Eric Grimsrud and John Amend Montana State University Bozeman. Montana 59717
Coulometry Experiments Using Simple Electronic Devices
Electrochemistry has in the past provided the basis for some of our most commonlv used analvtical methods. It has also been shown to he a powerful tool for synthesis and mechanism studies in the areas of oraanic and inorranic chemistrv ( I ) . In teaching and in learning electroche&istry, howeve;, a real challenge is presented by one of its greatest advantages-its versatility. It can be used in so many different ways. Also, the increased availability of inexpensive, but versatile electronic devices have been a boon to electrochemistry, hut perhaps an additional frustration to those trying to learn the field. Thus, in nrenarinp " chemists who can use electrochemistrv in its modern forms, we have tried to present a laboratory experience which illustrates a modular approach to electrochemical instrument design hut which is still readily understandable to students of limited electronics backgrounds. We wish to report two experiments in coulometry which we feel have been ~articularlvsuccessful. Within these experiments, one usinr H constantcurrent and the other a controlled potential, are illustrated many of the electrochemical and instrumental principles which lead quickly to an appreciation and understanding of the many other electroanalytical techniques. The circuits and cells are as simple as we could oossiblv make them and still achieve high performance. Simplicity is considered to be essential because the average student is heing challenged with several new chemical and instrumental concepts and is unlikely to appreciate subtle instrumental sophistications a t this time. Circuit hoards and assembly instructions for each of the instrument modules shown here can he obtained through the Montana State University ScienceIMath Resource Center, if desired. Cruder, but satisfactorv~. options to these can also be used. The electrochemical principles underlying these experiments can he found in several textbooks commonly used for Instrumental Analysis (2-5). Primary attention will he given here to the insturments and their functional elements. The most important electronic device in the circuits is the operational amplifier. (The reader unfamiliar with op-amps is encouraged to consult one of many sources (2-3) describing them.) Alwavs usina circuits confiaured for operational by each 06-amp here feedback (2, $,the t&k being can he quickly deduced from two simple rules which summarize the behavior of op-amps wired in the feedback configuration. These are
OPERATIONAL AMPLIFIER Rwl 0
+
- -
81110C1
. .
Rule 1) For practical purposes, the sum paint (Invertinginput) in an operational feedback circuit is maintained at the same potential as that present at the non-inverting input. Rule 2) The input resistance of an operational amplifier is quite high, and for practical purposes, little or no current flows into the amplifier inputs. The circuits are designed so that there is only one op-amp per function. so that the iob of each OD-ampconfiauration can he individually percrivcd. For nll funrlL~ns.TexasInstrument SN71741P opcrat~onal amplifiers were used and found to he adequate. hey are very inexpensive ($.30) and are rugged enough to withstand much abuse. When wiring an op-amp for use,it is our opinion that the connections for power and offset control constitute an undesirable complication in the circuits for the students with little practical experience in electronics. For this reason, special op-amp boxes were designed to contain these other
Figure 1. The printed circuit board panel of an operational amplifier module.
nermanent connections neath inside and to allow the students to concentrate on the function of the op-amp input and output connections. The top cover of the hox is a orinted circuit hoard, shown in Figure 1, on which the box's function is schematically symbolized, and to which the necessary electrical elements are attached. Each hox has two basic op-amps to which resistors or wires can be easily attached to spring clips. Also on each are two variable potential sources which are, in reality, two additional op-amps in the voltage follower (2,3) configuration. Each box is powered by two 9-V transistor radio batteries which are mounted inside the op-amp box. The offset control (which seldom requires adjustment) is also mounted on the PC board. Because they are tidy and their function is visually facilitated, we strongly recommend their use in student labs. Two of these boxes are usually required for each experiment. Boxes with more op-amps have been made, but in some cases separately grounded circuits are required. Constant Current Coulometry (Coulometric Titration) A coulometric titration is a titration in the sense that a chemical reaeent is nroduced a t an electrode bv passine a constant current (abdut 10 mA) through the cell. he amount of reagent required to complete the titration can be deduced very precisely from the time the current is allowed to flow. In the experiment to he described here, bromine is . generated a t the anode in a solution containing KBr 2Br-
-
Rrr
+ 2r-
The bromine then reacts with the sample, hydrazine 2Br2 + H2N-NH?
