Clerici solution

Clerici solution is an aqueous solution of equal parts of thallium formate (Tl(HCO₂)) and thallium malonate (Tl(C₃H₃O₄)). It is free-flowing and odorless. Its color fades from yellowish to colorless when diluted. At 4.25 g/cm³ at 20 °C (68 °F), saturated Clerici solution is one of the highest known density aqueous solutions. 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]

Saturated Clerici solution is more dense than spinel, garnet, diamond, and corundum, as well as many other minerals.[3] A saturated Clerici solution at 20 °C (68 °F) can separate densities up to 4.2 g/cm³, while a saturated solution at 90 °C (194 °F) can separate densities up to 5.0 g/cm³.[4] The change in density is due to the increased solubility of the heavy thallium salts at the higher temperature. A range of solution densities between 1.0 and 5.0 g/cm³ can be achieved by diluting with water. 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).[5]

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 do not reach as high in density as the Clerici solution.