Topics in..
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Chemical Instrumentation Edited by GALEN W. WING, Seton Hall University, So. Orange, N. J. 07079
These arlicles are intended to serve the readers O ~ T H I SJOURNAL
by calling allenlion Lo new deuelopmenls in the themy, design, or availability of chemical laboralory inslrnmentalim, or by presenling usefvl ingighls and explanalions of topics lhal are o j praclical imporlanee lo those who use, or leach the use oj, modem instrumenlalion and inslrumenlal lechniques. The edilor invites correspondence from prospectiue contribulors.
LXXIII.
Reference Electrodes (Concluded)
Roy D. Caton, Jr., Department of Chemistry, The University of New Mexico, Albuquerque, New Mexico 87131
COMMERCIAL REFERENCE ELECTRODES
$35, with prices ranging up to $100 for
Commercial reference electrodes are supplied with three basic internal elements and a host of junctions. The internal elements are the SCE, the Ag/AgCl (with KC1 or other chloride electrolyte of various concentrations), and the Hg/ HgSOn, sat'd K2S04. The most popular types of liquid junctions are the asbestos fiber, the porous plug (ceramic, fritted glass, quartz, sintered platinum), the glass sleeve, and the free flowing capillary. Figure 1 shows the eeneral amearanee of nwrt reference electrode.; hawng ellher i( pnroui plug or fiber pnrtiun, and Figure 2 shous t h e r a n t i r u r a t i o n ot n t h r r t y e r of junctions. The following discussion is essentially limited to aqueous electrodes supplied as a separate item, not in combination with a sensing electrode. However, much of the prior discussion on reference electrodes applies equally well t o the mternal elements in combination electrodes. The discussions on various electrodes will also be limited to those which are "nonstandard" in size and construction, or which are designed for industrial or novel laboratory applications. A "standard-configuration" electrode is about 5 inches long and % inch in diameter, and an example of one is shown in Figure 1. The reader should consult Tahle 3 for a summary of the types of electrodes available from a listing of manufacturers and suppliers. Whenever possible the author has attempted to give the reader an idea on how many different types and styles of electrodes each supplier offers. When making the counts from available catalogs and brochures, duplicate electrodes available with two or more sizes of leads were counted as one electrode. The author apologizes if a particular electrode was overlooked, or an erroneous tally made. The cost of reference electrodes varies widely, depending upon the construction, size, and application. Mast electrodes can
1. Beckman Instruments Beckman supplies twenty basically different reference electrodes with a variety of liquid junctions (see Tahle 3). A selection guide is available to assist the purchaser in selecting the proper electrode liquid junction for different applications (15). Far those who must make measurements in acid fluorides or other media which attack glass, an SCE with s Bakelite body and a Carhorundum frit liquid junction is available. Recently introduced is the Perma-Probe- electrode which has a permanent filling. The body of the eketrode is constructed of a polymeric material, and the lower half-inch consists of a large, porous junction area. Beckman states that the electrode is well suited for measurements in solutions containing suspensions as well as in strong redox solutions containing chromic acid or hypochlorite. The internal electrolyte is KC1, and the leakage rate a t room temperatures is about one pmole of KCl/hour. The ahsence of glass parts makes the electrode very rugged and well suited for process applications. All of the Beckman electrodes have integral leads with the exception of the FuturaTM line featurine - detachable cables to permit easy and less costly replacement of the electrode and/or cable. The Ag/AgCI models are autoclavahle a t 15 to 20 psig. far 15-30 minutes. Only the Futura'rM electrodes have either the quartz or the ceramic junction. The ceramic junction has a n electrolyte flow rate of approximately 0.1 m1/24 hr ( 1 to 4 pmoles C1-/hr) and the quartz junction a flaw rate of less than 40 pmoles af CI-/hr. The resistance of both junctions is nominally 20 kilohms when either a 4 M or saturated KC1 filling solution isused. Specialized electrodes include a probe SCE, about 0.5 cm diameter, a hydrogen electrode cell assembly for precise stan-
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be purchased a t prices ranging from $19 to those of more intricate design.
