Chemical Instrumentation S. 2. LEWIN, N e w Y o r k University, Washington Square, New Y o r k 3, N. Y.
T h i s series of articles presents a survey qf tlie basic principles, characteristics, and limitations of those instruments whirh find important applications i n chemical worlc. The emphasis i s on commercinll!, available equipment, and approrimate prices ard quoted to show the order qf magnitudeof cost of fhe uarious tjlpes of design and constrz,ction.
16. Electroanalytical Instrumentation El~rtroanalysisis that ares. of analyt,icnl chemistry in whieh the amount or eoncentration of a component is sensed in t,erms of either (1) the effect of that romponent upon an impressed voltage, (2) the effect of passage of current through the sample in changing the chemical state of the component, or (3) the effect of the component upon an electrode inserted in the sample. This field has shown tremendous vitality for many years, and it has participated fully in the elaboration of new and improved techniques based upon recent advanres in electronics and t,he srienre of instrumentation.
resistance s t the elrrtrode interface as a iunrt~on of time at c o n ~ t ~ ncurrent. t Since E = I X R, this is equivalent to the
2. When current flows from a power supply through an electrode into a chemical system, through that system to a second electrode, and finally back t o the power supply, electrorhemied ctanges are in general produced a t the electrode/ system interfaces, and concentration changes are produced in the bulk of the system. Electrogmuimetry is based upon the quantitat,ive deposition of a desired component from solution on an electrode, and weighing of the separated phase. Coalometry involves the determination of the total amount of charge that must, he passed through the system in order t o carry an electroehemiral reaction t o completion. The measurement of total charge ( = integral of current as a function of time) takes the place of the weighing operation which is necessary in eleetrogravimetry.
Types of Electroanalytical Techniques 1. When a voltage difference is applied t o a. rhemical system, a. current is observed t a flow which is determined by the impedance to the passage of charge through the eleetrode/system interfaces and the number and mobility of charge carriers present, in the bulk of the system. [It is assumed bhat the external resistance in the circuitry of t,he measuring equipment, or the intrrnal resistanre in the power supply, are not, limiting factors, since these variables can be adjusted through proper choicr of t,he instrumentation]. Conductmetry comprises those techniques in whieh the magnitude of the current is limited solely by the number and nature of the charge carriers in the bulk phase. The various forms of oolta~nmetryrepresent techniques in v~hihichthe currents flowing a t the applied voltages of interest are limited hy the state of polarizration existing st the electrode/system interface(8). Some of the suhdivisions of the general field of voltammetry are: Polaroyraphy-variation of current as a function of applied voltage in a system a t ronstant composition, containing a t least one polarized electrode. The current is measured a t such a time after the applicat,ion oi the voltage that the change of current with time is not fundamental to t,he interpretation of the data. Amperomelry-variation of current as a function of the composition of the system at a constant voltage of such magnitude t,hnt one electrode is polarized with respect to the component oi interest. Chronopotenlio~,,etry-vx~.iat.ion of the
Constant-potential Volla?,~ntet~.y-vsriation of eurrmt as a function of time a t constant applied voltage. The general relat,ionships hetween current, voltage, and romposition of a. polarisahle system nrr shown in the threedimensional representation of Figure 1.4; and the relationships bctncen current,, voltage, and time are shown in Figuro In. The various types of tm.0-dimensional variations observed in the diverse voltammetric t,echniques (e.g., current versus voltage; voltage versus time; e t a ) may be deduced from these figures by visualiaing the curve of intersection of an appropriate plane with the solid surface.
Figure I. A. Three-dimension01 representolion of the relotionrhip between current, voltage, and composition of a system when voltom. metric techniques are applied. The intersection with the solid surface of a plone perpendicular to the "yoOxidized" axir yields the current-vdtage curve obtained in polorography with a solution of conrtont camposition equal to the intercept of the plone on the "% Oxidized" axir. A plane corresponding to zero current interreds the d i d to yield the vruol potentiometric titrotion curve. A p l m e correrponding to conrtont voltage interrech the solid surface to give the omperometric titrotion curve. For a more detailed explanation, see Reilley, C. N., Cwke, W. D., and Furman, N. H. Anal. Chem., 23, 1 2 2 6 119511. B. Three-dimension01 representotion of the relationrhip between current, voltage, ond time in volt.mmetry. The intersection of o plane corresponding to eonrtont current giver the chr~nopotentiometric titation C w Y e 1e.g.. curve od in the Figure). For more detoilt, see Reinmuth, W. H., Anal. Chem., 32, 1 5 0 9 (1 960).
variation of the potential difference existing across the rhemiral system as a function of t h ~ duration af the flaw of thc constant rurrent through the system.
5. Whenever s n interface is formed between two phases, a. potential is generated that is a function of tho composition of the two ohases. The difference of potential between two such interfaces ronstitutes the EMF of an elertrorhemiral cell. Potentiomtry is based upon the quantitative relationship between the E M F of a cell, and the concentration of a component of interest. At least one of the elertrodes must he selected so that its potential is sensitive to the desired component.
Principles of Instrumentation All of the elertroznelytiritl techniques involve fundamentally similar instrumentation, the differences among them consisting principally in the kind of programming and control required to permit the necessary data. observation. The basic features of electroanalytical instrumentation are shown srhematically in Figure 2. The principal components llro ( I ) a powor supply for impressing a voltage or rurrent on the chemical system, ( 2 ) a controller or programmer, either for maintaining a constant impressed signal, or ior varying the voltage according t o a predptermined function, and ( 3 ) detection rir-
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Chemical Instrumentation ruitry for amplifying, modifying, and rending-out the chemic~lly-significantsignnl.
