Light Intensity Measurement by Means of Hydriodic Acid

Abstract

A good deal of reliance seems to have been placed by some investigators in Ecology on the decomposition of dilute solutions of potassium iodide and sulphuric acid in sunlight as a means of measuring light intensity; and as there appeared to be a certain amount of vagueness in the use of these reagents the present investigation was undertaken with a view to placing the subject on a more definite basis, the need for this having arisen out of some other work in hand on photosynthesis. A search through the literature showed that a considerable amount of work had been done on isolated aspects of the subject, the most useful general study being by A. v. Hemptinne (8) (1898), who tried the effect of light of different colours, various concentrations of acid, etc., but whose measurements on the whole are rather limited, and a very complete investigation by Plotnikoff (13) (1907), who, however, studied the reaction from the point of view of the concentration of the oxvgen present in solution. It seemwed worth while to re-study the influence of several factors under conditions which could be fulfilled readily in practice, and this has led, it is believed, to the establishment of some new results and also to the correlation of others which will be mentioned under the specific subject headings. It may be stated at once that the reaction is the oxidation of hydriodic acid by oxygen dissolved in the water (7), whereby iodine is liberated.