T H E INFLUENCE OF LIGHT OK T H E COLOR OF FERRIC CHLORIDE SOLUTIONS BY KENNETH S. RITCHIE
Several years ago, in connection with the development of a spectro-colorimeter, ferric chloride solutions were used for the production of standard colors. A stock solution of ferric chloride was first prepared by dissolving clean iron wire (Baker’s “Iron Wire for Standardizing-99.84% Fe”) in aqua regia, the excess nitric acid being subsequently destroyed by three evaporations with concentrated hydrochloric acid. The residue was then treated with excess dilute hydrochloric acid and brought to a concentration of approximately zoo gm. of dissolved iron per liter. In preparing this stock solution, C. P. acid was always employed. This stock solution, containing ferric chloride and excess hydrochloric acid, was kept in a bottle wrapped with black glazed paper and from time to time other ferric chloride solutions were prepared from it by suitable dilution. Under these conditions color tints of constant values were obtained and used as standards for colorimetric work over a period of two years. However, upon exposure to strong sunlight or to strong artificial light, color changes took place in the ferric chloride solutions. The direction of these changes was reversed as soon as the solutions were withdrawn from the influence of the light and in certain instances measurements showed that the original color had returned. As a search of the literature failed to reveal any record of similar observations by other investigators working with ferric chloride solutions, a preliminary study was undertaken to learn more about the effect of light on the color of these solutions. Some semi-quantitative experiments involving the effect of strong sunlight upon the stock solution and various dilutions thereof were first made. These showed an increase in the color intensity of about 157~ after one day’s exposure and somewhat smaller effects for succeeding days. These measurements were made with a Kober-Klett colorimeter but without temperature regulation during the exposure period. The original color intensity returned after the ferric chloride solutions had remained in the subdued light of the laboratory for several days. Later the development of the spectro-colorimeter’ afforded an opportunity to study the light sensitivity of the ferric chloride solutions in more detail and particularly to determine the spectral curves which show the changes occurring in different regions of the spectrum. I n one experiment, for instance, the stock solution diluted to one-fourth of its original concentration was employed. This liquid was exposed to the influence of the intense light from the lamp in the spectro-colorimeter, while a stream of air carried the A paper describing this instrument will probably be published during the coming year.
KENSETH S. RITCHIE
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heat resulting from the absorption of radiation away from the solution cup and thereby kept the temperature of the liquid practically constant. Under these conditions four sets of measurements were made after light exposures of 4, 13, 5 and 64 hours, respectively. The results obtained are shown graphically in Fig. I , in which the fraction of the incident light transmitted by the solution is plotted as ordinate against the (decreasing) wave length of the
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2 .70
'E n
I
L" 4
.60
FIG.I
light as abscissa. It is evident that there is a marked increase in the light absorption in various parts of the spectrum and that on the whole the effect of equal times of exposure becomes progressively less with increasing time. From these experiments, it is evident that exposure to intense light produces a photochemical change in the three-component system H20, HCl, FeC18. This change i., moreover, reversible and the solutions slowly return to their initial condition on removal of the light. A change, somewhat similar to this produced by radiation, was also observed when the ferric chloride solutions were heated t o 8j"C for a half-hour period. The resulting liquid, when cooled to room temperature, then gave an increased light absorption comparable to that obtained by I $ hour's exposure to light. In this latter
COLOR OF FERRIC CHLORIDE SOLUTIONS
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case, the effect of heating is possibly to partially hydrolyze the ferric chloride in solution; and, hence, the intense radiation may be doing the same a t a much lower temperature. While the present study is only preliminary, it has seemed best to publish the results thus obtained, as a continuation of the investigation in the immediate future is improbable and a search of the literature has failed to show any record of similar observations on the sensitivity of the color of ferric chloride solutions to light. Before concluding the author wishes to thank Professor S. W. Young for his kindly guidance in the research and Professor George S. Parks for advice in preparing the manuscript. Department o j Chemistry, Stanford University, Calijornia.
