ABSTRACT
Errors in the measurement of the absorbances of liquid filters result from instrumental and chemical uncertainties. This paper presents a systematic study of these variables on the absorbances of selected filters. Three types of liquid filters are discussed. These are (1) individual solutions of high purity compounds, (2) composite mixtures and (3) aqueous solutions of organic dyes. The accuracy of the absorptivity data is established using NBS-calibrated glass filters. The magnitude of the errors arising from spectral bandpass, beam geometry, stray light, internal multiple reflections, and refractive index are delineated. Finally, as a practical outgrowth of this study, the development and issuance of NBS Standard Reference Material 931, Liquid Absorbance Standards for Ultraviolet and Visible Spectrophotometry, is described.
ABSTRACT
In aqueous ethanol cobalt(II), nickel(II) and copper(I) react instantaneously with 2,3-quinoxalinedithiol at pH 2 to form strongly coloured complexes exhibiting absorption maxima at 510, 606 and 665, and 625 nm, respectively. At pH 6 the reaction of copper can be virtually eliminated, while the cobalt and nickel reactions show only small decreases in sensitivity. This behaviour has been made the basis of a rapid method for the simultaneous determination of these three elements. The absorbances of the complexes have been found to be additive for all combinations of these elements in which the individual concentrations have been varied from approximately 0.1 to 1 ppm. A least-squares estimate of the concentration of any of the elements may be readily obtained by using a simple linear equation based on the absorbance readings and a set of constants derived from the spectrophotometric data. The analyses of seven synthetic solutions and four NBS Standard Reference Materials yielded a maximum error of 6.0% and an average error of 1.6%.
