John 1. Stock University
of Connecticut Storrs, 06268
I
I
A Simple LOW-currentPotentiortat cou~oietritAnalysis
Several workers (1-5) have described solid-state potentiostats made from readily-available parts. Most of these instruments can supply a current of several amperes. Tackett's instrument will also control a very small current (5). Although a large current is often desirable in electrogravimetric analysis (4), removal of major constituents in metallurgical analysis ( 5 ) , or in preparative work (G), the maximum current in a controlled-potential coulometric analysis is often below 100 ma (7,s). I n fact, cells used for coulometric analysis are sometimes of fairly high resistance, duc to a long solution path that contains a t least one small-area fritted-glass septum (9). I n such cases, the maximum output voltage of the potentiostat may be more important than its maximum output current. Figure 1 shows the circuit of a simple device that was constructed for demonstrating controlled-potential reduction. With change of transistors and some circuit modifications, oxidation processes can be followed.
s,
: Figure I. Circuit of potentiorlot. ore half-watt.
RI, R ~ , . I ~ O I < RJ. 1.5K fi. 750-ohm
Rs, as
Rr, 180-ohm
R,, lo-ohm, 2 w R ~R. ~see , text
736
/
Except where specifled, fixed re%irtorr
BI. B,, 9." B2. Bi, 3-1. QI 2N647 (RCA) Q?:XI'S 363S.k (Motorola) IC, ChSOOO (RCA) M , 0-1 ma meter B,meter ranae s w i t c h
Journal of Chemiml Edumfion
The 31/2-in. milliammeter M is larger than the remainder of the potentiostat (excluding batteries). Despite its small size, the inexpensive integrated circuit differential amplifier IC incorporates five transistors, two diodes, and eleven resistors (10). Construction is thus simplified. This amplifier and the cheap transistors Q1 and Qp are mounted in their appropriate sockets after soldering has been con~pleted. Accidental bridging of the I C socket pins, which are only about 2 mm apart, can be avoided by slipping small pieces of insulating sleeving over adjacent pins before soldering (11). No connections are made to pins 5 and 7. The current drawn at the +3, -3, and - 12 v points does not exceed a few milliamperes. Two penlight cells and a 9 v pocket-radio battery such as the common Eveready 216 can therefore be used for B8 and B4, respectively. Dsize or larger cells are used to make up BI and Bz. Besides providing part of,the IC voltage, B2supplies the ~otential-setting control R1. This control carries a paper scale to permit the selection of any cathode potential up to about - 2 v with respect to the reference electrode. The -12 v supply causes a small reverse current to flow through the electrolysis cell, so that the net cell current can be brought to zero. Meter M has a maximum scale reading of 1 ma, but is provided with resistance-wire shunts Rs and R9. The cell current is 20 times the meter reading when switch 81is open, and 100 times this reading when S1is closed. Meters of greater or lower sensitivity can be used by appropriate adjustment of the shunts. Alternatively, the meter (which is likely to be the most expensive single component) may be omitted from the construction. A multirange test meteris then used to measure the cell current, A simulated checkout can he made by temporarily This apparatus was developed with the partial support of the National Science Foundation's program for the design of science teaching equipment.
connecting a 2 watt resistor of appr~ximat~ely 120 ohms between the anode and cathode binding posts. Switch S1should first be closed. A student-type potentiometer such as the Leeds & Northrup Model 765.5 is used to simulate the pot,ent.ial to be controlled. The negative post of the potentiometer is connected to the cathode post, while the positive post goes to the reference electrode connection. R3 is placed on a near minimum (wiper towards the right in Fig. 1) and the potent,iometer dials are set a t about 0.5 v. The reading of M should first remain at zero as the knob of Ra is slowly rotated, hut should move sharply to about 90y0 of full scale when R3 develops a voltage that is slightly greater than the potentiometer setting. With R3 set to give a half-scale meter reading, the latter should swing between zero and maximum as the potentiometer slidewire dial is slowly oscillated some 20 mv either side of its original position. The circuit of a line-operated unit that replaces all batteries is shown in Figure 2. This unit is actually a
Figure 2. Line-operated power supply. are me-won and copr~itorsore 25.".
