Studies in the Radiolysis of Ferrous Sulfate Solutions: III. Air-Free Solutions at Higher pH

Abstract

The present work extends the range of data of ferrous sulfate oxidation reported in the previous two papers (1, 2). The behavior of air-free solutions at pH levels of 1 to 2.1 has been found to be somewhat more complicated than at higher acidities. Experimental methods have been described in Part I (1). Figure 1 shows the 7-ray oxidation of air-free nitrogen-saturated 1 mM ferrous sulfate solutions in stoppered tubes containing no gas phase. Irradiations were made with and without added 1 mM chloride ion, at pH 1.57 and 2.10. Some additional points obtained at low doses are shown in the lowest curve of Fig. 3. It is seen that complete oxidation is never attained in these closed tubes; the rate of oxidation decreases with increasing dose until a steady state is eventually reached and no more change occurs. The addition of 1 mM chloride ion has no effect on the first part of the curve but markedly increases the rate in the later stages and leads to a considerably higher steadystate level of ferric iron. The initial values of G(FeIII) were 7.8 at pH 1.57 and 7.5 at pH 2.10. As explained in Part II (2), these values are interpreted as equal to the values of 2GH2 + 2GH, the yields of hydrogen atoms and molecules in the decomposition of water. We may assume GH2 = 0.45 at pH 1.5 and above, and 0.40 at lower pH (3, 4). Then we find for GH the values plotted in Fig. 2 as a function of pH, together with values of GH obtained in aerated solution and presented in Part II. Also shown is the value applying to pH 5 to 7, obtained from experiments on the combination of H2 and 02 in neutral solutions (5). The values from the aerated and air-free solutions agree well with one another and lie on a smooth curve. To ascertain the extent to which the growth of ferric iron in the solution is responsible for the decrease in rate with increasing dose, mixtures of ferric and ferrous ammonium sulfates were made up, freed of air by nitrogen bubbling, and irradiated with 7-rays. The oxidation of these mixtures is shown in Fig. 3. It is seen that in these solutions oxidation of iron always occurred, no net reduction being found even