Topics in..
.
Chemical Instrumentation
feature
Edited by GALEN W. EWING, Seton Hall University, So. Orange, N. J. 07079
These articles are intended to serue the readers o f ~ mJOURNAL s by calling attention to new developments i n the themy, design, ar availability of chemical laboratory inslrumentation, or by presenting useful insights and explanations of topics that are of praclical imporlance to those who use, OT Leach the use of, modern instrumentation and instrumental lechnipes. The editor invites correspondence from prospective contribulars.
LXXIII.
Reference Electrodes
Roy D. Caton, Jr., Department of Chemistry, The University of New Mexico, Albuquerque, New Mexico 87131 Within the past ten years there have been remarkable advances in the field of patentiometry, primarily brought about by the appearance of a host of ion-selective electrodes. Patentiametry and other electroanalytical techniques applied to nonaqueous systems have also seen phenomenal activity in the past few years. A perusal of recent reviews (1-3) on the above research areas certainly shows that much interesting work is being done. The exciting appearance of new sensing electrodes and new electrochemical findings in nonaqueous solvents seems, in a sense, to overshadow the silent partner of virtually all electrochemical measurements-the reference electrode, against which other electrode measurements are ultimately referred. There are few electrochemical techniques, whether they be static or dynamic in nature, where the POtential of an electrode is not a vital part of the experiment. Its value relative to a fixed reference potential must be known with an accuracy dictated by the experiment, and any inaccuracy in its measure-
Figure 1. A commercial reference electrode. (Fisher Scientific Co.) A. plastic cap 8. rubber sleeve covering filling hole C. reference element 0. KC1 solution E. porous plug
ment must not be attributable to the reference electrode. The author does not mean to imply that practicing electrochemists take their reference electrodes for granted, for they know only too well its importance. Indeed, when employing electrochemical techniques in nanaqueous solvents, the search far and testing of a suitable reference electrode may occupy a significant portion of the research, if it is not a separate problem in its own right. However, a person who is responsible for the monitoring of dilute ion species in ground waters, to give an example, may he very involved with the selection and use of specific ion electrodes and their maintenance, hut may tend t o take the reference electrode for granted. In part, this is due to the fact that commercially available reference electrodes can take considerable abuse before they fail, whereas the sensing electrode may need frequent cleaning, re~lenishment of electrolyte, and calibration. In any event, the correct choice of reference electrode for a particular experiment, a firm respect for its function, and some concern for its maintenance will greatly enhance the reliability of electrochemical measurements. An ideal reference electrode should possess the following properties: (1) have a stable potential; (2) meet the demands of charge transfer imposed by the measuring instrument without changing its potential (be nonpolarizable); (3) return to its fixed reference potential after accidental polarization; (4) obey the Nernst equation for some species in solution; and (5) if it is an electrode of the second kind, the solid compound must be only sparingly soluble in the electrolyte. The degree to which these roper ties must be adhered to depends upon the experiment, of course. Examples of electrodes which possess all of the above properties for use in aqueous solutions are the saturated calomel electrode (SCE) and the silver-silver chloride electrade (Ag/AgCl). In the majority of applications the internal element of a reference electrode (a silver wire coated with silver chloride, for ex-
Following undergraduate and master's work a t Fresno State College, and several years with the Army a t the Edgewood Arsenal, where he developed analytical methods for nerve gases and their intermediates and assisted in setting up a mobile laboratory, Roy Catan undertook doctoral work with Prof. Harry Freund a t Oregon State University, and was awarded the degree in 1962. Since then he has been a member of the faculty a t the University of New Mexico. Dr. Caton's research interests lie in the fields of electrochemistry in fused metaphosphate glasses, organic electrode mechanisms, the determination of diffusion coefficients by non-electrical means, and ionexchange separations in nonaqueaus media. He has put much effort into a new course for freshman chemistry majors and minors, including quantitative analysis and instrumental methods. ample) is isolated from the system being measured by means of a salt bridge which is an integral part of the electrode assembly, as shown in Figure 1. The salt bridge consists of the internal electrolyte necessary for the function of the reference electrade (a chloride solution in the case of the Ag/AgCl). An additional, external, salt bridge may be interposed between the electrode salt bridge and the test solution if the internal electrolyte must not contaminate the test solution. Such salt bridges are constructed to minimize, but not completely eliminate, the flow of electrolyte between the bridge junctures. The combination of a reference electrode salt bridge and the external bridge is called a "double junction" bridge. When solutions of different electrolyte composition are brought into contact a liquid junction potential arises. The magnitude of this potential depends upon the kinds of ions and their concentrations on each side of the junction, and it is caused by the diffusion of these ions at different rates and quantities across the junction
(Continued onpageA5741
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resulting in a separation of charges. The types of junctions and means of estimating their potentials have been discussed in detail by Lingane (4) and Bates (5a), and they are of considerable concern to the designers and manufacturers of reference electrodes (6, 7). The junction potential of a given reference electrode salt bridge may be considered as a part of the total reference potential. For example, the cell configuration for a SCE used with a test electrode may he written as: HelHe,CI,lKCl
metric titration, no standardizations need be performed unless physieochemieal data are to be obtained from the titration curve. Again, E should remain constant, hut the demands upon its constancy need not he as strict as in the other two experiments. Unfortunately, literature devoted strictly to the understanding, construction, and evaluation of reference electrodes is rare. Much of the knowlege which electrochemists have gained on the suhjeet is either relegated to brief discussions in publications
(sat'dll test solutionlter~telectrode
E,,,
=
E"
REFERENCE HALF CELL/KCI.,I,~I,,/ H + (activitv = 1)IH". Pt.