Except where specifled, rssistorr ener
diode. 12.". I.,"
lamp
ma, slur" blon
composite of two existing 12 v general-purpose regulated supplies, (a) and (b), respectively. Output potential dividers, Rla - Ru and Rls - RI9respectively, have been added. A further addition to (a), which has to supply the varying demands of the cell, is the large capacitor Cz. The simpler supply (b) can easily provide the small and approximately constant currents taken from the negative posts. Coulometric Analysis of 22-Dinitropropone
The polarographic wave height of 2,2-dinitropropane in dilute sodium hydroxide solution indicates an uptake of only two electrons per molecule. One possible reaction is: MenC(NO&
+ 2e
-
(MelC.NOz)-
+ NO;
The substance in parentheses is the anion of the aci
form of 2-nitropropane, and is not polarographically reducible. Support for this react,ion can be ohtained from the controlled-potential coulometric reduction of 2,2-dinitropropane, followed by the detection or determination of one or both of the reaction products (19). This example is attractive for teaching purposes, hecause nitrite ion at low concentrations is easily determinable. A tall-form 100 ml beaker can be used as a coulometric cell, as shown in Figure 3. The 6-hole rubber stopper carries guide tube A, anode compartment B, pH meter-type saturated calomel electrode C, and contact tube D for making connection to the mercury pool cathode E. A vent hole permits liquids to be introduced or withdrawn, while the sixth hole is for the stirrer F. The small spiral stirrer head just touches the mercury pool. Guide tube A is bent a t right angles at its center and allows the open end of the 0.038-in. diameter polythene nitrogen inlet tube G to he positioned at any desired depth in the solution. The anode compartment is a cut.down seal- Figure 3. Mercury-cothode elecing tube (Corning 39570) tro'Ys's with a 10-mm. diamet,cr medium porosity fritted disk flush with the lower end. A rubber stopper with a gas escape groove carries the tube upon which the platinum foil anode is mounted. A 5-ml capacity hydrogen-nitrogen coulometer similar to that described by Page and Lingane (13) was used in most of the present experiments. Procedure
1. Assemble the cell as shown in Figure 3. Pour mercury through the vent hole to fo1.m a pool that completely covers the platinum wire of thc pool contact. Measure 50 ml of approximately 0.1M sodium hydroxide into the cell, then a t once add the same solution to the anode compartment until the level is the same as in the cell. Thrust the nitrogen inlet tube well down into the solution and allow bubbling to continue until the experiment is terminated. 2. Fill the micro gas coulometer with 0.1M hydrazine sulfate and saturate the solution with gas by a preliminary electrolysis (13). Connect the coulometer and the cell in scries and attach to the appropriat,e binding posts of the potentiostat. The calomel electrode lead goes to the "Ref." binding post. 3. After bubbling has continued for about 25 min, switch on the stirrer, which should agitate the mercury pool without causing appreciable surging. Switch the potentiostat to the 20 ma range1, set the potential at
'The eell current depends upon eell geometry, rate and vigor of stirring, ete. If the meter needle goes off-scale, at once switch to the 100-ma range.
