1070-4272/01/7401-0164$25.00C2001 MAIK [Nauka/Interperiodica]
Russian Journal of Applied Chemistry, Vol. 74, No. 1, 2001, pp. 164 !166. Translated from Zhurnal Prikladnoi Khimii, Vol. 74, No. 1,
2001, pp. 159!161.
Original Russian Text Copyright + 2001 by Krichmar, Bardachev.
Microdosing on Evaporation from the Surface of a Porous Solid
S. I. Krichmar and Yu. N. Bardachev
Kherson State Technical University, Kherson, Ukraine
Received May 4, 2000
Abstract-The processes are examined that occur when dilute gas mixtures are prepared by the evaporation
of a solution of impurities in an indifferent liquid solvent from the surface of a porous solid. Relationships
between the concentrations of the impurities in the liquid and gas phases are given.
A procedure for dosing small amounts of volatile
impurities in gases was suggested in . This proce-
dure enhances the capabilities of the known technique
[2, 3] by increasing the surface area for the evapora-
tion on a liquid3vapor interface owing to the use of
a capillary-porous solid. The technique can be used on
a larger scale if highly volatile components are dis-
solved in an indifferent liquid solvent with a vapor
pressure lower than the vapor pressure of the impuri-
ties under study .
The aim of this work was to describe in more detail
the possibilities of the suggested dosing procedure.
We emphasize that this procedure can be used for the
preparation of gas mixtures of pollutants with a given
composition not only for calibration of gas sensors,
but also in modern technologies of obtaining pure
The operation of the leak device is the following.
A solution of impurities in a liquid solvent is placed
in narrow glass tube 2 with a widening in its top end
filled with capillary-porous solid 3 (Fig. 1). Its bottom
contacts the solution, and flat upper interface 4 is
directed to the space through which a diluent gas is
continuously flowing. The diameter of the glass tube
is chosen in such a manner that the liquid does not
flow out via its open lower end. The position of
the liquid meniscus in the lower part of the tube is
detected at regular time intervals with a cathetometer.
In a conventional case the limitations in the choice of
the tube diameter prevent measuring evaporation rates
of poorly volatile liquids. The evaporation rate is
proportional to the evaporation surface, all other
factors being equal. The surface of a finely porous
solid impregnated with a liquid is the most suitable
for technical use of this property. A fine liquid film
distributed over this solid evaporates evenly with a
rate considerably higher than the rate of the liquid
transport along the pores. Therefore, the process is
almost independent of the equilibrium vapor pressures
of the components.
In a steady-state mode the evaporation from the
surface of a porous solid and from the meniscus in the
tube results in changes in the position of the liquid
level. The rate of its migration along the measuring
tube (Fig. 1) is dl/dt, where l is the capillary length
and t is the time. If the capillary cross section is S
and the liquid density is r, the weight of substance
evaporating in unit time is Sr(Dl/Dt). When the space
velocity of the gaseous diluent is J
, the relative
weight concentration of the ith component in the
liquid containing several components will correspond
Fig. 1. Scheme of the leaking device: (1) liquid solution,
(2) transparent tube, (3) porous solid, (4) evaporation sur-
face (liquid3gas interface), (5) meniscus, and (6) gas space;
(s) effective diffusion layer and (l) capillary length (mm).