+ E"'
with the various contributions to the total cell potential indicated. Thus, another property for most reference electrodes is that the junction potential he stable and reproducible if E,.u is to he a reliable measurement. When making potentiametric measurements there are three basic experiments which are usually performed: (1) observation of a cell potential resulting in the determination of ion activity coefficients or cell reaction thermodynamics, (2) observation of a potential for the determination of test solution ion activities or concentrations, and (3) observation of a change in potential for determination of kinetic parameters or an equivalence point. In experiment (1) the absolute value of E" must be known if E"' is to be knawn, or E" must he forced to approach E' by adjusting experimental conditions or performing a series of experiments whose results can be extrapolated to eliminate Ej..,ti,.. In experiment (2) the cell can he standardized with knawn activities or eancentrations of the species of interest and an accurate graphical interpolation of the species concentration in the unknown test solution can he made. E" must be constant among all standardizations and between standardizations and unknown determinations, and any variations in
treating other subjects, or simply passed on by teacher to disciple in the laboratory. Mast physical chemistry laboratory texts have discussions and recipes for the construction and use of the normal hydrogen electrode (NHE) and the SCE, hut devote little space to the why of the matter. When the monograph edited by Ives and Janz (8) appeared in 1961, much of the mystery about reference electrodes was dispelled. Covering a large amount of material from over one thousand sources, the monograph contains chapters on all the important aqueous electrodes as well as chapters on nonaqueous and fused salt applications. A treatment of reference electrodes may he classified into two sections, aqueous and nonaqueous applications. The above discussion was applicable to both types, hut the following sections will treat the specifics of each type separately.
AQUEOUS REFERENCE ELECTRODES The reference electrodes used in the majority of all work continue to be the normal hydrogen electrode (NHE), the SCE, and the AzIAgCI. The NHE is used primarily in thermodynamic studies a n d ~ f o r the precise calibration of pH buffers,* and
Table 1. Data for the Common Aqueous Reference Electrodes
"
E" vs.
Electrode SCE Ag/AgCl, 1M K C l AgIAgCI, 4 MKC1 Ag AgCl, sat'd LC1 (4.17M a t 25'C) Hg/HgSO1, sat'd K~SOI
E" vs. N H E a t (dV/ 25"C,O dt)j.,c mV mV/"C 244.4 235.8 199.8
-0.70 -0.63 -0.78
198.8
-0.78
656
smin --- ...
wuE
E"vs. SCE,d mV (25°C)
0.1M NaOH, mV (25'C)
0 to 70 Oto80 Ota80
0.0 -8.6 -44.6
0.0 -12.1 -47.9
0.0 -1.6 -44.5
-45.6'
-45.6e
-45.6*
-10 to 100
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The potentials include any junction potential existing between the KC1 solution and the NHE. The SCE has long been one of the most reliable and popular reference electrodes, hut it suffers from two minor disadvantages. It has a rather large temperature coefficient caused chiefly by the changes in the solubilities of potassium and mercurous chlorides, and a t higher temperatures the mercurous chloride disproportionates slightly to yield mercury and HgCI2. Upon abruot cooline the electrode is no loneer a t equilibrium with regard to the soluhifity of the calomel, and the recombination of the disproportionation products and the return to equilibrium is slow. Nevertheless, compact and robust stable electrodes can he prepared, and their potentials are known to a millivolt or less from 0 to 100°C. In precise work the electrode, or the entire cell, should be kept a t constant temperature unless a thermoeompensatar is employed. The subject of temperature coefficients of reference electrodes and sensing electrodes, and temperature compensation, has been reviewed by Negus and Light (9). The AgIAgCl electrode has much better high temperature eharacteristics than the SCE, it can he made in very small, compact confiwrations, and it c& * A comprehensive discussion of the hydrogen electrode as reference has been puhlished by Biegler and Woods in this journal.7a (Note added in proof.)
