V O L U M E 2 2 , NO. 8, A U G U S T 1 9 5 0 il
+
0.741 (4.28) 0.322 (4.90) culated value a2
+
987
+ 0.0908 (2.46) = 4.97ra., cal-
+
(0.741)2(4.28) (0.322)*(4.90) (0.0908)2(2.46) = 2.88pa., calculated value Thus an error of +0.34% in io, of - 1.01%in i,, and of -0.34% in if resulted in errors of +2.8y0 in the acetaldehyde determination, - 12.0% in the ropionaldehyde determination, and +20.7% in the n-butyralfehyde determination.
The general method described above can, of course, be applied to the analysis of any homologous series where sufficient spread between LY values can be achieved by proper choice of solvents and where a precise method for measuring the total components in one of the layers has been developed. The major advantage is, of course, the fact that such an analysis is now polarographically possible. In addition, the time required for a binary system is usually short, 15 to 20 minutes, in contrast to many hours by the usual chemical procedures.
DISCUSSION OF LIMITATIONS AND ADVANTAGES
The results of these experiments have been summarized in Table IX. The results of the first two analyses are seen to be considerably better than would have been expected. I t is dangerous to draw many conclusions from so few data, in the light of the statistical nature of the computations of error, but it would seem that the calibration data are somewhat better than indicated by the precision of the data of Table VI and I X and that the diffusion currents have been measured to better than 1% in all cases. It seems possible that the back-extraction technique proposed by Tsai and Fu ( 1 5 ) might be used to increase the precision of determining the components which are preferentially extracted. A more attractive alternative involving less manipulation, which has not been investigated, is the possibility of measuring the extracted component in the organic phase. If possible experimentally, this would result in greatly improved precision f o r t h e extracted component and have the advantage over the back-extraction technique of reducing the number of experimental steps. Nonconstancy of LY us. C should not be considered a deterrent in the use of partition polarography as an analytical tool. If a smooth curve for LY us. C can be obtained with good precision, a method of calculation for C when (Y is not definitely known is available (2). This is the method of successive approximations and briefly would involve the following steps in a binary system. 1. Obtain io and in as described 2. Assume values for C Y A and C Y B 3. Solve for C 4. Using the values for C, obtain from the plots of CYA us. C A and C Y Bus. C Bnew values for CYA and C Y B 5. Solve for C again 6 Continue until there is no significant variation in C in two successive solutions
ACKNOWLEDGMENT
The authors gratefully acknowledge the valuable advice and criticism rendered by T. P. Wier, Jr., during the course of t h e experimental work. LITERATURE CITED
Behrens, W. V., 2.anal. Chem., 69, 17 (1926). Brattain, R. R., Rasmussen, R. S., and Cravath, A. M., J. Applied Phys., 14, 418 (1943).
Gnyubkin, V. I., Dobrinskaya, A. A., and Neiman, M. B., Acla Physicochim. (U.R.S.S.), 11, 701 (1939). Gosman, B. A., Collection Czechoslov. Chem. Communs., 7, 467 (1935).
Heyrovskj., J., C h m . Listy, 16, 256 (1922). Jahoda, F. G., Collection Czechoslov. Chem. Communs., 7, 416 (1935).
Knudsen, L.F.,and Grove, D. C.,IND.ENG.CAEM., ANAL.ED., 14, 556 (1942).
Kolthoff, I. M., and Lingane, J. J., “Polarography,” New York IntersciencePublishers, 1941. Matheson, L. A,, and Nichols, N., Trans. Am.Electrochem. Soc.. 73,193 (1938).
Osburn, C. L., and Werkman, C. H., IND.ENCI.CHEM., ANAL, ED.,3,264 (1931). Robinson, H.A., Dow Chemical Co., Midland, Mich., private communication. Sherwood, T. K., and Reed, C. E., “Applied Mathematics in Chemical Engineering,” 1st ed., p. 377,New York, McGrawHill Book Co., 1939. Smoler, I., Collection Czechoslov. Chem. Communs., 2, 699 (1930). Taylor, J. K.,ANAL.CHEM.,19, 368 (1947). Tsai, K.R., and Fu, Ying, Ibid., 21, 818 (1949). Warshowsky,B.,and Elving, P. J., IND. ENQ.CHEM., ANAL.ED., 18, 253 (1946).
RECEIVED December 27, 1849.
