BaTe, ionicity

Sankar, M. Bates, R. G., Buffers for the physiological pH range Thermodynamic constants of 3-(/V-morpholi no)propanesul Ionic acid from 5 to 50°C, Anal. Chem. 50, 1922-1924 (1978). 

It has been emphasized repeatedly that the individual activity coefficients cannot be measured experimentally. However, these values are required for a number of purposes, e.g. for calibration of ion-selective electrodes. Thus, a conventional scale of ionic activities must be defined on the basis of suitably selected standards. In addition, this definition must be consistent with the definition of the conventional activity scale for the oxonium ion, i.e. the definition of the practical pH scale. Similarly, the individual scales for the various ions must be mutually consistent, i.e. they must satisfy the relationship between the experimentally measurable mean activity of the electrolyte and the defined activities of the cation and anion in view of Eq. (1.1.11). Thus, by using galvanic cells without transport, e.g. a sodium-ion-selective glass electrode and a Cl -selective electrode in a NaCl solution, a series of (NaCl) is obtained from which the individual ion activity aNa+ is determined on the basis of the Bates-Guggenheim convention for acr (page 37). Table 6.1 lists three such standard solutions, where pNa = -logflNa+, etc. 

Recent developments for reactive C02 sorbents include sterically hindered amines such as 2-amino-2-methyl-l-propanol (AMP) and 1,8-p-methanediamine (MDA) and 2-piperidine ethanol (PE), which are claimed to have good reversible C02 capacity (Veawab et al., 1998) and low-temperature molten salts called ionic liquids (Bates et al., 2002). Ionic liquids are attractive due to their negligible vapor pressure up to their decomposition at... 

Merrigan, T.L., Bates, E.D., Dorman, S.C., and Davis, J.H., New fluorous ionic liquids function as surfactants in conventional room-temperature ionic liquids,... 

To obtain the pH, it is necessary to evaluate the activity coefficient of the chloride ion. So the acidity function is determined for at least three different molalities mci of added alkali chloride. In a subsequent step, the value of the acidity function at zero chloride molality, lg(flHyci)°, is determined by linear extrapolation. The activity of chloride is immeasurable. The activity coefficient of the chloride ion at zero chloride molality, yci, is calculated using the Bates-Guggenheim convention (Eq. 5) which is based on the Debye-Hiickel theory. The convention assumes that the product of constant B and ion size parameter a are equal to 1.5 (kg mol1)1/2 in a temperature range 5 to 50 °C and in all selected buffers at low ionic strength (I < 0.1 mol kg-1). 

Figure 9.1 Tetragonal tungsten-bronze structure of SiiBai-xNboOf, (Strontium barium nio-bate, SBN). Characteristic are the corner-linked niobium-oxygen-octahedra and 3 types of vacancies A,Bi,2,3 and C. Sr2+-ions are embedded in the A- and B2-positions and Ba2+ in the Bi-positions according to their ionic radii. The C-positions and B3-position remain empty.

It is appropriate at this point to discuss the "apparent" pH, which results from the sad fact that electrodes do not truly measure hydrogen ion activity. Influences such as the surface chemistry of the glass electrode and liquid junction potential between the reference electrode filling solution and seawater contribute to this complexity (see for example Bates, 1973). Also, commonly used NBS buffer standards have a much lower ionic strength than seawater, which further complicates the problem. One way in which this last problem has been attacked is to make up buffered artificial seawater solutions and very carefully determine the relation between measurements and actual hydrogen ion activities or concentrations. The most widely accepted approach is based on the work of Hansson (1973). pH values measured in seawater on his scale are generally close to 0.15 pH units lower than those based on NBS standards. These two different pH scales also demand their own sets of apparent constants. It is now clear that for very precise work in seawater the Hansson approach is best. 

Bates and Alfenar (23) proposed that the activity of the chloride ion in NaCl solutions be taken as the activity standard. Using this chloride convention together with the published values of mean activity coefficients, the activity of any simple ionic species can be estimated. The equation used to estimate values of yNo3 and thus aNo3 for nitric acid solutions is... 

Single Ion Activity Coefficients. The cation single ion activity coefficients were estimated using the model of Bates, Staples, and Robinson (19). Their model was derived specifically for chloride solutions, and was shown to give reasonable results in solutions of ionic strengths up to and including 6 molal. 

Bates, R. G., Staples, B. R., and Robinson, R. A. Ionic hydration and single ion activities in unassociated chlorides at high ionic strengths. Anal. Chem. 42, 867-871 (1970). 

Bates, J., Dudney, N., Neudecker, B., Ueda, A., and Evans, C., Thin-film lithium and lithium-ion batteries, Solid State Ionics, 135, 33, 2000. 

An operational definition endorsed by the International Union of Pure and Applied Chemistry (lUPAC) and based on the work of Bates determines pH relative to that of a standard buffer (where pH has been estimated in terms of p"H) from measurements on cells with liquid junctions the NBS (National Bureau of Standards) pH scale. This operational pH is not rigorously identical to p H defined in equation 30 because liquid junction potentials and single ion activities cannot be evaluated without nonthermodynamic assumptions. In dilute solutions of simple electrolytes (ionic strength, I < 0.1) the measured pH corresponds to within 0.02 to p H. Measurement of pH by emf methods is discussed in Chapter 8. 

The Macinnes convention has come under criticism, and other methods have been proposed. Scales of ionic activity at ionic strengths above 0.1 have also been developed based on considerations of the degree of hydration of the ions involved (Bates et al., 1970). 

El2.II The ionic radius of Be is 45 pro (Resource Section 1) and the ionic radius of Te is 207 pm. The radius ratio is 45 pm/207 pm = 0.22. According to Table 3.6, this should be close to ZnS-like structure. The ionic radius of Ba is 135 pro and the radius ratio is 135 pni/207 pro = 0.65, and BaTe should have NaCI-like structure. 

Bates ED, Mayton RD, Ntai I et al (2002) CO capture by a task-specific ionic liquid. J Am Chem Soc 124 926-927... 

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