Closed-system analyzer

Fig. 2.2. Example of a polythermal path. Fluid from a hydrothermal experiment is sampled at 300 °C and analyzed at room temperature. To reconstruct the fluid s pH at high temperature, the calculation equilibrates the fluid at 25 °C and then carries it as a closed system to the temperature of the experiment.

Polythermal reaction models (Section 14.1), however, are commonly applied to closed systems, as in studies of groundwater geothermometry (Chapter 23), and interpretations of laboratory experiments. In hydrothermal experiments, for example, researchers sample and analyze fluids from runs conducted at high temperature, but can determine pH only at room temperature (Fig. 2.2). To reconstruct the original pH (e.g., Reed and Spycher, 1984), assuming that gas did not escape from the fluid before it was analyzed, an experimentalist can calculate the equilibrium state at room temperature and follow a polythermal path to estimate the fluid chemistry at high temperature. 

Many, if not most, step polymerizations involve equilibrium reactions, and it becomes important to analyze how the equilibrium affects the extent of conversion and, more importantly, the polymer molecular weight. A polymerization in which the monomer(s) and polymer are in equilibrium is referred to as an equilibrium polymerization or reversible polymerization. A first consideration is whether an equilibrium polymerization will yield high-molecular-weight polymer if carried out in a closed system. By a closed system is meant one where none of the products of the forward reaction are removed. Nothing is done to push or drive the equilibrium point for the reaction system toward the polymer side. Under these conditions the concentrations of products (polymer and usually a small molecule such as water) build up until the rate of the reverse reaction becomes equal to the polymerization rate. The reverse reaction is referred to generally as a depolymerization reaction other terms such as hydrolysis or glycolysis may be used as applicable in specific systems. The polymer molecular weight is determined by the extent to which the forward reaction has proceeded when equilibrium is established. 

Besides two different hydrolysis methods (i.e., acid hydrolysis and pretreatment without the addition of acid), two different pieces of pretreatment equipment were used to perform the experiments (Fig. 4). Acid hydrolysis was conducted in a microwave oven, while pretreatment was performed in a steam pretreatment unit. The microwave oven provides a closed system where the amount of water added is fixed and there is no loss of material during the process (17-18). On the other hand, the sampleshave to be rather diluted for the microwave oven to be efficient. Another disadvantage is that the microwaves penetrate the material only a few center-meters, and therefore this method is not feasible on a large scale. The microwave oven may, however, still be of interest in the laboratory as a screening method to analyze the composition of feedstock as well as to determine a range of optimal conditions for steam pretreatment. 

The chemistry of an FIEM(A) of a finite collection of atoms is a finite closed system with a variety of well-defined properties. Thus its logical structure is easier to analyze than the logical structure of chemistry in general. Since any collection of atoms may serve as A, any qualitative relations that exist in an FIEM(A) are analogously also valid for chemistry as a whole [9]. 

N. HACKERMAN The products from this system are very hard to handle they change rapidly in air and thus must be analyzed in a closed system. If the transformation from Fe(OH)2 to magnetite can occur readily at 60°C, magnetite would be present, but we have not found it. 

Gas evolution from the hydrolysis of Grignard reagents can be used for the activity analysis. However, care must be taken in the calibration of standards, because the resulting gas will have some solubility in the solvent. Typically, the gas is analyzed by GC, taking the gas from the headspace of a closed system. The obvious limitation in this method is that only a selected amount of Grignard reagents (C4 or less) can be used, owing to the volatility of the hydrolysis products. 

Let us look now at some examples of applications of the energy balance for closed systems. Remember to follow the checklist presented in Chap. 2 in analyzing the problem. 

To Identify the foul smelling gas, a closed system was built in which the film was dissolved. The liberated gas was driven to a reaction chamber and oxidized. If the gas were phosphine, it would be converted to phosphate ion and as such could be analyzed. This test was positive for every plasma and plasma enhanced phosphorus doped oxide that was analyzed. Deductively it was concluded that the gas... 

Prior to each reaction test and spectroscopic measurement, the sample was treated with 100 Torr oxygen (1 Torr = 133.3 N m ) at 673 K for 1 h, followed by evacuation at 673 K for 1 h. The photooxidation of propene was performed with a conventional closed system (123 cm ). The sample (200 mg) was spread on the flat bottom (12.6 cm ) of the quartz vessel. Propene (100 pmol, 15 Torr) and oxygen (200 pmol, 30 Torr) were introduced into the vessel, and the sample was irradiated by a 200 W Xe lamp. After collecting the products in gas phase, the catalyst bed was heated at 573 K in vacuo to collect the products adsorbed on the catalyst by a liquid nitrogen trap. These products were separately analyzed by GC. The results presented here are the sum of each product yield. 

Closed automated systems designed for particular tests in an instrument have been developed. They are closed systems because the reagents and IA protocols were controlled by the manufacturer. Examples are the Roche Cobas FARA II , DuPont ACA , and Miles Immuno 1 analyzers. The reagents are expensive, and in many cases the applications are fixed and may not be adaptable for PK and PD study purposes. 

Evaporation, condensation, and the meaning of the term boiling point are all related to the concept of liquid vapor pressure. Consider the following example. A liquid, such as water, is placed in a sealed container. After a time the contents of the container are analyzed. Both liquid water and water vapor are found at room temperature, when we might expect water to be found only as a liquid. In this closed system, some of the liquid water was converted to a gas ... 

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