Hamed molality

Figure 2 presents the parameters of Equations 4 and 5 as functions of temperature and total molality. The experimental data compiled and discussed by Hamed and Owen (1 ) were used to calculate the experimental parameters shown in Figure 2. 

The activity of HCl in a 4-molal solution required for the AG i in Equation (20.39) was calculated from the mean activity coefficient, 1.762, taken from tables of Hamed and Owen [10], as follows ... 

Values of q> and y for many aqueous electrolytes at 25°C are reported in the books by Hamed and Owen [1], by Robinson and Stokes [2], in subsequent reports from the US National Bureau of Standards (now NIST) [3-8], and have been reevaluated more recently by Partanen and coworkers [9,10]. Values of the mean ionic activity coefficients of representative aqueous electrolytes at 25°C at several molalities m are shown in Table 7.1, in order for the trends with the natures of the ions making up these electrolytes to be seen, and some of these data are shown in Figure 7.1. In all the cases there is a decrease in y at low concentrations but the values tend to increase again at higher molalities. For the univalent cation chlorides, for instance, at 1 m the trend of the y values is HCl > LiCl > NaCl > KCl NH Cl > RbCl > CsCl. For the acids and lithium, sodium, and potassium salts, the trend at this molality among the anions is NOj" < CF < Br < CIO < F (KCIO is insoluble), but for rubidium and cesium the trend is F < Br (not shown) < CF. These trends are commented on in Section 7.2. The values of 1 2 and 2 1 electrolytes are smaller than those for 1 1 electrolytes and the diminution becomes larger as the charge numbers of the ions increase. 

The standard potential difference of the silver/silver chloride electrode, E°, is determined from a Hamed cell in which only HCl is present at a fixed molality (e.g. m = 0.01 mol kg" )... 

The quantity p(%Yci) = Ig (%YciX on the left hand side of (4), is called the acidity function (5). To obtain the quantity pH according to eqn. (2) from the acidity function, it is necessary to evaluate Ig g,- independently. This is done in two steps (i) the value of Ig (OhYci) t zero chloride molality, Ig (%Yci)°> is evaluated and (ii) a value for the activity of the chloride ion y°ci > nt zero chloride molality (sometimes referred to as the limiting or trace activity coefficient) is calculated using the Bates-Guggenheim convention (7). The value of Ig (anYa) corresponding to zero chloride molality is determined by linear extrapolation of measurements using Hamed cells with at least three added molalities of sodium or potassium chloride (/< 0.1 mol kg" ). 

The value of Ig (%Yci)° corresponding to zero chloride molality is determined by linear extrapolation of measurements using Hamed cells with at least three added molalities of sodium or potassium chloride (/ < 0.1 mol kg ) in accord with eqn. (7) ... 

The standard potential is determined according to Eq. (5.1.5) using cell 11. The Hamed ceU is fiUed with hydrochloric acid of known molality (/mhci = 0.01 mol kg ). The mean activity coefficient of HCl, y hci> various temperatures is taken from the literature [13]. 

The data given below give the EMF for the Hamed cell at 25 C and as a function of the molality, w, of the CV ion. Using the Debye-Hiickel limiting law, determine the standard electrode potential of the silver/silver chloride electrode. 

---