Shielded Dropping Mercury Electrode for Polarographic Analysis of Flowing Solutions Determination of Cyanide Ion WALTER H. JURA American Cyanamid Co., Stamford, Conn.
A cell has been designed for the simple, nonempirical, and continuous polarographic determination of electroactive materials in flowing solutions. Its practical application to the analysis of cyanide ions in air-saturated flowing solutions is illustrated. Dynamic polarograms obtained on flowing solutions are identical in every respect with those obtained by the static, classical technique, and the response time under the same conditions of measurement is rapid-98% of the equilibrium response time is reached in 2 minutes.
D
EVELOPMEST of polarographic equipment and techniques has led to interest in the application of the polarographic method to the continuous analysis of flowing streams such as might be encountered in product or process development and in plant control. Earlier workers ( 1 , 3, 8-10) devised continuous measuring systems using the dropping mercury electrode as the indicator rlectrode. Because the currents measured are not diffusion controlled, the polarographic waves cannot be represented by the IlkoviE equation, and it becomes difficult to define any one of these systems in exact quantities. Such measurements are empirical in nature and are more or less limited to the applications described. The work of Cooke ( 2 ) on a rotating silver amalgam electrode and, more recently, the studies of Marple and Rogers ( 7 ) on stationary mercury-plated platinum electrodes, suggest the application of solid wire electrodes to continuous measurements on flowing streams. .4t the present state of development, however, the dropping mercury electrode has very definite and practical advantages over solid wire and other novel electrode systems (6) such as: comparative simplicity of design and operation, absence of surface effects associated with other electrode systems, high hydrogen overvoltage, and the actual reproduction of a new and essentially identical electrode with each mercury drop.
Apparatus. A Leeds and Sorthrup Electro-Chemograph Type E, was used throughout this work. A Zenith metering pump (Zenith Products, West Newton 65, Mass.) was used to meter the product and supporting electrolyte streams. The dropping mercury electrode had the following characteristirs which were measured a t -0.60 volt us. S.C.E. in an airsaturated solution of 0.25M sodium chloride, 0.02M sodium hydroxide, and 0.0020M potassium cyanide: m = 1.060 mg. sec.-l, t = 5.45 sec., mz/s = 1.380 mg.Vn sec.-1/2. Stock Solutions. Air-saturated solutions of 0.25M sodium chloride and 0.02M sodium hydroxide which was 0.0010M to 0.010M in potassium cyanide. Air-saturated solutions of 0.020 to 0.060M potassium cyanide. These solutions were used t o standardize the Zenith metering Pump. Oxygen-free diluent. Twenty liters of 0.25.M sodium chloride were prepared in a glass carboy. The solution was kept under an atmosphere of prepurified Airco nitrogen, and 40 grams of anhydrous sodium sulfite were added. Several minutes after solution was effected, 200 in]. of 2.OM sodium hydroxide mere added. The oxygen introduced by the addition of the alkali was negligible. The solution was kept blanketed with nitrogen. Construction of Cell. The principal requirements were for a
cell with a reasonably rapid response time to changes in concentration of the electroactive material and a cell in which the currents measured would be diffusion controlled. It appeared that a cell with a shielded dropping mercury electrode, based on one designed b y Laitinen and Burdett (6) for amperometric titrations, but modified to handle flowing solutions, would meet these requirements. The cell (Figure 1) was constructed as follows: The main body of the cell was constructed from a borosilicate glass test tube having an inner diameter of 2.2 cm. and cut down to an over-all length of 11.5 em. A4one-way stopcock, to drain off accumulated mercury without interrupting the flow of solution through the cell, was attached 2.0 cm. above the bottom of the main body of the cell. The solution left the cell through a 1-em. (outer diameter) overflow arm attached 2.5 cm. from the top of the main body. The side arm, into which one end of a salt bridge from the saturated calomel electrode wasplaced, was attached 5.0 cm. below the top of the main body of the cell. The glass plug was made as follows: A cylinder of asbestos sheet was placed inside the main body of the cell to close off the opening where