of solution were used in each test tube, a huge amount of toxic waste was produced by the time the whole class had performed each experiment. We are very committed to protecting the environment, so we started thinking about how we could reduce the amount of chemicals used during the experiments.
Our first idea was to replace the test tubes with ‘dimple trays’ from empty plastic boxes of pills or chewing gum, using the indentations in the trays as the containers for the reactions (figure 1). This method, which we found described in a previous Science in School article (Kalogirou & Nicas, 2010), greatly reduced the amount of toxic chemicals used – but we wanted to go further.
Our next idea was to use filter paper as the reaction ‘container’ for precipitation reactions, using drops of the chemicals reacting together at the same spot. In these experiments, we used just one or two drops of each reagent – about a hundredth of the amount used in the test tube experiments. Because colourful compounds were used, the reactions were immediately visible and the products could be identified just from their colour.
We found it worked well if we soaked the filter paper in one of the non-toxic reagents, dried it, and then dripped the other reagents onto the paper. Several reactions can be carried out on the same filter paper if gaps are left between the reaction spots.
Figure 2 shows the reactions of iron(III) chloride: first with sodium hydroxide on the filter paper, producing a precipitate of iron(III) hydroxide; and then with potassium hexacyanoferrate on the filter paper, producing the characteristic Prussian blue precipitate. (For the equations for these reactions, please see Activity 1.)
The next method was inspired purely by chance when we found a used air-freshener box containing tiny, dried-up hydrogel balls. Hydrogels are super-absorbent polymers that shrink as they dry out, but swell up again when they are placed in water. These balls are easily available to buy, and are often used in floral displays as well as in air fresheners. As they swell, they retain their spherical shape, thus forming an aqueous bead in which reactions can take place.
Using the hydrogel balls, we first carried out precipitation reactions (as with the filter paper experiments). The results obtained with this method were interesting and convincing. Not only did we use a minimal amount of reagents, but due to the hydrogel balls’ spherical shape, they acted like magnifying lenses and made the reactions more visible.
We therefore tried out some other standard chemical reactions using the hydrogel balls – with mixed results. The acid-base reactions did not work well, because the colours of the indicators (universal, litmus, phenolphthalein and others) were not strong enough, and the procedure was rather complicated.
The electrochemical reactions, however, were a success: for example, electrolysis of silver nitrate solution, zinc iodide solution, water and other solutions all worked well (see Activity 2). Normally in such experiments, a thin layer of metal is deposited on the surface of an electrode, and this layer is often poorly visible. In the hydrogel balls, however, the reduced metal appears within the gel as a spot, where it is more visible and can be studied more easily.
To get the reagents into the hydrogel balls, we used syringes and hypodermic needles. For this reason, these experiments are suitable only for students aged 16 or older, and great care and disciplined behaviour are needed. For younger (or perhaps less well-behaved) students, the experiment can be carried out as a teacher demonstration using a computer webcam.
The time needed for this activity is approximately 10 minutes for each precipitation experiment, although the hydrogel balls need to be soaked in advance. Because so little reagent is used in each hydrogel ball, the filled hypodermic syringes can be reused in the next chemistry lesson.
This experiment is suitable strictly for students aged 16 or older, and with a high level of supervision by the teacher at all times. Hypodermic needles must be counted when given out and when given back, so that all are accounted for. Used hypodermic needles should be bent before disposal in a sharps bin. Safety glasses and disposable gloves need to be worn throughout.
Suitable additional precautions should be taken if any toxic reagents are used (e.g. mercury or lead compounds), with the hydrogel balls containing these compounds placed in a hazardous waste collector.
See also the general safety note on the Science in School website.
You will need the following materials for each student or group:
The method can also be used for other precipitation reactions, so the list of the materials here can be extended or adapted.
First, wash the hydrogel balls several times in distilled water, then leave them to swell in more distilled water for at least 2 hours. Approximately 500 ml of distilled water is needed to soak 30 hydrogel balls.
For each reaction:
The equations and colour changes for these reactions are:
For a class discussion, questions could include:
In these experiments, instead of injecting the swelled-up hydrogel balls with the electrolyte solutions, we placed the balls on filter paper soaked with the electrolyte and then inserted electrodes into the balls (figure 4). The electrolyte ions migrate from the filter paper into the balls, where the deposit forms. For the miniature electrodes, we used graphite leads from a mechanical pencil. In the electrolysis of water experiment, the bubbles of evolved gas (hydrogen at the cathode and oxygen at the anode) were very visible (figure 5).
Unlike the precipitation reactions, this experiment does not use hypodermic needles, and so it is suitable for students from age 14. The time needed for the activity is approximately 10 minutes for each of the reactions, if you are using already swelled-up hydrogel balls; otherwise add 2 hours soaking time.
You will need the following for each student or group:
The equations of the electrolysis reactions are:
Cathode (negative electrode): 2Ag+(aq) + 2e- → 2Ag(s)
Anode (positive electrode): H2O(l) → ½ O2(g) + 2H+(aq) + 2e-
Cathode (negative electrode): Zn2+(aq) + 2e- → Zn(s)
Anode (positive electrode): 2I-(aq) → I2(s) + 2e-
Cathode (negative electrode): 4H2O(l) + 4e- → 2H2(g) + 4OH-(aq)
Anode (positive electrode): 2H2O(l) → O2(g) + 4 H+(aq) + 4e-
All these experiments are inexpensive to carry out and encourage students to consider the environment. We also found one final advantage of not using test tubes and other laboratory glassware: we avoided the need to wash up after finishing the experiments – which saves time, water and effort.
Hydrogels are super-absorbent polymers, usually made from polymethacrylate polymers. These polymers are xerogellants, which means that when they are placed in water, they swell and significantly increase in size. They are used in agrochemistry to grow plants in hydroculture, and can be found in disposable nappies, potpourris, air fresheners and so on.
Chemically, xerogellants are ionic compounds. When placed into water or diluted salt solution, the ions become hydrated as they attract the polar water molecules, which is what makes the hydrogel balls grow in size. In distilled water, a hydrogel may absorb 300 times its weight, but in sodium chloride solution (at a concentration isotonic to human blood), the increase is only around 50-fold. The hydrogel keeps its original shape, thanks to the polymers’ cross links.
The author would like to thank the Doctoral School of Humanities of University of Debrecen, Hungary, for supporting her work.