Comment/Reply pubs.acs.org/jced
Comments on “Liquid−Liquid Equilibria for the Quaternary System H3PO4−NaCl−H2O−TBP at 298.15 K” A. Marcilla* and F. Ruiz-Beviá Department of Chemical Engineering, University of Alicante, P.O. Box 99, Alicante, 03690, Spain paper1 entitled “Improvement of Quality in Publication of Experimental Thermophysical Property Data: Challenges, Assessment Tools, Global Implementation, and Online Support” was recently published in the Journal of Chemical & Engineering Data. This article, to which we will refer as the reference paper throughout the text, describes a 10-year cooperative effort between the U.S. National Institute of Standards and Technology (NIST) and five major journals in the field of thermophysical and thermochemical properties to improve the quality of published reports of experimental data. The scientific community applauds such initiative that will definitively improve progress in found knowledge. Nevertheless, the NIST-journal cooperation is presently limited to “pure binary and ternary chemical systems” and extending such cooperation to other systems is becoming an urgent issue. The paper “Liquid−Liquid Equilibria for the Quaternary System H3PO4−NaCl−H2O−TBP at 298.15 K”, recently published in the Journal of Chemical and Engineering Data,2 to which we will refer as the paper2 throughout the text, is a clear example indicating that something else must be done. This article was published after the previously mentioned article providing clear recommendations regarding the type of data an article should provide. Nevertheless it accumulates a series of unfortunate examples of what should be avoided. Moreover, such examples are easy to detect, and it seems quite incredible that they were not noticed by the referees. The Statistics for Types of Problems Found section of the reference paper indicates that one of the most frequent problems is related to data that are inconsistent or show anomalous trends when plotted as a function of a particular variable. The authors present their Figures 10, 11, and 12 illustrating such problems. Returning to the paper,2 the first example, and the most readily visible among those selected in the present letter, refers to its Figures 1 to 3. The trends shown in the paper2 are by far more evident of anomalous behaviors than those selected by the authors of the reference paper to illustrate the problems. In the case of the paper,2 the lines even cross each other on several occasions. Moreover, the argument provided to explain the increase in D1 when increasing the concentration of phosphoric acid or sodium chloride (paper2 Figure 1) is somewhat weak and not proven at all. This Figure already shows several features about which the authors do not comment at all, such as the unexpected behavior of D1 exhibiting crossing curves. The discussion regarding their Figure 2 merits some comments. The Figure has been split into two in order to visualize the behavior of the separation factor S12, especially at the higher NaCl concentration where it shows a very high value. This behavior together with that reported in Figure 3, from our point of view is likely to be artifacts rather than real behaviors.
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© 2014 American Chemical Society
These anomalous trends, readily observable in the graphs, may to be a consequence of erroneous experimental data. Such deficient data can be easily detected if the data in Table 1 of the paper2 are analyzed in depth. Additionally, the nomenclature used in section 2.2 is confusing. The magnitudes mass fraction and mass percentage are mixed up throughout the text and the Tables. Additionally, the experimental methodology used does not seem very adequate. The authors report the composition of all components in both phases, but they only determine four experimentally, obtaining the rest (another four) by material balances. The authors run three replicates of each experiment and report a very low uncertainty that is inconsistent with the erratic behavior of the experimental data, as we will show later. They obtained the masses of both phases, but the data is not reported either. An adequate methodology would be to experimentally and independently determine the compositions of the four components in the conjugated phases and use the material balances to detect possible experimental errors. Moreover, the mass of the conjugated phases could also be experimentally determined allowing a higher level of confidence. The methodology used by the authors may lead to undesirable error propagations, as can be observed in the column corresponding to TBP in the aqueous phase (ω4 in Table 1). The oscillations in the values of ω4, when increasing the concentration of phosphoric acid, are inconsistent with the reported uncertainty of the ω data (u(ω) = 0.01 indicated in the footnote). Since the authors report the concentration of the initial mixtures, it is possible to obtain the amount of each phase by material balances, and they should obviously be the same regardless of the component selected to obtain them. We have done this exercise, and we have obtained the unexpected result that not all the lines fulfill the material balance or, in other words, that the amounts of the organic and aqueous phases in each experiment differ depending on the component selected to calculate them. This fact would reflect problems with the data that the authors do not seem to consider. The anomalies in the data reported of the composition of the conjugated phases are more evident if plotted in Cruickshank3 projections, frequently used for studying quaternary systems as is the case. We have plotted the data reported for the initial mixtures and the conjugated phases in the two Cruickshank projections (x/y and x′/y′, where x = x′ = ω1 + ω4, y = ω1 + ω2, and y′ = ω1 + ω3). Figures 1 and 2 are magnifications of such Received: March 4, 2014 Accepted: July 21, 2014 Published: July 25, 2014 2693
