[OLD] Dominant fields on log f O2 vs. pH

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From: Greg Anderson

Subject: dominant fields on log f O2 vs. pH

Recently I added data on polysulfides and sulfanes to the GWB database from various sources. The concentrations of polysulfides from Giggenbach's data are considerably higher than those in the database (thermo.com.V8.R6.full) which are from Shock & Helgeson (1988), but still far below H2S(aq). While doing this, I came upon a problem: on a log f O2(g) vs. pH diagram at 25 C, using my new database, Act2 shows a stability field for the polysulfide S5--, although on a log concentration vs. log f O2(g) plot (pH 6.5) made by React, this species lies far below the dominant species (H2S(aq)). I thought the stability fields on log f O2 vs pH were dominant species only. Using the old (LLNL) database, the field does not appear, although, as I said, it has a much lower concentration. I tried to make the Act2 and React compositions the same.

 

From: Craig Bethke

Subject: Re: dominant fields on log f O2 vs. pH

I looked at the calculations and at first glance they appear OK. I wonder if you remembered that since you are dealing with polymeric species you need to include the stoichiometric factor? In this case, for example, the diagram is drawn taking the activities of SO4-- and H2S(aq) to be .01, but the activity of S5-- is just 0 .002 (the stoichiometric factor is 5, since the polymer contains five sulfurs). The net effect is to increase the size of the stability field for S5-- relative to its size if we had set its activity to .01. As I remember there are two important reasons to remember the stoichiometric factor. First, if we omit it, the resulting diagram isn't in a "constant mass" plane (it really isn't anyway, in the strict sense, since activity coefficients vary from species to species). Second, without it the lines don't meet at triple points when polymeric species are considered. By the way, the quickest way to get to the bottom of questions like this is to look in the Act2_output.txt dataset. It contains all of the information used to construct the diagram. (If you set "equations = on", it will even contain all of the equilibrium lines, even those discarded.) This way, you don't ever have to think of Act2 as a "black box".