Determination of Microgram Quantities of Fluoride and Cyanide by Measurement of Current from Spontaneous Electrolysis BERTSIL B. BAKER and JOSEPH D. MORRISON Southern Research Institute, Birmingham 5, A l a .
Measurement of the current from an electrochemical cell which operates spontaneously may be used as a measure of one of the reactants in the cell. Cyanide can be determined in the cell 4 g 1 NaOH (0.1M) Pt and fluoride in the cell A1 1 CHJCOOH (0.2M) 1 Pt. The simplicity of the methods, combined with good sensitivity in the microgram range, suggests their use in portable detection devices for hydrogen cyanide or hydrogen fluoride vapors in industrial atmospheres.
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KTERNAL or spontaneous electrolysis, in which the cell reaction proceeds without any external applied potential, has been used for a long time as a technique in electrogravimetry. Such electrolyses represent a type of limited potential electrolysis, the choice of the electrodes allowing a certain degree of control over the available voltage. I n ordinary controlled potential electrolysis when 100% current efficiencies are obtained, a definite relation is to be expected between the instantaneous current and the bulk concentration of the limiting reactant (1). Calculations based on instantaneous current readings have been suggested as a substitute for the usual methods of current summation in controlled potential coulometric analysis ( 2 ) . The present work demonstrates that instantaneous current readings in spontaneous electrolysis may, under certain conditions, be an equally suitable measure of the concentration of the limiting reactant in solution. I n practice, the accuracy of results based on this relationship may be insufficient for widespread application to the determination of major constituents, but entirely adequate for trace amounts where a greater relative error is usually permissible. Good results have been obtained in the determination of cyanide by means of a silver anode and fluoride with an aluminum anode. A platinum wire may be employed conveniently as cathode.
aluminum suggested the use of an aluminum anode for the detection of fluoride. EXPERIMEYTAL
Estimation of Cyanide. Suitable electrodes consist of a spiral wound tightly from 18 inches of 18-gage silver wire, as anode and a similar spiral, from 10 inches of 0.03-inch diameter platinum wire, as cathode. The two spiralsmay be wound conveniently on the arms of a C-shaped, three-way, connecting tube (Kimble No. 45025) which provides the necessary permanent positioning of the electrodes. Sodium hydroxide (0.1.M) serves as the electrolytic solution. Stirring may be accomplished conveniently with a magnetic stirrer. Connection between the electrodes is made through a suitable precision microammeter ( \F7eston Model 430, 0 to 200 pa., 571 ohms internal resistance). When the electrodes are lowered into the hydroxide solution, a blank current of several microamperes flon s momentarily but decreases to nearly zero in about 1 minute. If a few micrograms of cyanide in 0 . l M sodium hydroxide solution are now added, a nearly exponential current-time curve similar t o that in Figure 1 results. The cell is calibrated by measurements on several standard solutions of cyanide, in the concentration range of interest, prepared in 0 . M sodium hydroxide. Five milliliters of 0.1M hydroxide are added to the cell and when the blank current has de-
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a 10 MICROGRAMS OF CYANIDE
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Figure 2. Typical calibration curve for determination of cyanide
Figure 1. Current-time relationship in determination of cyanide
The use of a silver anode was suggested by analogy with the polarographic determination of cyanide by its effect on the mercury dissolution wave. The use of a silver electrode in the potentiometric titration of cyanide is also a well recognized procedure. I n 1951, Roth (5)presented a paper concerning the detection of hydrogen cyanide by measurement of the potential of a silver electrode. The existence of strong fluoride complexes of
creased t o nearly zero, 5 ml. of the standard cyanide solution are added. After 1 minute the current is recorded. Then this cell solution is discarded and a similar procedure followed with another concentration of cyanide. A curve similar to t h a t in Figure 2 results from these data. The current is a function of the size of both electrodes (especially the anode), the cell dimensions, external resistance (microammeter), rate of stirring, and similar variables. The calibration curve often remains valid for days of use if none of these factors changes. However, changes sometimes occur for no apparent reason (probably the surface condition of the electrodes has been unknowingly altered) and therefore frequent checks are advisable. This is no serious handicap to the usefulness of the method, inasmuch as the process of recalibration is simple and rapid. On continued use the sensitivity of the cell may shovi a slow, steady decrease. I n this case cleaning of the platinum cathode with hot nitric acid often has been found t o restore the cell to nearly its original sensitivity. Cleaning of the silver seems unnecessary and even inadvisable, because silver anodes cleaned in acid exhibit erratic behavior for some time. Silver wire from several sources was used as received and little variation was noted. Ordinary silver wire, such as is supplied for medical purposes, seems to serve well, If the electrodes have stood for some while without use, making several measurements with a standard
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V O L U M E 2 7 , N O . 8, A U G U S T 1 9 5 5 cyanide solution uiitil reproducible values are obtained has been found t o be a good ~ v a yto clean and condition the electrodes. This device is admirably suited to the detection of hydrogen cyanide in air: sinve 0.1.11 sodium hydroxide serves well as a collection medium and then may be transferred directlj- to the cell. The cell may be used to monitor atmospheres for hydrogen cyanide by leading the air stream through the C-shaped connecting tube on vhich the electrodes are wound. I n this case the magnetic stirrer is unnecessary as sufficient stirring is provided by the air stream. Any increase in current above the base line (nearly zero) indicates the presence of cyanide and the rate of increase is a measure of the amount of cyanide present in the air. I n the presence of air the following chemical reaction may take place in the cell and result in consumption of cyanide and consequently smaller currents from the electrochemical reaction:
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1307 electrode (probably t o the extent that its history determines the condition of the surface). Therefore, determination of a calibration curve is necessary under the exact conditions of the analysis. The simplicity of the procedure makes this task easy. Cleaning and conditioning the aluminum for 2 minutes in 0.013P hydrofluoric acid before beginning a series of determinations are advisable. Kitrate, sulfate, and cyanide produced no interference in 1000-y amounts. Phosphate and sulfide cause some variation. but probably could be compensated by calibrating the cell n i t h approvimately the expected amounts of these anions present. Chloride (1000 y) caufies large currents and is a serious interference a t this concentration.
