The Thermoelectric Method of Measuring Vapor Pressure.

Abstract

The primary purpose of this paper is to acquaint physical chemists with the sensitivity and accuracy of Baldes' (1, 2, 3) modification of the thermoelectric method of measuring vapor pressure or of comparing differences in the vapor pressure of solutions. The method is one in which the difference in temperature of a drop of a reference or known solution and a drop of the unknown solution suspended in an humidified chamber is compared, by means of a thermocouple and a sensitive galvanometer, with the difference in temperature of drops of two reference solutions. Other conditions being equal, the difference in temperature of the two drops suspended in the humidified chamber is proportional to the difference in the rate of evaporation from, or condensation on, the drops, which in "turn is proportional to the difference in vapor pressure of the drops (3).The apparatus consists essentially of two thermocouples (permitting duplicate determinations) placed at right angles to each other, the thermo junctions being in the form of horizontal loops. The thermocouples are mounted in a watertight chamber suspended in a water bath and are connected to a sensitive galvanometer through double-pole, double-throw switches. The procedure consists in placing a reference solution, in the form of a small drop, on one of the loops of each thermocouple and the unknown solution on the opposite loop, the filter paper on the wall and floor of the chamber being moistened also with the reference solution or a solution of approximately the same vapor pressure. The deflection of the galvanometer to which the thermocouples are connected is proportional to the difference in vapor pressures of the two solutions and is calibrated by using two reference solutions, that is, solutions of known vapor pressures. SUMMARYData are presented to show the sensitivity which may be obtained with Baldes' modification of the thermoelectric method of measuring vapor pressures. Attention is called to a number of factors which must be taken into consideration in order to obtain an accuracy comparable to the sensitivity of the method. Evidence is presented to show that the temperature of a drop of solution placed on a thermocouple loop is, for practical purposes, independent of its size.

The Chemical Composition and the Osmotic Pressure of the Aqueous Humor and Plasma of the Rabbit

“…Roepke (21) has suggested that values for the osmotic pressure of viscous solutions may be incorrect because of the presence of surface films, difference in ~on-solvent volume, and various other factors associated with such solutions. These possibilities have been investigated by Roepke and Baldes (22) and by Roepke (21) who found that the error incurred with viscous mixtures of hemolyzed blood cells containing 35 per cent dry solids, or egg yolk containing 55 per cent dry material, was less than 2 per cent of the difference in vapor pressure. While these results seem to indicate the adequacy of the thermoelectric method for determining the osmotic pressure of plasma, the availability of crystalline human albumin which had been recently electrodialyzed and shown to be salt-free by conductivity measurements, made it possible to check the adequacy of the method in a viscous solution more nearly resembling plasma.…”

Charge stripes and antiferromagnetism in insulating nickelates and superconducting cuprates

“…If the spin and charge modulations are related to topological stripes, then one would expect the amplitudes of the modulations to be large. Although direct measurement of the amplitude of the charge modulation is difficult, local-probe measurements of the spin amplitude are available [29][30][31]. In particular, muon spin rotation (µSR) studies on samples of La 2−x−y Nd y Sr x CuO 4 , with x = 0.12, y = 0.4 [29], and with x = 0.15, y = 0.3-0.6 [30], indicate a maximum ordered moment approximately half that in La 2 CuO 4 , and an ordering temperature of ∼ 30 K. The magnitude of the ordered moments is substantially reduced compared to the antiferromagnetic insulator state, but at the same time, the ordering temperature is diminished by an even greater amount.…”

A New Method for the Determination of Osmotic Pressure

“…OP was determined by the general method of Hill ( 1930) as modified by Roepke (1942). Stem sections were frozen on dry ice, thawed, and expressed in a hypodermic syringe into stoppered tubes.…”

“…The sap was frozen, and thawed immediately prior to the OP determination. OP was determined by the general method of Hill ( 1930) as modified by Roepke (1942). In essence, this method consists of measuring the temperature increase due to heat of condensation when a drop of the cell sap is placed in a water-saturated atmosphere.…”