This vessel was capped with a watch glass in order to
avoid evaporation of the elements. Afterwards, the material was heated
to a temperature between 90 °C and 100 °C for 2min. Next, the content
was cooled to room temperature with subsequent addition of 50 mL of
deionized water and 15 mL of 5% (w/v) KMnO4 solution. The sample
was heated again under the same conditions for 30 min. After that,
after cooling the sample again, 50 mL of deionized water and 6 mL of