&E&
Recommended temp. range, "C
0.1M HCI, mV (25°C)
0 t o 100
"Data for the SCE and Ag/AgCl electrodes, with the exceptionoftemperature range, taken from ref. (9, 11); Hg/HgSO+ data from ref. (16b). Temperature ranges are conservative values; higher ranges possible dependine uoon electrode construction. OE" = Einternsl + EjUncti,.for the cell NHE/KCI ( M )SCE or AgJAgCl. Temperature coefficient of electrode when both NH E! and electrode are s t the same temperature. The temperature coefficient of the NHE is defined as zero. Using the appropriate KC1 bridge electrolyte, i.e., 1Mfor the 1M AgIAgC1, etc. Calculated value. No junction potential should exist for the cell AgIAgCI, sat'd KCI/O.l MNaOH or 0.1 M HCl/sat'd KCI, HgzClz/Hg because the two junctions are identical.
"
the other two shoulder the bulk of most applications. All three of these reference electrodes are treated definitively in Ives and Janz (8a, 8b, 8c), as well as other favorites such as the Hg/HgSO,, Hg/HgO, and Ag/AgX (X = Br, I) electrodes. The calomel half-cell may be used with saturated KC1 internal electrolyte (SCE) or with 1 M or 0.1 M KCI. Likewise, the Ag/AgCI internal may be found with varying concentrations of chloride ion. The potentials of these electrodes are knawn for cells' without transference, ones in which the junction potential is virtually nonexistent, as well as for cells having a salt bridge and a liquid junction. Table 1 lists the standard potentials of the most common reference electrodes referred against the NHE for cells having the configuration:
A Figure 2.
Reference electrode liqud junctions.
(Fisher Scientific C0.I A. Cracked beads:
8 . sleeve
be used in just about any orientation. Most industrial process electrodes have Ag/AgC1 internals because of the above desirable properties. Far precise work the methods of preparing the electrode given in Ives and Janz (8e) should be followed. An excellent review on the preparation and properties of the SCE and the Ag/ AgCl electrodes, along with a discussion of temperature coefficients and junction materials, may be found inBates ( 5 6 ) . More should be said about junction potentials, the design of bridge tubes or porous plugs used in commercial electrodes, and the maintenance of the bridge tubes. Until the availability of reliable specific ion electrodes and the expanded scale pIon meter, the demands upon commercial and homemade reference electrodes were not severe. Most of the reference electrodes were used in pH measurement and potentiometrie titration applications, and meter readings were usually made to +0.02 pH units and &2 mV respectively. Problems of irreproducibility and drift of reference electrode potentials were usually not noticed unless some grass malfunction occurred. Today it is quite common to make measurements to within +0.002 pH unit and to within +0.1 mV (about 11600 of a pH unit). Small bias ~otentialsand potential drifts are easily noticed under these conditions, and usually the trouble is due to a malfunction of the liquid junction salt bridge. Orion Research has commented on the problem in their Newsletter (6), and the subject has also been discussed by Covington (10) in a National Bureau of Standards monograph, and in
the Beckman Instruments Technical Report by Westeott (7). There are several causes of erratic junction potentials, and all of these have to do with plugged junctions or a poorly constructed junction: (1) for reference electrodes having saturated electrolytes, solid crystals of the electrolyte may form in the bridging element; (2) the use of saturated KC1 in the Ag/AgCI electrode increases the solubility of AgCl due to the formation of chlomcomplexes. (The solubility of AgCl in saturated KC1 is about 1.0 g/l, whereas in 1 M KC1 it is about 14 mg.) When the relatively high concentration of silver in the saturated KC1 comes in contact with solutions containing hydroxide, iodide, or sulfide, immediate precipitation of the less soluble silver salt occurs. The precipitation may also occur if the bridge solution is merely diluted with water, due to the dissociation of the chlorocomplex; (3) precipitation of the potassium ion by perchlorates getting into the junction; and (4) clogging of the junction by suspensions in the test solution. By using a mare dilute internal electrolyte (1) and (2) may be minimized, whereas (3) and (4) may be minimized by proper design of the junction to allow a positive flow of salt-bridge electrolyte. The use of a double junction, where a second salt bridge is interposed between the electrode and the test solution, also helps to eliminate (I), (2), and (3). A knowledge of these pitfalls has influenced manufacturers to design their electrodes with a variety of liquid junctions. Figure 1 shows a typical reference electrode with a porous ceramic plug in the tip, and Figure 2
M-
INNER RESTRICTOR
ISOLATION FLEXlBLE ONE SELF-CLEANING TIP
Figure 3.