dardization of buffer solutions, and a double junction SCE having inner and outer junctions of a fiber and glass sleeve respectively. Eleven pH combination electrodes with a Ag/AgCI reference are available, as well as a platinum-Ag/AgCI and a silver billet-Ag/AgCI combination. The latter has a double junction reference to allow the use of nonhalide outer filling salution. Beckman supplies two kinds of salt bridges. One is a tube which fits over the end o f their standard-sized electrodes and having a tubing nipple on the end, and the second fits their miniature 2.5 inch reference electrode. The latter has a curved tip to allow placement of the miniature reference close t o a n electrolysis electrode, and was designed for use as a n accessory to the Beckman Electmscan 30 electroanalytical instrument. This miniature salt bridge should he useful for any application that requires isolation of the reference from the test solution. The topics of salt bridge and special filling solutions for aqueous and nonaqueaus work are treated amply in the Beckman Technical Report by Westeatt (7). The characteristics of various junctions and the requirements to consider when selecting a filling solution are described as a basis for reference electrode selection and application. 2. Brinkmannlnstruments Brinkmann sells the Metrohm line of instruments and eleetrodes. All of their electrodes are available with either a straight
U Figure 7. Reference Electrode with Electrolyte Reservoir and Frit Tube Junction (Melrohm: supplied by Brinkmann Insbumeots.) A. reServOir 8. detachable reference electrode C. frit tube
(Continued on page A8) Volume 57, Number 7, January 1974
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Chemical Instrumentation
~ i g u r e8. Non-flowing Process Reference Electrode (The Foxboro C0.J. A. screw terminal B. plastic body C. Ag/AgCl internal D. porous Ceramic plug E. saturated KC1 saturated With AgCI salution
outer shank or one having a ground glass joint for use in sealed titration cell assemblies. Eight different standard reference electrodes, of wbich four are available with or without the ground glass joint, are supplied. Figure 7 shows one of their four Ag/ AgCl electrodes with a detachable frit tube junction and a KC1 reservoir with stopcock. The latter electrode, or their SCE with capillary tip and KC1 reservoir, would serve as an excellent laboratory standard because the junction is easily flushed out and renewed. When not in use these electrodes may be conveniently stored in a tube having a matching female joint. Both of their double junction Ag/ AgCl electrodes have inner and outer ceramic junctions. The outer junction is comprised of a tube with a ground glass joint which fits the ground joint on the shank of the inner reference electrode. Both parts are available as separate items. Of their five basically different combination electrodes, two are pH, one is a silver wire, and two are platinum electrodes. The silver wire combination contains a Ag/AgCI reference element having a KN03, saturated in AgC1, filling solution. A standard hydrogen electrode assembly is also available. All of them electrodes are supplied with standard Metrohm plugs fitting Swiss and German instruments only, but an adapter fitting the Beckman-type receptacle is available. 3. BroodleyJames In addition to standard laboratory reference electrodes, Broadley-James supplies process electrodes and twenty different pH-combination electrodes. Reference electrodes with fast-flow porous ceramic junctions are available, and custom electrades will he fabricated on special order. 4. Coleman ImtmmentsDiu. of PerkinElmer Coleman offers eight different reference electrodes of which one is a process SCE. Three of their electrodes are less than 4 A8
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inches long and have screw base terminals for mounting in their pH meter electrode holder. A miniature pH-combination with a coaxial salt bridge reservoir is also available.
NOTE: REMOVE PLUG BEFORE USING ELECTRODE
PART NO. IOENTIFICITION
5. Corning Coming offers eight basically different standard;eference electrodes with a variety of junctions. In addition, there are five types of process reference electrodes, all with porous ceramic junctions and Ag/ AgCl intemals, and six different pH-Ag/ AgCl combination electrades with porous ceramic junctions. Their newest addition to the line is a Ag/AgCI reference electrode with a permanent KC1 filling. The electrode is made of a plastic material that is highly resistant to coating and liquid junction blockage, and the junction consists of a porous plastic area which is approximately 350 times as large as the cross-sectional area of the conventional porous ceramie tip or fiber junction. Called the CorIastic'PM, the electrode is guaranteed for three years against breakage and one year against failure of the internal element.
