Microscale chemistry revisited

Looking at photographs of school chemistry laboratories before the second world war, you see Bunsen burners, tripods, and other very familiar apparatus still used today. Other sciences have seen more dramatic changes; physics with electronics, lasers and radioactivity and biology with aseptic techniques. Bunsen burners, tripods, burettes still work well but other practical methods are available which provide more variety in presenting practical chemistry to students.

Emergence

I remember a note coming to my chemistry department when I was teaching in the 1980s, to inform me that the demonstration involving passing hydrogen over hot copper(II) oxide to form copper and water was banned because there had been a serious incident resulting in a prosecution by the Health and Safety Executive. A hydrogen/air explosion had caused concentrated sulfuric(VI) acid (used to dry the hydrogen) to spray over the children, who were not wearing eye protection. Of course, it was not banned. It was the usual over-reaction to incidents caused by poor attention to detail by a teacher. However, the demonstration did fall from favour and, without a hydrogen cylinder, it is complicated to set up.

In the 1990s, 'out of Africa' came a kit (fig 1) from the Radmaste Institute¹ which allowed students to carry out this same reaction safely in microscale. The kit is available in this country and a number of enthusiastic users use it regularly.²

I developed another safe microscale method of reducing copper(II) oxide with hydrogen.³

In 2003 I saw Bruce Mattson demonstrate his microscale gas methods to teachers and technicians.⁴ He passed 60 cm³ of hydrogen over copper oxide in a Pasteur pipette, heated with spirit burner, to produce an exothermic reaction (fig 2). A video is available.⁵

The RSC produced a book in 1998 by John Skinner sent to every school in the country (it might be still on your shelves, possibly never opened!).⁶ This initial impetus to microscale however, did not mushroom as expected and enthusiasts met many objections.

10 common objections

1. Quantitative work is inaccurate
2. It's not what I expect with chemical equipment
3. It is too small and fiddly
4. There is no time to practice
5. The equipment and methods are not in our text books
6. Exam boards do not specify its use in practical examinations
7. The equipment is not available cheaply
8. It is cheap plastic equipment
9. Pupils will use pipettes and syringes as water pistols!
10. It is not spectacular enough to hold the attention of pupils

Many of these sweeping statements are made on one (often half-hearted) attempt, usually without help from an experienced practitioner.

Examples

I have many examples of successful microscale chemistry experiments but here are three of the most popular which often draw 'oohs' and 'aaahs' from the audience.

Drop chemistry

John Skinner showed that chemical reactions could be carried out on plastic overhead transparency sheets. These are not available now, but it is an easy jump to insert the written instructions in a plastic folder and carry out the experiment on the plastic surface (fig 3).

1 cm³ of water is added 0.2 g of iron(II) sulfate(VI)-7-water in a vial (the red circle in the figure). Drops from the vial are then placed in the circles on the plastic. In one circle, two magnesium turnings are added to the iron(II) sulfate(VI) solution. Iron is formed; bring a magnet close to it, the solid particles move about in the drop under the influence of the magnetic field (see the video⁷). Precipitation, redox (iron(II) to iron(III) and the reverse) and complex reactions can be completed in 15 minutes, leaving time to discuss the many observations.

Electrolysis

The electrolysis of metal chloride solutions produces chlorine (toxic) gas, a potentially unsafe procedure in an open laboratory. A microscale approach can solve the safety issues. It could be carried out by pupils (if they have no reaction to the 6 cm³ of chlorine produced) or it can be demonstrated and projected via a microscope or visualiser onto a screen.

The electrodes are made of carbon fibre, available from any online kite shop at about £5 for 1 m of 1 mm diameter rod. It is very flexible and strong and can be cut with scissors to the required length. Holes are drilled or burned though the opposite sides of a 90 mm diameter plastic Petri dish.

In the Petri dish, a piece of damp blue litmus, a drop of 0.1 M potassium iodide and a drop 0.5 M potassium bromide solution are used to detect the chlorine coming off (fig 4a). About 0.5 cm³ of 0.5 M copper chloride solution is placed between the electrodes and the lid is put on. The electrodes are attached to a low voltage supply or battery. Chlorine diffuses within the dish and reacts (fig 4b) with the solutions and litmus paper. Copper appears at the cathode (fig 4c). Obviously, other salt solutions can be used. A video is available.⁸

Indicators

The Comboplate (fig 5) from the original kit can be bought separately. To show the range of colours of indicators as shown in fig 5, solutions in the larger well plates are prepared by mixing (20 - x) drops of acid solution A with x drops of alkaline solution B delivered by a plastic 3 cm³ pipette. The value of x changes by 2 drops per well. After measuring the pH with a pH meter (optional), 3 drops of each well solution are transferred to the smaller wells above. The indicator is then added to obtain the beautiful array of colours. This procedure can be used not only at KS3 to demonstrate the colours of indicators but also (with adaption) at A-level to calculate pK_Invalues.

The upside

Microscale techniques will never replace our traditional approach but they do provide an extra dimension to our teaching strategies. Students, in extended studies, could compare the quantitative results with established methods to crucially examine sources of error. With jewellery balances weighing to 3 decimal places (capacity 20 g) available at £40, the study becomes feasible in all schools.

The procedures are extremely economical and answer environmental issues such as energy consumption and waste. As Stephen Breuer wrote 'if you need 100 mg, make 100 mg, don't make 5 g and throw away 4.9.⁹

If a microchemical approach satisfies one or more of the following points it should be seriously considered.

1. It allows a once-dangerous experiment to be carried out more safely
2. It shortens practical time so that lessons are not so rushed
3. It reduces the cost of equipment and consumable materials
4. Users report a higher level of concentration amongst pupils and mistakes are quickly rectified
5. It enables some stunning visual effects when filmed or projected onto a whiteboard
6. It reduces technician time in disposing and clearing up
7. It reduces waste, a factor which is becoming more important in the UK. (We would, in fact, consider many other countries to be draconian in their constraints on waste from a school laboratory.)
8. It shows equivalent or better quantitative results (although comparison of techniques is a useful exercise in error analysis).

As to dexterity; if students in other countries can cope, why cannot our pupils learn the techniques? It is often the teacher who lacks the techniques, experience and dexterity.

Janice Griffiths at the South East Learning Centre contacted some teachers who had attended a microscale course. All replies were positive and here is one statement:

The colour of indicators (fig 5) has provided possibly our first practical assessment for year 7s. They love it. Our science technician loves it and we promote the idea of reducing materials and potential risks. I think it should be used in teacher-training establishments. It's rare for me to feel I obtain value for money from an Inset but I was really inspired by this one.

Conclusion

Microscale techniques have been introduced as a matter of necessity in many countries because they do not have teaching laboratories. I came to the technique from a health and safety angle as a direct response to carrying out risk assessments.

Bob Worley is lead adviser on chemistry at CLEAPSS