Clerici solution

Clerici solution is an aqueous solution of equal parts of thallium formate (Tl(HCO₂)) and thallium malonate (Tl(C₃H₃O₄)). It is freely flowing and odorless. Its color fades from yellowish to colorless upon decreasing the concentration of the thallium salts. At 4.25 g/cm³ at 20 °C, the density of saturated Clerici solution is one of the highest known. The solution was invented in 1907 by the Italian chemist Enrico Clerici (1862–1938).^[1] Its value in mineralogy and gemology was reported in 1930s. It allows the separation of minerals by density with a traditional flotation method. Its advantages include transparency and an easily controllable density in the range 1–5 g/cm³.^[2][3][4]

Clerici solution is more dense than spinel, garnet, diamond, and corundum as well as many other minerals.^[3] The density of the saturated solution (thallium formate plus thallium malonate in water) increases from 4.25 to 5.0 g/cm³ upon heating from 20 to 90 °C (194 °F).^[4] (Note that density of water decreases from 1.0 to 0.96 g/cm³ in the same temperature range.^[5]) It can be reduced by diluting the solution with water down to 1 g/cm³. The refractive index shows significant, linear and well reproducible variation with the density; it changes from 1.44 for 2 g/cm³ to 1.70 for 4.28 g/cm³. Thus the density can be easily measured by optical techniques.^[2]

The color of the Clerici solution changes significantly upon minor dilution. In particular, at room temperature the concentrated solution with the density of 4.25 g/cm³ is amber-yellow. However, a minor dilution with water to the density of 4.0 g/cm³ makes it as transparent as glass or water (absorption threshold 350 nm).^[6]

Procedure for determining mineral density using the Clerici solution are available.^[2]

One drawback of the Clerici solution is its high toxicity and corrosiveness.^[2][3] Today sodium polytungstate has been introduced as a replacement, but its solutions will not reach as high in density as the Clerici solution.

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