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In light absorption spectrometry, plotting the data in the form of absorptancy against logarithm of concentration has important advantages over other plotting methods. For many instruments and operating techniques, the optimum range can be defined by inspection of the curve, as it is the concentration range in which it has its steepest slope. The maximum attainable accuracy is easily derived from the slope of the curve; the very flat portions of the curve at low and at high absorptancy emphasize the importance of selection of proper range if best accuracy is to be realized.Various methods of accomplishing measurement in the optimum range are discussed. For systems conforming to Beer's FROM time to time during the past several years, various authors have discussed the errors involved in photometric methods of analysis. Twyman and Lothian (30) showed that the minimum error occurs

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Cobalt(ll) reacts rapidly with 2,4,6-tris(2'-pyridyl)-$triazine (TPTZ) in water-alcohol solution at pH 8.5 to give a red-orange solution having absorption peaks at 485 and 404 nm. The color is stable for at least an hour. The system conforms to Beer's law. Optimum range for 1.00-cm optical path is 4 to 20 ppm of cobalt, determined with a relative standard deviation of about 1%. The molar absorptivity at 485 nm is 2.8 X 103. Ions of several transition elements interfere. Cobalt is separated from iron and nickel by an extraction procedure using tri-n-butyl phosphate. A reaction ratio of 1 to 2 for cobalt to TPTZ has been deduced from spectrophotometric data, and the perchlorate salt of the complex has been isolated and analyzed. The ionization constant of the reagent and the formation constant of the cobalt-TPTZ complex have been evaluated.