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8‘290, 4Ag 2H2O +4Ag(CN),40H- (1) This reaction apparently occurs t o an appreciable extent in solutions 2 X 10-4.11 in cyanide and 0.01M in hydroxide. However. in 0.1 or 1.11 liydroxide and 2 X l O - 5 X or lower cyanide concentration, little or no loss of cyanide results by this reaction.
Table I.
Effect of Various Substances on Estimation of Cyanide by Silver Anode
Substance Added
Equivalent t o 1000 y of
Current, Read after 30 See., pa. Blank Response current, current t o 2.6 y of C X 0.7 17.7 17.3 0.5 17.7 0.6 17.0 0.7 17.0 0.6 16.6 0.7 0.7 16.9 0.7 17.0 17.0 0.0 -12.0 3.0 Off scale positive ..
The effect of possible atmospheric contaminants on the behavior of the cell was investigated. Inasmuch as the electrolytic solution is concentrated sodium hydroxide, the effect of acidic gases could be determined conveniently by adding their sodium salts. Approximately 1000 y of the various substances (contained in a few tenths of a milliliter of solution) vxre added to a cell containing 10 ml. of 1M sodium hydroxide solution. When no added substance was present, the cell showed a blank current (read after 30 seconds) of 0.7 pa. and a response current of 17.7 pa, to 2.6 y of cyanide. Table I shows the values obtained in the presence of various contaminants. Of those tried, only hypochlorite (which would be obtained from chlorine) and hydrogen sulfide appear to be serious interferences a t the 1000-y level. Five micrograms of hypochlorite gave no interference. Five micrograms of sulfide produced a blank current of 3 pa. I n both cases, subsequent addition of 2.6 y of cyanide gave the usual additional 17 to 18 pa. of current. Determination of Fluoride. A cell similar to that described for cyanide may be used, with the exception that the anode should be duminum &-e of considerably smaller surface area than was the silver anode. -4 straight piece of 0.125-inch diameter, 99.997, aluminum wire (Aluminum Co. of America) was iound to be suitable. Ordinary aluminum (99.77,) can be used, but somewhat higher and more irregular blank currents are obtained. The wire should be coated where it enters the solution with a nonconducting film (-4piezon hard wax K is suitable) to avoid erratic current readings due to a variable surface exposed t o the solution. =\n exposed length of about 0.5 inch was suitable. Acetic acid (0.2M) serves satisfactorily as the electrolytic solution. When the electrode is immersed in the solution, a relatively large current flon-s monientarilj- and decreases to a constant value in 3 or 4 minutes. This blank current varies with the size and condition of the electrode, but usually iq 5 to 10 pa. If fluoride is now added t o the cell, the current rapidly increase3 and then decreases in :L regular fashion, as shown in Figure 3. The magnitude of this current is proportional to the amount of fluoride present, as is indicnted by Figure 4, where current value$, read after 2 minutes, are plotted against micrograms of fluoride pre3ent. A nearly saturated solution of benzoic acid ( 0 . 0 l i J f ) may be used instead of the 0.2.11 acetic acid. Benzoic acid gives a much smaller blank current and definitely is preferred when small amounts of fluoride (less than 5 y ) are being determined. As with the silver anode, the current is a function of rate of stirring, electrode size, and. to a certain extent, history of the
Current-time relationship in determination of fluoride
Figure 3.
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Inasmuch as the electrode shows good response t o 1 p.p.m. of fluoride, the method may be useful for determining fluoride in fluoridated public water supplies in cases where large amounts of chloride are not present. ACKNOWLEDGMENT
The authors are indebted t o the hluminum Co. of America for supplying the 99.99yo aluminum wire and to Ruby H. James, Virginia Jackson, and Carl 0. Thomas for assistance in the experimental work. The interest and encouragement of Killiam J. Barrett and Edlvard B. Dismukes are gratefully acknowledged. LITERATURE CITED
(1) Lingane, J. J., “Electroanalytical Chem~stry,”p. 194, Interscience, New York, 1953. ( 2 ) AIacSevin, TV. >I., and Baker, E. B., A x ~ L CHEW, . 24, 986 (1952). (3) Roth, H. H., 99th meeting of Electrochemical Society, 1951, unpublished data. RECEIVED for reriew December 17, 1954. Accepted March 11, 1955, Presented before the Division of -4nalytical Chemistry a t t h e 127th hleeting of the A J ~ E R I CCHEMICAL A~ SOCIETY, Cincinnati, Ohio, .\larch 1955.