the plug was t o be made. Borosilicate glass helices were then dropped into the side arm to form a loosely packed plug against the side of the asbestos cylinder. The side arm was heated just enough that the helices were fused t o the inside wall of the side 1 arm and the innermost helices would not fall out. This served as the support for the agar-agar potassium chloride plug. The inlet tube, for the incomn OUT ing solution, was made from 6-mm. borosilicate glass tubing. A small bulb was blown on one end. On the underside of the bulb, two portholes were blown out. This directed t h e i n c o m i n g solution downward into the cell, effectively displacing old solution from the bottom sections of the cell. The inlet tube was inserted into a rubber stopper, which, in turn, was inserted into the bottom of the main body of the cell a8 Figure 1. Construction of shown in Figure 1, a. Cell The outer shield for the a. Polarographic oell for handling dropping mercury elecflowing solutions trode was made from a b . Bottom view of outer shield 20.5-mm. (outer diameter) b o r o s i l i c a t e gl a s s teat tube. The closed end of the test tube was made concave to permit the mercury drops to flow out of this section. The test tube was then cut off 38 mm. from the bottom. Four equally spaced 8-mm. horizontal slits, 1.0 mm. wide, were cut as close and parallel to the bottom of the shield as possible mvith an S.S. White No. 32 diamond cutting tool. These slits were enlarged on the bottom side with an S.S. White N o . 16 diamond cutting tool as indicated in Figure 1, b. Four more equally spaced 8.0-mm. slits, 1.0 mm. wide, were cut 2.7 cm. from the bottom of the shield. These were cut parallel to, but not directly above, the bottom slits. Three small and equally spaced globules of glass were fused to the top 1121
ANALYTICAL CHEMISTRY
1122
edge of the shield to center the entire shield assembly when placed inside the main body of the cell. The inner shield measured 3.1 cm. in length and was made from 9-mm. (outer diameter) borosilicate glass tubing. Five vertical and equally spaced slits, 7 mm. long and 1 mm. wide, were cut at one end of the tube. These slits were enlarged just enough a t the bottom to prevent the formation of a ridge of fused glass when this shield is fused to the concave bottom of the outer shield. The glass ridge would prevent the mercury drops from readily escaping from the inside of the inner shield via the vertical slits. Two slits, 7 mm. long and 1 mm. wide, were cut at opposite sides 16 mm. from, and parallel to, the bottom of the outer shield. The inner and outer shield, now a unit, is rested on the top of the bulb of the inlet tube. RESULTS AND DISCUSSION
Determination of Cyanide Ion in the Presence of Oxygen. I t has been recommended that the cyanide ion be determined in alkaline solutions in the absence of air ( 4 ) . I t was of interest to learn if cyanide ion could be determined in alkaline solutions saturated with air, because this would simplify operations considerably. Polarograms were obtained on air-saturated solutions of 0.0010 t o 0.010M potassium cyanide in 0.25M sodium chloride, and 0.02111 sodium hydroxide, using a Heyrovskj. cell in which one end of a potassium chloride bridge from the saturated calomel electrode was immersed. Oxygen was subsequently removed from the same solutions with nitrogen and the electrode repolarized. The data obtained a t a temperature of 25 o Z!C 0.1 o C. are shown in Table I and indicate that the cyanide ion may be determined in the presence of oxygen under the experimental conditions described.
Cell Response to Changes in Concentration. After it was established that the dynamic current a t 95 ml. per minute was diffusion controlled, a measurement of the response time of the cell a t this rate was made. The response time represents the time lapse before the true diffusion current was recorded when switch ing from a solution of a given concentration of cyanide ion to oneof another concentration. Assume a switch from 1.0 to 2.0mM cyanide ion. With l.OmJ4 cyanide ion already in the cell (static), the e.m.f. a t which the true limiting current for 2.0mM cyanide ion is measured is applied to the dropping mercury electrode. At t = 0, the pinch clamp on the line to the 2.0mM cyanide ion reservoir is removed, and a continuous polarogram a t constant potential is plotted.
Figure 2.
Polarograms of Potassium Cyanide
A . LOmMKCN B. 2.0mMKCN Dynamic traces made a t a flow rate of 95 ml./min. are indicated by d , static traces by S.
Table I.
