Potassium hexacyanidochromate K3[Cr(CN)6] is a precursor to the coordination compounds contaning hexacyanochromate(III) anion that can bind to the central atoms in many different modes. It is a nice vivid yellow ionic compound forming needle-like crystals with molar mass of 325 g.mol-1 . The compound is soluble in water and insoluble in ethanol. It is stable in a neutral or slightly acidic enviroment. In alkaline solutions it is slowly converted into chromium(III) hydroxide. On heating it decomposes to leave chromium(III) oxide Cr2O3. It can be prepared in the four step synthesis described in available literature . As a starting substances potassium dichromate K2Cr2O7 (15 g, 50.99 mmol) was used.
1) Reduction of chromium(VI) to chromium(III)
Apparatus depicted below was used. Sulfur dioxide was produced by reacting powdered copper with concentrated sulfuric acid.t. Evolving SO2 gas was passed through potassium dichromate solution placed in a gas-washing bottle. A change of solution´s colour indicated the end-point of the reaction. (This step could be excluded if chromium(III) chloride is used instead of potassium dichromate). Cr2(SO4)3 is formed in this reaction.
2) Precipitation of chromium(III) hydroxide
A stochiometric amount of concentrated ammonia solution was added into the solution containing chromium sulfate. After precipitating chromium hydroxide 600 mL of distilled water was poured into the suspension and decanted after a while. This was repeated three times. The precipitate was filtered off using a frite G-4.
Cr3+ + 3OH– -> Cr(OH)3
3) Chromium(III) acetate preparation
Chromium(III) hydroxide was placed into a 500 mL beaker with 60 mL of glacial acetic acid. This was done under constant stirring (temperature about 100 °C). Reaction time about 240 minutes to get clear solution. The excess of acetic acid was removed by adding 350 mL of distilled water to the solution and evaporating the solution to the initial volume. (Warning!!! This step is important because in the last step it could lead to the formation of hydrogen caynide gas)
Cr(OH)3 + 3HC2H3O2 -> Cr(C2H3O2)3 + 3H2O
4) Addition of KCN – complex formation
Potassium cyanide KCN (50 g, ) was dissolved in 75 mL of distilled water. An excess of potassium cyanide was used as it supresses the hydrolysis of the final product. Under constant stirring, the chromium(III) acetate solution was added slowly through the funnel with a doubled filter paper (temperature 60 °C). After cooling to room temperature the final solution was filtered and placed in dark place to crystallize. The crude product was repeatedly recrystallized from 5 % KCN aqueous solution.
Cr(C2H3O2)3 + 6KCN -> K3[Cr(CN)6] + 3KC2H3O2