Oxygen Bleach under the Microscope: Microchemical Investigation and Gas-Volumetric Analysis of a Powdered Household Product

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• 作者：Fernando S. Lopes ; Alexandre L. B. Baccaro ; Mauro S. F. Santos ; I. G. R. Gutz
• 刊名：Journal of Chemical Education
• 出版年：2016
• 出版时间：January 12, 2016
• 年：2016
• 卷：93
• 期：1
• 页码：158-161
• 全文大小：230K
• ISSN：1938-1328

文摘

A closer look at particles of a solid oxygen-based bleach household product is proposed, both figuratively and literally, to help develop observation and critical thinking skills and to link everyday life to chemistry concepts like acid–base equilibrium, redox reactivity, gas laws, gas volumetry, stoichiometry, and morphology of a heterogeneous solid mixture. In a microchemical approach, students learn to discriminate particles by their morphology under an optical (or low-cost USB) microscope and to investigate how they react with (a) hypochlorite solution (or household chlorine bleach); and (b) acetic acid (or vinegar). Aware of the ingredients listed on the product’s label, and searching for the structure and formula of percarbonate (2Na₂CO₃·3H₂O₂), a hydrogen peroxide–sodium carbonate adduct, they end up concluding that the particles showing effervescence with both reagents consist of sodium percarbonate, and those bubbling only in contact with acetic acid are sodium carbonate, while other minor ingredients do not react visibly. In products with perborate replacing percarbonate as H₂O₂ source, bubbling occurs only with hypochlorite. Next, students are challenged to determine the content of sodium percarbonate and sodium carbonate in a weighed amount of bleach by performing the same reactions in scaled-up experiments of quantitative gas volumetry of O₂ and CO₂. A graduated cylinder filled with water and turned upside down into a beaker is suitable for the task of collecting the volume of O₂ or CO₂ evolving in a Kitasato flask containing the sample and coupled to a syringe for reactant injection. The volume of released O₂, measured at ambient conditions, allows the students to calculate the concentration of sodium percarbonate in the weighed sample by applying the ideal gas law and the reaction’s stoichiometry. The volume of CO₂ measured in excess to that originated only from the decomposition of the percarbonate lets them calculate the sodium carbonate content.