6. Fisher In their standard electrodes Fisher offers porous ceramic, cracked bead, and glass sleeve junctions, having flow rates of 5 al, 0.5-3 al, and 100 ~ l / h o u r respectively. Nine different reference electrodes, four pH-Ag/AgCI combinations with porous ceramic junctions, and one platinum-Ag/ AgCl combination are available. A remote reference junction, comprised of a standard size electrode shank with a porous ceramic junctian and a flexible tube connected to the top, can be used to isolate any reference electrode from the test solution.
7. Forboro The Foxboro Company manufactures two kinds of process reference electrodes, both with Ag/AgCI internals and porous ceramic junctions. Figure 8 shows their nonflowing (about 0.01 ml/day) electrode which generally has a life expectancy in excess of six months. The electrode mav be used from -5 to l 1 0 T at prucezs liquid prissures up to 150 prig, and it* resistance IS less than 10 kilohma 'Their ierond elec-
Figure 9. Process Electrode with Flowing Junction (The Foxboro C0.J.
trode, shown in Figure 9, is more conventional appearing and has a flow rate of one to two ml/day. It is operable between -5 and 130°C up to a differential pressure of 25 psi, and has a resistance less than 10 kilohms. Foxbom also supplies flowthrough and submersion electrode holders wbich can be equipped with the usual temperature compensators and an ultrasonic transducer for in-stream cleaning of fouled or coated electrodes.
8. Ingold Ineold undoubtedlv offers the ~urchaser the iargest assortment of electrodes and types of liquid junctions. Their catalog
Table 2. Technical Data lor Liquid Junctions Used on lngold Reference Electrodes
Junction type (see Fie. 10)
Approximate Approximate electrical electrolyte resistance Diameter, leakage, ml/day, kilohms mm 20°C. l m column
Application
Porous plug in tip Porous plug in tio Poraui plug, lateral
1.0
1
0.4
laboratory reference
2.5
0.6
0.04
micro-reference
1 .(1.5)
l(0.6)
0.4 (0.04)
Double plug
0.3
2x1
0.8
Triple plug
0.8
3 X 0.6
0.12
Large area
0.8
5
30
Glass sleeve
-
NS7/16
-
Interehaneeable "
1.5
5.5
industrial (high temperature) pH measurement in milk electrodes with semi-solid filling industrial reference for heavily polluted media low ion and polluted media heavily polluted media
1.5
(Continued onpageA14) Journal of Chemical Education
Chemical Instrumentation A
B
C
D
E
F
Figure 10. Types of Liquid Junctions on lngold Reference Electrodes (Ingold Electrodes, loc.) A. capillary tip 6. double porous plug C, large area trit D. interchangeable porous plug E. triple porous plug F. glass Sleeve
shows twenty-three different body styles, and most of these are available with SCE, Ag/AgCI, Hg/HgS04, and ArgentalTM intemals, the latter being a modified Ag/ AgCl electrode with an interior tube cantaining a proprietary mixture of silver and silver chloride in 3 M KC1. To provide even more flexibility, many of their electrodes are available with s variety of liquid junctions. Technical data for their nine standard liquid junction styles is given in Table 2, and Figure 10 shows six of the types. Special capillary tips, porous zirconia, and annular ring junctions can be supplied for surface measurements, for use in highly alkaline media, and for use in moving dilute ionic solutions, respectively. Upon special order, any Ingold reference electrode may be supplied with an electrolyte which has been solidified by addition of a high polymer material, but the advantage of no maintenance may be offset by a shorter electrode lifetime. The ArgentalTM electrode is worthy of more description because of its temperature stability, freedom from hysteresis, and law temperature coefficient of -0.17 mV/"C. Whereas an SCE equilibrated at 95°C and rapidly cooled to 25°C requires over one hour to retum to its equilibration the ArgentalTM returns to its proper potential in two minutes (160). It is used exclusively as the internal in all high temperature reference electrodes and autoclavable combination electrodes. At one atmosphere the temperature range is 0 to lM)'C, extending up to 130' a t about 3 atmospheres. The variety of combination electrodes is equally as impressive, and they are available with three kinds of porous plugs or the inverted glass sleeve junctions. Thirty basic body styles of pH-Ag/AgCI combinations are available, with a host of glass membrane styles to suit just about every application of pH measurement. For example, one of their five micro-combination electrodes has a "spear" glass membrane of 3 mm diameter and is used for puncture measurements in semi-solid substances. Three of the macro-combination electrodes are designed specifically for surface pH measurements. Ingold's series of Pt-, Au-, and Ag-Ag/AgCI combined electrodes should also satisfy all hut the most esateric needs of thelaboratory researcher. (Continued onpageA16) AI 4
