Capacitor Cell Modification of Chemical Oscillometer 0. A. NANCE, T. S. BURKHALTER: AMI 1’. H. MOSAGH4XV? Louisiana S t a t e Crniwrsity, Baton Roripe. L a .
ATTEMPTS to apply the Iictei,oclyiie t v l w
of chemical oscillometer developed by \\-est, Burkhalter. and Broussard ( 4 ) t o indubtrial analysis of systems of nonelectrolytes led to an increased demaiid for additional sensitivity and suggested the desirability of a snidlcr sample size. T h r volume of sample required by the origiiial instrumcilt (20 ml.) \vas i,atlier large \vheri the amount oi $ample tvits liiiiitctl or coritinuous sampling u i a reaction product \vas cicsired. Substitution of B capacitor cell f ~ i the coil t y p , of (Y.II involved problems of ckcuit design wk1ic.h are not imniediutc’ly obvious an(l \vhic.li indicated applications that 2
r . Lhe cells u r c d t v w c i~oristiucLcd111 t l i u glass-bloniiig S ~ J ~byJ flattening u secation of large-di:inieter g l : tuling ~ :ind attaching 8-mI. tubing to t h e tn-o cjnds of tlie eell. The over-:ill shape of the cell is that of :L disk. Sheet met:il plates w r e thenattached to the two flat surf:tees of the cell with polystyrene cement and the electrical connections w r e ni;idct firm l ~ ywrapping the leads u i d input tubes \vith wire. The cap:iritnnc.c>of the cells varied between 5 aud 8 micromicrofarads
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s oi this type is largely dependent on the “figure of niwit” or Q of the componen the ratio of rcactaiiw to radio-irequency re tice in clectronics engineering ( 3 ) requires that the diameter of the coil used in thr tank circuit he large compared to the height of t81iecoil. Ordiiiarily, the coil should be Jvound with wire having the maximum surface consistent Jrith the available space and the individual turns should lw spaced \vit,h a separation approsimately equal to the tiismeter of the nire. Thcse requirements suggest that tIi(i optiiiium coil for the frequency employed in these instrunwnts (:ibout 5 megacycles) should have a diameter of approximately 4 inches (10 cm.) and a height of about 2 inches. ~ - o u n d\\it11 wire of about gage 8. An inductor of this size rcquires H sample of about 200 ml. The difficulty imposed by increased sample size niay be :tvoided 13). const,ructing a cell ill which the smiiple is cont,ained between concentric ~ . a l land s COIIfined to the rcgioii imniediately inside the coil form. By this device. the sanipie may be decreased by a factor of 2 to 5 \vith’approximately 80of the effectiveness of a sample filling the whole interior ot’ th(, incluetor, but til? cells are somerhat difficult to reproducc and channeling or tioltlup of‘ flowing saniples may lead to erroneoli? results,
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Oscillator Circuit
The requirements for LL capacitor, on the other hand, are o n l ~ . that a high dielectric be maintained between the plates of the condenser and that tlie capacitance may be increased, for a given plate area, by decreasing the spacing between the plates. \\%le the first requirement is beyond the control of the designer, 1 ~ ~ 1 cause the diplectric material is the sample being studied, tlit. second requirement is precisely the one desired if the sample size is to be decreased. By a suitable design of the plates and thi. shape of the entrance clinmbers, channeling anti sample holdup may be controlled. 1 2
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Present addresi. S o r t l i T e u s State College, Denton, Tes. Present addre-, Humble Oil Co., Houston. Tex.
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BENZENE
’rypical Calibration Curve for IiexancBenzene Mixture
111 order to attain the maximuni frequency change tor a give11 material, it was necessary to change the LIC ratio and the size of the capacitors used in the oscillator, to make the capacitance of the cell as large a fraction as possible oi the total capacitance of the frequency-determining circuit. The capacitance of t’hecell having been fixed by its physical dimensions, satisfactory periormance could be attained only by careful circuit design. The best sensitivity attained with satisfactory stability required t’hat the oscillator operate very near the “overload” point at which the power drawn by the tank circuit was sufficient to stop the oscillations. The actual point of operation chosen depended on the range of dielectric constants encountered in the samplm investigated. I n the model discussed here, formamide and pyridine cuused the oscillations to cease. The modified portions of the tank circuit are illustrated in Figure 1: the complete circuit has been published ( 4 ) . A number of samples studied previously ivith the “coil cell“ riiodel were reinvestigated with the modified instrument. In all cttscs, a marked improvement was noted; the greatest improvement was in the frequency changes produced by materials that had formerly changed the frequency of the working oscillator by 1000 cycles or less and now produce changes that are greater by factors up t o 40. Materials of higher dielectric gave less markrdiy improved results, but the factors were still of the order oi 15 or more. The frequency differences were measured by supplying the output of the ojcillonieter to the vertical input of an oscilloscope a n d
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the output of a Ilenlitt-Packard oscillator, 1Iodel 200 C, to the liorizonial input. The circular Lissajous figure was used to determine the rquivalence point. The stability and drift of t,he new instrument, were approximately the same as those of the previous model ( 4 ) ,although it was difficult t o make exact measurements because of the wide range of frequency covered and t.he lack of test equipment of sufficient precision to guarantee that the obe r v e d drifts arose in the instrument discussed here and not in the coniparisori oscillator, The reproducibility of bhe nieasurements was within the calibration of the comparison oscillator. The data reported in this paper imply t’hat, for an average of three rneasurenients, beat frequency change values are accurate to uithin 50 cycles. The signal generator dial could not be read more accurately nor was its stability reliable within closer limits.
previous instrument, but the extent of the effect in tliitt case is to shift the beat frequencies of all points of an analytical calibratiori rurve hut to leave the new curve essentially parallel to the original. In the case of the capacitance cell instrument, the slope of the curve may change appreciably if the temperature is not controlled.