Cyanide Determination in Presence and ihsence of Oxygen
10.0
41 . O
4 10
41.3
4 13
Testing the Continuous Cell. To determine if the currents recorded with solution flowing through the cell were diffusion controlled and to measure the response time of the cell to changes in concentration of cyanide ion, two reservoirs containing different concentrations of cyanide ion were connected to the cell through a Y tube. Provisions were made for adjusting the flow of solution from either reservoir to the desired rate with s t o p cocks and pinch clamps. Dynamic versus Static Systems. When the stopcock was preset to give the desired flow rate, the pinch clamp was opened, and the dynamic polarogram was obtained by polarizing the dropping mercury electrode from -0.60 volt to 0.0 volt us. S.C.E. The static polarogram on the same solution was obtained by first closing the pinch clamp and repolarizing the electrode as indicated above. The results, obtained a t a flow rate of 95 ml. per minute with 1.0 and 2.0mrM cyanide ion, are shown in Figure 2. A critical examination of Figure 2 reveals no detectable differences betn-een dynamic and static polarograms. Similar observations were made on 4.0mJf and 6.0mM solutions of cyanide ion. With this particular cell, flow rates as high as 120 ml. per minute could be used. At higher flow rates, however-Le., > 120 ml. per minute-the dynamic current was no longer diffusion controlled, and the conditions under which the measurements were madr xere no longer exactly defined.
Recordings of the above and other changes are reproduced in Figure 3. All recordings were made a t 25 a & 0.5" C., and indicate that 79% of equilibrium response was reached after 1 minute and 98% after 2 minutes. Dilution of the Product Stream. For the continuous analysis of the product stream, the reaction product stream was continuously diluted with the supporting electrolyte. This was done with a Zenith metering pump, whereby an approximately tenfold dilution of the product stream was obtained. The actual dilution of the stream was determined by substituting volumetrically standardized potassium cyanide for the product stream and measuring the wave height of the resulting blend. After the dropping mercury electrode is calibrated with potassium cyanide, the concentration equivalent to the wave height measured is easily computed. Analysis of Product Streams. Initially, all continuous measurements were made with oxygen-free solution. The product stream, which was free of oxygen, was diluted approximately tenfold with oxygen-free supporting electrolyte. S o current was therefore recorded over the range of -0.60 to approximately -0.2 volt us. S.C.E., except for the small non-Faradic charging current. This was taken as the zero reference point; any deflection from this point, when a suitable potential was applied to the dropping electrode (diffusion current region of the potassium cyanide wave), was a measure of the cyanide ion content of the solution passing through the cell, which, in turn, is a function of the cyanide ion concentration in the product stream. Later it was shown that the oxygen content of the supporting electrolyte remained sufficiently constant to permit use of the cathodic peaks of the oxygen deflection, a t -0.55 volt us. S.C.E., as the zero reference point. Errors of no greater than =t2% were introduced by changes in the zero reference point due to changes in oxygen concentration. The deflection from the zero reference point, resulting from application of a suitable e.m.f. to the drop-
V O L U M E 26, NO. 7, J U L Y 1 9 5 4
1123
ping mercury electrode, was a measure of the cyanide ion content of the product stream. The deflection from the zero reference point so obtained actually contains a constant positive current (+pa.) owing to the reduction of the oxygen; this constant value may be subtracted from the apparent diffusion current-Le., &pa., depending upon the concentration of cyanide ion-to give the diffusion current due to cyanide with its proper sign (-pa.). If, instead, onc makes a plot of the concentration of cyanide ion versus current (with proper respect to sign, +Ma.), the calibration curve will cross the zero cyanide concentration line a t a diffusion current value (+pa.) equal to that produced bv the oxygen dissolved in the supporting electrolvte.
current of the cyanide ion in 0.25M sodium chloride and 0.02X sodium hydroxide, was found to increase bv 1.2% per O C. Measurement of the cyanide wave is complicated by two factors: A long flat plateau is not found in the limiting current region of the cyanide wave because of further dissolution of mercury due to the presence of hydroxyl ions, and the half-wave potential of the cyanide wave shifts by -0.030 volt per tenfold increase in the concentration of cyanide ion. The selection of a suitable potential for measuring the limiting current is, therefore, limited, and one must expect some error when measuring the cyanide ion a t constant potential. Table I1 shows the errors incurred when deviations of 1 5 0 or 60% from the desired operating level occur in a given process. The potential applied to the dropping mercury electrode, in each case, is the optimum value for each of the desired levels of operations
Table IT.