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Journal of Chemical Education
C/temica/ Instrumentation For those wishing to isolate a reference electrode from the test solution via a salt bridge, Ingold offers liquid junction tubes ranging from 3 mm to 12 mm diameter with single lateral, triple plug, inverted sleeve, large area, and interchangeable junctions. Tube4 are available with or without a glass ground joint that mates witb the joints on their electrodes. 9. Leeds and Northrup Leeds and Northrup offers eight stsndard electrodes and a new plastic fouling resistant reference electrode (FRRE).Five different pH combination electrades with fiber and ceramic junctions are available. The FRRE is a diffusion (nonflowing) electrade with a Ag/AgCl internal and packed with a KC1 slurry. The liquid junction is a porous plug mounted in a removable
threaded plug, making disassembly and cleaning the junction a simple matter. Designed primarily for long-term, maintenance-free service, the electrode is specifically recommended for industrial use. Operating temperatures may range from -5 to 110°C. and the maximum process stream pressure is 150 psig. Nominal resistance is 300 ohms. Since there is no pravisian for operating the electrode in excess of the process stream pressure, some encroachment of the process stream can occur over a long period of time. This may he evidenced by an excessive bias potential when standardizing, and the electrode should be removed for cleaning and refilling. Although not directly concerned witb a discussion of reference electrodes, Leeds and N o r t h p ' s new line of process stream monitoring equipment of the transmitter/ receiver design should be mentioned. Either flow or immersion electrode mount-
ings with a built-in .preamplifier (transmitter) are available. The immersion unit may be immersed up to 200 feet and should also be useful for remote measurements in lakes, rivers, etc. 10. London
The London Company supplies the Radiometer line of equipment. Radiometer offers eleven different styles of electrodes, some of which are available with different internals or different capillary tip junetions. A special SCE for polamgraphic applications can carry currents up to 200 pamp. It has an exchangeable calomel filling and is fitted with a plunger for easy renewal of the KC1 junction. Three of their capillary tip miniature electrodes are equipped with an electrolyte reservoir and stopcock, and the fourth with a plunger which permits easy and efficient renewal of the junction. A special SCE for pH measurements in cheese or other semisolid materials, a spear-type SCE for meat, a remote SCE that may be eonnected to a salt bridge with a porous pin junction, special electrodes for medical measurements, and an autoclavahle reference complete the line. The twelve pH combination electrodes have SCE internals and porous plug junctions. A combined Pt-SCE for redax titrations and a combined Ag-Hg/HgS04 for halide titrations are fitted with a built-in buret outlet for connection to any laboratory buret. This novel construction allows the analyst to locate the buret at same point other than the top of an overcrowded titration cell. All Radiometer reference electrodes come equipped with a banana plug which does not fit American instruments; how-
Figure 11. Orion Single Junction Reference Electrode (Orion Research, 1nc.j. A. cap 0. epoxy-coated spring C. cable D. O-ring E. electrode body F. outer sleeve G. filling solution chamber H. Ag/AgCI reference element I. vent J, filling hole K. inner Cone
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Journal of Chemical Education
ever, it can be easily replaced with an inexpensive pin plug. Also, their combination electrodes have the usual European style plug, and an adapter for Beckmantype jacks is necessary. 11. Markson Markson Science is the only company, to my knowledge, that supplies a Hg/HgO reference electrode for use in concentrated alkali solutions. The internal electrolyte is a hydroxide solution, and the electrode is uniquely well behaved. Other nonstandard configuration electrodes are a needle (fiber) tip electrode and a flow-through pH combination electrode. The latter has a tube running the full length of the electrode, and the pH sensitive glass and reference junction form a part of the tube. Markson also supplies two styles, glass body or plastic body, of nonrefillable reference electrodes having a gelled electrolyte and a large frit junetion.