Volume 45, Number 1 1 , November 1968
/
737
-0.80 volt and apply power2. The "residual current" indicated by the meter should not he more than about 1 ma. 4 . Tap the bottom of the coulometer to dislodge any bubbles that are clinging to the electrodes or walls. Note the initial reading. 5. Pipet 1.00 ml of 0.100M 2,2-dinitropropane in approximately 80% ethanol (1.34 g/100 ml) into the cell, starting a seconds timer when about half of the solution has been delivered. The current should rise abruptly' and then begin to fall. Take pairs of cnrrent and time readings a t intervals of about l ma. Terminate the electrolysis after 45 to 50 min, when the current should have fallen to or near the residual value. Note the final conlometer reading, the jacket temperature, and the barometric pressure. 6 . Withdraw 5.0 ml of the cell solution, transfer to a 100 ml volumetric flask, make up to the mark with water and mix well. Determine the amount of nitrite present in an aliquot of the diluted solution and calculate the total amount of nitrite formed during the electrol~sis.~ 7 . Plot the logarithm of the current against the corresponding time to obtain a linear graph. Find the slope (k) of the graph and extrapolate to find the zerotime current (&). Calculate the number of coulombs (Q) used in the electrolysis from Q = -i0/(2.303 X 10-3k). Although this method is not very accurate, it is useful for checking and gives a result for an electrolysis that is incomplete. The typical graph shown in and Q Figure 4 gives io = 16.6 ma, k = - 3.99 X = 18.1 coulombs (theoretical, 19.3). 8 . From the "standard temperature and pressure" volume (V) of gas collected during the run, calculate the number of coulombs (Q') from Q' = V / 0 .173R4. For the run partially depicted in Figure 4 , the values obtained were V = 3.58 ml and Q' = 18.6 coulombs. The When using batteries, the insertion of a &pole switch allows runs to be started and stopped without disturbing the battery connections. If s. switch is not used, make the z!z 3-volt connections first and the -12-volt connection last. Disconnect in the reverse order. aSpectrophotometrie measurement by the sulfanilic acid-lnaphthylamine (14) or the sulfanilamide-N(1-naphthyl). ethvlenediamine (16)method is suitable. he aqueous -&or pressure of 0.1M hydraeinesulfate must be deducted from the barometric pressure (IS). For a temperature of 2 5 T , a deduction of 24 mm of mercury is approximately correct.
738
/
Journol of Chemical Mumfion
100 micromoles of 2,2-dinitropropane placed in the cell yielded 96 micromoles of nitrite. 9. Take the cell apart and thoroughly rinse all components. Wash and recover the pool mercury, which may be used repeatedly provided that it does not exhibit "tailing." Literature Cited (1) WAD~WORT~, N. J., Analyst 85,673 (1960). F., AND DAVIS,J. B., Anal. Chem., 36, 11 (2) LINDSTR~M, (1964). (3) TACKETT, S. L., AND KNOWLES, J. A,, J. CHNM.EDUC.,43, 428 (1966). J. J., "Electroanalytical Chemistry," (2nd ed.), (4) LINGANE, Interscience Publishers (division of John Wiley & Sons, Inc.), New York, 1958, Ch. XV. J. J., 02). C Z ~Ch. . , XVI. (5) LINGANE, (6) ALLEN,M. J., "Organic Electrode Processes," Chapman & Hall, Ltd., London, 1958,p. 37. J. J., op. cit., Ch. XIX. (7) LINGANE, ( 8 ) KIES,H. L, J . Eledraanal. Chem., 4,257 (1962). (9) MEITES,L., Anal. Chem., 27, 1116 (1955). (10) "RCA Linesr Integrated Circuit Fundsmentsls," R d i o Corporation of America, Harrison, N. J., 1966, p.106. (11) GREENLEE, L. E., Radio-Eledmmics, 38 (7), 66 (1967). J. T., J. Chem. Sac., 1957,4532. (12) STOCK, J. J., A n d . Chim. Ada, 16, 175 (13) . . PAGE,J . A,, AND LINGANE, (1957). A. L., "Quantitative Analysis (14) DAY,R. A., AND UNDERWOOD, Laborat,ory Manual," Prentice-Hall, Inc., Englewood Cliffs, N. J., 1958,p. 149. (15) MILTON, R. F., AND WATERS, W. A,, "Methods of Quantitative Microanalysis," (2nd ed.), Edward Arnold, Ltd., London, 1955, p. 328.
O T ' " ' ~ " ' ' ' ~ " " ~ " 0 503 m 1 x 0 TIME, SEC.
Figure 4.
Eledrolyris c v n e f a 100 micromoles of 2.2-dinitropropane.