-
N1+ 4HBr
The endnoint of the titration occurs when traces of Br?- heein .. to accumulate in the solution. This endpoint is very sensitively indicated hv a second electrochemical techniaue. Twin Polarizable ~ l e c t r o d e (5). s The circuit for this experiment is shown in Fieure 2. The coulometric titration circuitry in Figure 2 can beseparated into its two separate functions-the coulometer is on Volume 56. Number 2 February 1979 1 131
Figure 2. The circuit for a coulometric tination wilh a twin polarizable electrode endooint.
the right associated with op-amp 1and the twin polarizahle electrode tecector is on the left. The system associated with op-amp 1 passes a constant current through the electrochemical cell in its attempt to satisfy Rule 1 of op-amps in feedback configuration. The op-amp does this by forcing a specific current around the feedback loop and the through the 150 resistor to ground. A voltage is developed across this resistor and is of the proper polar&y such that it exactly cancels the effect of the 1.5-V battery on the voltage difference between the op-amp inputs. his feedback cirrent is not dependent on the elements present in the feedback loop. For examole. . ,durine the titration. the cell voltaee mav.varv. .(due to changes in the concentrations of electrochemically active species), hut the current passed through it will remain constant. The cell current is determined only by the 1.5 V battery and the 150 fl resistor fro. which we expect a current of 10 mA. Since for practical purposes, no current flows to the op-amp inputs (Rule 2), this is then equal to the cell current. There are several other components of importance in the feedback loop. A precision resistor and a digital voltmeter are used to measure precisely the value of the constant current. The switch shown is the "stopcock" of the titration and also controls a clock which precisely accumulates the time during which the switch is closed. We have constructed an inexpensive device which performs these operations precisely. I t is a switch which, in addition to closing the feedback loop of the coulometer. activates a constant-freauencv oulse eenerator. are counted with a hinaq-coded:decink counter These and ~rovidea measure of accumulated time. The switch. pulse generator, and the counter are housed in two separatel;oxes because the counter is used in other experiments, such as Constant Potential Coulometry to he described later. The approximate cost of building these units is $50. With some sacrifice of accuracy a stop watch and simple on-off switch will suffice for this function. For the Twin Polarizable Electrode endpoint detector of Figure 2, a small voltage (0.2 V) is taken from the voltage source on an oo-amo box and anolied across the twin electrodes. With th'is s&ll applied Giltage a current will flow in this circuit onlv when both chemical forms of a reversible electrochemicoi couple are simulton~ouslypresent in the sample solution (5). In our application this condition is met only after the endpoint of the Bra-hydrazine coulometric titration, when traces of Brp first begin to accumulate in the sample solution which always contains an excess of Br-. The detector current is measured with the current-to-voltage convertor, op-amp configuration 2, and a VOM. An important subtlety associated with the current-to-voltage converter in Fieure 2 should he anoreciated. I t mav seem that on-amo 2 cohd he eliminated simply attachihg the 10K resistor to the electrode and to eound and then simolv measurine the voltage drop across i t k i t h a VTVM. The &-amp confi&ration will provide suoerior results. however. because its inverting input is maintained a t "virtual groind" a t all times (Rule I), and therefore, the entire 0.2 V applied will always
-
b;
132 1 Journal of Chemical Education
. .
Seconds Figure 3. A plot of detector current as a function of titration time in the coulometric titration of hydrazine.
-
he maintained across the twin electrodes reeardless of the level of current flowing in the circuit. The cell required for this experiment is very simple. The two pairs of electrodes needed are made of Pt. The anode of the coulometric pair is a P t mesh cvlinder (5 cm in leneht. " .2 cm diameter) ofthe type commoniy used for electrodepositions. A P t flag (1 X 1 cm) or spiral wire would also he adequate. The coulometric cathode was a P t flag and was housed in a fritted tube so that the generated reagent bromide will not come in contact with the cathode. The same electrolyte solution was added to the fritted tuhe. The twin polarizable electrode oair consisted of one P t wire. 1 cm lone. fused into a wire coiled soft-giass tube, and another much longer (20 around the glass tube and smaller electrode. I t is essential that the negative side of the 0.2 V signal be applied to the larger of the twin electrodes so that greatest sensitivity to the limited species, Brz, is achieved. All of the above and a magnetic stirring bar were contained in a 150-ml beaker and olaced on a magnetic stirrer. A glass tuhe (2 cm diameter) served as a convenient binding post to which the electrodes were secured with rubber hands. I t is essential that the ground shown in Figure 3 for electronics associated with the twin electrodes be separate from that of the coulometric circuit. Hydrazine was found to serve very well as a sample of unknown concentration because its rate of reaction with Brg is very fast and the analysis can be done in an aqueous media. (Phenol was also tested as an unknown, hut due to its slower reaction with bromine, achievement of a stable endpoint took about 30 min. Attemoted analvses of olefins in non-aaueous solution (6) were uns~ccessfulk t h our simple system hLcause the resulting cell resistance