Correlation of SO2 and Fe2+ measurements with new spectral data indicates that the Liebermann-Burchard (L-B) and Zak color reactions for cholesterol have similar oxidative mechanisms, each yielding, as oxidation products, a homologous series of conjugated cholestapolyenes. These studies further suggest that the colored species observed in these two systems are enylic carbonium ions formed by protonation of the parent polyenes. Thus, the red (λmax, 563 nm) product typically measured in the Zak reaction is evidently a cholestatetraenylic cation, and the blue-green product in the L-B reaction (λmax, near 620 nm) is evidently the pentaenylic cation. The effects of rate of carbonium ion formation and sulfuric acid concentration on sensitivity and color stability are discussed. A solvent extraction procedure is described for specifically converting cholesterol to 3,5-cholestadiene. Incorporating this step into the typical L-B method can increase the L-B sensitivity for cholesterol by several fold.

mosphere. The boat is subsequently pushed slowly into an 800 °C inlet. The results of this latter technique on the fuel oil samples exhibited much greater precision (±2.0% relative standard deviation) and good accuracy, so that for routine analysis the latter procedure is considered the more reliable. The precision of the reductive sulfur technique for industrial petroleum samples is summarized in Table VIII.All samples with sulfur greater than 500 ppm were diluted in toluene to approximately 50 ppm for the coulometric data shown here. The number of duplicate analyses per man-day is on the order of twenty (40 determinations), exclusive of dilution time. ACKNOWLEDGMENTThe authors thank Dean Hoggan of ARCO Chemical Co., Anaheim, Calif., for permission to use the ASTM results presented and for valuable help in coordinating this study.

Errors in the meas ureme nt of the absorbances of liquid filters res ult from instrumental and c hemical un ce rtainties. This paper presents a systematic study of these variables on the absorbances of selected filters. Three types of liquid filters are di sc ussed. These are (1) individual solutions of hi gh purity compounds, (2) composi te mixtures and (3) aqueous solutions of organic dyes. The accuracy of th e absorptivity data is established using NBS-calibrated glass filters. The magnitude of the errors ari sing from spectral bandpass, beam geometry, stray light, internal multipl e refl ections, and refractiv e index are delin eated. Finally, as a practical outgrowth of this study, th e development and iss uan ce of NBS Standard Refere nce Material 931, Liquid Absorbance Standards for Ultraviolet and Visible Spectrophotometry, is described.

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We investigated complexes formed between bilirubin and transition or rare-earth elements, and their relative stabilities. Relative rates of complex formation were analogous to metalloporphyrin and metal complex stabilities, according to the Irving—Williams series. Transition metals that formed strong, square planar complexes caused rapid bilirubin degradation. Iron(II) was oxidized to iron(III) in the bilirubin complex. Of the lanthanide complexes, only samarium(III) showed covalent bonding tendencies. Comparative molar absorptivities were calculated for several of the metal— bilirubin complexes. The stability of bilirubin in several solvents and under various experimental conditions is reported. At room temperature, bilirubin photooxidizes to biliverdin on exposure to laboratory light.