Resumen de Chemical Analysis of Household Oxygen-Based Powdered Bleach: An Engaging Approach to Teaching Sampling of Heterogeneous Materials and Addressing Statistics
Mauro S. F. Santos, Alexandre L. B. Baccaro, Guilherme L Batista, Fernando S. Lopes, I. G. R. Gutz
Crucial steps of a chemical analysis, such as sampling, determination, and statistical analysis of data, are addressed in a well-tested and stimulating laboratory activity that convincingly unveils the stratification of ingredients in a heterogeneous household product for laundry: a powdered oxygen bleach. Three different samples of bleach were taken from a single jar: one from the top, one from the bottom, and a more representative mixture collected by driving a core sampler (a graduated cylinder was found to be appropriate) throughout the full depth of the jar content. The resulting frequency distribution curves (histograms) of the H2O2 titrations with MnO4– presented widely separated maxima for the three samples, while no statistically significant difference was found for the titration of CO32– with HCl. These results intrigue the students, leading them to reflect until they understand that this is due to the presence of a single compound acting as a source of H2O2 (sodium percarbonate), while two ingredients in the formulation of the stratified bleach powder provide CO32– after dissolution. Large and light percarbonate grains are more frequent in the upper layer of the bleach jar, while smaller and denser Na2CO3 particles are richer at the bottom.
