Gas Evolution Oscillators. 2. A Reexamination of Formic Acid Dehydration

Article abstract of DOI:10.1021/j100232a013

At formic acid concentrations of about 0.3 M in warm concentrated sulfuric acid, carbon monoxide is evolved smoothly whether the solution is stirred or not.If such a solution is rapidly stirred, decay of formic acid obeys clean irreversible first-order kinetics.If the solution is not stirred, the concentration of dissolved carbon monoxide rises to a limit of about 0.07 M; this value is about 80 times the equilibrium solubility at 1 atm.In an unstirred solution, the system approaches a "pseudoequilibrium" in which the concentrations of dissolved HCOOH and CO are about equal.If the concentration of formic acid is increased to about 4 M, gas is evolved from a gently stirred solution in oscillatory pulses.The amount of gas evolved during a pulse decreases with successive pulses, the maximum change in dissolved-gas concentration being approximately 0.07 M per pulse.These observations indicate that the oscillations result from repetitive release of supersaturation by homogeneous nucleation; they invalidate the purely chemical explanation developed by Showalter and Noyes.Supersaturations of up to 80-fold suggest that formic acid in concentrated sulfuric acid can generate carbon monoxide in situ at concentrations that could otherwise only be attained with high-pressure apparatus.