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Figure 4. Typical Calibration Curve for o-Xylenep-Xylene Mixture
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Typical Calibration Curve f o r icetonrWater Mixture
‘l’he influence of temperature on the frequencA-of tlie individual oscillators and on the difference frequency was accentuated by the increase in sensitivity, and more careful temperature control n’as necessary than that formerly required. The temperature coeffirient for both sample and instrument was of the order of 40 1’) cles per degree centigrade. This temperature control was 1 eadily effected by storing the samples in a constant temperature bath for sufficient time to permit them to come to the predetermined operating temperature. Adoption of the instrument on a fairly wide scale and its manufacture would require attention to temperature-compensating components. S o attempts have been made in this laboratory to carry the engineering development of tlie instrument beyond the point required for laboratory use. The effect of varying sample temperature was noted in the
Table I.
Beat Frequency Changes
Substance Water Methanol Ethyl alcohol n-Amyl alcohol Acetone n-Hexaldehyde Methyl isobutyl ketone Propionic acid Methyl propionate Ethyl acetate Benzene Toluene Bromobenzene Nitrobenzene Aniline n-Heptane 2-Heptene
Beat Frequency Change Coil Condenser 2 10,000 7880 189,000 5560 178,000 4760 149,000 3370 168,500 4400 122,000 2520 146,5oc) 3230 89,000 1240 102,500 1890 100,000 1790 37,000 810 39,500 830 91,500 1670 192,000 5720 107,000 2040 ?7,500 670 33,000 780
Table I compares the frequency differences observed between t b c empty cell value and the value when the cell vias filled with a >ample and measured with the two instruments. The actual values of the beat frequency change are not to be taken as characteristic of the material, except for comparison purpose, because the deign of the cell, the circuit employed, and even variation iii individual components may cause different values to be observed with different models. In fact, almost any desired degree of senzitivity may be attained by appropriate choice of the relative niagnitudei o! inducLance and capacitance employed, but each change in sensitivity requires that the problem of stability be solved for the circuit values chosen. The order of change is not atrictly preseived in the two instruments; the answer to thiR problem lies in the fundamental physical constants being evaluated. This problem is, a t present, under investigation in these laboratories. Figures 2 and 3 illustrate calibration curves prepared in this laboratory for the analysis of mixtures of hexane in benzene and water in acetone. The curves are sufficiently linear ovei .
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PREPARATION OF 3,5-DINITROBENZOATES
T o 10 ml. of the aqueous solution in a 100-ml. glass-stoppered flask are added 0.1 ml. of redistilled pyridine and 1 ml. of benzene (commercial). The mixture is cooled in an ice bath, and 11 grams of anhydrous potassium carbonate are added a t such a rate that the temperature does not exceed 25” C. A solution of 0.5 gram of 3,5dinitrobenzoyl chloride (Eastman) in 2 ml. of benzene is added in portions a t room temperature with shaking. Three minutes after the addition of the acid chloride is complete, 30 ml. of sodium-dried ether are added and the mixture is shaken. The ether is decanted into a centrifuge tube. The extraction is repeated twice. The ether solutions are centrifuged, filtered through a dry filter paper, and evaporated a t atmospheric pressure. The residue is heated at 70’ to 80’ a t 20 mm. until the odor of pyridine can no longer be detected. The residue is extracted with 10 ml. of hot petroleum ether (ligroin). After cooling, this solution is applied to the chromatographic column. CHROMATOGRAPHIC SEPARATION
The column is a 1.8 X 40 cm. borosilicate glass tube which is joined a t the top to a 100-ml. bulb that serves as a solvent reservoir. The tube is constricted a t the bottom and joined to a borosilicate glass stopcock. The adsorbent is supported on a plug of
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Figure 1. Chromatographic Separation of Mixture of 100 Micrograms of Each of Seven 3,5-Dinitrohenzoates
The excellent chromatographic method described by White arid Dryden (8)for the separation of 3,5-dinitrobenzoates involves thc use of a fluorescent indicator on a silicic acid-diatomaceous earth column. I t requires approximately 1 mg. of a 3,s-dinitrobenzoate for detection. I n the present work the fluorescent indicator has been omitted. The eluate is collected systematically in fractions, and the concentration of 3,5-dinitrobenzoate i n each fraction is estimated spectrophotometrically. This makec possiblr the detection of as little as 30 niicrogranis of a 3.5-dinitrobenzoate
Figure 2. Chromatographic Separation of llIixture of 3,5-Dinitrobenzoates Prepared from Dilute Aqueous Solution of Alcohols A blank using 10 m l . of water showed t h a t t h e small peak a t 200 m l . is due t o a contaminant. I t was found i n all samples of 3.5-dinitrobenzoates prepared from aqueous solution and showed a n ahsorption maximum a t 255 mu
