Photronic Photoelectric Turbidimeter for Determining Hydrocyanic Acid in Solutions E. T. BARTHOLOMEW AND E. C. RABY, Citrus Experiment Station, Riverside, Calif.
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YDROCYANIC acid is used extensively in certain localities as a plant fumigant. Because of differences obtained when fumigating citrus trees, experiments
were conducted to determine the amounts of hydrocyanic acid that were absorbed by citrus foliage under varying environmental conditions. The Liebig silver nitrate method was used for determining the amounts of hydrocyanic acid that were recovered, but this method. which calls for the titration of the hydrocyanic acid in alkaline solution in a flask or beaker, was O M L not sufficiently accurate when dealing with such small @P . a m o u n t s of hydrocyanic acid. Taking a s u g g e s tion from C u p p l e s (I), a s p e c i a l l y devised visual nephelometric apparatus was then constructed and FIGURE1. DIAGRAM OF PHOTRONICused in a dark room. PHOTOELECTRIC TURBIDIMETER This m e t h o d g a v e 8 V . A C . 8-Volt alternating current decidedly b e t t e r AL. Auto lamp, 32 candlepower ML. Movable lens results but was far M, M. Mirrors P P Prisms from sa tisf a c t o r y . si: SC, solution cells In order to obviate Pd PC. WePton photronic cells VR: Decade resistance box, 0.1 ohm steps these troubles, after FR. Fixed resistance, 260,000 ohms DC. 1.5:Volt dry cell a few tests had been SW. Switch, double-pole single-throw made with the phoG. Galvanometer t r o n i c colorimeter described by Wilcox (g), it was decided to devise and construct a photronic photoelectric turbidimeter. The authors’ turbidimeter, which serves equally well as a colorimeter, has been found very satisfactory. However, if the apparatus were to be used as a colorimeter it would be well to use a storage battery in place of the dry cell to insure a constant e. m. f. across the potentiometer. It is the purpose of this paper to give a description of the turbidimeter and also enough data to prove its efficiency. Since in these tests the apparatus has been used as a turbidimeter only, it will be referred to as such in this paper.
LIP MAM
mirrors, M , M , is mounted in a rectangle of brass 1.1 cm. thick. By means of a threaded micrometer attachment the lens may be moved to the right or left, in order to throw the desired amount of light on each of the mirrors. These mirrors were made by a new process which calls for the evaporation of an alloy in vacuum, and are called “panchro” mirrors. They are excellent for this purpose because their nontarnishing front surface has a fairly high coefficient of reflection, which is uniform for all colors of the visible spectrum. The mirrors, which are on very thin glass, are cemented to a right-angle prism. This prism and the two rightangle prisms, P, P, are attached to a rectangle of brass 1.1 cm. thick, which is firmly attached to the baseboard of the apparatus. SC, SC are solution cells, preferably having two opposite, flat, polished walls parallel to each other. PC, PC are two paired, Model 594 Weston photronic photoelectric cells. (The energy output of this type of photoelectric cell is a function of the intensit and frequency of the light which falls upon its sensitive surface1 G is a box-type lamp and scale galvanometer. sensitive to 0.63 megohm. The positive terminals of the photranic cells are connected directly to the alvanometer leads. SW is a switch with which to operate t%e galvanometer circuit. The negative terminals of the photronic cells are connected to the potentiometric arrangement V R FR. V R is a decade resistance box with a maximum resistance of 9999.9 ohms, in 0.1-ohm steps. FR is a I-watt 250,000-ohm fixed resistance. DC is a 1.5-volt dry cell. When V R is set at zero, DC is shunted by FR. With V R a t zero and SW closed, the illumination may be so adjusted by the movable lenn, M L , that there will be no current flowing through the galvanometer.
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The wiring of the apparatus described has proved t o be very satisfactory. It is known that there is some polarizing effect on the photronic cells when current is allowed to flow through them. I n this system the cells are not shunted, but are directly opposed to each other. The system is potentiometrically balanced and calibrated so that there is no flow of current through the cells and therefore no polarization. The conditions described permit the cells to act at maximum sensitivity. In most of the photoelectric turbidimeters and colorimeters which have been described, only one photoelectric cell has been used, The authors have used two in order to exclude or reduce to a minimum the effects of possible fluctuations in line or battery voltage, possible errors due to light-effect changes in colored solutions or fatigue of the photronic cells, possible error in determining the end point in the presence of both color and turbidity in the solutions (S), and to eliminate the need for constructing a reference standard. Figure 2 is a photograph of the photronic turbidimeter. The galvanometer, resistance box, dry cell, variable resistance, and transformer are accessory and are not shown. The wiring indicated in Figure 1 is all on the under side of the base. The elevation to the right is the lamp housing which has a louvered top and base to permit ventilation. The two low elevations are the light tunnel (right) and the housing for the mirrors and prisms (left), with the movable lens and its micrometric adjuster between. The high elevation to the left contains two compartments, each 7.5 X 10.2 X 13.3 om. high, in which the solution cells are placed. The single cover for these two compartments is hinged at the back and in order to exclude external light is always kept closed while readings are being made. The two photronic cells are shown a t the extreme left, mounted in brass cylinders 4.5 om. long. All interior surfaces are flat black.
