Conversion measured using inerts

Catalytic rate measurements under potentiostatic or galvanostatic conditions were carried out using a galvanostat-potentiostat (Amel type 553). The reactant gas mixture was delivered at total flowrates of 1-2 x 10- mol sec, with partial pressures Pno. Pco. Ppropene varied between 0 - 6.5 k Pa, 0 - 1.5 k Pa, 0 - 0.4 k Pa, respectively with Phc, bringing the total pressure to 1 atmosphere in every case. Conversion of the reactants was typically =15%. Control experiments confirmed that the Au reference and counter electrodes were catalytically inert under all conditions. 

For the reaction a A + hB rR, with inerts il. Figs. 4.4 and 4.5 show the symbols commonly used to tell what is happening in the batch and flow reactors. These figures show that there are two related measures of the extent of reaction, the concentration and the conversion X. However, the relationship between Ca and is often not obvious but depends on a number of factors. This leads to three special cases, as follows. 

In these discussions we will thus use the following explicit definition of a chemical measurement in the atmosphere the collection of a definable atmospheric phase as well as the determination of a specific chemical moiety with definable precision and accuracy. This definition is required since most atmospheric pollutants are not inert gaseous and aerosol species with atmospheric concentrations determined by source strength and physical dispersion processes alone. Instead they may undergo gas-phase, liquid-phase, or surface-mediated conversions (some reversible) and, in certain cases, mass transfer between phases may be kinetically limited. Analytical methods for chemical species in the atmosphere must transcend these complications from chemical transformations and microphysical processes in order to be useful adjuncts to atmospheric chemistry studies. 

Using the Claus reaction as a model reaction, Sloot et al. [1990] has experimentally measured reaction conversions and verified the above concept of confining and shifting a reaction plane or zone for two opposing reactant streams inside a porous catalytic membrane. The agreement between the experimentally observed and calculated (based on the dusty-gas model) molar flux of H2S is reasonably good [Sloot et al., 1992]. The simplified model based on Equation (10-101) is a good approximation for dilute systems where the mole fractions of the reactants and products are lower than that of the inert gas. 

Coulometric determinations can be carried out in which no physical separation occurs but simply a quantitative change in oxidation state. For example, MacNevin and Baker determined iron and arsenic by anodic oxidation of iron(II) to iron(III) and arsenic(III) to arsenic(V). The reduction of titanium(IV) to titanium(Hl) and the reverse oxidation have been used for the analysis of titanium alloys. Conversely, the output current from a cell made from a silver-gauze cathode and a lead anode with potassium hydroxide electrolyte can be used to measure low concentrations of oxygen in inert gases. ... 

Catalytic activity was determined in a tubular packed b isothermal reactor at 500 K and 1 atm. A gas mixture was fed to the reactor at 350 cm min (CO 3%, HjO 26% Hj 48% N2 23% v/v) the catalyst weight was 0.04 g with a particle size of 0.177-0.250 mm. Reactants were analyzed by gas chromatography, using a thermal conductivity detector. Two packed columns were employed to analyze the reaction mixture. One was packed with 5A molecular sieve to separate hydrogen, nitrogen and CO, while COj was analyzed in a column packed with Porapak Q. Absence of diffusional control was experimentally verified by measuring the reaction conversion with catalyst particles of various sizes. The b was diluted (D%=10% v/v) with inert particles to provide isothermal conditions. 

Cyanide is metabolized by the ubiquitous enzyme rhodanase to thiocyanate (SCN"), drawing on the body s sulfur-donor pool for substrate to convert CN" to SCN". Thiocyanate is relatively inert and is cleared by the kidney. The conversion of CN" to SCN" occurs slowly relative to the pharmacological action of CN", so measurement of SCN" is of use in monitoring clearance but not very useful in assessing acute CN" exposure. In an acute exposure, the patient experiences symptoms of toxicity with high blood CN" levels, but the serum SCN" level remains low until 12 to 24 hours later. 

Parallel coupling constitutes another type of coupled enzyme reactions. This includes the competition of two enzymes for a common substrate as well as the conversion of alternative substrates and the competitive binding of a substrate and an inhibitor to an enzyme. Thus, analytes become measurable even though they cannot be converted to readily detectable products. Coupled enzyme reactions can also be used to eliminate disturbances of the enzyme or transducer reaction caused by constituents of the sample. Compounds interfering with the signal transduction can be transformed into inert products by reacting them with an (eliminator) enzyme which can be coimmobilized with the analyte-converting (indicator) enzyme in the vicinity of the transducer. On the other hand, constituents of the sample which are at the same time intermediate products of coupled enzyme reactions and will thus... 

Such interferences can be compensated by using difference measurements, which usually require a reference transducer. An elegant alternative is the use of enzymatic anti-interference systems containing enzymes in front of the sensor that catalyze the conversion of the disturbing compounds to inert products. Such systems have been developed in conjunction with analytical enzyme reactors as well as enzyme electrodes. 

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