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 tory. D u r i n g t h i s time many nearly parallel. Another diaphragm, 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.
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tube 11, between which the readings on I did not vary from 50, was taken as the correct value. If an exact balance is not attained, for the sake of simplicity in handling the data, it is better to secure it by again polishing the bottoms of the tubes. The same procedure is followed in taking the reading of a colored solution in tube 11. Tube I is filled with the blank solution and the microammeter always set at 50. By having the blank solution contain everything except the color-producing substance, errors due to the absorption of light by other substances in solution are automatically eliminated. Setting the light intensity to a definite value with the blank makes it unnecessary to have a sensitive voltmeter, since an exact knowledge of the voltage applied is not required. f
-
1
- ,e
FIGURE 2. DIAGRAM OF APPARATUS \>TUBES.The tubes (Figure 3) are made from precision-bore tubing of clear glass and have optically plane, fused-on bottoms to prevent distortion of the beam. The inside height to the graduation mark is 100 mm. and the volume is 100 ml. These tubes can be made so that the volume variation is a negli ible fraction of a milliliter (100.04 ml. is the volume of each o f t h e two tubes a t hand). This provides a very convenient volume for ordinary work. On the bottom of each tube is MECHANICAL CONSTRUCTION. cemented a brass ring into which a slot has been cut to coincide with a pin, so that the tube is always held in the same position. A turntable, mounted on smooth bearings, carries the two tubes and successively rotates them into position in the light path. A ratchet device with the two fixed points insures the correct position of the tube for a reading. At the base of the cylinder containing the turntable, a convenient slide permits the insertion of color filters just above the lens (e, Figure 2). When a filter is not used, a clear optical flat is kept in the slide to protect the lens and to keep out dust. A sliding door on the side of the cylinder opens to permit insertion of the tubes and when closed keeps out stray light and dust. The lens, lamp, and reflector are all rigidly mounted in a lightalloy casting. The lamp socket is adjustable to permit accurate centering of the filament with respect to the lens, and the reflector can be focused by means of a screw, thus permitting realignment of the beam should a bulb have to be replaced. A pair of adapters (Figure 3) can be mounted on the turntable, sus ended from the shoulders that hold the tops of the large tubes in L e d position. The adapters hold microtubes (5 mi.) so that the instrument can be used in cases where only very small volumes of liquid are available, as is often the case in biochemical and biological work.
The photronic cell has an inherent objectionable defect which may be classified as a “fatigue” effect, and can produce an appreciable error unless the proper care is observed in operation. When the cell is first exposed to light its reading is high, but after a very short time it drops to a constant value. As long as the solution is dilute and transmits almost as much light as the blank, the fatigue effect is negligible, but when the solution is concentrated, it is necessary to wait for about a minute before taking the reading of the blank. This delay may allow the voltage across the lamp to change, since the battery is continuously discharging and the reading may be in error by as much as 1 pa, This voltage change may be reduced greatly by using a battery with a high ampere-hour capacity or by using two batteries connected in parallel. However, for work of the highest accuracy, it is necessary to determine the approximate concentration of the unknown and then make u p a standard of that concentration. Setting the light intensity to the correct reading of the standard, the reading of the unknown is taken and the relationship between the readings and the Concentrations is given by Equation 5 .
Operation The first and a very important step is cleaning the tubes and securing a photometric balance. The tubes are first
FIGURE 3. TUBESAND ADAPTERS
cleaned inside with sulfuric acid-dichromate cleaning solution and then rinsed very thoroughly with distilled water. The bottoms are occasionally washed with a soap solution free from abrasive, but ordinarily polishing them with lens paper is sufficient.
In case solutions to be studied must be made up with a colored reagent or contain colored substances which do not interfere by chemical reaction, color filters may be used to eliminate their absorption. If a color filter having the same spectrophotometric transmission curve as the interfering colored substance be inserted in the light path and the blank set t o a reading of 50 pa as before, the absorption observed will be due almost entirely to the substance whose concentration is to be measured. It is thus unnecessary to know the concentration of the interfering colored substance so long as it remains below the limit of the depth of color of the filter. When the microcells are used, the reading of the blank is set a t 25 rather than 50 because a smaller area of the photronic cell is exposed and sufficient intensity to cause full scale deflection would discharge the battery so rapidly that constancy of the blank reading could be maintained only with great difficulty.
