Article Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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Heat Capacities of Aqueous Solutions of K4Fe(CN)6, K3Fe(CN)6, K3Co(CN)6, K2Ni(CN)4, and KAg(CN)2 at 298.15 K Yaser Kianinia,† Lubomir Hnedkovsky,*,‡ Gamini Senanayake,‡ Chandrika Akilan,‡ Mohammad Reza Khalesi,† Mahmoud Abdollahy,† Ahmad Khodadadi Darban,† and Glenn Hefter‡ †
Department of Mining Engineering, Tarbiat Modares University, Tehran, Iran Chemical & Metallurgical Engineering & Chemistry, Murdoch University, Murdoch, Western Australia 6150, Australia
‡
S Supporting Information *
ABSTRACT: Isobaric volumetric heat capacities of aqueous solutions of K4Fe(CN)6, K3Fe(CN)6, K3Co(CN)6, K2Ni(CN)4, and KAg(CN)2 have been measured at 298.15 K over the approximate concentration range 0.02 to 0.4 mol· kg−1 using a Picker flow calorimeter. These data were combined with measured densities to calculate the corresponding apparent molar isobaric heat capacities, Cpϕ. The values so obtained were fitted as a function of concentration using an extended Redlich−Rosenfeld−Meyer-type equation to provide the standard state (infinite dilution) quantities, Copϕ, for each salt. The Cpϕ values for all the salts studied showed similar dependences on concentration, with a slight upward curvature at higher molalities, possibly due to anion aggregation. Values of Copϕ for the aquated cyanometallate anions were estimated using the tetraphenylphosphonium tetraphenylborate extrathermodynamic assumption and were little affected by ion size but were strongly dependent on ionic charge, ranging from −191 J·K−1·mol−1 for [Fe(CN)6]4−(aq) to +178 J·K−1·mol−1 for [Ag(CN)2]−(aq). This indicates that the differences between the anions are mostly due to their effect on the surrounding water molecules.
1. INTRODUCTION The cyanide ion (CN−) binds with many metal ions (Mx+) to produce1 highly stable complex cyanometallate anions, M(CN)n(n−x)−. Interestingly, the “soft” CN− forms these complex anions with both “soft” and (at least some) “hard” metal ions, although the complexes with the former are generally more stable.2 Cyanometallate ions are significant in a variety of industrial situations. Most importantly, they form routinely during the extraction of gold from its ores via the ubiquitous cyanide leaching process.1,3 Specific potassium cyanometallate salts, KyM(CN)n also have a host of practical applications. For example, K4Fe(CN)6 is used as an anticaking and purifying agent in some foodstuffs,4−6 as a fertilizer in agriculture,7 and as a precursor for the production of Prussian blue dyes in the textile industry.8,9 The corresponding Fe(III) salt, K3Fe(CN)6, finds application in photography,10 for tempering steel and electroplating in the metals industry,11 in the production of paper12 and pigments,13 and as a mild industrial oxidizing agent.14,15 The salt KAg(CN)2 is used as a bactericide16 and in silver electroplating, K2Ni(CN)4 has been employed as a catalyst17 and titrant,18 and K3Co(CN)6 has been used as an antidote for cyanide poisoning.19 While many of the physicochemical properties of the aqueous solutions of cyanometallate salts are reasonably well characterized, rather little is known about their heat capacities. Such data are especially important in the mineral processing industry, due to the increasing use of chemical speciation modeling for process control and optimization.20 Heat capacity © XXXX American Chemical Society
data are also required for engineering calculations of processes that involve significant temperature variation.21 As part of our ongoing studies into the behavior of metalion/cyanide systems,22 this paper reports the heat capacities of aqueous solutions of five salts: K4Fe(CN)6, K3Fe(CN)6, K3Co(CN)6, K2Ni(CN)4, and KAg(CN)2 at 298.15 K and 0.1 MPa as a function of concentration. To the best of our knowledge, no such data have been published for the last three of these salts and, while some heat capacity data are available in the literature for K3Fe(CN)6 and K4Fe(CN)6, they have not been replicated.
2. EXPERIMENTAL SECTION 2.1. Reagents. The salts used were commercial samples of the highest purity available. Their origin and treatment are summarized in Table 1. Briefly, K4Fe(CN)6, K3Co(CN)6, and K2Ni(CN)4 were recrystallized twice from hot aqueous solution. The K2Ni(CN)4 salt was subsequently dehydrated at 383 K under reduced pressure (∼15 Pa), revealing a water content of w = 0.0225. The water content of K4Fe(CN)6·3H2O, determined by dehydration at 383 K and ∼15 Pa, was w = 0.1246 (theoretical, w = 0.1279). The remaining salts were used as received. A stock solution of ∼0.4 mol·kg−1 was prepared for each salt and filtered (Millipore, 0.45 μm). Approximately 13 test Received: January 24, 2018 Accepted: April 18, 2018
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DOI: 10.1021/acs.jced.8b00078 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
Journal of Chemical & Engineering Data
Article
Table 1. Sample Sources and Purities initial mass fraction purity
chemical name
CASRN
source
potassium hexacyanoferrate(II) hydrate
14459-95-1
Chem-Supply
>0.985
potassium hexacyanoferrate(III) potassium hexacyanocobaltate(III)
13746-66-2 13963-58-1
Baker Analyzed Hopkin & Williams
0.999 0.995
potassium tetracyanonickelate(II) hydrate potassium dicyanoargentate(I)
14220-17-8
Aldrich
>0.99
506-61-6
Aldrich
not stated
0.999 0.999 0.995
⎪
⎪
⎪
⎪
(4)
Differentiation then leads to Uc(Cpϕ) = [{(M + 1/m)u(cp)}2 + {(cp − cp w )/m2u(m)}2 ]1/2
(5)
The standard uncertainties in molality and heat capacity inserted into eq 5 were: u(m) = 0.0005m and u(cp) = 0.00015 + 0.004(cp − cpw), respectively. The uncertainties so calculated are listed with the Cpϕ values in Tables 2 to 6 and are also shown as error bars in Figures 1 to 4.
3. RESULTS AND DISCUSSION Densities and volumetric isobaric heat capacities, and the corresponding cp and Cpϕ values calculated from them, are reported in Tables 2 to 6 for the solutions of the five potassium cyanometallate salts studied. For convenience, the values of Copϕ obtained via eq 3 are collected into Table 7, along with the relevant ω values and the adjustable parameters BC and CC and their standard errors (SEs). Although the present results, being limited to m ≳ 0.02 mol·kg−1, are not ideal for determining Copϕ, it is well established27 that extrapolation via RRM-type equations of Cpϕ data at modest solute concentrations provides reasonable estimates of Copϕ. The ability of eq 3 to fit the present Cpϕ(m) values is illustrated in the deviation plots shown in Figure S1 of the Supporting Information. For each set of solutions, the deviations were essentially random with solute concentration. The average deviations for K3Fe(CN)6 and KAg(CN)2 were