Y
-
Process reference electrode ( 7 7 ) .
shows other means of establishing the liquid junction. Consideration of the desirable properties of liquid junctions led Light (11) to design the process electrode shown in Figure 3. A steady, but small, flow of electrolyte into the test solution is maintained, and the flexible tip furnishes a contact area which is sufficiently large to permit a well-defined junction. The inner restrictor limits the flaw of electrolyte from the electrode, but it is sufficient to prevent the test solution from entering the isolation zone. All liquid junctions must allow a flow, however small, of the internal or bridge (Continued onpageA576)
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Chemical instrumentation
Figure 4. Laboratory reference saturated calomel electrode ( 1 1 )
electrolyte into the test solution. This flow may take place by diffusion a t one eatreme, or i t may consist of s streaming flow a t the other extreme. To ensure that the flow tokes place in the direction of the test s o l u t i o ~and not from the test solution into the electrode, the electrode electrolyte level should always be higher than the test solution leuel, and any filling hole or sidearm on the upper part of the electrode should be open to the otmosphere to maintain pressure equalization. Both thk inner and outer junctions of a double junction should meet the requirements of good liquid junctions. Aside from a liquid junction malfunction, the internal reference element can malfunction or simply become broken. Troubleshooting to determine the cause of drift or a large bias potential is quite sim-
ple. Westcott (7) recommends the following procedure: The questionable eleetrode and one which has the same type of internal are connected to a pH meter or a Poggendorf potentiometer, and millivolt readings are taken with bath electrodes placed in a buffer solution and then in a KC1 solution. If the potential between the two electrodes in each of the solutions does not differ by more than *5 mV the electrode is good. A potential of zero (+5 mV) in the KC1 solution coupled with a nonzero ( > +5 mV) reading in the buffer indicates a faulty junction, and readings greater than zero in both solutions indicate a faulty internal. Light (11) has proposed a test sequence which is more stringent, and his procedure should he used for accurate work with expanded scale meters. Using a laboratory standard SCE, shown in Figure 4, the suspect reference electrode is measured in each of three electrolyte solutions: (1) KC1 solution of the same concentration as the electrode internal electrolyte, (2) 0.1 M NaOH, and (3) 0.1 M HCI. The potentials observed are then compared with established values for the electrode in each of the three electrolytes. If the millivolt readings observed in the acid and basic solutions agree with established values, then the liquid junction is good. If the readings observed in the KC1 solution agree with established values, then the internal element is good. A contaminated or partially plugged liquid junctmn will cause large deviations to be observed in the acidic and basic solutions. Acceptable tolerances for the readings will depend upon the application involved, but agreement to within +1 mV may be observed under laboratory conditions. The last three columns in Table 1 list the established values for the difference potentials observed by Light. It is goad practice to have a laboratory reference SCE around to perform the traubleshaating described above. Glassware similar to that shown in Fimre 4 is avail-
Figure 5. Reference electrodes for student use. I. AaiAaCl internal. -, A. copper wire 5. Solder joint C. silver wire coated with AgCl D. epoxy seal I I. Assembled reference electrode. A. Ag/AgCI internal 5. KC1 Solution C. flexible tubing filled with KC1 solution. The ends are plugged with Gooch asbestos or cotton wadding i l l . Assembled reference electrode A. Ag/AgCI internal 8. KC1 solution C. glass tube with frltted glass disc D. vent IV. Reference electrode with self-contained Salt bridge. A. Ag/AgCI internal B. rubber sleeve C. KC1 solution D. glass tube with Gooch asbestos or cotton wadding junction in tip V. External salt bridge (to be connected to electrode 11). A, flexible tubing with asbestos or cotton
-
Plugs 8. inert electrolyte
