Y. C. Wu

Molality-based primary standards of electrolytic conductivity (IUPAC Technical Report)

New values of electrolytic conductivity were determined for aqueous KCl solutions with molalities of 0.01, 0.1, and 1.0 mol/kg in the temperature range 0 to 50 °C, at 5 K intervals. Expanded uncertainties, 2uc, were also calculated in accordance with the presently accepted protocol for the treatment of uncertainty. The new conductivity values are recommended as primary standards of electrolytic conductivity based on molality. They replace the previous values, based on the nonstandard demal scale, which were determined only at 0, 18, and 25 °C. The accuracy of the technique used was evaluated by repeating the determination of the previously recommended demal-based IUPAC standards of electrolytic conductivity and through comparison with other absolute measurements.

Absolute determination of electrolytic conductivity for primary standard KCl solutions from 0 to 50°C

An absolute determination of aqueous electrolytic conductivity has been made for primary standards 0.01D and 0.1D (demal) potassium chloride solutions over the temperature range of 0 to 50°C in 5 degree intervals. A cell with a removable center section of accurately known length and area was used for the measurements. Values were adjusted to be in conformity with the ITS-90 temperature scale. The overall uncertainty over the entire temperature range is estimated to be 0.03%. Values at 25°C for 0.01D and 0.1D KCl solutions are 0.0014086 and 0.012852 S-cm−1, respectively.

Review of electrolytic conductance standards

Measurements of aqueous electrolytic conductance are performed routinely in a variety of disciplines and industries. Conductivity is a measure of the ionic content in solution and thus has applications in pharmaceuticals, power plants, rainwater, lake surveys, and oceanography, to name a few. A thorough review of the measurement of and standards for aqueous electrolytic conductance is herein presented. At present, the most precise and accurate standards have been set forth by the International Organization of Legal Metrology (OIML), and have been adopted by most other standards organizations. However, the uncertainty assigned to these standards, especially the secondary standards, is somewhat larger than would be expected from the physical aspects of the measurement. Several changes in the units and measurement scales, including temperature, volume, molar mass, resistance, and concentration obfuscate the accuracy of these standards. In addition to the review, research is proposed, using a conductance cell with variable length, to establish new standards for aqueous electrolytic conductance.

The second dissociation constant of sulfuric acid at various temperatures by the conductometric method

The thermodynamic second dissociation constant K2 for sulfuric acid has been determined by conductivity measurements of aqueous sulfuric acid solutions at various temperatures. The data are treated by using two different methods developed with two independent assumptions due to Noyes et al. and Shedlovsky. Both methods require the knowledge of relevant ionic conductivities, which may be calculated from the Onsager limiting law. The values for K2 obtained with these two methods show excellent agreement. The value of 0.0103 mol-L−1 at 25°C agrees with the ‘best’ literature value of 0.0102 mol-L−1 within the experimental uncertainty, as also does the enthalpy of dissociation which is derived from the derivative of the temperature coefficient of K2.

Evaluation of liquid junction potentials and determination of pH values of strong acids at moderate ionic strengths

A new experimental approach is proposed for the evaluation of liquid junction potentials and single ion activities. Neither of these quantities can be experimentally measured without assumption. Furthermore, they are concentration dependent but without clearly defined functional relationships. Thus, a given assumption may be satisfactory to obtain these quantities at a given concentration, but will fail at another concentration. It is the intent of this paper to establish a functional relationship between the ratio of activity coefficients and the ionic strength, through which the liquid junction potentials may be computed, as well as the pH values at moderate ionic strengths (<1.0 m). Experimental results are presented and uncertainties are discussed.

Determination of the absolute specific conductance of primary standard KCl solutions

A determination of the absolute specific conductance of KCl solutions is demonstrated. The measurement is based on the conductance cell with a well defined geometry, having a difference in the removable center tube of accurately measured dimensions. The specific conductance of the solution is obtained from the measured resistances of the cell with and without the center tube and the measured l/A ratio of the center tube. Specific conductances obtained using the cell agree with the previously accepted standards for 0.1 demal and 0.01 demal solutions within 0.02%. Results are also presented for solutions based on molality. The temperature control, bridge, and detector technology used to obtain results of this accuracy are described.

First and second dissociation constants of deuterio-o-phthalic acid in D2O from 5 to 50°C

The first and second dissociation constants of deuterio-o-phthalic acid in deuterium oxide have been determined by the emf method over the temperature range of 5 to 50°C. The pD values for potassium deuterium phthalate have been calculated from these two constants and experimentally verified. The thermodynamic properties for the dissociation of deuterio-o-phthalic acid have been evaluated. At 25°C, these values in the molality scale are: pK1A=3.505, pK2A=5.890, and pD=4.518. From K1A and K2A, respectively: ΔGo=20.003, 33.582 kJ-mol−1; ΔHo=2.851, 2.208 kJ-mol−1; ΔSo=−76.7, −105.2 J-mol−1-K−1; and ΔCpo=−52.7, −315.6 J-mol−1-K−1. The isotope effect is discussed.

Investigation of the interaction of HCl and three amino acids, HEPES, MOPSO and glycine, by EMF measurements

Ionic interaction in the three systems Hcl-HEPES (N-2-hydroxyethylpiperazine-N′-2-ethanesulfonic acid), HCl-MOPSO (3-(N-Morpholino)-2-hydroxypropanesulfonic acid), and HCl-glycine have been studied in terms of their mutual influence on the respective activity coefficients of each component. Activity coefficients for each component of the three systems are calculated from emf measurements of solutions containing HCl and the amino acid in a H2/solution/AgCl,Ag cell at 25°C. The first dissociation constant of HEPES at 25°C has also been determined using emf and pH titration measurements.

Thermodynamic properties of DCl in D2O solution from 5 to 50°C

The themodynamic properties of solutions of deuterium chloride (DCl) in deuterium oxide (D2O) have been determined from emf measurements of the electrochemical cell without transference from 5 to 50°C, and from 0.002 to 1.0 mol-kg−1. The standard potential of the silver/silver chloride electrode relative to the platinum/deuterium electrode has been determined. An equation for the Gibbs energy as a function of temperature has been derived from which the enthalpy, entropy, and heat capacity have been computed. Equations for the activity coefficient and the osmotic coefficient of DCl in D2O have been developed. The excess Gibbs energy of the solution and the excess partial molar free energy as a function of temperature have been calculated, from which the other excess thermodynamic properties have been computed. The values for the heat capacity and the apparent molar heat capacity have been compared with calorimetric data in the literature. The relative partial molar enthalpy has been calculated. The solvent isotope effect on the excess thermodynamic functions is discussed.

An update on the electrolytic conductivity values for the primary standard KCl solutions: Conversion to the ITS-90 temperature scale

A new international temperature scale has replaced the 1968 scale. Here, we report the changes required in our previous results for primary conductivity standards for 0.01, 0.1 and 1.0 Demal KCl solutions at 0,18 and 25°C.