Measurement of Concentrations near an Interface during Liquid-Liquid Mass Transfer Alan Traherl and Donald J. Kirwan" Department of Chemical Engineering, Cniversity of Virginia, Charlottesville, Vu. 22901
A simple microinterferometric technique for measuring interfacial concentrations and transfer rates during liquid-liquid extraction is described. Illustrative data for the system acetone-carbon tetrachloride-water are presented. Significant interfacial resistance was observed for this system.
T h e purpose of this paper is to describe a simple, rapid method of measuring interfacial solute coiicentrations during the transfer of solute between two immiscible liquid phases (Traher, 1971). The method is a microinterferometric technique employing an optical wedge. It is similar to that used to measure concentration profiles about growing crystals (Berg, 1938; Humphreys-Owen, 1948; Kirwan and Pigford, 1969) aiid to measure liquid-liquid diffusion coefficients (Sishijima and Oster, 1986; Secor, 1965). Details of the theory of the optical wedge can be found elsewhere (Searle, 1946; Tolansky, 1948) and oiily a brief summary of the principles and their application to the study of liquid-liquid extraction will be given here. The basis of the method caii be explained with reference t'o the schematic drawing in Figure 1. An optical wedge is formed by two partially alumiiiized microscope slides placed a t a slight angle to one another and illuminated with parallel, monochromatic light. If the material between the slides is of uniform refractive index, the interference pattern will consist of straight parallel fringes. When drops of two immiscible fluids containing a transferable solute come iiito contact within the wedge, a solute coiicentratioii change (therefore, a refractive index change) occurs in both phases near the interface. Siiice a fringe is a locus of equal optical path, Le., the product of refractive index and wedge thickness is constant, one obtains a pattern of curved fringes as shown in the photomicrograph in Figure 2 . A knowledge of the wedge thickness, tlie refractive indexcoiiceritratioii relationships, and the original (bulk) solute concentrations allows the calculatioii of the conceiitratioii and concentration gradient a t aiiy point in the field from t'he observed fringe shifts. Experimental Details
The relationship between the concentration change and the fringe shift is Ac = A,v(ho/2t)(dc/dn)
(1)
where Ac is tlie concentration change from the bulk phase due to a friiige shift, AAV, iois the wavelength of light being used, t is wedge thickness, and (dcldn) is the change of coiiceiitratioii with refractive index. The wedge thickness, t , can be obtained by carefully focusing the microscope on the lower face of the upper slide and then 011 the upper face of the lower 1 Present address, Department of Chemical Engineering, University of Massachusetts, Amherst, rvlass.
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slide using a calibrat'ed microscope focusing knob. Alternately, the wedge t'hickiiess can be calculated from the geometry of the wedge aiid measurements of the distance from t,he apex of the wedge, X, and the fringe spacing
t
=
S tan
e
=
Xho;2d,n,
(2)
n, is the refractive index in a bulk phase, d, is the friiige spacing in a bulk phase, and e is the wedge angle, which is very small (
