Activity of arsenic in molten lead
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Apb-As, and AAs-As, over an extended composition range for
M.E. SCHLESINGER and D.C. LYNCH A review of the literature has yielded several studies of the Pb-As system. The validity of these studies has not been examined through comparison of results. This is due in part to the fact that the various authors have focused their attention on one species and neglected the other. The problem is further compounded by selection of a standard state for arsenic and confusion over the molecular composition of elemental arsenic vapor. In this review of the behavior of arsenic in lead alloys, care has been taken to ensure that the results are compared using the same standard state, namely, pure liquid arsenic. It has also been our goal to try to compare all vapor-melt equilibria studies using the same vapor pressure data. In this regard we have not been totally successful due to the limited results reported in some studies. The investigations reviewed in this communication are listed in Table I, together with the technique employed and the respective temperature and composition ranges studied. The results obtained in the two emf studies conducted by Zaleska 1 and Suleimanov e t al. z are the most extensive, and their data are suitable for extrapolation to conditions employed in other investigations. In both studies the oxidation potentials were measured against a pure molten Pb reference electrode and reported as a function of temperature. Using these results, and the Gibbs-Duhem equation, it is possible to calculate the activity of arsenic. The emf data were used to predict the natural logarithm of y ~ at the desired temperature and at various compositions. These results were curve-fitted to the subregular equation, lnypb = X 2 s / R T { 2 X p b a A s . P b + (1 - 2Xpb)apb-As + Xpb(2 -- 3Xpb)AAs_A~},
[1]
using a least squares routine. Eq. [1] was differentiated with respect to the mole fraction of lead and then multiplied by dXr~ to facilitate its substitution into the Gibbs-Duhem equation. Integration of the latter equation yields: lnyAs = X 2 b / R T { 2 X A s A p b _ A s + (1 -- 2XAs)AAs-Pb + XA,(2 -- 3XA~)AA~_As}+ C
[2]
where C is an integration constant. Normally this constant is zero when the integration is started at XAs equal to 1. However, the normal procedure cannot be used with the Pb-As system because it would involve integrating the Gibbs-Duhem equation, which contains the constants AAs-Pb, Table I. Investigator Zaleska' Suleimanov et al. z Itagaki et al. 4 Predel and Emam9 McClincy and Larson 1~
y*=
9 a g*~ / X g~
o = (PAJPA~4)
,14
, /XA,
[3]
where X*s is the mole fraction at the liquidus line, P~s 4 is the vapor pressure of the tetratomic species in the chosen standard state, and PAs4 is the actual vapor pressure over the melt, evaluated as noted above. With this information the integration constant in Eq. [2] can be determined. In this study the hypothetical liquid standard state for arsenic was selected for comparison of results. Values of P ~ , have been calculated using the following equation: 4As
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