was too high t o allow the passage of sufficient current). The hrominating solution contained 0.2 F KBr and 0.1 F HC1. The students were given a hydrazine solution of unknown concentratim, and were told to add 5 ml or this to IOU ml of their hrominatiny solution. At this point M: The the hvdrazine concentration was about 1.5 X detector current-time curve of a typical analysis is shown in Fieure 3. The extremelv sharo endnoint observed eives evid e k e to the high sensitkty of'the endpoint detection method and also to the verv fast rate of reaction between bromine and hydrazine. Prior to adding sample, the coulometric titration is commenced until the detector current reaches 30 mA. an arbitrary value. The11 thr sample 1s i l d d ~ d,~ndthr titrptiun continued until the same currenr level is reached after tht: endpoint.The coulometric equivalent to the sample is taken as the time passed between the two detector current maxima shown in Figure 3. This procedure has two desirable features. One is that errors caused by any impurities in the hrominating solution which mav consume hromine or hv traces of excess hr mine left from a previous analysis will he eliminated. Another is that in the analysis of several samples, no data need
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he collected other than the time passed between the detector current maxima. In the set of 12 determinations reported by students the relative standard deviation was f1.5%. T h e accuracy of the mean was within the f3%level, a maximum to be expected considering the purity of the source from which the unknown was made. Thus. this exoeriment illustrates well an important virtue of the ~ o u l o m e i r i Titration-the c high orecision attainable in the analvsis of relatively dilute solutes. Controlled Potential Coulometry In controlled notential coulometrv. the ootential of the working electrodeis held a t a constant yalue sifficient to cause the reduction or oxidation of a samole comoonent. The number of n ~ u l u m l ~thus s passed after the inmplr omponent has been exhaustiwlv rrducrd or midiled is used for qunntitation. We have found that with the circuit shown in Figure 4. controlled ootential coulometric analyses of high . quality . rnn In. performed, and, yet, it is readily runsrr~trtrrland untlrrstwd hv the student. Oo.amr, 1 ,c.rves ns tht, p~m*ntio:rat cornparingits input voltagks and provides the feedback necessary to equalize them by causing concentration polarization a t the working electrode. The signal generator here is simple 1000-ohm potentiometer powered by a 1.5-V penlight hattery. Initial currents on the order of 100 mA or greater are common in Controlled Potential Coulometry. ~nfortunately,simple OD-amos . . are incaoable of nrovidine this much feedback current (-20 mA maximumj. ~herefGre,the output of the potentiostat (OD-amol) is used instead to control the flow of current from n 6-\' uutn battery ihruugh s transistc,r tu the rrll In a mmner which will satisfy its inl,uts. .\s shown in Firure 4, the circuit is set for analises in;olving oxidation a t t h e working electrode. Thus, the negative pole of the 6-V hattery is attached to the collector of a PNP-type transistor. The system can he altered to perform analysis by reduction a t the workine electrode simnlv terminal . . hv, attachine.. the oositive . of the 6-V hattery to the cullcwor o f an NPK-type transistor. The trsnsisturs used a t w PNP-lvoe 2K.1502 fur uxidarions and NPN-type 2N2219 for reductions each costing about $50. T h e ammeter in the ffedhack looo . .orovides a convenient meilsure ol' the progress of earh exlwrlment. Hersuse the current rhaneei (decreases ex~unentiwllvwith time), the task of measuring coulombs canbose somedifficulty. Perhaps the simplest way would he to pass the cell current to ground through the 1fl resistor as shown in Figure 4 and continuously record this voltage drop with a strip-chart recorder. The area under the resulting curve would he proportional to coulomhs passed. We have chosen to perform this function by the use of a voltage-to-frequency converter and a counter. T h e advantages to this approach include superior accuracv. .. ease of data collection and. most imoortantlv, .. the introduction of its usefulness. he voltage:to-frequency convertor is an Analog Devices AD-456 and costs $34. A divide-hy-ten presralvr 17490, iipprcmmare cost, $.GO, waq added to ihr V-to-F outout to orevent rhe wunler firm oeertli8wmg. The counter is the same one used for the coulometric titration. We used the above apparatus for the analysis of the Fez+ content in unknown ferrous crystals by oxidation to Fe3+. This experiment is a convenient choice in that a simple cell with P t electrodes and no necessity for the removal of dissolved oxygen will suffice. The cell was similar to that previously described except that instead of the twin polarizable electrodes. a calomel reference electrode was oresent along with the lsrie Pt working n n d e and the isolaied I't flag n,untrr elerrrude. The an& should he as large as posrible t o minimize tht: time required for an analysis. 'l'he sturlt:nti were ankrd tu dissolve l-g samples in 250 ml of 1.0 F H('1 solution, and an~~
~
~
Q-H-I
conrertor
-
C0Y"l.I
-
Figure 4. A circuit for controlled potential couiomatry (for oxidation at the working electrode, as shown).
alyze 25-ml aliquots of this several times. Using as small a beaker as possible, the solution volume is increased just to cover the anode. With the solution continuouslv stirred and the voltage applied to the potentiostat set a t -08 V, the oxidation of Fez+ to Fe"+ is commenced. Starting- a t about 100 mA, a n rxpmt.ntial (lecren~ein the current with time is ubserved. Aiter abuur 20 min the ntrrenr had drcrcased 1
