Atmospheric pressure chemical calibration

An early intercomparison of LPA and CTM measurements of HO within a chemical reactor has been reported (120, 121). The measurements were made by directing the beam transverse to the detection axis within the reactor at atmospheric pressure N2 was replaced by He to improve fluorescence efficiency. A folded-path LPA measurement within a calibration chamber would have obvious advantages in an instrumental intercomparison and seems within the realm of possibility. 

Of course the preceding thought-experiment, in which calibration of the balance is conducted in a vacuum, is an unrealistic example of the buoyancy effect. Consider now a realistic case where the calibration of the balance and the weighing of the nnknown object are both conducted at atmospheric pressure now the variation arises because the densities (and thus the volumes) of the standard mass and the unknown are different. The buoyancy force on the steel standard is stiU the same (equivalent to the gravitational force on 1.5 x 10 g), but in this case this buoyancy force is accounted for in the calibration procedure. Assume that the object to be weighed is a powdered or liquid chemical with density 1 g.cm and also of true mass 10.0000g, so its volume is lO.Ocm the mass of air that it displaces is thus 12 x 10 g. In other words, the upward (buoyancy) force on the sample is greater than that on the steel standard used to calibrate the balance by an amount equivalent to a mass of (12 — 1.5 = 10.5) X 10 g, and the indicated mass on... 

Radex Safety Calorimeter. The Radex calorimeter is a modular instrument that can simultaneously evaluate six different samples (size range 0.5 to 5 ml), or one substance under a variety of conditions. Each module is a separate entity with its own calibrated oven capable of being operated under an open, closed, or pressurized condition, with all temperature differences between the sample and the oven being stored in a microprocessor for further analysis. The Radex calorimeter is very versatile samples can be tested in either an isothermal or ramp mode. In the isothermal mode, each oven is heated to a preset temperature and held at that temperature throughout the experiment. In the ramp mode of operation, the oven is heated linearly to a preset temperature, or can be maintained at a given temperature for a predetermined time. The flexibility of oven function in the Radex calorimeter enables the user to determine the intrinsic stability of a chemical and to also compare the impact of such parameters as temperature, atmosphere, and impurities on the stability of a given substance. 

The success of studies on ocean chemistry usually hinges on the availability of adequate analytical methodology and expertise. The nature of the chemical problems involved the size and heterogeneity of the ocean the extremes of concentration, composition, temperature, and pressure and the dynamic interactions at its interfaces with the lithosphere, atmosphere, and biosphere create challenging and often unique analytical problems. Current analytical methods are limited, and improved capability in referee, routine, monitoring, and in situ analyses is needed. Valid sampling, the use of reliable standards in calibration, and the collaborative evaluation of methods under real-world conditions are important. 

The boiling point serves as an indicator of a chemical s physical state at ambient conditions. Mostly it is not used as an autonomous parameter in environmental hazard assessments, but for estimating further properties such as the vapour pressure. The boiling point is defined as the temperature at which the vapour pressure of a liquid is equal to the pressure of the atmosphere on the liquid (Rechsteiner, 1990). The measurement of boiling points is in principal simple, requiring only a calibrated thermometer, but the results may be severely affected by ... 

Probe Considerations. Experimental results have shown that although this is a simple technique, several considerations must be made for accurate use. The probe baseline drift with time must be corrected by frequent calibrations, especially during initial sensor service. The probe also loses this baseline when it is removed from vacuum for an extended period of time. This is due to the long time required for the solid probe to return to chemical equilibrium at the low pressure. Return to the original baseline may take up to two weeks, depending on the length of time which the probe was exposed to the atmosphere. In tfiis study, the probe was maintained under vacuum to prevent baseline drift. 

A related colligative technique is vapor-pressure osmometry. Two thermistors are placed in a carefully thermostatted chamber that contains a pure-solvent reservoir so that the atmosphere is saturated with solvent vapor. A drop of solvent is placed on one thermistor and a drop of polymer solution is placed on the other. Because of the solvent s lower chemical potential in the solution, solvent vapor condenses on the solution drop, giving up its heat of condensation, warming the solution drop relative to the pure solvent drop. In principle, the equilibrium AT is thermodynamically related to the molar solution concentration, thereby allowing calculation of M . In practice, heat losses (mainly along the thermistor leads) require that the instrument be calibrated for precise results, really making it a relative technique (as opposed to absolute). On a routine basis, commercial instruments are probably limited to maximum M values of 40,000-50,000 [4]. 

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