The tubes are filled to the mark with distilled water and then placed in position in the instrument. The sliding panel is closed and the light switch turned on. The coarse resistance is adjusted so that the voltage reading is approximately 3 volts. The instrument must then be allowed to stand for 2 or 3 minutes to permit the fiIament and the rheostats to warm up so that their resistance becomes fairly constant. Then the photocell switch is turned on and the intensity of the light adjusted by means of the coarse and fine resistances so that the microammeter reads 50 with tube I in position. Tube I1 is then quickly turned into position. The reading should be 50, when the needle has come to rest. It is necessary to turn I back into position to make certain that the reading is still 50. The first reading is the least reliable and in this work it was discarded and the mean of three readings on
JULY 15, 1935
ANALYTICAL EDITION
Calibration The instrument must be calibrated for the quantitative determination of a substance by means of a given color reaction. Two methods may be used for handling the data, the choice depending upon the accuracy demanded and the time available. The first, and simpler, method is a plot of ammeter readings against concentrations (Curve I, Figure 4) or a curve of the decrease in ammeter readings against concentrations (Curve 11, Figure 4) gives a positive slope
C@KCEITR*TlOh’S
(P.P.U.
OF
mate Beer’s law conditions. If the blank solution is found to show a measurable absorption, it must be used as the blank in tube I, but if it shows no absorption or a a t e r can be found which will match its absorption, distilled water may be used in tube I and thereby greatly simplify the procedure. The colored solution on which the most extensive investigation was carried out was ammoniacal cupric sulfate [Cu(NH&SO*]. It was chosen because it is a relatively poor colorimetric system for visual study and hence would give the instrument as rigid a test as possible and still remain within the limits where comparison with visual methods could be made. Ammonia solutions exhibit an appreciable absorption in the visible region; hence it was necessary t o use as a blank an ammonia solution of the same strength as the standards in which an excess of very large magnitude was used. Less extensive studies on two other systems gave comparable results, so that the results which follow may be considered as typical, but it must be remembered that many colorimetric reactions give solutions of the order of one hundred times as intense a coloration a t a given concentration and the sensitivity to concentration change is therefore of the order of one hundred times as great.
Results
CUI
FIGURE 4. CALIBRATION CURVES and may be preferable. I n either case, the concentrations may be read directly from the graph. Using this method, however, precludes the possibility of eliminating the error due to the fatigue effect mentioned previously. The second method of handling the data is based on two assumptions: the validity of Beer’s law and proportionality of current production to light intensity. Beer’s law in its most familiar form is:
I, = l o . l O - “ ~
283
The stock copper solution was prepared from reagent grade CuSOe.5H20 by dissolving 39.282 grams in a liter of solution a t 20” C. Then 100 ml. of this solution were diluted to a liter to give a solution which contained 1mg. of copper per ml.
(1)
taking logarithms
log I , = log l o - “CZ -4ssumimgdirect proportionality of current to light intensity, R = KI (2) where 61 is the ammeter reading. Since the depth of solution z is constant (100 mm.) a and 5 may be combined into another constant k R log a- = kc (3) Rc
A plot of log Ro/Rc against c should give a straight line whose dope is k. That such is the case, a t least within certain limits, can be seen from Figure 5. Plotting the curve and evaluating the slope makes it possible to calculate the concentration from the relation: 1 k
c = - log
50
-
R
(4)
From this the relationship between two concentrations and their readings follows: (5)
A table of the values of log 50/R for values of R from 50 to YO a t intervals of one-tenth was made up and much time saved in the calculations. The f i s t step in studying a given color reaction is to determine what the blank solution must contain. It is advisable to choose a fixed amount of the color-developing reagent, or reagents, of sufficient quantity to give a large excess for all concentrations that will be encountered. If the excess be very large, the amount used up in the reaction will be negligible in comparison and the variation in percentage excess will be small, so that the system will more closely approxi-
