A NOVEL OXIDATION-REDUCTION REACTION BETWEEN FORMALDEHYDE AND CYANIDE: I. STOICHIOMETRY, THERMODYNAMICS AND MECHANISMS. II. APPLICATION TO FORMALDEHYDE ANALYSIS BY ENTHALPIMETRIC METHODS (THERMOMETRIC, ORGANIC).

摘要

Potentiometric titration of formaldehyde with cyanide at pH 12 using a platinum indicator electrode yielded well-defined titration.;H(,2)CO + CN('-) + H(,2)O = CH(,3)OH + CNO('-) (1).;(TABLE ENDS).;This reaction is remarkable, as formaldehyde functions as an electron acceptor, whereas it is normally a reducing agent.;curves. The analytic geometry of these curves substantiated occurrence of the redox process (UNFORMATTED TABLE FOLLOWS).;Formal heat of reaction, Gibbs free energy, and entropy were evaluated at 25(DEGREES)C in 0.01 mol dm('-3) aqueous sodium hydroxide by.;combining calorimetric and potentiometric measurements. The following assignments were obtained: (UNFORMATTED TABLE FOLLOWS).;(DELTA)H('o)' = -82.1 (+OR-) 1.1 kJ mol('-1).;(DELTA)G('o)' = -67.5 (+OR-) 1.9 kJ mol('-1).;(DELTA)S('o)' = 50.0 (+OR-) 0.5 J mol('-1)K('-1).;(TABLE ENDS).;The corresponding equilibrium constant calculated from the Gibbs free energy of Reaction 1 is (UNFORMATTED TABLE FOLLOWS).;(' a)CH(,3)OH ('a)CNO('-).;K = = 7 x 10".;(' a)H(,2)CO ('a)CN('-.;)(TABLE ENDS).;where a denotes activities of each species. This relationship predicts that the redox reaction proceeds to virtual (99%) completion when formaldehyde and cyanide concentrations are greater than or equal to 10('-5) mol dm('-3). This was verified experimentally. Moreover, thermodynamic equilibrium was rapidly attained.;Methods for formaldehyde determination based on thermometric enthalpy titration and direct injection enthalpimetry were developed, predicated on Reaction 1. Solutions as dilute as 10('-5) mol dm('-3) were successfully analyzed with relative precision.