Reaction kinetics of sonochemical oxidation of potassium hexacyanoferrate(II) in aqueous solutions

•Iron in hexacyanoferrate(II) complex is efficiently oxidised by ultrasonic treatment at 536 kHz.•Cyanide groups are not oxidised by ultrasonication of hexacyanoferrate(II) at 536 kHz.•High formation rate of hydroxyl radicals is obtained by focusing ultrasonic waves. We studied sonochemical reactions resulting from ultrasonic treatment of potassium hexacyanoferrate(II) in aqueous solutions using a custom-built apparatus working at 536 kHz. We concluded that primary reactions are completely dominated by oxidation of Fe(II) to Fe(III) and did not find any evidences for degradation of cyanide. At the highest concentration used in the present study (0.1 M) we detected formation of pentacyanoaquaferrate(II) complex, which is most probably formed in reactions between hexacyanoferrate(III) anions and hydrogen atoms or hydrated electrons formed in sonochemical processes. We also determined that hydroxyl radicals formation rate in our system, (8.7 ± 1.5)∙10−8 M∙s−1, is relatively high compared to other reported experiments. We attribute this to focusing of the ultrasonic wave in the sample vessel. Finally, we suggest that oxidation rate of hexacyanoferrate(II) anions can be a convenient benchmark of efficiency of sonochemical reactors.