Mobile Infrared Spectrometer -
Fo r Rap id, on the-Spo t A nalys es R. L. CHAPMAN AND R. E. TORLEY American Cyanamid Company, Stamford Research Laboratories, Stamford, Conn.
T
HE development of the modulated beam type of infrared spectrometer has permitted the operation of this instrument under conditions of fairly widely ambient room temperatures without any appreciable drift of thc instrument zero point. A littlerealized value of this property is that it now permits the analyst to set up his instrument for on-the-spot analyses at the location of the chemical experiment. This is especially important where the critical steps in the experiment involve reactions in the gaseous state or where they may be followed by the observation of changing compositions of gaseous mixtures. Heretofore, the chemist was required to withdraw samples from his reaction system periodically and send them to the epectroscopy laboratory for analysis. To eliminate this transfer of material and its associated delays when the infrared spectral absorption was the basis of the analy-
sis, the infrared gas analyzers (5) and, more recently, a multicomponent analyzer ( 4 ) were developed. Neither of these methods, however, fulfills the requirement of versatility when the various intermediates or by-products of a reaction are unknown. T h e mobile unit herein described waa developed for the purpose of providing rapid, on-the-spot analyses involving any region of the infrared spectrum normally covered. Because it is essentially a spectrometer, either spot checks of the concentration of several components may be made or a continuous record may be obtained of the concentration of a single component. SETUP OF INSTRUMENT
Basically, the instrument consists of a Perkin-Elmer Model 12C spectrometer which, along with all the necessary electrical
988
ANALYTICAL CHEMISTRY
and optical wmponents, has been mounted a8 a single unit on B w h i l e carriage. I n addition to the standard facilities supplied with the instrument, there has been added a means of rapidly determining absorbance values witbont rewurse to calculations. No wmplete description of this instrument exists in theliterature, but a description of the optical system (1) and of a somewhat modified electrical system (3)may be found. Detailed deacrip tions are given in the manufacturer's bulletins. The phpical and electrical layouts are illustrated in Figure 1, which shows the locations of the various elements of the complete unit.
ing and chassis were made electrically common and a separate ground lead WBB included in the main power cable. Several changes were made on the original equipment, inoluding the addition of a null-balance circuit similar to the one in use on the earlier model of the Perkin-Elmer Spectrometer (8)in this laboratory and some modifications of the rewrding system. The normal power supply for the globar consisted of a Sola voltageregulating transformer and a Powerstat. By introducing a V a r i x before the Sols and operating on a power plateau, mme added source stabilization was obtained. Checks on the noise level of the unit indicated that the peak-to-peak noise ws8 about
A
Figure 1. Phantom Drawing of Complete Mobile Spectrometer U n i t Showing Location of Component 6. Amplifier m e r supply H . Null eimouif potentiometer I . AmpE6sr I. Variec end powexsfst K. Sola d t m g s regulator L. Filter oircuit mntmle
Swctmmstu Rccorder
A.
6.
W p f t m e t e for G l o b u Gelvenomner Reoordios aysantrole Null circvit mntmls
C. D.
E. F.
The framework consists of 2 Y( 2 inch pine, trussed a t the front and back with airplttne cable. while the aides. top. and shelves are live-ply p l p o o d ~pauelstr, aid irr rtrengthmi~e,rne structure. Aluminum panels, liiripd a t the l~otrom.providc spare ior the wntrols with r a w arrcssibilitv of the elrmtnrs hrhinrl rhrm. The carriage is miunted on locking casters whioh serve as legs when the instrument is in use. A detaobilble notebook shelf is also included to serve as a oonvenienoe to the onerat,or. &s is ~ _ ~~. ~ shown in Figure 2. This, along with other supplementary equi ment such as absorption cells, cell helders, and the like, msy stored in a compahmcrrt henmth 11w r r . c ~ l r r . The smkllrr aluininuin panrl srrvcs as a door for this romparrmmr and a l j o as a mount for thr rcsDonse coimols oi the t i l t w vircuir. One electrical outlet plus w&r and sink facilities is needed for operation. The entire unit measures 2 feet, 2 inches by 4 feet, 8 inches and is 3 feet, 2 inches high, a size which permits passage through standard door frames. ~
~~
~~
~~~
~~
~
~
~
~
~
~
,
&
In order to minimize pickup, the amplifier wan placed as far as possible from the two power supplies and, in particular, the Sola voltage stabilizer. For thiq reason also the thermocouple lead waa kept BB short aa possible) although it was still found necessary to wrap it with Mu metal strips for further shielding. The interconnections were made with shielded cable using airplane-type connectors for flexibility and easy maintenance. All cable shield-
Figure 2.
