April 15, 1943
ANALYTICAL EDITION
277
wave lengths is due to the reabsorption of the shorter wave lengths of fluoresced light.
energy and the position of the maximum are independent of concentration of the fluorescing substance.
Summary
Literature Cited
,4simple modification of the CenCO SpeCtroPhotometer Permits the measurement of reflection and fluorescence spectra Ivith moderate resolution. The arrangement whereby fluorescence is observed from the same direction as the exciting light is superior to other possible arrangements, in that the shape of the curve representing the spectral distribution Of
Dutton, H.
J., bfanning. W. M.,and Duggar, B. M., J . Phys. Chem. (in press), (2) General Electric Co., Schenectady, N. Y., “Electrical Measurement of Color”. (3) Sheard, C., and States, M. N., J . Optical SOC.Am., 31, 64 (1941). (4) Wood, R. Phil. ,Mag., 774 (1925). (5) Zscheile, F. P., and Harris, D. G., private communication.
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Ultraviolet Photometer for Analvsis of Solutions J
IRVING RI. KLOTZ, Northwestern University, Evanston, Ill.
A simple and inexpensive ultraviolet photometer, suitable for the analysis of solutions, is outlined in detail. Results obtained with phenol, sulfanilamide, and potassium nitrate are described, and applications to other substances, particularly aromatic compounds, are discussed.
AXY instruments and methods of analysis based on
Design and Operation of Apparatus A schematic diagram of the optical and electrical system is given in Figure 1. LIGHTSOURCE. A General Electric T-IO germicidal lamp was used in the manner described by Hanson (4). After a preliminary warm-up period of about an hour, the lamp maintained an intensity of radiation constant to wit,hin 0.1 t o 0.2 per cent. PHOTOELECTRIC DETECTOR.An RCA C-7032 phototube served as the light-sensitive element. The photocurrents were amplified by an RCA 954 acorn tube used in the manner described by Gabus and Pool ( S ) , but in a somewhat modified circuit (6). With a potentiometer in the grid circuit and a galvanometer in the plate circuit, the amplifier can be used as a sensitive null point indicator, and difficulties due to nonlinearity in response may be eliminated. The changes in the plate resistance and the elimination of a separate battery for the photocell have increased the stability of the electrical system. The light intensities are measured by the following procedure. Tl’ith the shutter closed and the potentiometer, P , set at zero, the plate resistnnces are varied until the galvanometer shows a null reading. One of the absorption cells is then placed in the path of the light beam and the shutter is opened. The potential drop across the grid leak produced by the flow of the photocurrent through the high resistance is counterbalanced by increasing the potential imposed by the potentiometer until the plate meter has returned to its original reading, The reading of the potentiometer is then directly proportional to the intensity of the light incident on the phototube. For a precision of 0.1 to 0.2 per cent, a Leeds &: Northrup student potentiometer may be used to measure grid potentials. For less precise work a rheostat-potentiometer is suitable.
the absorption of light have been described in t h e literature of the past few years. Photometric procedures have been found t o be very convenient, particularly in routine testing and control, because of their sensitivity and rapidity. A description of some common types of apparatus using visible light has been given recently by Muller (6). Some instruments have also been developed t o utilize the absorption of ultraviolet light to measure concentrations in solutions. Demarest (b), for example, has described a photometer which uses light of about 3300 A. for the analysis of vitamin A. Similarly, Buswell and Dunlop ( I ) , using a spectrograph and photographic plate, measured the concentrations of aqueous solutions of phenol by the absorption of radiation of approximately 2700 A. This paper describes a photoelectric SHUTTER AND photometer which may be used in DI A PH RAG Y the analysis of so1;tions which RCA 9 5 4 RCA absorb light of 2537 A. The construction of such a n apparatus has been facilitated recently by the development of inexpensive ultraviolet-sensitive photocel!s and high-silica, ultraviolet-transmitting glass. These have been combined 1.000 n with a commercial germicidal lam , CURRENT a convenient source of 2537 . ND V O L T A G E T A B IL I Z L R S radiation, to yield a simple in50.00 n strument composed of readily available materials. Instruments similar in principle to the one described in this paper have been developed by Woodson (7‘) and by Hanson (4) for the determination of concentrations of FIGURE 1. DIAGRAM OF ULTRAVIOLET PHOTOMETER vapor in air.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
278
ABSORPTIONCELLS. The body of the cells consisted of Pyrex tubing with suitable side arms for the introduction of solution. The windows, however, were cut from Corning No. 791 ultraviolet-transmittin glass, which transmits about 50 per cent of radiation of 2500 wave length, and were sealed to the body of the cell by means of Sealstix cement. The absorption cells were 10.02 cm. in length.