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Journal of Chemical Education
able from several &ply houses, and one can construct the SCE according to established procedures given in Ives and Janz (8b) or Bates (5b). Alternatively an SCE with a replenishable salt bridge may be purchased as noted below in the discussion on commercial reference electrodes. For student use homemade Ag/AgCI electrodes may be used to advantage hecause of their ruggedness. Figure 5 shows the electrode components which have been in use in our undergraduate analytical laboratories for over six years. The silver internals are made by ehloridizing the silver wire far about 30 minutes in 0.1 M HCI a t a current density of 1 mA/cm2. Before chloridizing, the wire should be cleaned with concentrated ammonia and washed well with distilled water. The electrodes are washed well and stored in distilled water for several days after chloridizing. They are t h e n immersed in a beaker filled with 1 M KC1 saturated with AgCl and tested against a commercial Ag/ AgCl electrode dipping into the same beaker. Electrodes exhibiting bias potentials greater than zt2 mV are cleaned again and rechloridized. The reliable electrodes are then mounted in the various assemblies shown in the figure. The addition af the salt bridge will contribute a small junction potential, and the assembled electrodes should he checked to be certain that stable mV readings are still obtained. The assembled electrode bridge should always be mounted with its liquid level above the test solution level. The salt bridges should be checked, cleaned, and refilled when necessary.
drogen electrode and the Ag/Ag(I) eket r d e find greatest use. Butler lists some 1W applications of the silver reference electrode in a large number of aprotic solvents. Construction of room temperature nonaqueous electrodes parallels that of aqueous electrodes, and in many applications a dry Ag/AgCI commercial electrode can be filled with the desired nonaqueous solvent containing an appropriate electrolyte sueh as tetrabutylammonium chloride. Most Ag/Ag(I) electrodes consist of a silver wire dipping into a nonaqueous solution of 0.01-0.1 M AgN03 or AgCIOn with a higher concentration of an inert electrolyte such as tetrabutylammonium perchlorate to reduce junction potentials. Salt bridges usually consist of fine porosity fritted glass, the usual asbestos fiber, or a porous ceramic plug. The design of reference electrodes far high temperature melts requires a completely new bag of tricks. Here the prohlem is one of materials because of the high temperatures and the reactivity of many fused salts with most materials. The chapter in Ives and Janz (8e) is quite informative in this regard. Both a suitable housing and a structurally sound salt bridge material must he found. The electrode half-cell, or internal, usually poses no problem. Many half-cells are suitable as reference electrodes in fused salts since the high temperature favors rapid electrode kinetics. Materials used to form or contain the salt bridge include asbestos fiber, glass or quartz frits, capillary tips, porous ceramic and refractories, and graphite sleeves. At
Figure 6. Ag/Ag(I) Reference electrode lor use in fused alkali metaphosphates (13). A.
epoxy seal
B. Corning z1720glass. 6 mm OD.
C. Corning ~ 7 7 4 0glass D. platinum wire E. vent to argon F. silver wire G. sol~tion01 AQPOJin melt temperatures in excess of 400'C silicate glasses are ionic conductors and serve as excellent salt bridges and the internal reference element can thus be isolated from the test solution by the salt bridge membrane. Caton and Walfe (13) used this technique for their Ag/Ag(I) electrode, shown in Figure 6. At 700°C the glass was sufficiently rigid, but yet conducted ions quite well. Their electrode obeyed the (Continued on page A578)