12. Orion Orion has followed the approach that one basic electrode design having a very reliable junction is to be preferred over a multiplicity of offerings for special applications. Since Orion was one of the early pioneers in the specific ion electrode field. they were naturally concerned with reference electrodes which function properly with their specific ion electmdes. During the early years that Orion marketed their electrodes approximately 80% of their customer complaints were traced to problems with reference electrodes. As mentioned earlier in this review, prablems of drift and junction potential fluctuations were
Table 3. Summary of available aqueous reference electrodes
Supplier
SCE
Beckman
DJ, FA, PPC, PPCarb, PPG, S, G(FA, P P Carb, S)
DJ, FA,
Brinkmann (Metrahm)
C, FA, S
BmadleyJames
PPC, FA
DJ (PPC) Frit tube, PPC PPC, FA
Coleman Coming
FA. S DJ(PPC), F, PPC, S, G(FA)
FA FA, PPC, Corlastic
Fisher
CB, PPC, S G(FA, PPC)
PPC, S
S
PPC, PPQ, S
Ingold
C, DJ, PPC, S, frit
PPC, nonflowing DJ, S, PPC, frit
Leeds and Northrup
DJ, F, PPC, PPG, S
DJ, F, PPC, PPG, S
Foxboro
Hg/ H~SOI
Ag/AgCI
C, FA
Remarks and Special Electrodes Futura line is autoclavable; Perma~robehas ~olymeric body Gith porou;polymeric junction; probe, miniature electrodes; SCE for HF solutions; metallic and OH COMB; omeess electrod&; special &der electrodes, RRJ; H pH and metallic COMB Ag/ AgCI, SCE, and Hg/HgSO1 with integral reservoirs; H Process electrodes: soecial order
Process SCE; pH COMB. Corlastic electrode has polymeric body with porous polymeric junction; process electrodes; pH COMB. RRJ,pH and metallic COMB. Process electrodes only PPC, S
RRJ (several types of junctlons); micro; high temp.; autoelavable; pH and metallic COMB. soeeial order electrodes; b k s s electrodes; dismantled electrodes Process electrodes (fouling resistant, plastic body); micmreference probe (Continued onpageA18)
Volume 51. Number 1, January 1974
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Table 3. Summary of available aqueous reference electrodes (continued) Supplier
SCE
London (Radiometer)
C, PP, S
Marksan
PPG, G(PPC)
L. G. Nester
PPC
AdAnCI
Hgi H~SO,
Remarks and Special Electrodes
electrodes
Orion Photovolt SargentWelch Arthur H. Thomas
C, DJ, FA, S G(S. FA) PP, S, G(PP)
DJ, PPC,
PP
PP
S J , DJ FA, S
flow-through) pH COMB. See Figure 11 pH COMB.
PPC
Process ( p COMB); ~ H
S, G(PPC)
Type of liquid junction; C, capillary CB. cracked bead
Other symbols: RRJ, remote reference junction G. miniature electrodes for Beckman
FA, asbestos fiber S, sleeve
not noticed to any great extent hefare the use of expanded scale pion meters. Orion often recommended the use of ordinary SCE's with fiber junctions, later changing their recommendation to electrodes with glass sleeve junctions. Noting the problems of clogged junctions and AgCl precip-
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Journal of Chemical Education
itation in the junction, Orion engineers optimized electbode stability and incorporated features which they felt were essential. The outcome was their single junction and double junction electrodes which possess the advantages of the glass sleeve junction, but not the disadvantage of fragility.
