C. E. Hedrick University
of
Pennsylvania Philadelphia
Some Analytical Applications of Photoconductive Cells
chemists who have performed photometric titrations may have wished for a simple electrode-like device which could he inserted into the solution to provide a measurement of the optical qualities of the solution during the titration. A step in this direction is the use of miniature cadmium sulfide or cadmium selenide photoconductive cells as the sensing elements in such an indicator probe, or "photoprobe." Cadmium sulfide photoconductive cells are used in sensitive photographic exposure meters, and as "electric eyes" in modern cameras. Rost (1) used a cadmium sulfide photoconductive cell to construct a miniature filter photometer designed to act as aremotemonitor for radioactive systems. Flaschka and Sawyer designed a filter photometer using a silicon photodiode (t), and also used the photodiode as the sensing element in a microtitration apparatus (3, 4). In this paper, some analytical applications of miniature, glass-encapsulated cadmium selenide photoconductive cells will be illustrated. Literature sources which might be of interest to the reader include a book concerning the photoconductivity of the elements (6) and two works concerning photometric titrations (6, 7). Construction and Operation of Photoprobes
The photoprobes used in this work are designed to show the simplicity and sensitivity of miniature photoconductive cells. Clairex CL-602 cadmium selenide photoconductive cells were obtained from Newark Electronics, Inc., New York, N. Y. 10010. The glassencapsulated cells measure 0.25 in. in diameter and 0.50 in. long. The peak spectral response lies a t 515 millimicrons. The cells were found to have a resistance of 4000 ohms when placed six inches from a 100-watt lamp. At six feet from a 100-watt lamp, the resistance increased to 200,000 ohms, and the dark resistance measured in excess of 1000 megohms. The simplest photoprobe is constructed as follows: A small square of No. 2 microscope coverglass is cemented
to the end of a six-inch length of 9-mm id Pyrex tubing. Long, flexible wire leads are soldered to the photoconductive cell, and the cell is pushed into the glass tube (Fig. 1). The wires may be fastened at the top of the probe using cement or a small rubber stopper. Alternative designs include an I.-shaped probe, to take advantage of light entering the titration vessel from the side. I n most photometric measurements, the incident light must be filtered or monochromatic. The problem of light filtration can be handled in a number of ways. A small piece of filter material can be placed in the photoprobe; a dye which has the proper optical transmission characteristics can be dissolved in an organic solvent of low conductivity, such as nitrobenzene or chloroform, and the photoprobe sensing cell immersed in the liquid to a depth of about 5 mrn; the light source itself can be filtered; a monochromator can he used; the titration vessel can he coated or wrapped with a transparent filtering material; or, finally, an "internal filter" consisting of an inert dye can be added directly to the solution to be measured. The simplest light source used in this work is unfiltered room light. Our laboratory is lighted by a large window (4.5 X 6.5 ft) and by six double five-foot fluorescent lights. This lighting was more than sufficient for all hut fluorescence-titrations, or other measurements requiring an ultraviolet light source. The power supply for the photoprobes is a 1.4-volt mercury "D" battery, connected in series with the probe. "Button" type miniature mercury cells might also be used because of the very low current drain. The measuring equipment used is a Photovolt Model 43 recorder. The output of the photoprobe ranged from approximately 100 microamperes to nearly zero current, and the recorder displays this range over a ten-inch chart span. Also, the recorder is capable of presenting the logarithm of the input, so that measurements corresponding to units of absorbance can be plotted. A microammeter which measures this current range should be useful for recording transmittance data when continuous recording of the photoprobe output is not practical. Colorimetric Applications
Figure 1. Photograph show the rmoll sire of the components of the photoconductive probe.
