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.

Motivating students in analytical chemistry can be challenging, in part because of the complexity and breadth of topics involved. Some methods that help encourage students and convey real-world relevancy of the material include incorporating environmental issues, research-based lab experiments, and service learning projects. In this paper, we describe an approach that combines all three of these methods by integrating environmental research-based activities into the second-year undergraduate analytical chemistry course. We discuss the development, implementation, and preliminary evaluation of the research-based labs employed during the new summer analytical chemistry course. Students perform environmental investigations of sites on Beaver Island, Michigan, and prepare reports to contribute to an ongoing research project analyzing these locations that began in the 1970s. Preliminary impacts on analytical students were examined using pre-and postsurveys, including the Chemistry Attitudes and Experiences Questionnaire, and a new survey and questionnaires developed for this work. Responses and grades were compared across three summers, and to those from students in the traditional analytical course. Results suggest that the research-based activities positively impacted aspects of student attitudes, their perceptions of how chemistry knowledge influences understanding of environment issues, and their perceptions of how analytical techniques are applied in the real-world. Students indicated that the new labs provided real-world applications of class content, helped them learn new concepts and gain skills working with others, and helped them feel more confident conducting chemistry-related experiments. ■ INTRODUCTIONAnalytical chemistry offers unique challenges in motivating students; the entire course requires continued fluency in algebra and students can easily be overwhelmed with its breadth and complexity of topics. Conveying a relationship between the content and the real-world is a crucial factor in increasing student motivation and attitudes toward the subject. 1,2 The integration of environmental topics provides a great opportunity to illustrate the relevancy of the content and its real life applications. 3−8 The incorporation of research-based activities into the chemistry laboratory has been shown to have positive impacts on student attitudes and skills. 9−20 Furthermore, projects with service learning components are known to have positive effects on student outcomes including student critical thinking skills, efficacy for civic engagement, and the ability to integrate theory and experience. 21−27 We wondered how the combination of these approaches for incorporating environmental issues, research activities, and service learning would affect analytical chemistry students. We previously described the preliminary impacts of a researchbased lab in our honors general chemistry laboratory, and now we have expanded the work to the undergraduate analytical chemistry course. 28 In this paper we discuss the developm...

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.

. The absorbances of five concentrations of potassium dichromate in 0.001 ]l.f perchloric aCid have been determined at eight wavelengths in the ultraviolet on the National Bureau of Standards Institute for Materials Research high-accuracy spectrophotometer. Four of the wavelengths-235, 257, 313, and 3.50 nm-correspond to absorbance maxima or minima in the HCrO,-spectrum and are useful wavelengths for checking the accuracy of the absorbance scale of narrow bandpass speetrophotometers. Although partial dimerization of nCrO,-to Cr207-prod~c.e~ small positive deviations from Beer's law at these wayelengths, the apparent absorptlvltles calculated for each concentration arc reproducible to one part in a thousand. The estimated uncertainties in the absorptivity values are ± 0.7 percent at 0.1 absorbance (A) and ± 0.2 percent ncar A = 1. These uncertainties include all known sources of possible systematic error and the 95 percent confidence level for the mean. The remaining four wavelengths used for measurement arc ncar two predicted isosbestie points in the HCrO,~/Cr207-sp~ctra . The absorptivities at 345 nm arc sufficiently independent of concentratIOn that thiS wavelength can be used for checking absorbance linearity to one part in a thousand over the range A = 0.2-1.~cy words: ~bsorbance linearity; accuracy; acidic potassium dichromate solutions; calibratIOn of ultravIOlet speetrophotometers; liquid filters; transfer standards; ultraviolet absorbance standards.

A new method of estimating the amount of heterochromatic stray light in UV spectrophotometers is described. The method uses the same solution filters with sharp UV absorption edges as ASTM Test Method E387, but one measures the apparent absorbance of a 10-mm path-length cell in the sample beam relative to a 5-mm cell in the reference beam. Scanning toward shorter wavelengths, one records an apparent absorbance maximum which is a direct measure of the stray light. This method was found to be in satisfactory agreement with the ASTM method in comparative tests of several spectrophotometers at different wavelengths between 200 and 390 nm, using KC1, KI, Nal, acetone, and NaNO(2) solution filters. The new method proved to be simpler, the main advantage being that the apparent absorbance maximum occurs at considerably lower scale values than the corresponding absorbance plateau measured by the ASTM method. This reduces the need for successive attenuations of the reference beam every time the spectrophotometer runs off scale. In many instances the new method required no attenuation at all.