Treasure of the Past VI: Standard Potential of the Silver-Silver-Chloride Electrode from 0° to 95° C and the Thermodynamic Properties of Dilute Hydrochloric Acid Solutions.

From electromotive-force measurements of the cell without liquid junction: [Formula: see text]through the range 0° to 95° C, calculations have been made of (1) the standard potential of the silver-silver-chloride electrode, (2) the activity coefficient of hydrochloric acid in aqueous solutions from m (molality) =0 to m=0.1 and from 0° to 90° C, (3) the relative partial molal heat content of hydrochloric acid, and (4) the relative partial molal heat capacity of hydrochloric acid. The extrapolations were made by the method of least squares with the aid of punch-card techniques. Data from at least 24 cells were analyzed at each temperature, and 81 cells were studied at 25° C. The value of the standard potential was found to be 0.22234 absolute volt at 25° C, and the standard deviation was 0.02 millivolt at 0° C, 0.01 millivolt at 25° C, and 0.09 millivolt at 95° C. The results from 0° to 60° C are compared with earlier determinations of the standard potential and other quantities derived from the electromotive force.

Recalculation of the Faraday Constant Due to A New Value for the Atomic Weight of Silver.

A report of the Faraday constant as determined at NBS via silver coulometry and atomic weight measurements is presented. The uncertainty of the reported result represents a five-fold improvement over measurements made at NBS 20 years ago. The result should contribute to an analysis of the self-consistency of several other fundamental constants measurements. Experimental details have been reported in other publications which are cited in the text.

A Practical Test of the Air Density Equation in Standards Laboratories at Differing Altitude.

A number of weighings of kilogram artifacts have been completed at sites of differing altitude. The artifacts and altitude difference were chosen to amplify the role of the necessary buoyancy corrections and thereby to uncover systematic errors in those corrections as they are usually applied. Small systematic effects were discovered but these are not explainable by buoyancy errors. Rather, we suggest their source is a lack of thermal equilibrium between the artifacts and the balance chamber.

The Electrochemical Equivalent of Pure Silver-A Value of the Faraday.

Oxygen-free high-purity samples were used in a precise determination of the electrochemical equivalent of silver. A comprehensive mass spectrometric analysis for impurities was performed. Our value agrees well with prior measurements of the same quantity at the National Bureau of Standards (NBS) by Craig and coworkers. We find the electrochemical equivalent of pure silver to be 1.1179648 mg F = 96486.33 ( 24 ) A NBS ⋅ s ⋅ mol - 1 ( 2.5 ppm ) . Attached to this figure is an uncertainty whose random component (standard deviation of the mean of 8 determinations) is 9.5 × 10-7 mg C-1 (0.85 ppm). The root-sum-square of systematic uncertainties of known origin is 1.07 × 10-6 mg C-1 (0.96 ppm). The above value for the electrochemical equivalent of silver leads us to calculate the Faraday to be: F = 96486.33 (24) A NBS · s · mol-1 (2.5 ppm).

A Novel Method for Analyzing Silver Sediment With High Precision.

A technique has been devised which is sufficiently accurate to aid in an electrochemical determination of the Faraday constant using the silver coulometer. The technique is used to recover the silver residue which falls from the anode during operation of the silver coulometer. In contrast to previous efforts at recovery, which involved weighing of the silver residue, the method here described is to convert the silver atoms to ions and then to plate the silver onto a cathode held at constant potential with respect to a reference electrode. The current involved in the electrolysis is integrated electronically. An overall standard deviation of 5 µg is achieved for samples ranging in size from 400 µg to 1.8 mg.

Direct Determination of Air Density in a Balance Through Artifacts Characterized in an Evacuated Weighing Chamber.

This paper describes a simple device which permits mass comparisons in air without appeal to the correction for air buoyancy. The device consists of a canister which is evacuated and weighed on a laboratory balance with a mass inside. A second weighing of another mass in the evacuated canister provides the desired mass comparison. The method was used to determine the mass difference between two stainless steel weights of widely differing densities. With knowledge of this mass difference and of the volume difference one may, by a simple air weighing of the two objects, determine directly the density of the air in the balance case. Densities of air determined by this method were compared with those calculated from the barometric pressure, the temperature, and the relative humidity of the laboratory air. The experimental and calculated values agree throughout to within 1.0 µg cm-3 (where the normal air density is about 1.2 mg cm-3). The calculated and experimental values of day-to-day fluctuations in air density agree to within 0.5 µg cm-3.

Properties of Aqueous Mixtures of Pure Salts. Thermodynamics of the Ternary System: Water-Sodium Chloride-Calcium Chloride at 25 °C.

Isopiestic vapor pressure measurements have been made to determine some thermodynamic properties of the system: water-sodium chloride-calcium chloride at 25 °C. Equations are derived for the activity coefficient of each salt in the presence of the other when the total ionic strength is kept constant. The limiting conditions in very dilute solutions are considered and "trace" activity coefficients calculated. A comparison with previous work is made. The excess free energy of mixing is calculated.

Properties of Aqueous Mixtures of Pure Salts. Thermodynamics of the Ternary System: Water-Calcium Chloride-Magnesium Chloride at 25°C.

Isopiestic vapor pressure measurements have been made on the system water-calcium chloride-magnesium chloride at 25°C. Activity coefficients have been evaluated for each salt in the presence of the other in systems of constant total ionic strength. The excess free energy of mixing has been calculated and compared with the values for analogous systems.