V O L U M E 21, N O . 6, J U N E 1 9 4 9
743
ACKNOWLEDGMENTS
The authors wish to thank Melvin M. Fink for his contributions in the design of the corrosion test unit, Hugo Martinson and G. IT. Sichols for construction of the apparatus, and Arthur Klingel, Jr., and the Automotive Group of the Sohio Research Laboratory for the engine data. Appreciation is extended to the management of the Standard Oil Company of Ohio for permission to publish this v o r k . LITERATURE CITED
(1) Backoff, TT. J., S.A.E. Quart. Trans., 2, No. 1, 88-93 (1948). (2) Bartleson, J. D., U. S.Patent 2,403,894 (July 9 , 1946). (3) Bartleson, J. D., and T'eatch, F., I b i d . , 2,403,474 (July 9, 1946). (4) Bassett, W.B , S a t l . Petroleum n'ews, 36, No. 27, R-450 (1944). (5) Bowhay, E. C., and Koenig, E. F., S.A E . Quart. Trans., 2, SO. 1, 132-47 (1948)
(6) Burk, R. E., Hughes, E. C., Scovill, W. E., and Bartleson, J. D., IND.ENG.CHEM., ANAL.ED., 17, 302-9 (1945).
(7) "Coordinating Research Committee, Handbook," New York, J. J. Little & Ives Co., 1946. (8) Larsen, R. G., ANAL.CHEM.,20, 547-55 (1948). (9) MacCoull, Ryder, and Schlop, S.A.E. Journal, 50, 338-45 (1942). (10) Pigott, R. J. S., Ibid., 48, 165-73 (1941). (11) Raymond, L., Ibid., 50, 533-7 (1942). (12) Talley, S.K., Larsen, R. G., and Webb, W. L4.,IXD.ENG.CHEY., ANAL.ED.,17, 168 (1945). (13) Underwood, A , S . A . E . Journal, 43, 385-92 (1938). (14) Waters, G. W., and Larson, E. C., ISD. Esc. CHmf., ANAL.ED. 15, 550-9 (1943). RECEIVEDJuly 30, 1948. Presented before the Division of Petroleum SOCIETY, Chemistry a t the 113th Meeting of t h e . ~ \ I K X I C . A S CHEMICAL Chicago, Ill.
Recording Differential Refractometer DAVID ZAUKELIES
AND ARTHUR
A. FROST, Northwestern University, Euanston, ZZl.
An improved and simplified recording differential refractometer has been constructed. Its sensitivity is of the same order of magnitude as obtained with an interferometer. The previously used beam-splitting method, as described by Claesson, has been simplified by the use of a twin-cathode phototube. Pen and ink recording is used.
R
EFRACTIVE index measurements are of great value in folloning the results of a fractionation process such as a fractional distillation or an adsorption fractionation. They are of particular value in the latter process, xhere thermometry is not applicable. Claesson (1, 2 ) has described an automatically recording differential refractometer for adsorption analysis and n ith its aid has been successful in applying chromatographic methods to colorless solutes and in putting these methods on a more quantitative basis. His technique involves the deflection of a beam of light as it traverses a diagonal boundary beta een a solution and the corresponding solvent. The beam of light is split by a hexagonal prism : the two wparate beams fall on separate barrier-laver
photocellswhichareconnected in opposition, so that the galvanometer deflection is proportional to the deflection of the beam in the refractometer cell but greatly amplified. Such differential refractometer cells for visual observation have also been described recently by Dutton ( 4 ) , Debye (S), and Kegeles ( 5 ) . The purpose of the present work is to improve or simplify the Claesson apparatus in the following ways: to use pen and ink recording in place of the less convenient photographic recording, to use a simple twin-cathode photocell in place of the more complicated prism and photocell arrangement for splitting the beam, to use a photoemissive type of cell in place of the barrier-layer cell so as to avoid possible fatigue effects, and to increase the sensitivity by using a suitable electronic circuit. The volume or weight of effluent is not recorded in this apparatus. DESCRII'TIOX OF APPARATUS
The Refractometer Cell is shonn in detail in Figure 1, which is a top view.