Vol. 15, No. 4
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Calibration To avoid frequent calibrations with mixtures of known composition, the author used the exponential relationship between light intensity and concentration log l o l l = e c d where lo = intensity of light leaving pure solvent I = intensity of light leaving solution e = extinction coefficient c = concentration of solute d = length of absorption cell
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The transmission, Z/Zo, was determined from the ratio of the potentiometer readings with solution and solvent, respectively, in the path of the light beam. With d and c known, one can calculate E very readily. The “effective” extinction coefficient observed with the photometer differed somewhat from that determined with a spectrophotometer because of the large amount of stray radiation in the light source of the photometer. With sulfanilamide, for example, the “effective” e was 10,100 liter mole+ cm.-l, whcreas that determined with monochromatic radiation of 2537 A. was 14,700. Nevertheless, this discrepancy has no significant effect on the order of magnitude of the sensitivity obtained. The presence of stray light was also exhibited by the variation of the extinction coefficient with concentration. Since Beer’s law was not applicable, it was necessary to construct a calibration chart of log lO/Z us. concentration for each substance of interest. TABLEI. DEPENDEKCE OF SENSITIVITY ON EXTINCTION COEFFICIENT Extinction Coe5cient Liter mole-1 cm. -1 10,000 1,000 100 10 1
Xinimum Detectable Concentration Mole lifer-’ 3 x 103 x 10-7 3 x 10-6 3 x 10-6 3 x lo-‘
Applications Figure 2 shows three typical calibration curves obtained with the ultraviolet photometer. The sensitivity of the ultraviolet photometer toward phenol is probably comparable with that of any chemical method, for with a precision of 0.2 per cent in the light intensity it is possible to determine changes of approximately 3 X gram of phenol per liter of water. This type of instrument may be very useful, therefore, in the analysis of phenol in water. The limit of detection of sulfanilamide is even lower than that for phenol and is of the order of 2 X lo-’ gram per liter. A direct analysis of biological fluids is not possible, however, because of the presence of many other interfering substances which also absorb ultraviolet light. Nevertheless, the instrument will be very useful in a study of the adsorption and diffusion of sulfa compounds in various artscial media. The results obtained with phenol and sulfanilamide are typical examples of the sensitivity one may expect with aromatic substances. Ultraviolet photometry would also be applicable to the analysis of organic substances such as ketones, conjugated hydrocarbons, heterocyclic compounds, and other ultraviolet-absorbing substances when they are dissolved in nonabsorbing solvents.
0 005 00002 02
0010 0 0004 0.4
0015 0.0006 06
A
B C
G R A M S PER L I T E R
FIGURE 2. CALIBRATION CURVES A.
Phenol B . Sulfanilamide C.
Potaaiium nitrate
An application to inorganic analysis, the calibration curve for potassium nitrate, is also illustrated in Figure 2. In this case the sensitivity, about 3 X gram per liter, is not so high as that obtained with aromatic compounds, but the instrument would still afford a very rapid and convenient method for the analysis of nitrates. These examples are only a few illustrations of the applications of this photometer. Many other involved and tedious chemical analyses, particularly of aromatic substances, can be replaced readily by ultraviolet photometry. To determine the sensitivity toward any particular substance, i t is merely necessary to estimate the extinction coefficient from data usually available in the literature and to apply Equation 1 to calculate the minimum detectable concentration. A series of such estimates has been summarized in Table I to enable one to tell a t a glance whether the desired sensitivity can be attained with a given compound.
Acknowledgment The author is indebted to H. Campaigne, Chemistry Department mechanic, for advice and assistance in the construction of the carriage for the absorption cells. Literature Cited (1) Buswell, A. M., and Dunlop, E. C., paper presented at t h e 103rd Meeting of the AMERICAN CHEMICAL SOCIBTY, Memphis, Tenn., April, 1942. (2) Demarest, B., IND. ENG.CHEM., ANAL.ED., 13, 374 (1941). (3) Gabus, G . H., and Pool, M. L., Rev. Sci. Instruments, 8 , 196 (1937). (4) Hanson, V. F., IND.ENG. CHEM.,ANAL.ED.,13, 119 (1941). (5) Klots, I. M.,Ph.D. dissertation, University of Chicago, 1940. (6) Mdler, R. H., IND.ENG.CHEM.,ANAL.ED.,11, 1 (1939). (7) Woodson, T. T., Rev. Sci. Instruments, 10, 308 (1939).
