Normal seawater contains about 2660 ppm (0.028m) of sulfur in the form of sulfate. Sulfur in this form has a valence of +6, meaning that it has six fewer electrons per atom than has native sulfur, which exists (though not in the ocean) as a yellow crystalline solid. In some parts of the ocean, however, sulfur exists in the form of dissolved hydrogen sulfide, H2S(aq). Sulfur in this form has a valence of -2, meaning that it has two extra electrons compared with native sulfur, and in this form it is a deadly poison. Those parts of the ocean containing this electron-rich form of sulfur contain no living organisms other than a few kinds of bacteria. Obviously, the number of electrons that each sulfur atom has is not a question of interest only to atomic physicists. Changing sulfate-sulfur to H2S-sulfur or vice versa involves transferring electrons from one to the other, and this electron transfer is the basic element of redox (reduction–oxidation) reactions.
Many naturally occurring elements in addition to sulfur show similar variations in their number of electrons, with similarly large differences in their chemical properties. It would be difficult to overemphasize the importance to us of these variations in valence, or numbers of electrons per atom. Biochemistry, for example, is in large part a study of redox reactions. Because natural environments show great variability in their redox state, we need to develop some kind of measurement, an index, which will be useful in characterizing these redox states, much as we use pH as a measurement or index to characterize the acidity of various states, or temperature as a measurement or an index of the hotness of states. In this chapter we develop two such indexes of redox state.
Electron Transfer Reactions
You may not have noticed it, but we have considered two kinds of reactions in previous chapters. In some, such as (9.3),
all elements on the right side have the same number of electrons that they have on the left side – there is no change in valence of any element. In others, such as (9.16),
there is such a change. For example, the carbon in CH4 is C4-, and the carbon in CO2 is C4+.