Experiments with static systems

Brennan and Fletcher [5] found, for the atomisation of hydrogen over tungsten at about 1200 K, that dPV2/dt is constant over the pressure range 9.3 x 10-3 to 1.52 x 10-6 torr, as shown in Fig. 9. Similar behaviour was observed for H2—Pt, H2—Au and 02—Pt [5, 7, 37]. Kislyuk and 

Tret yakov [111] did not find half-order kinetics for the atomisation of hydrogen by gold under these conditions. On the contrary, they reported first-order kinetics. They attributed the observation of half-order kinetics by Brennan and Fletcher for this system to the presence of surface impurities capable of rapidly dissociating molecular hydrogen, whereas molecular adsorption on the clean surface, being activated, was considered by them to be rate-determining. We will return to the H2—Au system in Sect. 3.2.1(d). Nomes and Donaldson [8] have demonstrated half-order behaviour for the N2—W system, but over a narrower pressure range. 

The effect of varying the temperature is exemplified in Fig. 10 using data for the 02—Pt and N2—W systems. Similar plots were obtained for the other systems studied and all can be accommodated in a relation of the form 

Experimental rates of formation (ca) of atoms per unit area of surface in the region of half-order kinetics at 1200 K, expressed in the form ua = b(Pi)X12 exp (— EJRT), Values of si(s2/s2)12 have been calculated from the theoretical equation, eqn. (48), and the values of its parameters given in Table 2. 

Kislyuk and Tret yakov [111] have reported that the atomisation of hydrogen (and deuterium) over gold is first-order in the temperature range 950—1250 K at pressures in the range 10 3 to 10 5 torr. Their result for 

---

Laboratory experiments with model systems under static conditions have been aimed at the determination of the retention mechanisms of colloids and pseudocolloids (association of a heavy element with a colloid) onto mineral surfaces. This will give a better understanding of the fate of radioelements associated with colloids upon interaction with mineral surfaces as it will occur in the water fiow across fissures and fractures around a radwaste repository. In these studies, polished cm-sized monoliths are used to simulate macroscopic surfaces of fine particles or as mineral surfaces. Rutherford Backscattering Spectrometry BS) is the technique chosen to determine accurately the amount of elements fixed on the monolith. 

To be consistent the value of E2 should be corrected to constant pressure so that it represents AH for the process involved (flow system studies and static system work with excess inhibitor are essentially constant pressure experiments). Then D < E < D+RT. In the present work a reasonable estimate gives D — E—0J = 57.0 kcal.mole-1. Similarly, D2+D2 should be corrected to 0 °K, giving an estimated value of 59.0 kcal.mole-1. This gives D2 = 2.0 kcal.mole-1. Such corrections are normally within the limits of experimental error, so that experimental values of E are associated directly with dissociation energies, and thermochemical data at 25 °C are used. 

Before considering this mechanism and comparing it with other proposals an important fact should be noted. This work was carried out over the approximate temperature range 380-440 °C. Most of the special experiments carried out to test the mechanism were done above 400 °C. All other significant static system work has been done at much lower temperatures, 265-366 °C. 

Poth and Focht (1985) reported on a series of experiments with N. europaea in which [ NJnitrite or [ N]nitrate was mixed with NH, and the rates of formation and isotope compositions of nitrite and N2O were followed. The systems were either well oxygenated (shaken) or poorly oxygenated (static). When shaken, no N2O was produced by the cultures. With static cultures, N2O but not N2 was produced in the presence of nitrite but not nitrate, and nitrite was the preferred source of N for N2O. The observed N20/ N20 ratio was 0.25 rather... 

Eq. (5a) with v(T) = 1 and Eo(T) = 0 i.e. r0(T) = rv is identical with Eq. (2a), obtained previously by Casper and Schulz and proposed for the mathematical description of the PDC-calibration curves. It was shown above that this is not possible even in the vicinity of the theta point, if a large range of P is considered. Independently, the same result was obtained by Wolf and Breitenbach 10) from static separation experiments on the system polystyrene/cyclohexane (PS/CHX). In their notation... 

It must be remembered that irradiation experiments can be performed with monophasic systems (in the absence of an aqueous phase) or in extraction conditions in the presence of an aqueous phase, in static or dynamic conditions (organic and aqueous phases continuously stirred with a dedicated stirrer unit (20), magnetic stirrer, or sparging with air). 

The accuracy of Eq. (1), when dealing with real systems like polystyrene spheres in water, has been usually tested indirectly. Theoretical calculations and/or computer simulated experiments, of static properties such as the static structure factor, are carried out assuming such functional form of the pair... 

Preliminary experiments indicated that the rate of reaction of gaseous NF3 with caustic soda solution in a static system was first order with respect to NF3 with little dependence on the initial concentration of the base. Although these observations are apparently consistent with the first-order Equations 3 and 4, more detailed studies show that these results are misleading. 

These operators induce transitions between different spin states so that by applying an rf-field to nuclear spins in the presence of a large static magnetic field close to the Larmor frequency, the spin distribution between the energy levels is perturbed away from thermodynamic equilibrium. In pulsed NMR experiments the spin system is excited with a short if pulse near resonance and the system is measured afterwards. The total external Hamiltonian from the applied fields is then... 

The control over the uptake experiment in a chromatographic column and interpretation of results is even more difficult than with static experiments. For example the pH in dynamic experiments varies not only as a function of time but also spatially (as a function of the position in a column). On the other hand, chromatography is an efficient method to entirely remove certain solutes from solution, it is also used to simulate migration of pollutants in natural systems. 

---