CONTROLLER PROCRAMMER
u Figure 2. Fundomento1 components of electraon.1ytic.l inrtrument.tion.
power supply is not needed, since the chemical syst,em itself generates the voltage signal.
Power Supplies
A v a r i e t , ~of Dower snoulies is used in
power pzwks. I k poner wpplies, such as hatterics or more elnborste rectified electronio power parks, are usually not employed in the design of eondurtometric instruments, since extensive electrolysis and polarization effects oemr a t the electrodes when a steady voltage is applied t o a cell. However, s method of dr ronduetanee measurement is nevrrthrless possihlo, if a fomelertrodr rrll is employed. In one example of this approach, a high dde voltage is employed, and the conductance cell is placed in series m-ith a high resistance, R. IJnder these randitions, a pract,ieally constant current will flow through the cell, for the eell resistance, r,,rr, including the resistances a t t.he two current-carrying t:lnctrode/solut~ioninterfaces, is effectively negligible compared with the t,ot.al resistame, and:
If r. second pair of electrodes is inserted1in the eell, as shown in Figure 3, the IR-drop betwecn thoso auxiliary electrodes can he sensed hy means of a voltmeter. Since I is constant, thc voltmet,er readings are proportional to the resistance (i.e., inverse of eondurtance) of thc chemical system. Dc pox-er supplies are, on t,he other
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Journal of Chemical Educotion
Chemical Instrumentation
Figure 3. The dc condudonce technique. Electrodes A ond B a r e the current-carrying eiectmder; C and D ore the tensing electrodes. R i s very large compared to reell.
hand, essential in eoulornetry and electrogrsvimetry. Simple, dry-cell batteries may be employed, hut sinac the resistance of the electrolysis cell increases as the electrodepaition prweeds, the cell current decreases proportionately. This necessitate> long periods of electrolysis for quantitative separations, m d tedious recording and integration of currents if coulomet,ry is involved. These difficulties can he circumvented by the use of electronically controlled power supplies (see below).
RCELL
CCFLL SUPPLY
47' DETECTOR
Figure 4. Schematic diagram of a simple ac Wheohtone bridge circuit. The balancing resistor, R,. and copacimr, C, ore adjusted until the null detector show a minimum rignol. At this point, C, hor balanced CCln,and the conductance of the cell contents may be determined from the value of R,
Ae power supplies are employed in conductance bridges, t,he hnsic circuitry nf which is shown in Figure 4. Since the conductance cell contains nleetradrs sepa-
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Chemical Instrumentation rated by a dielectric, there may he appreciable cell caparitnnce, and provision is generally mado for hnlnnring out this capacitance, and provision is generally made for balancing out this capacitance by means of a variable capacitor in parallel with one of the arms of the hridgc. The ac resistance, or, more correctly, the eapaeitioe reactance, X,, of a rnpacitor, C, is inversely proportional to the frequency, ,f, of the alternating signal, as shown hy the equstion:
where X , is in ohms, if J" is in cyrlcs per second, and C is in farads. Thus, for a. given cell design, the capacitive reactance decreases as j increases. The equivalent. circuit of n eell with immersed electrodes is shown in Figure 5A; thr rcll r e p x i t ~ n e e is in parallel with the rell r~sistance. If the cell capaeitanre is not balunrrd out in the process of achieving the resisbancn halance in the bridge, the resist,t~nrevaluc obtained for the eell will not bc cqual to the dc resistance of tho ccll contents. At low frequenrics, such as GO rps, the capacit,ive reactance is generally vcry l s r ~ e[if C = 100 picofarads = 100 mirromicrnfards = 10-'Ofnmds, and j = 60 rps, S,= 26.6 megohmsl and the ellertivr resistance of the parallel combination of (' m ~ dR is practically equal to that oil3 :tlonc. However, a t audio frcqwnr,irs ( 1 - 100 kilo-
egrles prr acrond), the cnpactiv~react,:~nw may be import,nnt. At radio froqurneirn, the capacitive rcactanm tends to br so small t,hilt rdinhle oondurtanre moasurrments c:tn hc uhtnined without even insrrb ing t h r elcrtrodes in the solution to hr measured. Figum 5B shows t,hc equivxlent cirruit, for n wll with cxternnl clcrtrodes: if (; = 1 picofarad and j = 10 mcgxryclrs per seronrl, S,,is only 16 kilahms.
Figure 5
A.
Equivalent circuit of a coninternal lirnrnerredl eledrodes. 8 . Equivalent circuit of a conductonce rell with extern01 eledroder.
d~ctoncecell with
Conductilnre bridges are designed for op~rat.ionwith ac powcr supplies of ill1 types, ranging from 60 c / s to 30 Mr/s. The rhoiec of frequency is determind t,? the cell design and t,he cleetrirnl charnrtrristirn of t,he n . s t m ~to lr, mmsureil.
The typcs of power supplies e~nphyrxl in polarography range iron, simpk,, drycrll bnt.trrios shunted by voltage-dividing resistors vieldine veriable dc vnlt,:txrs. .. . 11, ., rnudio-frrquoncy sawtooth, squaw, and sinc K:WCsignal gcnerstors. ~
~~~
Power Supply Controllers I n onlcr 10 maintain a constan! potcntiirl at n wurking electrode in m elwtrolysis r d l , it is usually nec.essary to add ;I third, n
