Reactant concentration monitoring

Sensitivity The sensitivity for a one-point fixed-time integral method of analysis is improved by making measurements under conditions in which the concentration of the monitored species is larger rather than smaller. When the analyte s concentration, or the concentration of any other reactant, is monitored, measurements are best made early in the reaction before its concentration has substantially decreased. On the other hand, when a product is used to monitor the reaction, measurements are more appropriately made at longer times. For a two-point fixed-time integral method, sensitivity is improved by increasing the difference between times t and f2. As discussed earlier, the sensitivity of a rate method improves when using the initial rate. 

For an industrial application it is clear that flow cells will be highly advantageous since they allow continuous operation, the use of external heat exchangers and continuous monitoring of quality. In addition the reactant concentration remains invariant with time, allowing the cell to be run under true steady-state conditions, and the lower residence time of the products reduces further reactions. 

The kinetics of the oxidation of Fe(II) by Ce(IV) in aqueous perchloric acid have been studied, using reactant concentrations in the range 10 to 10 M (ref. 242). A quenching method was utilised to monitor the disappearance of Fe(II). The reaction conforms to a 1 1 stoichiometry and is of simple second order, viz. 

Recently, such a temperature oscillation was also observed by Zhang et al (27,28) with nickel foils. Furthermore, Basile et al (29) used IR thermography to monitor the surface temperature of the nickel foil during the methane partial oxidation reaction by following its changes with the residence time and reactant concentration. Their results demonstrate that the surface temperature profile was strongly dependent on the catalyst composition and the tendency of nickel to be oxidized. Simulations of the kinetics (30) indicated that the effective thermal conductivity of the catalyst bed influences the hot-spot temperature. 

Suppose that an irreversible reaction between A and B leading to >95% product or products, designated D, is examined in the usual way. One of the reactants, B, is held in excess and the loss of A monitored. It is likely that the loss will be a first-order process (rate constant k). At low concentrations of B (but still > [A]), the value of k may be proportional to the concentration of B. At higher concentrations of B however this direct proportionality may disappear and eventually k will become independent of [B]. Obviously, a second-order reaction at low reactant concentrations has lost its simplicity at higher reactant concentrations and eventually turned over to first-order in A alone. Such a situation is accommodated by a rate law of the form... 

Several experimental approaches have been used to obtain information concerning the identity, concentrations, and reactivities of intermediates in catalytic reactions. Tamaru 136), during measurements of catalytic activity, concurrently determined the total quantity of gas adsorbed. One possible limitation to this method is that a proportion of the material bonded to the surface may not be involved in the surface reactions (57), The use of labeled reactants and monitoring the radioactivity in the region of the active solid may (95) provide a potentially useful technique for the estimation of Cj and/or c2. An alternative, and perhaps complementary, approach is through the individual investigation of the kinetics of product formation from reactions of known amounts of adsorbed material. This method has been used to elucidate some of the elementary steps in the breakdown of methanol on platinum (80). The independent preparation of a postulated intermediate,... 

Equation 3-199 infers that the absorbance approaches the value at the end of the reaction (infinity value) with the same rate constant k as that for the reaction expressed in terms of the reactant concentration. The required rate constant can be determined from the slope of a plot of In (Dm - D0) versus time. The same equations can be written for reactions monitored in terms of optical rotation or conductance. 

For reactions with essentially instantaneous kinetics, the reaction rate is limited by the feed addition rate. For other reactions, particularly if the reactor is operated at too low of a temperature, a reactant concentration can build up, eventually reaching an unsafe level that could lead to a rapid temperature rise and explosion. It is important for these reactions to monitor the heat flow to confirm that the reactant concentration is not increasing to unacceptable levels. 

To determine the branching ratios in multi-channel reactions requires measurements to be made of the rates of appearance of the various products. Unfortunately this is usually a far more difficult task than measuring the rate parameters for removal of the reactants. The reasons for this are discussed in Chapter 1 briefly, using pseudo first-order conditions for reactant removal, the overall rate constant can be determined without a knowledge of the absolute concentration of the reactant being monitored, whereas to determine the rate constant for appearance of a product, requires a method for the detection of what may be a small product yield and a means of calibrating the signal monitored, either in terms of the absolute concentration of the product, or its concentration relative to that of the minority reactant. Consequently, few studies have been made of rates of product appearance. 

The approximate initial reactant concentrations in both reactors (in ppmv units) were as follows NO2, 50-100 alkyl iodide 10-30 and 2-iodopropane 10-30 (1 ppmV = 2.46 x lo " molecule cm at 298 K and atmospheric pressure). The concentration-time behaviom of the alkyl iodides were monitored over a 15-30 min irradiation period in both the kinetic and product studies. 

Again, note the negative values for the reactants and the positive values for the products (usually written without the plus sign). Figure 16.4 shows a plot of the simultaneous monitoring of one reactant and one product. Because, in this case, product concentration increases at the same rate that reactant concentration decreases, the curves have the same shapes but are inverted. 

The kinetic reaction profile (Figure 3.1) we discussed for this reaction was one for which the initial concentrations of C10 and Br were relatively small and similar in magnitude (3.230 x 10 moldm 3 and 2.508 x 10 moldm 3, respectively). However, the reaction can be investigated over a much wider range of reactant concentrations a specific example is shown in Figure 5.7. (The temperature is reduced to 15 °C so that the reaction is slow enough to be monitored by conventional techniques.)... 

The experimental rate law can be determined by monitoring the concentration of one of the reactants or products as a function of time using spectroscopic means. For instance, the Beer-Lambert law states that the absorbance of a colored compound is directly proportional to its concentration (for optically dilute solutions anyway), so that the absorbance can be measured as the course of the reaction proceeds. The data are then fit to a model, such as the function that results when integrating one of the differential rate law equations. The integrated rate laws for some commonly occurring kinetics are listed in Table 17.1. Half-life equations are also included for some of the reactions in this table, where the half-life ftyi) is defined as the length of time that it takes for half of the initial reactant concentration to disappear. 

---