Errors Resulting from Measuring C>-aiiideIon a t Constant Potentials
Desired Operating Level 2 0m.M KCN 4 OmW KCh6 OmM K C S
Deviation from Desired Operating Level a n d Resulting Error, Yo +50 -60 +60 +2 8 -1 1 -3 2 -3 0 $6 6 -1 5% -50
CONCLUSIONS
2
I
0
-1
-2
-3
.4
-5
-6
Current. U P
Figure 3. Cell Response t o Changes in Concentration Change from 2.0 to l.OrnM KCN, US. S.C.E. Change from 1.0 to 2.0m.M KCN, Ed.c. = -0.21 volt US. S.C.E. C. Reduction of oxygen a t Ed = -0.60 volt US. S.C.E.
A.
B.
Ed.e. = -0.23 volt
...
Electrical Noise. Considerable difficulty with electrical noise was experienced whenever a continuous conducting medium (supporting electrolyte) extended between the dropping mercury electrode and the blending pump. The most satisfactory solution to this problem was to feed the solution to the cell by gravity. The diluted product stream was allowed to fall from a height of 6 to 8 inches above a funnel connected to the continuous cell. .4t any time t , breaks occurring along the falling column of liquid effectively isolated the polarograph and cell-electrode system from the metering pump. Accuracy. Under ideal conditions, the polarographic determination of cyanide ion in a given aqueous solution is accurate within approximately i l to 2% of the material present'. Because the dynamic polarograms were identical in every respect with the static polarograms, the accuracy will largely depend upon the accuracy with which the product stream can be diluted with the supporting electrolyte. The accuracy of the Zenith pump was found to be within &2% as determined b y diluting known concentrations of potassium cyanide by the method outlined. The degree of temperature control exercised on the solution pa3Ping through the cell also affects the accuracy. The diffuEion
.4 shielded dropping mercury electrode-cell system was found suitable for the continuous analysis of a flowing alkaline solution of cyanide ion. Polarograms reproduced indicate that the waves produced with solution flowing through the cell are identical with those obtained by the classical polarographic method and, therefore, expressible by the IlkoviE equation. The response time of the cell to changes in concentration of the species measured fall8 within reasonable limits. Since the indicator electrode sees no difference between static and dynamic conditions, the method could, undoubtedly, be extended to the continuous analysis of any ionic or molecular species which gives suitable oxidation or reduction waves a t the dropping mercury electrode by the classical technique. This, of course, would be limited by the polarographic complexity of the solution examined. Because the conditions under which the currents are measured are exactly defined-Le., diffusion controlled-the technique may be extremely useful in research and engineering development studies and plant process control. ACKNOWLEDGMENT
Acknowledgment is made to several members of these laboratories-particularly, R. P. Chapman, R. E. Torley, and Paul Giesecke for advice and encouragement during the course of this work. LITERATURE CITED
(1) Beckman, P., Chemistrg & Industry, 1948, 791. (2) Cooke, W. D., ANAL.CHEM.,25, 215 (1953). (3) IngOlS, R. I N D . ENG.C H E X . , ilNAL. ED.,14, 256 (1942). (4) Kolthoff, I. M., and Lingane, J. J., "Polarography," p. 541, New York, Interscience Publishers, 1952. (5) Laitinen, H. A . , and Burdett, L. W., ANAL. CHEM.,22, 833 (1950). (6) Lee, T. S., J. Am. Chem. Soc., 74, 5001 (1952). (7) Marple, T. L., and Rogers, L. B., ANAL.CHEM., 25, 1351 (1953). (8) Spoor, W. A., Science, 108,421 (1948). (9) Wilson, L. D., and Smith, R. J., ANAL.CHEM., 25,218 (1953). (10) Wise, W.S., Chemistry & Industry, 1948, 37.
s.,
RECEIVED for review January 9, 1984. Accepted April 15, 1954
