able number of points are already available within this range. This of course does not mean that points outside these limits are valueless for other purposes; they can be very useful, for example, in characterizing a situation with strong deviations from linearity.
Accuracy of reproduction is limited by the accuracy of the X-Y recorder slidewire and the accuracy with which the various controls can be set, and is affected by the sweep speed used. A sweep speed of 50 or 20 inches/sec is about right for typical spectral curves, with lower speeds for sharp fine-structure-type peaks and greater speeds for flattish curves. With the Moseley components described, the accuracy is such that if unit scaling factors are used, the reproduced curve will be in register with the curve being reproduced within ca. 0.5 mm at worst. As a further test of accuracy, experimental visible absorption spectral curves of green-blue Cr(en)(OH2)2Br2+ and magenta Cr(enXOH2)3Br2+ (5) were converted by the curve expander to plots of molar absorbance index aM vs.wavelength on a common scale and superimposed to locate the isosbestic points to be expected in the conversion of the dibromo to the monobromo complex. These predicted values, namely, 430 (24.5), 492 (24.5), and 580 µ (32.0 M-1 cm-1), agreed well with the experimental values found (6) by scanning the changing spectra of a solution of the dibromo complex during its aquation to the monobromo complex,(5) R. G. Hughes and C. S. Garner, Inorg. Chem., 6, 1519 (1967.namely, 432 (25.0), 492 (25.0), and 585 µ (31.5 M-1 cm-1). The values in parentheses are the molar absorbance indices, and the average estimated probable errors in predicted and found values are ± 2 µ and ± 0.5 M-1cm-1.
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