Photoelectric Photometers for Use in Colorimetry I
CH. ZINZADZE’ New Jersey Experiment Station, New Brunswick, N. J.
T
HE recent development of photoelectric cells ( Q , l Q68) , has made it possible to avoid many limitations that characterize the use of photometric colorimeters read by means of direct ocular observation. Some of the photoelectric photometers thus far described have single photocells (1, 5, 6, 7, 10, 16, 14-18, ZI), while others (9, 3, 4, 8, 11, 13, 20,66) have two. The two-celled form seems to the writer to be preferable. I n this paper are described two newly developed instruments of the two-cell form, which have proved satisfactory for the determination of potassium, phosphorus, and arsenic in solutions (94-27). I n the first instrument two photoelectric cells are on opposite sides of the light source, and between light and photocell, on each side, are an adjustable diaphragm (from a Pulfrich photometer, made by Carl Zeiss, Inc.) and also a solution cell. While the two solution cells contain the same known
to decide upon the general arrangement of parts, including size of cabinet and distance from light source to photocells. Then a suitable light source and galvanometer are chosen and, finally, two photocells are selected, which have been matched by the manufacturer for the required light intensity, resistances, etc.
Set-Up of Photometer Weston photronic cells, model 594, mounted firmly in the cabinet by means of UX-type radio sockets, have given excellent results. They are connected so that terminals of o posite polarity are joined, with the galvanometer in parallel wit[ these cells; Wood (23) showed that this arrangement has advantages with respect t o sensitivity, linearity of response, and stability, and it has been employed by Shook and Scrivener (20) and by Brice (4). The potentiometers are of the General Radio Company’s type 374-A, each equipped with that company’s No. 717-C calibrated dial and with a vernier added by the writer; it is desirable to choose the resistances so as to permit direct readings of light transmission (4). The light source is a singlefilament, 6- to 8-volt Mazda bulb of the type used for automobile headlights, mounted on the rear wall of the cabinet so as to be midway between the two photocells. A large rectangular opening in the bottom of the cabinet beneath the bulb permits free air circulation around the latter, for the cabinet itself stands on four rubber knobs or legs about 1 om. high. Ordinary house current, with or without transformer, proved unsatisfactory, notably because of fluctuating voltage, and a 10-volt battery was finally employed as current source. A voltmeter with capacity of 8 volts and a rheostat with capacity of 0.5 ohm are in series with battery and bulb, so that the light intensity may readily be kept constant. Because the cement of cemented glass solution cells may be attacked by the solutions to be studied, specially blown flatsided glass flasks were developed for the solution cells. The body of each flask is a horizontal cylinder about 5 om. in diameter, with the ends closed. A vertical neck extends upward about 6 cm. from the cylindrical body. To permit different thicknesses of solution layer to be interposed between bulb and photocell, solution flasks with different lengths of cylindrical body (from about 1 om. to about 6 om.) are provided, holding about 10, 25, 50, and 100 cc. of solution, respectively. The necks of the larger flasks are about 1 cm. in diameter, but when the main cavity is less than about 1 om. thick the neck is narrower. Each flask is held in a rectangular case of Bakelite (or wood), the two like arts of which are screwed together after the flask is in place. Each part is provided with a circular window having the same diameter as the sensitive surfaces of the photocells. A rubber ring around the neck of each flask supports the flask in its case. All cases for the left side of the cabinet are alike in external size and shape but those for the right side, although also alike, are slightly different from those for the left side, and the two openings in the cabinet, which receive the cases, are correspondingly slightly different. This is because the flat sides of the flasks are not accurately plane, and consequently each flask must have its own position in the cabinet, either at left or at right. Light from the bulb shines horizontally through the cylindrical body of the solution flask to the photocell on the same side. When the light beam is to be shut off or narrowed, suitable blocks of Bakelite or wood, with or without circular openings, are employed as convenient diaphragms.
