A quasiequilibrium model of the process of atomization of metal oxides in electrother- mal (ET) atomic absorption (AA) analysis with tubular graphite furnaces ...
STUDY OF THE ATOMIZATION OF METAL OXIDES IN ELECTROTHERMAL ATOMIC-ABSORPTION ANALYSIS D. A. Katskov and V. A. Kopeikin
UDC 543.42
A quasiequilibrium model of the process of atomization of metal oxides in electrothermal (ET) atomic absorption (AA) analysis with tubular graphite furnaces was proposed in [i, 2]. In this paper we describe an experimental method for checking the theoretical results and we present the results. It is proposed in [i, 2] that when an analyte material evaporates from the surface of a tubular furnace into the cavity collisions of the evaporating particles with molecules of the shielding gas in the boundary layer above the sample at a distance close to the meanfree path length (A) establish thermodynamic (TD) equilibrium for all condensed M s, MnOmS , and gaseous Mg, MnOmg, 02 components of the thermal-dissociation reaction of the oxide MnOmS. The composition of the vapors in the boundary layer is controlled by the partial pressure of oxygen, flowing into the boundary layer, p02(A). If the pressure of oxygen formed as a result of the reaction I
s
s
11'1.
Kp 2n/m is greater than p02(A), then a metal phase forms on the surface of the sample, while the equilibrium vapor pressure of the metal in the boundary layer p(A) is close to the sat!! K p___ uration pressure p0. For po~(A)>K~ p(A)~p~ and the boundary layer contains molecular
vapor in addition to atomic vapor.
Vapors of the sample diffuse from the boundary layer through the transilluminated cavity of the furnace to the ends of the furnace. The diffusion flow depends on the total vapor pressure of the free and bound metal in the boundary layer with the temperature of the evaporation surface T, the area of evaporation o, and the parameters of the atomizer (At), characterizing the mass transfer: the length Lr and radius R c of the cavity, and the temperature of the furnace Tf. If it is assumed that with stationary evaporation and diffusion transport of particles under the condition o 10 -6 g and a small area wetted by the solution (~3-10 -6 m 2) one would expect that the atoms would be distributed in a multilayered fashion, the evaporation area o would be constant, and the character of the dependence A = f(Tf) would be reproducible in the course of the measurements. This is not what happened inpractice. The position of the curves A = f(Tf) changes gradually as the experiments are repeated - the curves shift toward high temperatures. To eliminate the arbitrariness in the calculations in all cases the values of Tf, A corresponding to the lowest-temperature curve were chosen. The conditions of the experiments and the values of Tf, A found experimentally are presented in Table 1 together with the data necessary to calculate the parameters aT, and the adopted values of the saturated vapor pressures of ~he metals pTf,A ~ [9-12]. K e was calculated based on the data of [7] for decomposition reactions for the most stable oxides [13] with the help of the expression lgKe--
1
AS~ 298 -- . . . . .
,
(6)
where hSf,2~s ~ hHf,29 a o are tile changes in the entropy and enthalpy in the reaction (i) under standard conditions. The list of reactions is presented in Table 2, which also gives the values of AS ~ and AH ~ as well as the results of the calculations of Po=(A) and A t . To evaluate the oxygen content in the boundary layer above the sample and on the a~is of the furnace we employed the value L c = 2 cm. The results of the calculation of the parameter ~ for an experiment with Lc/R c = 9 and PO2 ~ ~ i'i0 -s based on the method of [2] show that z ~ I for the elements presented in Tables 1 and 2, with the exception of AI, Sr, and Ba, for which ~ = 0.95; for this reason, in the calculation of A t it was assumed for all elements that % = i. 14
TABLE 2. Decomposition Reactions of Oxides, Their Thermodynamic Characteristics at Tf, A = 0.043, and the Results of the Calculation of po2(A), p(A), log A t o -lg ~ - ~ m --AHL298, AS~, 298, --lgEa -- ~-~-n lgpo~ J/g-atom
Reaction
1
~
gg-
1
-lg p(A) -lg At
02
15,5
34,3
--1,0
3,7
.