PHOTRONIC TURBIDIMETER The construction of the photronic turbidimeter is diagrammatically represented in Figure 1. A transformer is inserted between the regular laboratory convenience outlet and the 32-candlepower auto lamp, AL, which reduces the voltage to 8 volts, The lamp is attached to a heavy metal base (duralumin) to prevent any possible warping effects due to changes in temperature. The metal base is movable to permit focusing. The lens, M L , which throws parallel rays of light on the 68
January 15, 1935
ANALYTICAL EDITION
OPERATIONOF PHOTRONIC TURBIDIMETER The first step in the operation of the photronic turbidimeter is to insert the solution cells, each containing a duplicate aliquot of the solution to be tested. These cells should be of such a height that they will take an aliquot which is large enough to bring the surface of the solution well above the upper edges of the photronic cells. This is important. If the surface of the solution is too low, light passing upward will be reflected onto the sensitive surface of the photronic cells and thus give an erroneous reading, especially a t the end of titration when the surfaces of the liquids in the two solution cells would be of unequal heights.
69 RESULTB
Accurate determinations of small amounts of hydrocyanic acid could be made with the photronic turbidimeter even though the solutions were not clear before titration began. For example, on adding like amounts of liquid hydrocyanic acid to clear alkaline solutions and to alkaline solutions that had been made light yellow and slightly turbid by immersing and shaking the leaves in them, practically the same titration end-point readings were obtained. With the photronic turbidimeter determinations could be made which were accurate to within 0.00054 mg. of hydrocyanic acid, but for the authors’ purpose, determinations to within 0.027 mg. were considered sufficiently accurate. Table I shows the accuracy with which given amounts of hydrocyanic acid could be recovered from the solutions. The presence of color and slight turbidity in the solutions before titration was begun did not materially affect the accuracy of the end-point reading. TABLEI. RECOVERY OF HYDROCYANIC ACIDFBOM CLEAR AND NON-CLEAR SOLUTIONS
FIGURE2. PHOTOGRAPH OF TURBIDIMETER, WITHOUT ACCESSORIES When the solution cells have been inserted, the lid has been closed, and V R set a t zero, the switch which operates both turbidimeter and galvanometer lamps is turned on. After the photronic cells and the galvanometer have become approximately adjusted (perhaps 10 to 15 minutes) the switch, SW, is closed and the light beam on the galvanometer scale is brought to the null point, by shifting the movable lens so that t h e light falling on the photronic cells is properly divided. As soon as the photronic cells have become completely adjusted and the galvanometer beam remains permanently a t the null point, titration is begun. A quantity of silver nitrate is added to one of the solution aliquots, thoroughly stirred, the lid closed, and the switch thrown in. If there is no deflection from the null point, the process is repeated until a deflection is obtained. The deflection signifies that the end point has been reached-that is, that silver cyanide has been precipitated, that the solution has become turbid, and that, as a result, the energy output of the corresponding photronic cell has been decreased. The nature of the precipitate particle is not a factor in these measurements, because it is the beginning and not the degree of turbidity which is being determined. The size of the particles causing the slight turbidity in the solution before titration was begun was such that there was no appreciable settling between the time of stirring and the taking of the reading. The amount of hydrocyanic acid in the aliquot is computed from the amount of silver nitrate necessary t o produce initial turbidity. This completes the operation, unless one wishes in a general way to determine the increase in turbidity as additional amounts of silver nitrate are added. In this case sufficient resistance from V B is added each time to bring the galvanometer beam back to the null point. The variable resistance would be used also when the instrument was being employed as a colorimeter. The usual rules of titration, such as those relating to stirring and the time element, not only apply here but their effect can be more easily detected with this instrument than by the usual methods.
H C N RECOVERED Clettr Non-clesr HCN USED solution solutiona Gram Gram Gram 0.001456 0.001458 0.001458 0.01550 0.015490 0.015486 0.031185 0.031185 0.03120 0.030691 0.030688 0.03070 0.031382 0.031377 0.03140 a Solution which had been filtered after immersing and shaking the leavea in it.
Table I1 further indicates what small amounts of hydrocyanic acid may be detected and measured with the photoelectric turbidimeter. The figures in columns 2 and 3 represent the amounts of hydrocyanic acid in samples of air aspirated from the fumatorium 2 minutes after the beginning and 2 minutes before the close of the fumigation period. The air and leaf samples represent equivalent volumes. The actual amount of hydrocyanic acid to which the leaves had access in the fumatorium was 3528 times as much as is indicated in the figures in column 2. The variations in the amounts of hydrocyanic acid in the fumatorium and in the amounts absorbed by the different lots of leaves will be explained when a more detailed report is published. TABLE11. ACCUMULATION OF HYDROCYANIC ACIDIN FRESH, MATUREVALENCIA ORANGE LEAVESDURING 45-MINUTE FUMIGATION PERIOD TE0T
1 2 3 4
HYDROCYANIC ACIDRECOVERED Fumatorium Fumatorium 200-gram air, air, lots of sample 1 sample 2 leaves Gram Gram Gram 0,0002376 0.01728 0.0004176 0,01584 0.0002742 0,0002020 0.0002304 0.01368 0.0002953 0.0001940 0.01656 0.0002590
RATIO Column 4: Column 4: column 2 column 3 41.4 57.8 46.3 63.9
72.7 78.4 59.4 85.4
ACKNOWLEDGMENT The authors with to express their appreciation to H. W. Edwards, of the University of California a t Los Angeles, for making the “panchro” mirrors, and to H. H. Bliss, of the Riverside, Calif ., Junior College, for assistance in working out a satisfactory electrical circuit for the turbidimeter. LITERATURE CITED (1) Cupples, H.L.,IND. ENG.CHEM.,Anal. Ed., 5, 50-2 (1933). (2) Wilcox, L.V., Ib{d., 6,167-9 (1934). Sattler, L., and Lorge, I., Ibid., 6,178-83 (1934). (3) Zerban, F.W., RECEIVED October 23, 1934. Paper No. 313, University of California Citrus Experiment Station and Graduate School of Tropical Agriculture, Riveraide, Calif.
