Small-scale chemistry in the school laboratory - small is beautiful, green is more beautiful Introduction Small-scale chemistry - what is it?

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1 Small-scale chemistry in the school laboratory - small is beautiful, green is more beautiful Orla Kelly, O. E. Finlayson, School of Chemical sciences, Dublin City University Introduction Practical work is now a major part of the Leaving Certificate Chemistry syllabus. However, it is a real challenge to find the time to do all the experiments, act as a laboratory technician by preparing for practical work and maintaining stock levels, and last but certainly not least, keeping safety a priority in the laboratory. This is one of the motivations behind this research. The other is that within the School of Chemical Sciences in DCU, there has been a major push towards a reduction in waste over the last few years. This has been done using small-scale lab techniques due to waste disposal, safety and financial issues. Both these issues initiated my research into small-scale chemistry in the school laboratory. Small-scale chemistry - what is it? Reduction of chemical use to the minimum level at which experiments can be effectively performed is known as microscale or small-scale chemistry. It is an environmentally safe pollution prevention method of performing chemical processes using small quantities of chemicals without compromising the quality, precision or accuracy. i In this set of experiments, there is at least an 80% decrease in reagent quantities for all experiments and a 90% decrease for most. Our experience of microscale at undergraduate level in DCU shows that students find it more challenging but have better lab techniques. Their accuracy and precision is improved. They realise that losing ¼ of a gram during the experiment will greatly affect their end result and so take much more care. The experiments require safe and easy manipulative skills and improve students dexterity. Since experiment times are reduced there is more time for investigative work and theory. It is also more studentfriendly. For example, how many times have you seen students kneeling on stools to fill their burettes? Countless I m sure. Using a micro-burette, this hazard is removed. In terms of the academic and technical staff, they found laboratory techniques improved with accurate and precise results. There has been a phenomenal decrease in waste generated and therefore financial benefits, since it often costs more now to dispose of a chemical then it does to purchase it in the first place. ii Students are more careful and there is a safer laboratory environment due to less clutter on the benches. There is also an improvement in air quality and a decrease in hazards associated with large-scale synthesis. By decreasing hazards, microscale opens up the possibilities of using chemicals too hazardous to contemplate on a larger scale, therefore increasing students experience of practical chemistry. Less storage space is required for both apparatus and chemicals, and there is a decrease in preparation times. Finally, students increase their learning due to reduced experimental times. Unlike times before, this does not require the purchase of kits, which have received mixed responses from teachers, including Many experiments require specialised glassware and although it might be possible for schools to purchase 1 or 2 kits, it is unlikely that they would be prepared to equip a whole class. Also breakages are costly to repair. However, the apparatus which is used throughout these experiments are mostly already in a normal scale laboratory, such as well-plates, and petri dishes.

Some if it is made, such as the microburette, which will be further discussed later. The rest is normal scale glassware but on a smaller scale, available from regular laboratory suppliers.

2 Some if it is made, such as the microburette, which will be further discussed later. The rest is normal scale glassware but on a smaller scale, available from regular laboratory suppliers. Figure 1 Figure 1 shows the organic synthesis apparatus for normal and microscale. Titrations make up a large proportion of the syllabus, and microburette is used. It is made using a 2ml graduated glass pipette and a 5ml plastic syringe. 25ml volumetric and conical flasks are also used. Other apparatus requires are well-plates, petri dishes, small volume pipettes, and sand baths. A sand bath is the preferred method for safe heating of organic reactions. Leaving Certificate syllabus - a quick reminder What experiments are on the syllabus? titrations rate of reaction studies characterisation tests gas preparations organic synthesis We will now take a closer look at these on a microscale. Some of you might be wondering how accurate and precise the microburette is. Tests were carried out to compare the two burettes. The % ethanoic acid in vinegar titration was used. In this trial, the same stock solutions were used and therefore the burettes were the only variables. To analyse precision the standard deviation was calculated for each trial. The microburette favoured comparably to the normal with values from compared to for the normal. To measure accuracy, the % acidity was measured. The results were % compared to % for the normal. The actual % acidity was quoted as 5%. This shows a relative accuracy between the two burettes. The next set of trials looked at the overall microscale of a titration. The %w/v of hypochlorite in bleach titration was used for this trial. It involved making up the

3 solutions in the 25ml volumetric flasks and using the microburette and small volume pipette (2mls). In comparison to the normal scale preparation and glassware. The concentration of the reagents was also reduced by a factor of ten and a second micro titration carried out. The table shows the concentration of the reagents and the apparatus used. The standard deviation and %w/v for each trial is also shown. It can be seen that the precision of both micro titrations favour comparably to the normal titration. The %w/v shows a relative accuracy between the 3 since the %w/v was quoted as not more than 5%. An important thing to note here is the two-fold benefit of microscale. There is less starting material but also a decrease in concentration. Table 1 Macro Micro 1/10 concentration Conc. Na 2 S 2 O 3 0.1M 0.1M 0.01M Bleach sample 1/10 dilution 1/10 dilution 1/100 dilution Volumetric volume 250cm 3 25cm 3 25cm 3 Burette volume 25cm 3 2cm 3 2cm 3 Standard deviation %w/v I m sure you are all familiar with the sodium thiosulphate and hydrochloric acid reaction, which forms sulpher, aswell as other products. This reaction is often used for rate of reaction studies as the sulpher forms a dense yellow cloud in the flask and therefore the time taken for an X marked under the flask to disappear can be measured as a function of concentration and temperature. Normally conical flasks are used and a large amount of smelly sulpher containing acidic waste is produced. On a microscale, it is done in a well-plate using no more than a ml or two of reagent in each well. This is very effective and very quick. The results obtained for a study on the effect of concentration on the rate is shown. Figure 2 Time 03: : : : : :00.0 Effect of concentration on rate of reaction % Concentration of Thiosulphate Well-plates can also be used for various characterisation tests. On a normal scale, the testing of anions, for example, requires a large number of test-tubes. However, only a well-plate and a few pasteur pipettes are required on a microscale. The table below shows a procedure for the identification and confirmation of carbonates, hydrogencarbonates, chlorides, sulphates, sulphites, and phosphates. Lets take a