-
LIGHT B E A M
s
I @---D ---A-
.-----
--- -.-
I
Figure 1. Refractometer Cell
f
The cell is coniposed of four brass parts 5 X 5 em. (2 X 2 inches) in cross section, bolted together and holding between them microscope slide cover glasses, C, to serve as end windows and diagonal windows of the cell. The two main blocks of the cell mith the diagonal interface have holes A and B drilled through them to form the half cells for solvent and solution, respectively, and for passage of the beam of light from right t o left. The opening, B , of 0.125-inch diameter is smaller than il, vhich has a 3 / , 6 inch diameter, so as to keep the volume of the solution half cell as sniall as possible and also so that the beam of light after being deflected a t the diagonal boundary will not be interrupted by the side of opening A . The E AM length of each half of the cell averages 0.5 inch. The volume of B is 0.10 ml. which is somewhat less than Claesson's volume of 0.23 ml. Inlets and outlets of 1,116 inch diameter for the solution and solvent are a t top and sides, respectively, and so positioned lengthwise of each half cell that the fluid in flax-ing through the half cell enters a t one end and goes out at the other with very little dead space. The solvent for reference is conveniently held stationary in its half cell, and, although the glass separator is not sealed in any way, no difficulty has arisen due to possible leakage of solution into the solvent compartment. The two end plates, which hold the outer windows in place, are soldered to brass tubes of 1 inch inside diameter which serve aa
ANALYTICAL CHEMISTRY
744
TOP V I E W
Figure 2.
Complete Optical Assembly
FRONT V I E W
light shields and also hold the other optical parts rigidly to the refractometer cell. The diagonal boundary is at an angle of 30" fro? the axis of the light beam, giving an angle of incidence of 60 . This angle was chosen greater than Claesson's angle of 45' so as to increase the sensitivity which is proportional to the tangent of this angle ( 5 ) . The cell is thermostated by passing thermostat water through 16 inch holes, D,drilled in each main half of the cell block. The w&%rentrance and exit are a t the bottom of each part and the same water can be circulated through a jacket on the adsorptlon column or other devil-e with which the refractometer is used. The Optical System as well as the general layout is shown in Figure 2.
.
White light from a lamp, A , is focused by the condensing lenses, B , on the entrance to the refractometer cell. A plate of heat-absorbing glass, C, decreases the heating of the solution in the cell. After passage through the cell the light diverges and partially fills the two cathodes of the photocell a t D. The deflection of the beam due to the difference in refractive index between the solution and solvent usually will be small enough so that the full beam remains on the photocell cathodes. However, a large deflection can be handled through a lateral motion of the photocell by means of the micrometer screw, E, which may also be used to center the photocell for zero response when the same fluid is in both halves of the refractometer cell. The Lamp is a 6-watt "exciter" tungsten lamp operating on 6 volts from a storage battery. The filament is tightly coiled, to give a nearly uniform intensity when imaged on the refractometer cell. Because of the relatively large image distance the exact position of the lamp is critical. However, adjustment of the lamp by hand with a screw for claniping in the desired position has been found satisfactory. .An opening in the tube near the'refractometer entrance and ordinarily covered by a sliding sleeve makes it possibsle to observe the image when making adjustments of focus and position of lamp. A current of air for cooling is passed through the lamp housing. Monochromatic light might be desirable, but for the applications in mind a t present, white light is just as satisfactory; the response of the instrument is dependent on an average refractive index difference for the wave lengths involved. The Photocell is an R.C.;1. S o . 920. The use of such a txin-cathode photocell is an important feature in the simplification of the apparatus. Not only does this replace two cells by one but it also eliminates a prism or mirror arrangement for splitting the beam. The
shape of the diverging beam as it strikes the photocell is circular because it is essentially an image of the condensing lens formed by the refractometer cell acting as a pinhole camera. This beam falls on both cathodes Fvith only a slight loss of light due to the approximately '/,e inch separation between them. A deflection of the beam due t'o a change in refractive index causes an increased photocurrent from one cathode and a decreased current from the other. The amount of light lost between the cathodes may change and cause a slight nonlinearity between the response and the refractive index. Xonlinearities may also occur through nonuniformity of the beam. The apparatus is held together rigidly by the two brass tubes on either side, the distance between centers being 12.75 inches from lamp to refractometer cell and 8.75 inches from the latter to the photocell. Despite the great sensitivity, no special precau50 MEG A
I I
I 50 MEG A
I
I
I
I
-
I
6F0
I
~
20,000
0.IMFD
T I
'MEGn
T O RECORDER
Figure 3.