w
+ -
FIGURE1. ARRANGEMENT OF PARTS OF TwoCELLPHOTOELECTRIC PHOTOMETER WITH POTENTIOMETERS
L,light source R,resistance (0.5 ohm) V ,voltmeter (8 volts) B . heavv-dutv batterv (10volts) F;,Fz, color filters
C1, Cz, openings in cabinet, each with its own solution cell (SCI SCZ)
PC1, PCz: photoelectric cells PI, Pa, potentiometers with calibrated dials G, galvanometer
solution, the diaphragms are adjusted so that the current output from both photocells is the same, as shown by means of a galvanometer. Then the known solution in one solution cell is replaced by the solution to be studied and the diaphragm on the opposite side is closed until the galvanometer again stands a t zero. The reading for the unknown solution is taken from that diaphragm scale, which shows the amount of closure reauired to balance the two beams after the unknown solution is introduced. The second instrument is like the first, except that adjustments and readings are made by means of calibrated potentiometers instead of diaphragms. Its essentials are shown in Figure 1 I n building either of these photometers it is advisable first 1
A calibration curve (W, 19, 90) is made from readings on a series of different known concentrations of the solute to be studied and readings obtained with unknown solutions of that solute are referred to that curve. When the unknown solution to be studied is slightly turbid or when its color tone is somewhat different from that of the standard solution series represented by the calibration curve, it is advisable
Present address, Johns Hopkins University, Baltimore, Md.
280
JULY 15, 1935
ANALYTICAL EDITIOR’
281
t o add (2, 12, 20) a known quantity of known standard solution and t o compare the resulting mixture photometrically with another sample of the unmodified unknown solution.
(10) (11) (12) (13)
Kramer, K., 2. Biol., 95, 126-34 (1934). Lance, B., Chem. Fabrik, 7, 45-7 (1934). Muller, R. H., Mikrochemie, 11, 353-68 (1932). Naumann, E., and Neumann, K., 2. anal. Chem., 97, 81-6
Acknowledgment
(14) (15) (16) (17)
Oltman, R. E., Plant Physiol., 8, 321-6 (1933). Osborn, R. A., J . Assoc. Oficial Agr. Chem., 17, 135-47 (1934). Rumpf, P., Bull. soc. chim., (4) 53, 84-95 (1932). Samuel, B. L., and Shockey, H. H., J . Assoc. Oficial Agr.
(1934).
The writer is grateful to W. C. Colby, of the New Jersey Experiment Station, for valuable practical assistance and to Burton E. Livingston, of the Johns Hopkins University, for milch helpful literary suggestion and criticism. Literature Cited Aten, il. H. W., Galema, N., and Goethals, C. A., Chem. Weekblad, 31, 258-64 (1934).
Bartholomew, E. T., and Raby, E. C., IND.ENG.CHEM.,Anal. Ed., 7, 68-9 (1935). Bendig, M., and Hirschmuller, H., Z . anal. Chem., 9 2 , l - 7 (1933). Brice, B. A., J . Optical SOC.Am., 24, 162-3 (1934). Ellis, M. M., Science, 80, 37-8 (1934). Frear, D. E. H., and Haley, D. H., Penn. State College, Tech. Bull. 304, 1-8 (1934).
Gibson, K. S., “Photoelectric Cells and Their Applications,” Physical and Optical Societies, London, England, 1, 157-71 (1930).
Halban, H,
and Siedenhpf, K,, z. physik. Chem,, 100,
208-30 (1922). Kofman, Th., BuZl. soc. chim. biol., 15, 623-36 (1933).
