chemical thermodynamics of neptunium and plutonium - U.S. ...

574 A. Discussion of selected referencesFigure A.1: Calculated log 10 K for average data with the adjustment of E by the author.The line is log 10 K = 0.39 + 0.145 [NO − 3 ]1.00.80.6log K0.40.20.00.0 0.5 1.0 1.5 2.0[NO 3 ]Since in a spectrophotometric study the properties of the complex are measured,its existence is established. Hence the speciation proposed by the author together withthe value calculated for the constant at I = 2 M are accepted in the present review.Properly, in order that the value can be converted to the molal scale and used in thestandard SIT procedure (see Appendix B), the constant should be extrapolated so asto apply to trace nitrate concentrations at I =2M(HClO 4 ). For this reason the valueestimated by the author using the assumption that the activity coefficients, and alsothe constant K (A.1, I = 2M), do not change while the medium is changed from2MHClO 4 to2MHNO 3 , is accepted. However, for this reason (and because thestudy was carried out at 20 ◦ C rather than 25 ◦ C) the estimated uncertainty is increasedto 0.20. If an attempt is made to take account of the variation in the ionic mediumat constant ionic strength using the SIT, a linear dependence of the constant on theconcentration of nitrate is predicted. As can be seen from Figure A.1,thisseemstobethe case, and from the intercept of the line at zero nitrate, a value of log 10 K(A.1, 2MHClO 4 ) = 0.39 ± 0.1 is estimated. The results for the mixtures (9.1 M NH 4 NO 3+ 1 M HNO 3 )and(9.1MCa(NO 3 ) 2 + 0.1 M HNO 3 ) are further complicated by theinteraction of the negatively charged hexanitrato complex ion with NH + 4 and Ca2+ .The results from the measurements at high ionic strength from the work of Hindman[49HIN] are unlikely to provide thermodynamic data that can be used for the purposesof the present review, and are not considered further here.