Mobile Spectrometer Unit Showing Detachable Notebook Shelf in Place
V O L U M E 2 2 , NO. 8, A U G U S T 1 9 5 0
989
A compact mounting containing the Perkin-Elmer Model 12C infrared spectmmeter complete with the necessary power supplies, amplifier, and recorder is described. Also included is a null-balance circuit, the use of which enables the operator to obtain absorbance values directly without recourse to calculations. The entire unit is mounted on casters to facilitate the performance of infrared analyses at the site of the laboratory experiment.
10 X volt, a value sufficiently close to the recommended 8 X 1O-O volt for normal operation. Because it was desired to operate the instrument with a null-balance densitometer circuit in conjunction with either a modulated or unmodulated beam, a sixteen-pole, three-position switch was included for the purpose of making the necessary circuit changes. The advantage of the incorporation of the null-balance circuit is most apparent in routine analyses where all the components of the mixtures being analyzed are known. With it the operator is able to read the appropriate absorbance values directly from a scale attached to the potentiometer of a voltage dividing circuit used to buck out the thermocouple signal before amplification. A simple galvanometer connected to the output of the amplifier serves to indicate the null condition. In the further interest of speed and ease of operation, the controls required for this circuit were located under the sample space and slit control of the spectrometer, thereby making them readily accessible. Because of the modulation of the beam in the Model 12C instrument, it was necessary to introduce the bucking voltage of the null circuit a t such a point in the amplifier that it as well aa the zero control and test signal would be interrupted in phase with the beam modulation. It was further required that the null circuit should not interact with either the positioning or polarity of the zero balance control. To accomplish these purposes it was necessary to introduce a second 1.5-volt battery and use both poles of the double-pde, single-throw switch individually. The modified amplifier circuit is shown in Figure 3. Normally, in the case of modulated beam operation, the “dark current” is zero and the direct current feed-back circuit of the filter unit is used to compensate for the thermocouple direct current signal. However, an overdamping of the galvanometer occurred when this filter unit was used with the null circuit, and other means had to be found to compensate for this thermocouple signal to prevent its appearance as an alternating signal superimposed on the rectified amplifier output. This was accomplished by disconnecting the zero balance leads from the switch on the modulator shaft and shorting out terminals 1 and 2 of the amplifier, thus su plying an adjustable direct current voltage suzcient to buck out the direct current thermocouple signal before amplification. The necessary circuit changes were made by use of the selector switch, S,, shown in the switching circuit given in Figure 4. Reading clockwise, position 1 of this switch was chosen for the normal operation of the spectrometer, and 2 and 3 for the unmodulated and modulated beam operation of the null-balance unit, respectively.
The voltage divider, which again is an integral part of the Perkin-Elmer filter, also had to be replaced in the null circuit and, as indicated in Figure 4, it was done by using a variable 1000-ohm potentiometer, RIO,the center tap of which was grounded, having a 950-ohm fixed resistor a t each end to give a h e r control in its adjustment. The balance of the system may be checked by introducing a standard direct current
Figure 4.
I AI I I
II I
I I
I I
I
I
I
TO POWER SUPPLY
Figure 3.
Amplifier Circuit for Use with Null-Balance Unit
signal as from the test micro-volts and adjusting the potentiometer until the galvanometer pointer is a t its mechanical zero. The slight ripple which remains during operation, causing a flickering of the galvanometer, was minimized by placing a 500-mfd. condenser across the galvanometer terminals. A fair but not objec-
Null-Balance Circuit and Connections with Chopper, Amplifier, and Filter Units
ANALYTICAL CHEMISTRY
990 tionahle amount of sluggishness of the galvanometer was caused by the introduction of this wndenser. When the unit was operated with an unmodulated beam, this sluggishness was not often spparent because of the much higher signal levels normally encountered.
