THE RUDOLF REPORT

GERMAR RUDOLF · THE RUDOLF REPORTof chemisorption 313 on solid surfaces deserves being mentioned, wherethe hydrogen cyanide releases its proton (H + ) to an alkaline oxide andis itself attached to a metal ion.Absorption and dissociation of the superbly soluble hydrogen cyanide(see chapter 6.5.4.) is clearly superior to chemisorption. Furthermore,the aqueous solution (as solvent) is indispensable for the complexformation and redox reactions of the cyanide with Fe 3+ . Additionally,the aqueous medium makes the reacting agents mobile, which donot always form at the same location. And finally, the moisture containedin the solid material works as a trap for hydrogen cyanide, becauseit intensely binds the hydrogen cyanide. Or the other wayaround: the drier a solid material is, the easier hydrogen cyanide,which was ad-/absorbed before, will be released back into the environment.Therefore, a relatively high water content of the solid materialwill accelerate the reaction.Experiments with reactions of hydrogen cyanide (some 4 g per m³in air, 15°C, 75% rel. humidity) with mixtures of Fe(OH) 2 -Fe(OH) 3attached to wet paper strips showed that a blue discoloration occurredafter 30 min at a pH value 342 of 2 to 3, since at such low values almostno hydrogen cyanide dissociates to the reactive cyanide (see chapter6.5.5.). At pH values of 7 to 9, a visible blue discoloration occurredafter a few minutes of inserting the sample. At higher pH values, thistime span grew again, because the initially absorbed hydrogen cyanidehad to lower the pH value first, before it could form the pigment (seechapter 6.6.1., pH Sensitivity).These experiments show clearly that undissociated, gaseous HCNor HCN dissolved as gas shows no reactivity. An addition of smallamounts of KCN to an aqueous sulfuric acid solution of Fe 2+ /Fe 3+ ,however, results in the immediate precipitation of the pigment. Thecyanide obviously reacts faster with the iron salts than it is protonatedby sulfuric acid, i.e., converted into hydrogen cyanide.342 pH (pondus hydrogenii) is a measure for the acid content of aqueous solutions (negative,decadic logarithm of H 3 O + concentration: -lg 10 (c(H 3 O + ))): pH < 7: acidpH = 7: neutralpH > 7: alkaline162