The Kinetics of Penetration Xiv. the Penetration of Iodide into Valonia

Former p~pers from this laboratory indicate that the penetration of a weak base, ammonia,* and of a strong base, guanidine, 2 is probably preceded by a chemical reaction with a constituent of the protoplasm. On the other hand the penetration of a weak acid, hydrogen sulfide, s appears to take place by simple diffusion of molecular hydrogen sulfide through the non-aqueous protoplasm. What is the method of penetration of a strong acid? The present paper is an attempt to throw light on this problem by an investigation of the rate of entrance of iodide into Valonia macrophysa, Ktitz. The choice of hydrogen iodide for this investigation was a practical necessity, since of the other available acids, nitric ~ and hydrobromic, the former is already present in fairly large, but variable amounts, and the latter presents analytical difficulties in the presence of the high concentration of chloride ion in Valonia. Hydrogen iodide presents some theoretical difficulties owing to the possibility of oxidation or reduction in the cell. The reversible reaction, I, + 2 e ~ 2I-, goes with the greatest ease in either direction according to conditions, while with much greater difficulty any free iodine produced may be oxidized to iodate. This will be discussed later. On the other hand, hydriodic acid is as strong an acid as hydrochloric, and with proper care it and its salts can be determined accu-