NONAQUEOUS REFERENCE ELECTRODES The electrochemistry of species in nonaqueous solvents is a field of ever increasing activity. Water is but one of thousands of solvents, and it is only natural to seek other solvents which are more useful for electrochemical work. Aside from the novel and unique reactions which may occur in nonaqueous solvents, detailed electmchemical studies yield useful electroanalytical methods, new methods of preparative chemistry, and fundamental physicochemical data. The chapter in Ives and Janz (ad) serves as an excellent introduction to the subject and a recent review by Butler (12) treats the use of aqueous and nonaqueous reference electrodes in aprotic solvents quite thoroughly. When performing electmchemical investigations in sueh solvents there are two routes followed concerning the reference electrode question: (1) use a reliable aqueous electrode in s nonaqueous solvent, and (2) construct and/or test a truly nonaqueous reference electrode and then use it. The first choice is obviously a poor one when doing work in high temperature melts, and it is often a poor choice for many mom temperature solvents because of the large and nonreproducible liquid junction potentials which occur a t the juncture of two very dissimilar solvents. For example, the nature of the salt bridge can influence E" for the SCE by as much as 40 mV in acetonitrile. Nevertheless, the aqueous SCE is still the most papular electrode used in studies of aprotic solvents. Next to the aqueous SCE, the hyVolume 50, Number 12, December 1973 1 A577
Chemical instrumentation Nernst equation over the concentration range of 0.04 m to 0.7 m, and after polarization the electrode returned quickly to its equilibrium potential. Tests for asymmetry potential should always be made when membrane salt bridges are used to isolate electrodes from the test solution. This potential is the difference between the potentials generated a t the inner and outer surfaces of the membrane under identical conditions, and i t is usually caused by differences in the capacity of the two surfaces to exchange or adsorb ions from the melt. To measure it two identical electrodes are immersed in two portions of the same solution, the two portions being separated by the membrane. Thus, Caton and Wolfe (13) made measurements of the cell Ag/Ag(I), (m, in KP03-Nap03 melt)/Corning :I720 glass/ Ag(I), (ml in KP03-Nap03 melt)/Ag to determine the potential difference between the two surfaces of the glass bulb. The asymmetry potential was found to be 17 *2 mV, and the outer part of the bulb was always positive with respect to the inner surface. The Ag/Ag(I) half cell has continued to be a favorite among fused salt workers, as well as the Pt/Pt(II) half cell which has found considerable application in fused alkali chlorides. To make their reference electrode for studies in LiCI-KC1 euteetic, Laitinen and Ferguson (14) immersed platinum wires into compartments having a fritted glass disc a t the bottom and generated Pt(I1) a t constant current until the desired concentration was obtained. Other reference electrodes frequently used are the Zn(liq)/ZnClz, the C,Clz/chloride melt, and the Pt, Oz/oxide or onyanion melt.
Literature Cited 1. Buck,R. P.,Awal. Chem., 44,27OR(19721. 2. Toren. E. C.. Jr.. and Buck. R. P.. A m 1 Chem... 42.. 284k (1970'1. 3. Broming, D. R.. Ed.. "Eleefmmatrie Methods." McGrau-Hill Bmk Co.. Maidenhead, England, ,"em Lam.
4. Lingam, J. J., "El.ctmanalyfi~al Chemistry." Interscience Publishers, Ine., New York. 1958, pp. 59-
,
.
.. .
Orion Research. I"".. Csmbtidge, Mas.. vo1, 4. 1969. 7. Wetcott, C. C.. "Selection of a Referenee Ekefmde." Applications RPsearch Technical Report No. 543. Beckman In~trumenta.1ne. Scientific I " ~ strumentaDiuision, Fullerton. Calif. 7a. Biegler. T., and Wmds, R.. J. Cham. Educ., 50. M 4 (1971) ,. ...,.
8. Ima, D. J. 0.. and Jsnz. G. J.. (Ed.) "Reference EiocMdes, Academic Ress. New York. 1961; (a1 Hills, G. J. and 1ves D. J. G.. 'The Hydmge" Electrode." Chapt. 2: (bl Hills. G. J . and lves, D. J. G., " T h Calomel Electmde and Other Mercury-Mercurous Salt Eloctmdes," Chapt. 3: (r) Jan., G. J.. "Silver-Silver Halide Electmdes, Chspt. 4; (dl Hills. G. J.. "RoferenccElaetmdes in Nonaqueous Solutions.'' Chapt 10; and Is1 Laity, R. W., "Electrode. in Fused Salt Systems." Chapt. 12. 9. Negua. L. E. and Light, T . S., Instrvm. Tech.. l9,23 (1972). 10. Cavmgton, A. K., "Referenee Electrodes." Not. Bur Stand. (U. S.J. Spec Pubi,314, Chsgt. 4. 11. Light,T. S..Awl.lmlrum.. 8.1 (19701. 12. Butlor. J. N., "Advances in Electmchemistry and Electroehemica1 Engineering," P. Delehay. Ed.. Inteneienco, N. Y.. 1970, Vol. 7, p. 77. 13. Caton, R. D., Jr.. and W o k C. R., Awl. C h m . .
~ . . ~ ~ ~
llfiFn(19711~ ,~~
14. Laifinen. H. A , and Ferguson. W. S.. Anal. Cham.. 29.4(19571.
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(To be continued in January 1974 issue)
