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Comment on the Standard Molar Volumes for Citric Acid published in J. Chem. Eng. Data, 2011, 56, 3285–3290
Journal of Chemical & Engineering Data ( IF 2.298 ) Pub Date : 2020-01-14 , DOI: 10.1021/acs.jced.8b01218
Pedro Bernal

Re: Roy et al.: Densities and Viscosities of Citric Acid in Aqueous Cetrimonium Bromide Solutions with Reference to the Manifestation of Solvation.(4) In the article cited above, the limiting (standard) molar volumes for citric acid in water, at various temperatures, are given in Table 2 as 203.55; 213.29; and 223.41 cm3 mol–1 at 25, 35, and 45 °C, respectively, and in aqueous solutions of cetrimonium bromide (CB) as follows: in 0.001 M CB, 230.91, 236.46, and 242.32 cm3 mol–1; in 0.003 M CB, 233.36, 243.04, and 253.20 cm3 mol–1; and in 0.005 M CB, 238.95, 247.97, and 257.39 cm3 mol–1 at 25, 35, and 45 °C, respectively. Given that the molecular weight of citric acid is 192.12 g mol–1 (210.14 g mol–1 for the monohydrate) and that the specific volume of analogous compounds tends to be around 0.6–0.7 cm3 g–1, the standard molar volumes reported by Roy et al. are certainly high. To illustrate the point, here are some values reported in the literature for citric acid in pure water (this is by no means an exhaustive list): 112.44 cm3 mol–1 at 25 °C (ref (1)); 115.2, 116.7, and 118.9 cm3 mol–1 at 25, 35 and 45 °C, respectively (ref (2)); 115.20 cm3 mol–1 at 20 °C (ref (3)). The values reported in refs (2and3) are not strictly standard molar volumes. They are, rather,the results obtained at the lowest concentration reported (0.03 m) (ref (2)) and at 20 °C and m = 0.161 (ref (3)), in that order. These minor variations are not enough to account for the differences observed between the volumes reported by Roy et al. and references (1−3). Additional evidence which shows that the temperature dependence of the standard molar volumes is also in error, can be provided. In going from 25 to 45 °C, the standard values for citric acid (in pure water) reported by Roy et al. increase by about 20 cm3 mol–1 (∼203–223) which is a change of about 1.0 cm3 mol–1 K–1. That expansion is much higher than anything previously reported. The values in ref (2) increase by 3.7 cm3 mol–1 over the same temperature range, which leads to an expansion, assuming linearity, of about 0.18 cm3 mol–1 K–1, a number consistent with results reported for similar compounds. The expansion reported by Roy et al. is about five times higher. The fact that Roy et al. worked with the monohydrate will change the numbers a bit but not enough to account for the observed differences. The author declares no competing financial interest. This article references 4 other publications.
更新日期:2020-01-15

 

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