Single-ion activities of H(+) and Cl(-) in aqueous hydrochloric acid solutions up to 500 mmol·dm(-3) HCl have been determined, independently of each other, with reasonable accuracy, 95% confidence interval of log(10)γ being ±0.012 at 25 °C, by use of an ionic liquid salt bridge inserted in a Harned cell. The geometric averages of those activities agree well with the literature values of the mean ionic activity of HCl determined with a Harned cell up to an ionic strength of 500 mmol·dm(-3). The agreement supports the assumption used in estimating the single-ion activities, that is, the liquid junction potentials on both sides of the ionic liquid salt bridge stay constant, within the confidence interval of the cell voltage, ± 1 mV, over the concentration range of HCl between 10 and 500 mmol·dm(-3). The single-ion activity coefficient values of H(+) and Cl(-) over the concentration range studied show fairly good agreement with the recent theoretical predictions by the "smaller-ion-shell" model proposed by Fraenkel (Mol. Phys. 2010, 108, 1435; J. Phys. Chem. B 2011, 115, 557), which fact is significant and implicative in physical and analytical chemistry of electrolyte solutions.
The activities of hydrogen ions in 20-200 μmol dm(-3) H(2)SO(4) solution were estimated by use of an ionic liquid salt bridge (ILSB), made of tributyl(2-methoxyethyl)phosphonium bis(pentafluoroethanesulfonyl)amide (TBMOEPC(2)C(2)N), sandwiched by two hydrogen electrodes. The experimental pH values (pH = -log a(H), where a(H) is the activity of hydrogen ions) were in good agreement, within 0.01 pH unit, with those calculated using the Pitzer model. The difference between the experimental and theoretical pH values at 50 μmol dm(-3) H(2)SO(4) solution was much smaller than that obtained by use of a glass electrode in combination with a reference electrode with a concentrated KCl salt bridge. The source of the small deviation can be explained by the residual diffusion potential due to the dissolution of TBMOEPC(2)C(2)N in the H(2)SO(4) solution (W) and the resultant increase in the ionic strength of W. The use of a reference electrode equipped with an ILSB opens the way to accurately estimate the pH in dilute aqueous solutions, for which we have not had effective means.
1. Introduction Titanium metal has superior properties such as high specific strength, high corrosion resistance, biocompatibility, etc. In addition to these properties, crustal abundance of titanium is more than 40 times larger than those of commonly used copper and nickel. However, titanium is not used widely and commonly due to two problems: the high cost of present smelting method and the poor workability of itself. Therefore, both a new smelting method and a processing method are required. Plating titanium metal on an inexpensive substrate is one of the methods to resolve these problems. Electrodeposition of titanium is a promising plating method from the viewpoints of the cost and the flexibility in shape of substrate. Thus, electrodeposition of titanium metal has been investigated for a long time using high-temperature molten salts [1–4]. We have already reported that compact, smooth, and adherent titanium films are electrodeposited in molten KF–KCl and LiF–LiCl containing Ti(III) ions at 923 K [5–8]. However, crystal grains of titanium become large with the increase of a film thickness, which results in a rough surface. Concerning the crystal grain size, Wei et al. electrodeposited titanium in molten LiCl–NaCl–KCl–K2TiF6 (+ Ti metal) at 723–973 K and reported that the grain size became small as the temperature decreased [1]. This report suggests that titanium films are expected to have a smoother surface at a lower electrodeposition temperature. Therefore, in the present study, we investigated the effect of temperature on morphology and smoothness of electrodeposited titanium films in LiF–LiCl eutectic melt at 823–973 K. The effect of temperature on the electrochemical behavior of Ti(III) ions was also studied. 2. Experimental The experiments were conducted in LiF–LiCl eutectic melt (LiF:LiCl = 30:70 mol%, m.p.: 774 K [9]) in an Ar glovebox. The experimental temperature was varied from 823 K to 973 K. Li2TiF6(2.00 mol%) and Ti sponge (1.33 mol%) were added to the bath and Ti(III) ions were prepared by comproportionation reaction. Ni plate, Mo flag, and Au flag electrodes were used as the working electrodes. The counter and reference electrodes were Ti rods. The potential of the reference electrode was calibrated by Li+/Li potential measured at a Mo flag electrode. Samples prepared by galvanostatic electrolysis of Ni plate substrates were analyzed by SEM/EDX after washing with distilled water to remove adhered salts. 3. Results and Discussion Galvanostatic electrolysis was conducted at 823, 873, 923, and 973 K. Fig. 1 shows the optical and surface SEM images of the samples. The cathodic current density and electrolysis time were 100 mA cm−2 and 10 min, respectively. The surface roughness (Sa) was measured by SEM, which is also shown in Fig. 1. All the samples have a metallic luster. The sample at 823 K has the highest brightness with metallic luster. The surface SEM observations indicate that the crystal grain size increases as the temperature rises. This tendency is reasonably explained by a previous work on the temperature dependent grain growth of Ti [10]. The value of Sa also increases as the temperature increases. The smoothest Ti film with Sa = 2.05 ± 0.22 μm was obtained at 823 K. These results conclude that Ti film with smoother surface can be electrodeposited at a lower temperature by suppressing the grain growth of Ti. In the presentation, more detailed analysis results and the effect of temperature on the electrochemical behavior of Ti(III) ions will be discussed. Acknowledgement A part of this work was supported by JSPS Fellows grant number 19J15015. The present address of Kouji Yasuda is Graduate School of Engineering, Kyoto Univ. References [1] D. Wei, M. Okido, and T. Oki, J. Appl. Electrochem., 24, 923 (1994). [2] H.Takamura, I. Ohno, and H. Numata, J. Jpn. Inst. Metals, 60, 388 (1996). [in Japanese] [3] A. Robin and R. B. Ribeiro, J. Appl. Electrochem., 30, 239 (2000). [4] V. V. Malyshev and D. B. Shakhnin, Mater. Sci., 50, 80 (2014) [5] Y. Norikawa, K. Yasuda, and T. Nohira, Mater. Trans., 58, 390 (2017). [6] Y. Norikawa, K. Yasuda, and T. Nohira, Electrochemistry, 86, 99 (2018). [7] Y. Norikawa, K. Yasuda, and T. Nohira, J. Electrochem. Soc., 166, D755 (2019). [8] Y. Norikawa, K. Yasuda, and T. Nohira, J. Electrochem. Soc., 167, 082502 (2020). [9] J. Sangster and A. D. Pelton, J. Phys. Chem. Ref. Data, 16, 509 (1987). [10] K. Okazaki and H. Conrad, Metall. Trans., 3, 2411 (1972). Figure 1
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