Chem., 17, 141-6 (1933). (18) Sanford, A. H., Sheard, Ch., and Osterberg, A. E., Am. J . Clin. Path., 3, 405-20 (1933). (19) Sarp, C. H., and Eckweiler, H. J., J . Optical SOC.Am., 23, 246-50 (1933). (20) Shook, G. A., and Scrivener, B. T., School Sei. Math., 32, 845-51 (1932). (21) Weil, A., Science, 79, 593-4 (1934). (22) Wilcox, L. V., IND.ENQ.CHEM.,Anal. Ed., 6, 167-9 (1934). (23) Wood, L. A., Rev. Sci. Instruments, 5, 295-9 (1934). (24) Zinzadze, Ch., Ann. agron., 1, 321-36 (1931). (25) Zineadze, Ch., Chimie & Industrie, 27 (Special No.), 841-3 (March, 1932). (26) Zineadze, Ch., IND.ENG.CHEM.,Anal. Ed., 7, 227 (1935). (27) Ibid., p. 230. (28) Zworykin, V. K., and Wilson, E. D., “Photocells and Their Application,” 2nd ed., New York, John Wiley & Sons, 1934. RECEIVBDFebruary 15, 1935. A brief account of these photometers was presented before the Association of Official Agricultural Chemists at Washington, D. C., October 30, 1934.
A Photoelectric Colorimeter JOHN H. YOE AND THOMAS B. CRUMPLER University of Virginia, University, Va.
URING the past few years much interest has been taken in the use Of photoelectric in photometric
rather than the e. m. f. as is usual in the ordinary sources of electrical energy. The cell used here is a photronic cell, Model 594, manufactured by the Weston Electrical Instrument Corp. (1). chemical analysis. The chief advantages are greater It is connected in series with a microammeter ( M A , Figure 2) havsensitivity and the elimination of errors due to eye fatigue ing 50 ohms resistance and reading UP to 50 microamperes. A and to the inability of the observer to judge color intensity special scale permits an accurate estimation to 0.1 microampere. LIGHTCIRCUIT. The source of energy is a 6-volt, 17-plate accurately. Some of the colorimeters described in the lead storage battery and the lamp used is a 6- to &volt, singleliterature use a single photocell, others employ two. The filament auto headlight bulb. The lamp, b, is connected to the battery terminals through a pair of resistance$, RI and Rz, in photoinetric balancing or comparison has generally been acparallel, one for coarse and the other for fine adjustment. Across complished either by means of adjustable diaphragms and a the lamp is connected a small voltmeter, V , reading up to 8 galvanometer or by the use of a potentiometer. The chief for approximate adjustment of the resistances. objections to some of these colorimeters are their lack of OPTICAL SYSTEM. Below the lamp is placed a spherical metal reflector, a, with the filament a t its compactness and portability and center of curvature, so that light rays the need of expensive resistances, are reflected back upon themselves to potentiometers, galvanometers, etc. approximately double the intensity of The photoelectric colorimeter dethe beam traveling upward through diaphragm, c, which defines the beam. scribed in this paper (Figure 1) is a Above the filament a lens, d-diameself-contained instrument of moderter, 2.5 cm.; focal length, 5 cm.-is ate cost and is the result of several placed a t its focal distance, so that years of investigation in this laborathe beam striking it is rendered very nearly parallel. Another diaphragm, tory. D u r i n g t h i s time many f, limits the parallel beam before it arrangements of both the optical enters the tube containing the liquid. and the electrical s y s t e m s w e r e The unabsorbed light then strikes the tried with varying success. The surface of the photocell exposed by instrument herein described has the aperture of diaphragm, g. The beam is rigidly defined in order to proved very satisfactory. eliminate as far as possible errors due to stray reflections from the sides of Description of Apparatus the tubes, which are never entirely PHOTOCELL CIRCUIT. Only one regular; condensing lens effect of photoelectric cell is used and it is the drops a t the tops of the tubes; rep h o t o v o l t a i c type which acts as a flections from finger prints on the outsource of current without the aid of side of the tubes; and the diverging an external e. m. f. The fundamental lens effect of the meniscus, an error characteristic of this type of cell is the which is here reduced to a minimum current, which is almost exact1 proby having the diameter of the beam portiorial to the light intensit 6 r low small in comparison with the diameter r e s i s t a n c e s in the externar circuit, FIGURE1. PHOTOELECTRIC COLORIMETER of the meniscus.