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The first application is an example of a photometric titration of iron(II1) with EDTA [disodium ethylene (dinitri1o)tetraacetatel. Unfiltered room light and a straight unfiltered photoprobe were used to obtain the results presented in Figure 2. The solution volume was 100 ml, with 50 mg of ammonium thiocyanate as the indicator. The distance between the protoprohe and the reflecting surface was5-7 om. In the first titration, 5.00 ml of 0.1 M iron(II1) chloride was titrated with 0.1
traced from the chart record obtained when an unfiltered photoprobe is inserted into approximately 100 ml of 0.1 M EDTA a t pH 5.3 containing 150 mg of chromium(II1) nitrate. After one or two minutes, 100 mg of sodium carbonate is added to the solution, and the rapid formation of the color is recorded. Turbidimetric and Fluorescence Tihmtions
Figwe 2. Photometric titration of iron(lli1 with EDTA. Ammonium thio~ ~ m a is t ethe indicator, and a Cioirex CI-602 photoprobe is "red to meosvre the transmittance imicroampereJ and absorbance [log (microa m p e r e ~ ]of the rolution. (Tho m i b of the scales ore arbitrarily thore of recorder response.)
M EDTA a t pH 2, adjusted with HC1 and trichloroacetic acid. The container was a 150-ml beaker, resting on a magnetic stirrer covered with white paper to reflect the room light. Care was taken to prevent the operator's shadow from interfering with the light entering the beaker. Microamperes were recorded in the first titration to show the general response of the photoprohe under the stated conditions. The second titration was performed in the same way, except that the recorder was switched to the logarithmic mode, and the results were directly presented in units representing "log microamperes," or absorbance. As can be seen in Figure 2, the plot becomes linear when this is done. Photoconductive cells are not noted for fast response (0.5-3 milliseconds), but the response is fast enough to record kinetic phenomena occurring over periods varying from a few seconds to several hours. An example of such an application is presented in Figure 3. Kameswara, Sundar, and Sastri found that the formation of the deep purple chromium-EDTA complex is catalyzed by the bicarbonate anion (8). The chromium-EDTA complex normally forms rather slowly at room temperature. When carbonate or bicarbonate salts are added, the complex is formed in minutes. Figure 3 shows data
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2
3
9
Somewhat more rigid systems are necessary in order to perform turbidimetric and fluorescence titrations with the photoprobes. If ordinary unfiltered room light is used to illuminate the titration vessel, poor results are likely to be obtained in the case of a turbidimetric titration. The photocell senses a decrease in transmitted light as the opacity of the suspension increases, but scattered light tends to cancel out this effect. As a result, the slope of the titration curve tends to be small. If the titration vessel is shielded using black paper and a black cover, and room light is reflected into the bottom of the vessel with a mirror or white paper, good turbidirnetric titrations can be obtained using either unfiltered or filtered light. Figure 4 illustrates a series of turbidimetric titrations of ammonium sulfate with barium chloride in 40% isopropanol. When 0.1 millimole of sulfate was titrated with 0.02 M barium chloride in 100 ml of solution, a change of only 0.1 to 0.2 absorbance units was recorded. When the concentration of the reagents was increased five-fold, much better curves were obtained, as shown in Figure 4. The presence or absence of malachite green as an internal filter did not seem to increase the slope of the titration curve to a useful degree. The concentration of the malachite green was approximately 3 X lo-= M in the screened solution. Excellent results were obtained when the photoprohe was used to record fluorescence titrations. The titration vessel was surrounded by reflecting foil to take advantage of all the light produced. The light source was a small portable long-wave ultraviolet unit obtained from Edmund Scientific Corp., Barrington, N. J. The photoprobe was filtered with a 7-mm layer of nitrobenzene inside the probe tube because the cadmium selenide cell is sensitive to ultra-violet light. The light source was placed beneath the titration vessel, and an airbubbler was used to stir the solution.
I
5
mlnYteJ
Figure 3. An unfiltered photoprobe is "red to meowre the increase in absorbance of a chromiumlllll-EDTA solution. Bicarbonate onion catalyzes the formation of the purple chromiumlllll-EDTA complex.
Figure 4. Turbidimetric titrotion of suifmte with borium chloride. A CI-602 photoprobe measures r w m light reRected into the bottom of an otherwise light-shielded vessel.
