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.

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Because of questions concerning the suitability of potassium nitrate as a dynamic temperature standard for DTA, the relation between the experimental procedure and the resulting curves was ascertained for the Standard Reference Material KNO 8. The material behaves differently on cooling in open pans than in cylindrical holders because confinement in the latter case initiates reversion to the room temperature form. Under the conditions of use as a dynamic temperature reference material, the curves are accurately reproducible.Recently, Deshpande, Karkhanavala and Rao [1] in their thermal analysis studies called attention to the behavior of B.D.H. AnalaR potassium nitrate. As a result, they have questioned the suitability of potassium nitrate in general as a Standard Reference Material. The material, as is typical of many crystalline materials, may display pronounced variability of thermal behavior depending on its origin. This may be due to trace impurities, thermal or mechanical history or to a combination of factors.For this reason, and as a result of earlier studies by various members of the Standards Committee of the International Confederation of Thermal Analysis (ICTA) and the National Bureau of Standards (NBS), the certification of the temperature transition [2] was based on selected batches of materials of known homogeneity. The materials were selected to provide reproducible (_+0.5~ crystallographic transitions or melting points. The dynamic transition temperatures given for each were based on mean values which were obtained on a variety of instruments [2]. Thus, the joint certification of these and similar materials serves to facilitate the communication of relatable data among research laboratories. Their thermodynamic enthalpy of transition data were not certified, and the Standard Reference Materials at present are not intended for this purpose. It was gratifying to find satisfactory agreement between the dynamic experimental temperatures and the equilibrium transition temperature [2].There is an apparent dissimilarity between the AnalaR potassium nitrate and the ICTA-NBS Standard Reference Material, potassium nitrate. The latter undergoes two clearly defined transitions on cooling of a compacted sample. The peak areas on reheating have been found to be visually identical to those of the first heating. Deshpande et al. found little evidence that the AnalaR material 2