dx.doi.org/10.1021/je500210z | J. Chem. Eng. Data 2014, 59, 2693−2694
Journal of Chemical & Engineering Data
Comment/Reply
quaternary system is the partially miscible system and contains salt, which belongs to the strong polarity of the system. Using NRTL is relatively appropriate. In this model, the first material is phosphoric acid, the second is sodium chloride, and the third is water.” We believe this paragraph of the authors is extremely confusing and hardly understandable and merits no additional comments. Moreover, the results obtained for the correlation show that no set of data including salt is regressed by NRTL. We wonder how the authors have attempted the regression of the quaternary system using only three components. Additionally, the paper2 has many minor errors and seems as it has not been proofread. Many sentences are hardly understandable and are incomplete, such as that in the abstract that we reproduce here: “... . Moreover, it was found that the mass ratio of ternary system H3PO4-NaCl-H2O to the saturation point....”. Comments such as that in the second paragraph of the Results and Discussion section are somewhat spurious: “The combination of phosphoric acid with TBP is more than sodium chloride. “ What does the combination mean? In what phase?, Why the combination of both if the concentration of phosphoric acid is higher than that of sodium chloride?. The definition of the “extraction rate” i.e.: “The extraction rate refers to the extraction of material into the organic phase of the two-phase percentage of the total. Indicates the extent of the extraction”, is hardly possible to be understood. Moreover, this defined parameter is not used anymore in the article, but the separation factor. Summarizing: Though the procedures outlined in the Scope and Statistics section of the reference paper are limited to “pure binary and ternary chemical systems” and do not directly apply to quaternary systems as that reported in the paper, in our opinion this paper should had never have been considered for publication in this form, and such a conclusion should have been the result of the reviewing procedure. This example stresses the need to extend the scope of the NIST-Journal cooperation to develop adequate procedures valid for quaternary and higher order systems.
representations to show the erratic and inexplicable behavior of the organic phase, with multiple lines crossing over.
Figure 1. Magnification of the organic phase in Cruickshank projection I. The legend indicates (according to the authors) the weight fraction of NaCl in the initial aqueous phase.
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Figure 2. Magnification of the organic phase in Cruickshank projection II. The legend indicates (according to the authors) the weight fraction of NaCl in the initial aqueous phase.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
There are also errors in the experimental densities of the organic phases determinations, as could be observed if they were plotted versus the concentration of phosphoric acid in the initial mixture. It can be observed that the behavior of the aqueous phase is quite reasonable, but that of the organic phases is erratic. The reference paper discusses an example showing densities in the following terms: “... An example is shown in Figure 11, where densities of (dibutyl phthalate + ethenyl acetate) are shown as a f unction of composition for three isotherms. When plotted, such data are generally a series of parallel curves, as seen in the upper lef t of Figure 11. The densities of the upper curve on the right side of the Figure are anomalous. This manuscript was rejected at the approve stage for this reason...”. Data in that rejected paper had apparently less anomalies than those in the paper.2 Finally, when the authors mention the NRTL equation, they cite an article from 1967 to justify: “The (sic) nonrandom twoliquid (NRTL) model of Renon and Praustnitz can be used for the association of the strongly polar system.19 The biggest advantage is that it can be used for correlation evaluation of the partially miscible system. The H3PO4−NaCl−H2O−TBP
The authors declare no competing financial interest.
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REFERENCES
(1) Chirico, R. D.; Frenkel, M.; Magee, J. W.; Diky, V; Muzny, C. D.; Kazakov, A. F.; Kroenlein, K.; Abdulagatov, I.; Hardin, G. R.; Acree, W. E., Jr.; Brenneke, J. B.; Brown, P. D.; Cummings, P. T.; de Loos, T. W; Friend, D. G.; Goodwin, A. R. H.; Hansen, L. D.; Haynes, W. M.; Koga, N.; Mandelis, A.; Marsh, K. N.; Mathias, P. M.; McCabe, C; O’Connell, J. P.; Pádua, A.; Rives, V.; Schick, C.; Trusler, J. P. M.; Vyazovkin, S.; Weir, R. D.; Wu, J. Improvement of Quality in Publication of Experimental Thermophysical Property Data: Challenges, Assessment Tools, Global Implementation, and Online Support. J. Chem. Eng. Data 2013, 58 (10), 2699−2716. (2) Liu, C.; Ren, Y.; Wang, Y. Liquid−Liquid Equilibria for the Quaternary System H3PO4−NaCl−H2O−TBP at 298.15 K. J. Chem. Eng. Data 2014, 59, 70−75. (3) Cruickshank, A. J. B.; Haertsch, N. Hunter, T. G. Liquid−Liquid Equilibria of Four-Component Systems. Ind. Eng. Chem., 42−10, 2154−2158.
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