daylight compared to darkness-also showed no effect. ,4 precision within 2% was obtained when the conditions prescribed for color development iTere adhered to strictly.Nature of Colored Compound. Little has been learned about the nature of the color reaction. The literature is nearly devoid of mention of color formation between halogens and vitamins D or related sterols. Although the reaction of Tortelli and Jaff6 ( I f ) may be related in principle to the reaction disclosed here, the techniques used are different, and the resulting colors are dissimilar. I blue-green color observed by Green ( 5 ) between his iodine trichloride reagent and p-carotene may be related also. Green postulated a mesomeric change in the carotene moleculd under the influence of IC&, C1-, or Ciasimilar to that produced by antimony trichloride on carotenoids and vitamin D. Although no direct evidence for a mechanism was observed in the authors' experiments, it is suggested that a loose union occurs between iodine and the unsaturated center of the D vitamins. More specifically, because traces of bromine increased both €he intensity of color and the speed with which maximum color was attained, it appears that iodide ion may be the active form which unites with the vitamin to form the colored product. As the enhancing agents did not change qualitatively the spectral ab-sorption properties of the colored vitamin D complex, their function may be to promote the reaction by increasing the concentration of the iodide ion.The condensation of urea with acid diacetyl monoxime (Fearon reaction) with concomitant oxidation by arsenic acid has been extensively studied in an effort to improve reproducibility and the linearity of response of the reaction. The concentration of mineral acid and oxidizing arsenic acid was found to be critical. By performing the reaction in 3.8N hydrochloric acid and 0.08V arsenic acid maximum color is produced which conforms to Beer's law at urea concentrations up to 60 y per 10-ml. reaction volume. Dilution of the reaction mixture results in a deviation from Beer's law, and the urea response curve no longer passes through the origin. By the study, a rapid and accurate method for the determination of urea in blood and urine has been developed. Comparative studies with existing methods and recovery studies have shown the suitability of the procedure. Analysis of a sample in duplicate requires less than 1 hour.H E need for a direct, simple, and accurate method for de-T termining urea in blood and biological fluids has resulted in the development of a variety of direct and indirect procedures. The indirect methods, which are the most widely used, depend on the hydrolysis of urea with the enzyme, urease, to form ammonia. The liberated ammonia is usually determined by direct nesslerization, by aeration and nesslerization, or by aeration and titration (3, 7 , 9, If). The methods for the direct determination of urea depend upon the condensation of urea with a-isonitrosopropiophenone (1) or diacetyl derivatives (2, 5, f2, f3) in th...