4 closer look at the testing of sulphates and sulphites. Initially 10 drops of each test solution is added to the wells. Then 10 drops of Barium Chloride is added and if a white precipitate forms, it suggests the presence of either sulphates or sulphites. To confirm this, hydrochloric acid is added (10 drops). The sulphate precipitate is insoluble, whereas the sulphite precipitate is soluble. These are also very quick and very effective. Table 2 Cell A1 A2 A3 A4 A5 A6 Add 10* of CO 3 HCO 3 Cl SO 4 SO 3 PO 4 Add MgSO 4 ** MgSO 4 ** 5 of Silver Nitrate BaCl BaCl BaNO 3 ** Add 15 of NH 3 ** HCl ** HCl ** * Number of drops of solution ** Confirmatory test There are 2 gas preparations on the syllabus - the preparation of ethyne and the preparation of ethyne. The normal scale synthesis of ethyne involves dropping funnels, two hole stoppered conical flasks, test-tubes and troughs of water. On a microscale, 60mls plastic syringes, syringe cap fittings, and plastic pipettes are all that s required. Only 0.2g of CaC 2 is used per student. This is a huge safety benefit since CaC 2 is a pyrophoric substance and the smaller the amount used the better. The gas is produced within the syringe in a controlled sealed environment. Potassium permanganate is added tot he syringe. A colour change from purple to brown is observed, showing the presence of unsaturated compounds. This procedure was taken from a Microscale Gas Chemistry website. iii There are 4 organic synthesis reactions preparation of soap prepartion of ethanal prepartion of ethanoc acid steam distillation of clove oil (so far this has proved too difficult to microscale). The preparation of soap on a microscale involves only 0.2g of sodium hydroxide, 0.2g of lard and 1.5mls of a 50/50 ethanol/water mix, compared to 4g of sodium hydroxide, 4g of lard and 50mls of the ethanol/water mix on a normal scale. The procedure followed is the same as that for normal synthesis and typical yields are g of soap. The picture below (figure 3) shows a slight adaptation to the normal apparatus. Instead of a liebig condenser, which uses cold water to condense gases being formed, this uses an air condenser. It is basically a long tube connected to the round bottom flask, allowing air to condense the gases. It proved to be as effective as the typical apparatus giving yields of between g of soap.

Figure 3 Figure 4 The small scale synthesis of ethanal involves 2.5g of sodium dichromate compared to 12g and 2mls of ethanol compared to 10mls. The typical yield was between 2-3mls.

4mls of concentrated sulphuric acid compared to 12g, 5ml, and 2mls respectively for the normal scale synthesis. Again the typical yield was 2-3mls.

5 Figure 3 Figure 4 The small scale synthesis of ethanal involves 2.5g of sodium dichromate compared to 12g and 2mls of ethanol compared to 10mls. The typical yield was between 2-3mls. The apparatus is shown in figure 4. The small scale synthesis of ethanoic acid involves only 2.4g of sodium dichromate, 1mls of ethanol and 0.4mls of concentrated sulphuric acid compared to 12g, 5ml, and 2mls respectively for the normal scale synthesis. Again the typical yield was 2-3mls. For both the ethanal and ethanoic acid, the yields obtained were enough to carry out the necessary characterisation tests on a microscale. The table below gives an overall view of the cost of running the leaving certificate experiments. To set up a normal scale and a microscale lab from scratch we can see that it is slightly less for the microscale. However, we notice that there is a saving from year to year on chemicals of about 450euro. These figures were obtained from Lennox. The amount of waste for disposal is greatly reduced. Table 3 Microscale Macroscale Apparatus 22,811 24,355 Chemicals Organic waste 280mls 1600mls Dichromate waste 130mls 700mls Transition year microscale chemistry modules are being developed at present. So far, a polymer and an alcohol module have been completed. These are meant to introduce students to leaving certificate chemistry and to act as fuels for learning about topical aspects within the syllabus. Well-plates and petri dishes alone were used for the 4 experiments in the alcohol module. They are all quick, easy and effective. The 4 experiments are: 1. Test for primary, secondary and tertiary alcohols 2. Test to differentiate between methanol and ethanol

6 3. Preparation of an alcohol in a petri dish - namely 2,4,6-trichlorohydroxybenzene (commonly known as TCP) 4. Purity of alcohols Conclusion What have people said about microscale? Students said it was chemistry for barbies, more challenging but different, fun, and quick and easy. Technicians say a lot safer, waste dramatically reduced, and less work for us. The teachers and academics say more skill involved, accuracy and precision improved, safer lab environment and manipulative skills improved. So all thats left to say is that it saves money, its safer and students like it, so why not try it! i ii Skinner, J., Microscale Chemistry Experiments in Miniature iii

Microscale Chemistry. Microscale Chemistry The Royal Society of Chemistry. Experiments in miniature

Microscale Chemistry. Microscale Chemistry The Royal Society of Chemistry. Experiments in miniature Filling titration apparatus Microscale Hoffman apparatus Sampling a bottle of hydroxybenzene Developing microscale chemistry experiments, using small quantities of chemicals and simple equipment, has been