Volumetric titrations in the modern curriculum

Volumetric titrations have been used in chemistry since the 1790s, when the first burette (similar to a graduated cylinder) was developed by Descroizilles, with the first true burette being developed by Henry in 1845. Titrations measure the volume of a liquid phase reagent that is required to completely and exactly react with a second reagent. The amount of one reagent is known, enabling the determination of the (unknown) amount of the second reagent. Volumetric titrations have been recognised as a quantitative analytical method since at least Mohr’s classic Textbook of analytical chemistry titration methods in the 1850s. Even so, a hundred years ago, while titrations were being more common in the chemistry syllabus, most volumetric analyses were still based on the gravimetric methods, based on measurements of mass. Since then, volumetric titrations have become a standard part of the curriculum, but students often have difficulty understanding their intricacies. In addition to the manipulative skills of using volumetric glassware, other learning outcomes include being able to read an upside-down burette scale correctly, choose an appropriate indicator, identify the endpoint, and do the titration calculations. These calculations are repetitive applications of the amount–volume–concentration relationship, but involve numbers of differing orders of magnitude, unit conversions, using the correct volume value at the appropriate calculation step, and many other details. Novices are usually presented with a long list of detailed instructions. It is unsurprising that titrations are often perceived to be recipe-driven, boring and mindless activities that do not engage students. ‘The sheer pointlessness of it – and the repetition – turned me right off chemistry’ is a common complaint from students. The point at which reaction is complete can also be detected using pH, potentiometric, amperometric, coulometric and photovoltaic (spectrometric) means. The electrical output from these detection methods, in combination with a motorised syringe pump and electronic controls, is the basis of automated titrations. The Australian Curriculum requires students to know that ‘techniques such as auto titration … are used to measure the chemical composition …’. Why should titrations be retained in the modern syllabus, when there are more advanced quantitative analytical methods available? At some universities, there has been a tendency to replace titrations and other classical manual techniques with instrumental methods. The primary advantages of titrations are that they are relatively simple and use equipment that is much cheaper than more advanced quantitative methods. While automated titrations are being used in many industrial contexts, manual titrations are still widely used in industry and research. For example, the Rymill winery in the Coonawarra region of South Australia uses auto titrations for total acidity and manual titrations for SO2, and proudly displays its analysis laboratory within its visitors’ centre. Titrations can be used for several educational outcomes. The obvious outcomes are to visualise when a reaction is complete, and to provide student data to determine the concentration of a solution. Less obvious outcomes are to correctly use glassware; for example, to adjust clamps to fingertight-ness, which will be a useful skill for clamping glassware in distillation, reflux, filtration and so on. Vygotsky argued the zone of proximal development (ZPD) is that area of learning that students cannot complete by themselves, but can be achieved with guidance or assistance. Activities that lie within the ZPD present doable challenges, while activities that lie beyond the ZPD result in frustration, with loss of motivation and engagement. The problem with titrations is not the lack of relevance, but that they are usually presented as an activity that is beyond the ZPD of many students. The solution is not to remove titrations from the syllabus, but to adjust the activity so that it lies within the ZPD. For example, as part of the ASELL for Schools project (blogs.deakin.edu.au/asell-for-schools-vic), we developed a fruit-juice titration activity for year 10, set in the context of determining which fruit juice might be the best in lowering some risk factors for kidney stones. Students compare the citric acid content of different fruit juices. The titration volume for total acid content is used as a proxy measure of citric acid concentration, without the need for any calculations. Typical student results for the comparison of fresh organic fruit juice and reconstituted fruit juice are so strikingly different (average titration volumes of 17.60 and 14.74 mL, respectively) that even incorrect rinsing of the glassware and other not-quite-right procedural steps will not affect the fundamental result. Classical methods, such as titrations, can offer rich learning opportunities for students. When the desired learning outcomes are not achieved, we should think about adjusting the laboratory learning activity, instead of throwing the baby out with the bathwater.