Round-Table Discussion
Chemical Analysis by High Frequency Methods Digest of round-table discussion held by Division of Analytical Chemistry 'at 119th Meeting, American Chemical Society, Cleveland, Ohio, April 1951 Moderator: Panel:
W. J. BLAEDEL, University of Wisconsin, Madison, Wis. T . S. BURKHALTER, North Texas State College D. G. FLOM, Pennsylvania State College GEORGE HARE, Beckman Instruments, Znc. F. W. JENSEN, Agricultural and Mechanical College of Texas
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It has also been established that the solution is couple- sapacitively into the oscillating circuit, and that considerably greater sensitivity results from loading a condenser-type circuit element with the solution, rather than an inductive-type element such as a coil (1, S). The frequency-measuring type of instrument lends itself very readily to a recording system. The frequency change may be converted to a direct current voltage whose magnitude is proportional to the change, and whose sign is dependent upon the direction of the change. This voltage may be recorded with a standard instrument, such as a Brown recorder. The application of this type of instrument to monitor industrial process solutions is apparent. A recording instrument might also be used in theoretical work to decrease the effort associated with measuring reaction rates ( 1 ) . I t was stated that it would be very desirable to have an instrument that would differentiate between the resistance and capacitance of the solution, and would measure each. Work is progressing on two such instruments, but these were not described in detail (2, 6).
INCE widespread attention \vas called to the high frequency method of chemical analysis several years ago (5), it has
created much interest. among analytical chemists. Briefly, the method is one for measuring solution concentrations without material contact of any sort with the solution. When placed within the coils or condensers of a high frequency oscillator, the solution affects the characteristics of that oscillator t o an extent determined by the nature and concentration of the solution. These characteristics, such as plate or grid current, plate or grid voltage, or frequency, are easily measurable, and therefore any of them serve as a measure of the concentration of the solution. n
THEORY OF THE RESPONSE
The equivalent circuit of the solution shown in Figure 2 seems adequate to explain the responses of both types of instruments described above ( 3 ) . I n this circuit, L represents the inductance of the network, and C1 represents the series capacitance, , iiP , which includes that through the walls of the solution container. C2 is the capacitance of the solution itself, which is constant within a few per cent over a wide frequency range, as the dielectric constant of water is independent of frequency, except a t very high frequencies. Also, CZis independent of electrolyte concentration. R is the resistance of the solution, inversely proportional to the conductivity Figure 2* and varying greatly as the solution Circuit Of changes from dilute to concentrated. To explain the response of the kind in curves 3 and 4, Figure 1, it should be realized that as R is varied by changing solution concentration, the other circuit constants remaining roughly constant, the power absorption of the solution passes through a maximum, and therefore so does the plate current. At high concentrations, R is small and passes current with little absorption. Absorption increases with R until a certain limit is reached, above which (2% begins to shunt greater portions of the current. When R is very high, energy absorption is very small because the current in R is small, and losses in R decrease as the solution is diluted. To explain the S-shaped curves, it need only be realized that when the impedance of R becomes small compared to that of CZ, the latter is almost shorted out, and the frequency approaches ( 1 / 2 7 r ) d P in concentrated solutions. I n dilute solutions, where the impedance of R is large compared to that of C2, t h e
-3 -2 -I 0 I LOG HCl OR NaCl MOLARITY. Figure 1. Response of Two Different Types of High Frequency Instruments for Solutions of Electrolytes -5
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Till recently, attention has been centered largely upon instrumentation. There are several good instruments now available, and increasing interest is being shown in the theoretical aspects of the method and in the possible applications.
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INSTRUMENTS
The instruments described in the literature are of two types, according to the nature of the response to an electrolytic solution. The currenbmeasuring type has a response which passes through a maximum with concentration of the electrolyte (curves 3 and 4, Figure 1). The frequency-measuring type has a response which is S-shaped (curves 1 and 2, Figure 1). For both types, there are certain regions of'concentration wherein the instruments are insensitive t o changes of concentration, while in other regions the response varies almost linearly with concentration of the electrolyte. The location of the maxima and points of inflection of these response curves depends almost entirely upon the operating frequency, and the regions of adequate sensitivity-Le., those regions where response changes adequately with concentrationmay be selected by selecting the frequency (3). 198
V O L U M E 2 4 , N O . 1, J A N U A R Y 1 9 5 2
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f i e uency approaches ( 1 / 2 x ) d , ( C 1 C2)/LClC2. I n the intermeliate regions, where the two impedances are the same order of .magnitude, the frequency varies nionotonically with concentration between these two limits The equivalent circuit of Figure 2 explains quantitatively data obtained with both kinds of instruments. The concentration corresponding to maximum absorption may be derived as a function of frequency from the circuit of Figure 2, and the relation is the same as that obtained using Falkrnhagen’s equation for the rela\;:ition time of ionic solutions APPLlCATION TO TITRATIONS
A desirable situation exists when an unloading of the oscillator takes place during titration ( 4 ) due to the formation of un-ionized or insoluble substances, follo\ved liy an increase in loading caused by :in excess of titrating agent beyond the end point. The choice of the titrating agent which produceE an insoluble substance frequently presents problems, in that the precipitate must be quickly :tiid quantitatively fornied and must show low ahsorption properties. Many oscillating circuits :ire not respousive t o clianges in heavily loading solutions, whem the ion concentrations are high, as i n niarine maters. Modificatioiis m:ty he niade so that the response varies linearly with c*onc.ciitr:rtionin these cases, The titration of c:~lciumn.ith st:tiid:~i~(lsoap solution vas disAlso presented were preliminary data on the saponification of ethyl acetate t o shovi the feasibility of following r e a d o n rates by the high frequency method. Readings of concentration changes nixy be made instantaneouply as reactmionproceeds. APPLICABILITY O F HIGH FREQUENCY METHODS TO ORGANIC S Y S T E M S
1Iaiiy organic solutions do not sholv the decreasing sensitivity nit11 increasing concentratioti th:rt ionic. solutions do ( I , 2 ) . For
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such noncouducting solutions, where the impedance of R is always much greater than that of Cp, examination of Figure 2 shows that the response is inherently simpler than that of conducting systems, for all changes are essentially capacitive. If the particular instrument, temperature, and volume of solution are held constant, the frequency change upon insertion of any particular substance is a fixed property of that substance, and is measurable (1). This property, called the frequency change, may he handled in the course of analysis as though it were additive for ideal solutions. Ideality and a linear response are not necessary for analysis, however (1, 2 ) . By use of a working curve, it is possible t o determine the lvater content of alcohol (1). The composition oi‘ o-xylene in p-xylene may be determined with an accuracy of 1% (1). Even ternary systems may be so analyzed .4n example is ~vater-benzene-methyl ethyl ketone. The frequency change upon removal of water by almorption in calcium chloride is dependent only upon the water content, and independent of the benzenemethyl ethyl ketone ratio. The remaining binary system is analyzed as described above (1). By forming a condenser of two semicircular plates wrapped around the bottom of a chromatographic adsorption column, the high frequent>- method ma^ be used to detect the passage of colorless substances down the column. Solutions of aniline, acetone, phenol, ethanol, and methanol were consecutively developed on the same Fluoroqil column (1). REFERENCES
(I) Burkhalter, T. S.,reported i r i meeting. 12) Elving, P. J., Flom. D. G . , and Coleman .\, I . , reported i n meet-
ing. (3) Hare, George, reported iri meeting. (4) Jensen, F IT., reported in meeting. ( 5 ) Jensen, F. K,, and Parrack, A . I,., 1x1).1,:si:.C’tIE>f.3 .-IN.