The single junction model is shown in Figure 11. The junction is quickly cleaned by merely pushing the outer sleeve up into the spring-loaded cap. The design minimizes junction potential variations and clogging by providing a relatively high leakage rate of filling solution. It has a Ag/ AgCI internal with a dilute electrolyte solution designed to yield a reference potential equal to that of the SCE. The filling electrolyte is a n equitransferent solution containing the proper concentrations of potassium, sodium, chloride and nitrate ions. Other chloride electrolyte solutions may be used if so desired. The single junetion electrode is also available with a side nipple for attaching a filling solution reservoir. A special flow-through reference was designed to be used with their flowthrough specific ion electrodes. Orion combination electrodes are available for fluoride, chloride, sodium and redm determinations. Each electrode has the sensing element and a sleeve-type reference mounted in a single body. 13. Photouolt Photovo~toffers fourteen different eleetrodes with two types of internals and several types of junctions. Their high temperature electrode (-5 to 130°C) has a Ag/ AgCl internal and a pressure connection. Twenty-dne pH combination electrodes are available. 14. Sargent-Welch Ssrgent-Welch offers a special prohe SCE for use in small titration assemblies. About the size of a pencil, hut smaller in diameter, the electrode has a resistance of 1 kilohm. For those wishing to make their
o w SCE, Sargent-Welch offers the glass-
ware needed. Once assembled, such SCE's should sewe as excellent laboratory standards. Their standard-configuration electrodes have electrolyte flow rates of about 25 pl/hr. They offer two styles of pH combination electrodes, a miniature one with a Ag/AgCl reference and a standard-sized one with a thallium amalgam reference. The latter electrode has been described elsewhere (17). 15. Arthur H. Thomas Arthur H. Thomas supplies six types of laboratory electrodes and seven pH combination electrodes, of which two are for industrial use. All of their combination eleetrodes have Ag/AgCI internals.
References 16. " M m n e e Eleetmde Selection Guide," Suppiarnent to Bulletin 7686. Beckmsn Imlrumentn. Ine.. L i. eotific tnstruments ~ivisian.Fullenon. calif. 16. Catalog E-12348, Ingold Electmdos, Lexington, Mass..fa) p. 27, fbl p. 23. 17. Baucke. F. G. K..J. E l e r t m w I . Chem.. 33. 135 (1971).
MANUFACTURERS AND SUPPLIERS Beckman Instruments, Inc., Scientific Instruments Div. 2500 Harbor Blvd. Fullerton, Calif. 92634 Brinkmann Instruments, Inc Cantiague Rd. Westbury, N. Y. 11590 Braadley-James Corp. 1714 So. Lyon St. Santa Ana, Calif. 92705
Coleman Instruments Div., The PerkinElmer Corp. 20W York Rd. Oak Brook. Ill. 60521 Corning Glass Works Medfield Industrial Park Medfield, Mass. 02052 Fisher Scientific Co. 711 Forbes Ave. Pittsburgh, Pa. 15219 The Foaboro Co. 38 Neponset Ave. Foxbaro, Mass. 02035 Ingold Electrodes, Inc. 113 Hartwell Ave. Lexington, Mass. 02173 Leeds & Narthrup Co. Sumneytown Pike North Wales, Pa. 19454 The London Co. 811 Sharon Drive Westlake, Ohio44145 Markson Science, Ine. P. 0. Box 767 Del Mar, Calif. 92014 L. G. Nester Co. P. 0. Box 666 Millville, N. J. 08332 Orion Rerearch, Tnc. 11 Rlackstonr St. Carnbrrdac, .Mas*. 02139 Photovolt C o p . 1115 Broadway New York, N. Y. 10010 Sargent-Welch Scientific Ca. 7300 N. Linder Ave. Skokie, Ill. 60076
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