Volume 42, Number 12, December 1965
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The system chosen for study was the direct titration of 0.5 mM of copper(I1) in 100 rnl 2y0 ammonium acetate using 0.1 M EDTA and calcein as the fluorescent indicator. Calcein is a metallochromic indicator prepared by condensing formaldehyde, iminodiacetic acid, and fluorescein. The indicator retains the acid-base and fluorescence properties of fluorescein, but also responds to metal ions. Copper quenches the fluorescence, and when the copper is complexed using EDTA a t pH 6-7, the fluorescence returns. Fuorescent indicators are useful for titrating systems which contain highly colored substances or suspended matter, but darkened rooms or special ultraviolet titration boxes are needed to make use of the indicators.
..
Titration of C o ~ ~ Usino e r " 0.1 M EDTA'
ml 0.1 M EDTA (0.02 ml increments) I.
Log (microamperes)
5.00 5.27
1.90 0.75 (EP)b 2.00 1.40 0.935 (EP) 0.930
0 . 0 [microlitersof 0.1 1M copper(II)] 1.0 2.0 3.0
" See text for conditions. The endpoint is chosen as that volume after which no sienificilnt change in fluorescence signal is observed. ' Titration of the indicator blank using 0.1 M capper(I1) and a 0-10 microliter syringe (see Fig. 6).
Literature Cited
0 rnh",*S
Figure 5. The fluorescent indicator cdcein reaches maximum inten+ slowly a t the endpoint of the titrotion of copper(lf) with EDTA. This short trosine shows the slow kinetics of the colrein-copper endpoint, measured with o nitrobenzene-flltered photoprobe.
When copper is titrated with 0.1 M EDTA using calcein (100ml solution), a curious phenomenon is observed. At the endpoint of the titration, the fluorescence of the solution abruptly begins to increase. If the titration is stopped, the fluorescence intensity rises to a maximum value over a period of 1-2 minutes. This phenomenon is recorded on the chart tracing in Figure 5. The first fluorescence increase is observed a t 55.7 ml, and the titration is stopped. After the fluorescence intensity reaches its maximum value, more EDTA is added to test the completeness of the titration, when no further increase in fluorescence intensity is observed. The endpoints are approached cautiously in increments of 0.010.02 ml to prevent over-titration, and typical results are presented in the table. The table also contains the data taken when the indicator blank was titrated using a 10-microliter syringe and 0.1 M copper(I1) solution. In this case, the equilibrium is established quickly, and the quenching of the calcein fluorescence can be plotted as a fluorescence titration, as shown in Figure 6. Acknowledgment. The graphs in this paper were drawn by Charles Searles, and the photograph is by C. F.. Hedrick, Sr.
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lournol of Chemical Education
(1) ROST,G. A,, Anal. Chem., 33, 736 (1961). P. O., Talanta, 8,521 (1961). (2) FLAS~HKA, H., AND SAWYER, (3) FLASCKXA, H., AND SAWYER, P. O., Microchem. J., Symp. S ~ T . , 2,783 (1962). H., Mierochem. J., Symp. (4) SAWYER, P. O., AND FLASCAKA, Ser.. 2.825 (1962). (5) ~ o s s , 'S., ~ :"Phot&onduetivity in the Elements," Academic Press.,New York. 1952. -~ (6) HEADRIDQE, J. B., "Photometric Titrations," Pergrtmon Press, Long Island City, N. Y., 1961. (7) UNDERWOOD, A. L., "Photometric Titration," in Advan. Anal. Chem. Instr., 3 , 31 (1964). (8) KAMESWARA, K., SUNDAR, D. S., AND SASTRI, M. N., ChemistAnalyat, 54,86 (1965). ~
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Figure 6. The colcein indicator blank for the titration of copperllll with EDTA is measured using 0.1 M copper(l1) and 0 ten-microliter syringe os the buret. The flvolercence of the indicator is quenched b y the copper.
