Choosing a Malachite Synthesis

Numerous syntheses of the green painter’s pigment malachite, Cu₂(OH)₂CO₃, are proposed in the literature, some yielding distinctly blue products. To help instructors choose a method that will produce a green product, a laboratory experiment was designed to test eight procedures for making malachite. All involve copper(II) sulfate or nitrate and a carbonate source: Na₂CO₃, CaCO₃, NaHCO₃, or carbonated water (seltzer water). Some reactant mixtures are heated, and others are aged for a few days at room or low temperatures before filtration. Students evaluated the colors of the products by comparing their CIELAB color space, measured using a reflectance spectrophotometer, to that of a sample of natural malachite. As the data were displayed in plots, the students’ ability to interpret graphical information was tested. Five of the eight syntheses yielded green products, more vividly green than natural malachite, while three yielded blue. Thermal decomposition of the green samples to CuO gave results consistent with the formula of malachite. Infrared spectra support identification of the green products but not the blue products with malachite.