Aqueous solutions of organic media were examined by the extinction point method, using a geometrical series of concentrations, in a search for media having high gradation of absorption in the ultraviolet region.
Curves of log (concentration) against frequency ν of extinction point are shown for thirty-two compounds, grouped as to spectral region in which the curves have high slope. Five compounds had not been studied previously: gallic and tannic acids, aniline arsenate, arsanilic acid, and p-dichlorobenzene.
The quantity a, the slope of the curve of log i against ν, is shown to be the sole significant factor in determining gradation of absorption. Media having rectilinear log i curves are shown to have constant gradation of absorption of a degree proportional to the slope of the curve.
Curves of log b, log c, and log log 1/T are shown to have an invariant form, that of the curve of log i. It is shown that use of these curves locates directly the frequencies at which a is highest; leads to simple methods of testing Beer’s Law, of determining low solubility, and of finding “true half breadth” of absorption lines; and permits partially graphical solution of other absorption problems. The relations between the curve of a mixture of two solutes and the curves of the components are developed.
The extinction point method is shown to have certain advantages over other methods because of the low value of transmittancy involved, and to be adequate for determination of a when a is high. A modification of it, involving use of a constant low transmittancy T, is suggested.
© 1927 Optical Society of AmericaFull Article | PDF Article
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