Harned cells

The review of Martynova (18) covers solubilities of a variety of salts and oxides up to 10 kbar and 700 C and also available steam-water distribution coefficients. That of Lietzke (19) reviews measurements of standard electrode potentials and ionic activity coefficients using Harned cells up to 175-200 C. The review of Mesmer, Sweeton, Hitch and Baes (20) covers a range of protolytic dissociation reactions up to 300°C at SVP. Apart from the work on Fe304 solubility by Sweeton and Baes (23), the only references to hydrolysis and complexing reactions by transition metals above 100 C were to aluminium hydrolysis (20) and nickel hydrolysis (24) both to 150 C. Nikolaeva (24) was one of several at the conference who discussed the problems arising when hydrolysis and complexing occur simultaneously. There appear to be no experimental studies of solution phase redox equilibria above 100°C. 

Abstract The primary method for pH is based on the measurement of the potential difference of an electrochemical cell containing a platinum hydrogen electrode and a silver/silver chloride reference electrode, often called a Harned cell. Assumptions must be made to relate the operation of this cell to the thermodynamic definition of pH. National metrology institutes use the primary method to assign pH values to a limited number of primary standards (PS). The required comparability of pH can be ensured only if the buffers used for the calibration of pH meter-electrode assemblies are traceable to... 

The primary method for pH is based on the measurement of the potential difference of the electrochemical cell without a liquid junction involving a selected buffer solution, a platinum hydrogen gas electrode and a silver/silver chloride reference electrode, often also referred to as a Harned cell. 

To summarize pH measurement consists of a hierarchical system of three metrologically defined steps (1) the definition of pH by primary pH(PS) values assigned to primary buffer solutions (PS) in a Harned cell defined as a primary method of measurement, (2) the assignment of pH(SS) to secondary buffer solutions (SS) by tracing them back to pH(PS), and (3) measurement of pH(X) of unknown solutions (X) by comparison with... 

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

The essential feature of a primary method is that it must operate according to a well-defined measurement equation in which aU of the variables can be determined experimentaUy 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 traceabUity 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 Harned cell (5), fulfils the definition of a primary method for the measurement of the acidity function, p( h7ci)> subsequently of the pH of buffer solutions. 

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

The typical values in Table 2 should not be used in place of the certified value (from a Harned 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 tire 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-Gu enheim 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 Harned cell measurements are calculated (1) to be 0.004 in pH at NMIs, with typical variation between batches of primary standard buffers of 0.003. 

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 Harned cell is used... 

TABLE 3. Values of pH(SS) of Some Secondary Standards from Harned Cell 1 Measurements... 

If in the Harned cell the fixed molality of HCl is applied, then... 

In practice, the pH measurements of the so-called secondary standard solutions are often used for calibrating the measuring electrodes. The pH values of these are established in comparison with the primary standards. Their pH values must be consistent with the pH values of the primary standards. The measurements are carried out with the hydrogen electrode and in cells without transference in determination of the pH of the secondary standard solution, if it is possible. The calibrating standards are commercially available, and their pH values at different temperatures are certified by the manufacturer. In Table 7.2, the pH values of a few secondary standard solutions are listed. They were determined with the Harned cell for different temperatures. The table is taken from the report of lUPAC 2002 [35]. 

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. Batch-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 Harned cell measurement) for a specific batch of buffer material. 

TABLE 3. VALUES OF pH(SS) OF SOME SECONDARY STANDARDS FROM HARNED CELL I... 

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