Electronic Circuit
V O L U M E 2 1 , N O . 6, J U N E 1 9 4 9
745
tions to avoid strain are needed, and the apparatus is clamped by Bunsen clamps to an ordinary frame support. The Electrical Circuit is shown in Figure 3. The two halves of the photocell are connected in a bridge circuit with 50-megohm resistors mounted in the photocell compartment. The differential output is applied to one grid of 6F8 twin triode, and the other triode grid is held a t constant potential as determined by the setting of the potential divider, P I . The differential output of the two triodes is tapped by the potential divider, Pz,and this voltage is applied to a Brown strip-chart potentiometric recorder of 5-mv. range. The vacuum tube is used here to match the high impedance of the photocell to the low impedance of the recorder. Batteries are used to supply all voltages so as to be as free as possible from fluctuations. Pl is useful in changing the zero setting on the recorder while Pn varies the sensitivity. OPERATIOY 4YD TYPICAL RESULTS
The sensitivity is limited by the instability of the apparatus. The recorder fluctuates by an amount which corresponds to about 2 X 10-6 in refractive index change. A drift also takes place during the warm-up period of 0.5 to 1 hour. The behavior of the apparatus was tested by introducing solutions of known refractive index and observing the response of the recorder. The effect of shifting the position of the photocell by a given amount was observed and compared with that to be expected on the basis of the optical theorv ( 6 ) . The theory predicts that for small deviations of the light beam, 8, the angle in radians is 0 = An tan 01
tion is made directly in terms of the relation between recorder response and concentration of solutions being used and so avoids the problem of just exactly what kind of an average refractive index is involved. Because the minimum detectable An is about 2 X 10-6, the greatest sensitivity that could be used for 170 accuracy of a fullper recorder unit or 2 X scale deflection would be 2 X for full scale deflection, as there are 100 divisions across the chart. The minimum sensitivity is determined by the size of the spot of light in relation to the photocell cathodes. This results in a A n of about 1 X for full scale deflection. The minimum detectable refractive index change of 2 X 10-6 is about one fifth as great as with Claesson’s recording differential refractometer and about trvice as great as with Tiselius and Claesson’s (1, 7) nonrecording interferometric method. Figures 4 and 5 show typical recorder tracings obtained in cnnnrction with adsorption columns.
B
A A
-+n-&.L+-
where A n is the change in refractive index and a is the angle of incidence of the beam of light on the diagonal separator.
1,
2
0
AIR
-
1
TIME
8------0
Figure 5.
Frontal C u r v e
Figure 4 is for a 0.5-ml. sample of a 10% ethanol in carbori tetrachloride solution adsorbed on an alumina column and then eluted with carbon tetrachloride. Full scale deflect,ionof 0.91 X 10-3 for An corresponded to 0.0070 ethanol and the wave as it came through had a peak Concentration of 0.407,. Figure 5 shows how the differential refractometer can be used in the study of gas mixtures. A sample of natural gas as distributed in gas mains was slowly passed through a bed of charcoal. previously saturated with air. The successive steps in the curve are typical of the “frontal analysis” method. The full scale deflection was 2.4 X lOU4forAn.
-+I i
8
Figure 4.
Elution C u r v e
As an example, a solution of ethanol in carbon tetrachloride l.0070 by volume and differing in refractive index for sodium
light from pure carbon tetrachloride by 1.01 X 10-3 was found a t a particular recorder sensitivity setting to give a recorder deflecchange tion of 111 recorder units. This amounted to 0.91 X in refractive index per recorder unit. It was found that shifting the photocell 0.010 inch caused the recorder to move 68.5 units. radian and according to This shift amounts to a e of 1.14 X This the equation An then would be @/tan60” or 6.6 X expressed as A n per recorder would be a sensitivity of 0.96 X unit as compared with 0.91 X 10-5 found above. The agreement is all that could be expected, considering nonlinearities in response and the use of white light with a different average refractive index. I n actual use the recorder response must be calibrated against known solutions to obtain quantitative results. Such a calibra-
ACKNOWLEDGMENT
The authors are pleased to acknowledge the assistance of John Kamper in the construction of the apparatus. A similar apparatus has been built and used by G. R. Thomas (6) and C. D. Hurd in this laboratory. LITERATURE CITED (1)
(2) (3) (4)
(5) (6)
Claesson, S., d r k i v KemiXineral. Geol., 23A, No. 1, 1-133 (1946) Claesson, S.,The Svedberg (1LIem. Vol.), 1944, 82-93. Debye, P. P., J . Applied Phys., 17, 392 (1946). Dutton, H. J., J . Phys. Chem., 48, 179 (1944). Kegeles, G., J. Am. Chem. Soc., 69, 1302 (1947). Thomas, G. R., Ph.D. thesis, Northwestern University, Evanstori,
Ill., 1948. (7) Tiselius, A., and Claesson, S . , Arkiv Kemi Mineral. Geol., 15B,No. 18.1-7 (1942). RECEIVED October 13, 1948. Presented before t h e Division of Analytical and Micro Chemistry a t t h e l l l t h l l e e t i n g of t h e AMERICAN CHEMICAL SOCIETS, S t . Louis, 1\10,
