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Nov. 5. 1962 RHODIUM SALTS AS BUTADIENE P O L Y M E R I Z A T I O N C a T A L Y S T S 4145 (iii) A portion of the reactants disappears to give material other than CbHg isomers a t a rate which is nearly independent of time. Evidently both cis and trans isomers participate in this reaction which presumably gives high boiling material.(iv) Beyond the maximum in the concentration of cis-pentadiene, the reaction ck-CHs= CHCH=CHCHa + hv d ~uLEs-CH~=CHCH=CHCHI (10) may account for the decrease in the cis isomer.These results give more direct evidence for the statement made in an earlier section that the general reaction2 is probably rapid when compared to the rates of rotation around the carbon-carbon bonds in the molecule. Reaction 8 may be pic-$2 V tured as proceeding through an excited state (V) which is probably the same as the state which leads to the cis isomer through rotation around the bond between C3 and C4. The latter operation has no energy barrier in the excited state up to an angle of r/214 and its rate may be comparable to the rate of rotation around the bond between CZ and Cp. Since the rate of reaction 8 is observed to be faster than reaction 9, it follows that the former is favored by a suitable configuration in the ground state in the molecules which undergo the process. l5Acknowledgment.-The author wishes to thank Dr. Harold L. Friedman for his advice and encouragement during the course of this work. He is grateful to Drs. J. Kumamoto and J. C. Powers for many helpful discussions.

PHOTOCHEMICAL STUDIES. XV 4143 ever, the ferric iron may be detected qualitatively in the presence of much higher concentrations of copper. For example, a small excess of reagent was added to a solution containing Fe+++ 8 p. p. m. and Cu++ 40 p. p. m. A white precipitate was formed which settled upon standing for a few minutes, giving a dark green supernatant solution showing the presence of ferric iron. Salts that easily hydrolyze, such as those of tin or titanium, interfere with the color matching unless their concentration is very low.We desire to express our thanks to G. D. Searle and Co., Chicago, 111., who furnished us with the 7-iodo-8-hydroxyquinoline-5-sulfonic acid used in this investigation. J. D. Riedel-E. deHaén A. G., Hannover, Germany, supplied a number of pure salts of the rare elements, for which we are very grateful. Summary 7-Iodo-8-hydroxyquinoline-5-sulfonic acid is a very sensitive colorimetric reagent for ferric iron, its yellow aqueous solution changing to blue or green upon the addition of ferric ions. It gives no color reaction with ferrous ions nor with a large number of other ions. The reagent is sensitive to 1 part of ferric iron in 10 million parts of solution when the observation is made in a 50-ml. (210 mm. to mark) Nessler tube. The reaction is best carried out in a solution acid to methyl orange paper. The color is stable to light (an advantage over the thiocyanate method) but is destroyed by strong acids and bases. Salts that hydrolyze easily or yield colored ions should not be present unless in very low concentration.University, Virginia