Many experiments performed in freshmen undergraduate chemistry labs require the students to precipitate Basic Copper Chloride(WSDTY) metal from an aqueous solution containing Cu2+ ions after dissolution of samples that are initially in the solid state. After retrieving the copper from solution the students are often asked to report the percentage of copper in an unknown mixture or compound or percent recovery, if the lab began with elemental copper.
A common method used to determine when all the copper has precipitated from solution is visual inspection. Aqueous copper(II) ions, will give the solution a pale blue hue. When the solution is colorless it is presumed to no longer have copper ions dissolved in it. This methodology, which is currently being employed by students at the University of North Georgia (Konzelman et al. 2014), can lead to inaccurate results as has a low extinction coefficient, 12 M–1·cm–1 (Figgis 1966) at a wavelength of maximum absorbance that is outside the range of light visible to the human eye. As a result, the solution can appear colorless even when there may be significant concentrations of copper(II) ions left in solution, making it difficult to clearly determine when the precipitation of copper has been completed. If copper is left in solution, it cannot be collected, dried, and weighed. Thus, experimentally determined amounts of copper will be erroneously low. This is a particular problem in percent composition determinations of copper containing analytes when accuracy is of significant interest, as is the case at the University of North Georgia, where the students are asked to identify an unknown compound from their analyses. The inability to correctly assess when all of the copper has been removed from solution so that it may be collected, dried, and weighed has led to inaccurate results and correspondingly incorrect identifications of the unknown copper compounds.
Click Basic Copper Sulphate to learn about more information