Measurement of partial pressures

Lubbers D.W., Opitz N., The pC02-/p02 optode, a new probe for measurement of partial pressure of carbon dioxide or partial pressure of oxygen in fluids and gases, Z. Naturforsch. 1975 30C 532. 

Measurements of partial pressure of oxygen (pC>2) and of carbon dioxide (PC02) are described in the following sections. 

Gases/vapors must get into solution in the thin fluid film in the alveoli for systemic absorption to occur. For this reason doses are often a measurement of partial pressures, which is important for gases/vapors. 

The blood gas cell can be modified to optimize either the measurements of partial pressures or contents. Decreasing the permeability of the membrane by an order of magnitude will effectively reduce all boundary layer effects, allowing a linear calibration for P02. At the other extreme, increasing the membrane permeability (e.g., silicone rubber membranes) would permit a rapid depletion of the blood gases, facilitating content measurements. 

Transcutaneous measurement of partial pressures is based on the gas permeability of human skin. An electrochemical sensor is placed on the skin which is heated to increase arterial blood in superficial blood vessels [1]. 

For measurements of partial pressures in tissues and in liquids such as serum or blood, sensors according to Severinghaus are used. A membrane separates the external liquid or gas phase from the electrolyte surrounding a glass electrode, the potential of which is measured against an Ag/AgCl electrode (see Sections 23.3 and 23.4). 

The measurement of partial pressures over a liquid or solid mixture of two metals is not as simple. Mostly, it is restricted to higher temperatures or even to the molten phase. The direct measurement can be done, for example, in high or ultra high vacuum, using a Knudsen cell and a mass spectrometer for selective pressure determination. Dynamic measurements were developed, e.g., transportation methods. A steady stream of a reactive gas is passed over the sample transporting the reactive component to a cooled region of the apparatus. From the measured mass of the transported metal and the flow rate the vapor pressure can be calculated. Kubaschewski et alP have given a detailed description of the experimental possibilities. 

Galvanic cells with solid electrolytes can be used for direct measurement of partial pressures in gases and concentrations in liquids and melts. An important example is cell I, which contains doped zirconium dioxide as solid electrolyte. By using cells of this type a wide range of oxygen partial pressures in gases (down to 10 atm) can be determined. The zirconium dioxide probe for such work is used at temperatures between about 500 and 1000°C. 

Californium is the heaviest actinide for which data like the enthalpy of sublimation have been determined directly with bulk quantities of about 2 mg of pure metal. Due to the limited availability of the heaviest actinides down to the one-atom-at-a-time scale, the preparation of the metals becomes an integral part of an experiment for studying the metals. Unusual experimental approaches like the measurement of partial pressures of the actinide under study over an alloy, studies of diffusion of actinide atoms in metals, and adsorption studies of actinide atoms onto metal surfaces by thermochromatography have been reported. 

If the solute is one such as sugar, whose vapour pressure is minute, it will be impossible to measure its partial pressure along the curve AS of Fig. 39. However, we can continue to regard the tangent A W as defining the ideal solution. Although this tangent now almost coincides with the axis AJ7, the activity coefficient of component B remains equal to the ratio BS/BWy and can be determined by methods other than the direct measurement of partial pressure, as will be described in 9 8. 

In case of multi-component mixtures, total isosteres can be measured at constant sorption-phase composition by changing, in successive steps, the total amount of gas mixture sorbed at constant mole fractions of sorption phase [31]. On the other hand, a point-bypoint measurement of partial pressures of a multi-coinKtnent mixture sorbed leads to partial mixture-sorption isosteres that can be evaluated further by solution thermodynamics [57]. 

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