Hamed cell

For unknown activities the measurement of the standard electrode potential is more complicated. The standard electrode potential is defined at /ra = 1 mol kg with the hypothetical activity coefficient of y = 1 (ideal diluted solution). First principal experimental determinations of standard potentials may only be made by extrapolation to this hypothetical value. For measurements, selected cell arrangements are used with complete elimination of the diffusion potential and with diluted electrolytes. For the correction of the activity the Debye-Hiickel approximation (Eq. (1.15)) may be used, for example, for the Hamed cell Ag/AgCl, HCl (m+)/Pt(H2). A concentration corrected potential value is plotted versus the square root of the molaUty. The extrapolation to 7ra+ = 0 gives the standard potential of the Ag/AgCl electrode (Figure 3.5). Using this electrode as reference electrode other standard potentials can be determined. 

The essential feature of a primary method is that it must operate according to a well-defined measurement equation in which all of the variables can be determined experimentally in terms of SI units. Any limitation in the determination of the experimental variables, or in the theory, must be included within the estimated uncertainty of the method if traceability to the SI is to be established. If a convention were used without an estimate of its uncertainty, true traceability to SI would not be established. The electrochemical cell without liquid junction, known as the Hamed cell (5), fulfils the definition of a primary method for the measurement of the acidity function, p(aH7oi)> subsequently of the pH of buffer solutions. 

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) ... 

As there can be significant variations in the purity of samples of a buffer of the same nominal chemical composition, it is essential that the primary buffer material used has been certified with values that have been measured with Cell I. The Hamed cell is used by many national metrological institutes for accurate measurements of pH of buffer solutions. 

Typical values of the pH(PS) of the seven solutions from the six accepted primary standard reference buffers, which meet the conditions stated above, are listed in Table 2. Balch-to-batch variations in purity can result in changes in the pH value of samples of at most 0.003. The typical values in Table 2 should not be used in place of the certified value (from a Hamed cell measurement) for a specific batch of buffer material. 

The required attributes listed above effectively limit the range of primary buffers available to between pH 3 and 10 (at 25 C). Calcium hydroxide and potassium tetraoxalate are excluded because the contribution of hydroxide or hydrogen ions to the ionic strength is significant. Also excluded are the nitrogen bases of the type BH (such as tris(hydroxymethyl)aminomethane and piperazine phosphate) and the zwitterionic buffers (e.g. HEPES and MOPS (10)). These do not comply because either the Bates-Guggenheim convention is not applicable, or the liquid junction potentials are high. This means the choice of primary standards is restricted to buffers derived from oxy-carbon, -phosphorus, -boron and mono, di- and tri-protic carboxylic acids. The uncertainties (11) associated with Hamed cell measurements are calculated (1) to be 0.004 in pH at NMIs, with typical variation between batches of primary standard buffers of0.003. 

Fig. 5.1.1 Platinum hydrogen cell (Hamed cell) designed at Physikalisch-Technische Bundesanstalt (PTB) and produced by Fa. Rettberg, Gottingen...

In the primary method for pH the hydrogen electrode is combined with a silver/ silver chloride reference electrode in a so-called Hamed cell [14]. 

Fig. 5.1.2 Schematic representation of the platinum hydrogen cell (Hamed cell). (1) Platinum hydrogen electrode, (2) inlet bubbler,...

Write down the cell reaction for the Hamed cell,... 

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. 

If an electrochemical cell consists of an SHE and a Ag/AgCl electrode imbedded in the same solntion of HCl(aq), that is, a cell without transfer, then this cell is called the Harned cell. The Hamed cell can be used for measuring the standard thermodynamic properties and mean activity coefficients of HCl(aq) without any complications/uncertainties due to a salt bridge and corresponding diffusion potential in the cell with transfer. The traditional electrochemical diagram of the Harned cell is as follows ... 

Using CRC Handbook, find the mean activity coefficient of 0.1 mol kg" HCl(aq), and calculate the potential difference of the Hamed cell (Ag/AgCl electrode on the right) at a temperature of 25°C and at hydrogen partial pressure of 1 bar. 

Box 4.1 The measoKBiait of acBvity coeffidmlB In Chapter 2, the variation of iurtivity co Sdents with electrolyte concraitiation was discussed, Figme 2.8 showed the concentration dependence for several solutions. Following the discussion in Section 4.1 we can now see the basis for the e qierimental determination of sdution non-ideality and the measurement of data such as that in Fig. 2.8. As an illustration conrider the Hamed cell,... 

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