Since this installation was made the Perkin-Elmer Corporation has developed an improved amplifier and detector system. I n this the beam is chopped to produce a 13-cycle modulation and the output of the thermocouple is fed through a short lead to B preamplifier. This serves to increase the signal by a factor of %bout 10,Mx) hefore i t is transmitted to the main amplifier, thereby minimizing noise from extraneous impulses and stray 6O-cycle signals and rendering the instrument more favorable for operation in “noisy” locations. The essential parts of this a y e tern are used in the newer Perkin-Elmer Model 12C instrument (3). The use of this amplifier is, therefore, t o he recommended over the breaker-type amplifier used in the instrument described herein. OPERATION OF NULLBALANCE CIRCUIT
Referring to Figure 4,with S, in osition 2, reading clockwise, the dark current resent in unmodnyated beam o eration is comvensated for bv $e balance control on the a m v h e r vanel with
117L7
Figure 5.
4 0 MFD 600V
Clicker U n i t for I’roducing Fiducial M a r k s on Recorder Chart
Because a current A o ~ in s the null-balance circuit when it is set
at the galvmometer null point, the angular diaplscenient of the potentiometer, Rs is not a linear function of the transmittance. This, of coume, wuld be compensated far by using B nonlinear scale but, hemuuse this nonlinearity wss found to amount to less than 0.670, a linear male w m used sati.tisfaetorily. An inexpensive potentiometer was used, so adjusted that the runoffs a t the ends did not produce any diswntinuity in the resistance. When teated, i t wa8 found that the nonlinearity of this potentionieter didnotexceed +0.2%. The original system for placing fiducial marks on the recorder chart was less satisfactory than was desired because the pen would not quickly return to the spectral curve after its excursions. A better system consists of a adenoid a m a g e d to give a sudden jerk to the pen cable of the recorder. This solenoid is operated b y the discharge of a condenser whose capacity ia such t,hat the release is inehntaneous. Figure 5 is B photograph of the back of the hinged element of the Brown recorder showing this unit, and the circuit is given in Figure 6. Two other minor alterations were made in line with the operational procedures used in this laboratory. Because portions of the spectral traoinge are normally removed from the recorder as s w n BB they are completed, i t was Convenient to make the rewrder door easily removable rather than to leave it open. To accomplish this the light wires and bracket were removed from the door and the upper hinge pin was replaced with a removable one having a knob on ita upper end. Again, it has been found convenient to divide the rock salt spectrum into six sections and to use four successive scanning speeds. In order to obtain Ratisfactory spectra in a minimum of time, the over-all speed of the Perkin-Elmer motor drive was almost doubled by the simple substitution of two gears in the gear train of the motor drive unit. ~~
5 LE. SOLENOID Figure 6. Clicker Unit Circuit The range of transmittance spanned b the potentiometer slide wire can he decreased for measurine a n a 5 differences in relativelv hiah absorbances by making the s h wider than normal and adj u h g the coarse a d finr vannhle rcsistom, K, and Isr, with Spin poArion 1 whrn making thc crll-out adjusrmmt. After this fir31 adiurtmrnt of the wsi.u11s I t , and K,. the owrations are similar to‘thase in the preceding partapraDhkxceptt’thst S, is moved to position 1 rathPr t h a n 5 lor &VI; cell-olit adpstmmt. Switch positions 2 and 4 ilre idrntical ior corrvcnimce nhen a span of 1ew than loO% tranmirrarrce i+uwd. ACKNOWLEDGMENT
The authors wish to express their gratitude to Max D. Liston and other members of the Perkin-Elmer Corporation for their advice in the design of this unit and the electrical circuits invblved. They &.lso thank Charles Ssleman of these laboratories for his aid in rewiring the amplifier. LITERATURE CITED
(1) Barnes. R. B.. MoDonald. R. 5.. and Williams. V. Z., J . Applied Phys., 16, No. 2,77-86 (1945). (2) Chapman, R. L.. Colthup. N. 8.. and R a n d , R. J., Reo. Sci. Zn8trummfs. 19.116 (1948). (3) Listan. M. D.. and White. J. U., J . Opticol Soc. Am.. 40, No. 1. 3G41 (1950). (4) White, ....J...U., ... Listan. .-. M. D., and Simrd, R. G.,ANAL. can^.. 21. Il5ti-til (1Ll4YJ.
(5) Williams.
V. Z., Re”. Sci. Inah.umenta, 19, 135-178
RECEIVED J*~W.W 26, 1850.
(1948).