CORCEFTMTIONS
1 P.P.U.
FIGURE 3. CALIBRATION CURVES This was then checked by electrolytic deposition and -was found to be exact. The test solutions were made up over a range of 5 to 100 p. p. m. of Cu+-, the proper number of milliliters of the Cut’ solution (1 mg. per ml,) being added to 25 ml. of 15 M ammonium hydroxide and diluting to a liter. Thus in the most dilute solution the amount of ammonia present was 1200 times that required for the reaction
+ 4 NHa = Cu(NHa)a++
CU++
and in the most concentrated, 60 times the theoreticaI amount. The blank was made by diluting 25 ml. of the same ammonium hydroxide solution to a liter. Following the procedure outlined under “operation,” the following results were obtained with the Cu(NH;04* solutions of concentrations as indicated. TABLE I Copper, p. p. m. 5 10 15 20 25 35 50 75 100
R
1 46.8 43.8 40.9 38.0 35.4 30.8 25.1 18.1 13.8
2 46.8 43.7 40.9 38.0 35.3 30.8 25.2 18.1 13.8
3 46.8 43.7 40.9 38.0 35.3 30.8 25.1 18.1 13.8
Mean 46.8 43.7 40.9 38.0 35.3 30.8 25.1 18.1 13.8
50-R log 60/R 3.2 0,029 6.3 0.059 9.1 0.087 0.119 12.0 14.7 0.151 19.2 0.210 24.9 0.299 31.9 0.441 36.2 0.559
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284
These data are plotted in Figures 4 and 5. From Figure 5 it can be seen that the linear relationship predicted by Equation 3 holds over a fairly wide range of concentrations, the value of k being 0.00599. In repeating the above with two new sets of solutions, the values of k obtained were 0.00603 and 0.00597, respectively. A reliable average of these values is k = 0.0060. A solution, whose concentration was unknown to the observer, was placed in tube II and gave a reading of 44.7 pa. From the curves of Figure 4,this means a concentration of 8.2 p. p. m. Or, the following calculation gives c =
log 50/44.7 0.0060
= -0.0486 0*0060 =
tinguished only with difficulty from 7 or 9 p. p. m., and with certainty from 6 or 10 p. p. m.
Acknowledgments The authors are indebted to Frederick L. Brown of the Rouss Physical Laboratory for many helpful suggestions and to Robert H. Kean of this laboratory for assistance in studying some of the early models of the colorimeter. Our thanks are due the American Instrument Company, Washington, D. C., for building the apparatus and for assistance in solving some of the problems of design.
Literature Cited
8.1 P. P. m.
The correct concentration was 8.0 p. p. m. In comparison with visual methods, by means of Xessler tubes in a roulette comparator (g), a solution containing 8 p. p. m. can be dis-
VOL. 7, NO. 4
(1) Weston
Electrical Instrument Corp., Newark, N. J., Technical
Data, B-1001-A. (2) Yoe and Crumpler, IND. ENG.CHEM.,Anal. Ed., 7 , 7 8 (1935).
R~~~~~~~ April 16, 1935.
A Pressure Regulator for Vacuum Distillation 0. J. SCHIERHOLTZ, Ontario Research Foundation, Toronto, Ontario, Canada
A
NUMBER of devices have been described in recent years for the control of pressure, the most recent being those of Palkin and Nelson (2) and Jacobs (1). The apparatus described below is very simple in construction and is rugged and foolproof. There are no ground joints subject to wear and leakage, no liquids likely to cause corrosion, no electrical contacts subject to disturbance, and no fragile glass parts which cannot be replaced in a very few minutes. The action of the regulator is not affected by traces of impurities in the mercury in the U-tube. The regulator is actuated directly by the vacuum itself without the aid of a secondary outside agency. The only wearing part requiring replacement is the rubber valve seat and this can be cut, without special tools, from any reasonably good rubber sheet. This device is not intended to furnish extreme accuracy, b u t it will control pressures within the accuracy of laboratory thermometer readings over a wide range, with a minimum of attention in the way of adjustment and repairs. While this regulator operates on the flutter-valve principle, the valve never shuts off entirely while in operation (except when the vacuum is first being built up), but the rubber valve seat “floats” an infinitesimal distance from the surface ’of the glass orifice. This makes for smooth operation.
In Figure I (a side elevation of the apparatus) A is a small square of sheet rubber cemented t o a metal plate which in turn is riveted to lever C. B is a glass capillary tube about 4 cm. long and ground down to the edge of the orifice at its upper end. The lower end fits into pressure tubing held by clamp E, the position of which is adjustable by knurled nut G, and which is connected to the vacuum line preferably as near as possible t o the distilling flask. When A closes down on B, the influx of air stops and the vacuum builds up, and vice versa. Lever C revolves on pivot F and has attached to it a threaded stud carrying a counterweight, D, which serves as a fine adjustment for the valve assembly. The metal plate carrying A rojects beyond the rubber square and engages a slot in metal bractet H . K is the main bracket which is bolted t o an instrument board at L. In Figure I1 (an elevation taken at right angles to that of Figure I) M is a wooden beam which swings on knife edge N , hun on bracket P, which is also bolted to the instrument board. To t%e wooden beam is attached U-tube &, the horizontal ortion and right leg of which consists of Pyrex capillary tubing oF3-mm. bore and is sealed at the end, like a closed-end manometer. The left leg consists of Pyrex tubing of 5-mm. bore and is attached to the vacuum line by means of a piece of pure gum tubing filled with short glass cylinders to prevent its coIlapse under vacuum, and at the same time allowing the greatest possible degree of flexibility. Sensitivity can be increased somewhat by increasing the difference between the bores of the two legs of the U-tube. The U-tube is filled with mercury for the full length of the 3-mm. capillary.
