going beyond. going further
edited by
-George Washington High School
N. H. ETTINGER
Bustieton Ave. and Verree Road Philadelphia. PA 19116
Knives, Forks, and Beer Cans as Potentiometric Sensors Walter S.Sellg Lawrence Livermore National Laboratory, University of California, Livermore, CA 94550 reaeent-made chemicals. The titrants were 0.01 M lead nitrate and har& nitrate, O.05 M silver nitrate, 0.0133 M lanthanum arrording.to nitrate, and 0.01:13 M cerous solution. prepared . . Taulli and Irani (5). The borate buffer contained 0.12 M sodium tetraborate and 0.04 M boric acid and had a pH of 9.25. The ammonium acetate buffer was 0.5 M ammonium acetate, adjusted to pH 8.9 with ammonia.
Potentiometric titrations can be carried out with verv simple equipment: a pH/millivolt meter, a buret, a sensing electrode, and a reference electrode. I previously discussed in THIS JOURNAL (I)pencil and graphite sensors costing $1.50 or less. As a continuation of this work I have investigated metallic sensors such as copper, stainless steel, and aluminum for the potentiometric titration of some common anions: fluoride, halides @), sulfate (3), and orthophosphate (4). In potentiometric titrations, the endpoint is signalled by a change in the electromotive force (emf) between two electrodes, the sensor and the reference, immersed in the solution being titrated. Depending on the ion being titrated, various kinds of sensors are used to detect the endpoint of a potentiometric titration. In acidhase titrations. the electrodes are usually made of glass, in oidation/reducti& titrations, of gold or platinum. Ion-selective electrodes (ISE), developed recently (since 1966) are used to sense specific ions. Potentiometric endpoints are often better defined than endpoints obtained by other detection methods. For example, endpoints indicated by color changes can be indefinite, or totally obscured in a colored or opaque sample. The accuracy of measurement in a ~otentiometrictitration may be 10 times that obtainable by direct measurement (from plot of emf versus concentration) based on the Nernstian logarithmic relationship between emf and activity. In addition, since it is the change in emf rather than the absolute value of the emf which is of interest, the influence of liquid junction potentials and activity coefficients have little or no effect. The endpoint of a potentiometric titration occurs at the point of maximum potential change, which is where AEIAV is at maximum, or where the second derivative, A2ElAV2is zero.
Procedures In most instances, determinations of 0.025 mmol of analyte were made in a volume of 25 ml. Excentions were chloride (0.020 mmol), fluoride (O.lOmmol), and h a t e (0.005 -01). When a d i v methanolic medium was used. time was 81lowed to dissipate the heat generated upon mixing of methanol with the aaueous solution. Potentiometric titrations mav be performed using an automatic titration apparatus, or t ~ y manually adding increments of titrnnt. ktting the emf equilibrate, taking emf readings, etc., and finally drawing a titration curve. It this l a t ~ e rmethod is used, it 1s helpful for endpoint calculation toadd equal increments01 titrant in the endpoint region nccording to the method of Lingane ( 6 ) . The metal rode and wires were abraded with 310 emery paper prior to use. Several conditioning runs were done prior t o the actual experiments. This is good practice also when using ion-selective electrodes (ISE's).
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Measurements Anv convenient nH/millivolt meter can be used as the measiring instrument. The reference electrode was a plastic single-junction silverlsilver chloride electrode containing a 0.1 N sodium nitrate salt bridge. In many cases, metal wires or rods may also be used as "reference." This has the disadvantage of no a priori knowledge of the direction of the endpoint breaks. Most of the materials used as sensing electrodes were previously described (3,4). Chemicals Solutions of the ions determined were prepared from ACS
Portions of this paper were presented at the 22nd Eastern Analytical Symposium in New York, November 18, 1983.
Fluoride Titratlons I recentlv discussed various t m e s of manhite and nlatinum as endpoiit sensors in rhe tiirkion i f h o r i d e v i lanthaSince then 1 discovered that mimv numllll) and thorium (71. ISE'~besides the fluori'de ISE, as well as some metal rods and wires. mav be used as sensors in this titration. A com~arison of endpoint breaks for this titration using various sensors is presented in the table. Only sensors of aluminum surpass the magnitude of the endpoint breaks of the fluoride ISE. Because of the outstanding ~erformanceof aluminum in this titration i t seemed logical 6 attempt using an aluminum container both
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Going Beyond, Gohg Furihwrespondsto a need long felt by those who teach or suowvise hioh schwl science .oroarams. While student interest often is piqued by involvernerd in science fairs and competiiions. students are often at a loss for suitable projects-particularly in
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chemistry.Teachers. too, find )difficultto choose a topic that is at an appropriate intellectual level, is acceptably safe, is reasanably inexpensive, and leadsthe s M e m tolwmer research. In *is featurewe hope to aWress such concerns. Many high scharl science teachers have "pet chernisby projects. If you feel that you have a pmlect mat meets the above criteria. please submit it to the featureBditor.
Volume 62 Number 5
May 1985
431
A. Fluoride ISE
B. Aluminum wire
-i
im
Sensor: C. Aluminum can1
Aluminum rod
0. Reference: Aluminum can
1400
: 2 p ] 1160
1090 1000 0
ISg L
1.2
130 100
3.6 0 1.2 Titrant volume (ml)
2.4
2.4
3.6
Poiari~reversed F i v e 2. Tibation fn0.1 mmol of flmidevenus 0.013 Mlanthanum(l1l) in 60% meihanol: comparison of fluoride ISE and aluminum sensors.
1. Tiiation assembly using an aluminum can as titration vess%laod sensa (4)(reprintedby permission, International ScientificCommunications,kc.). F-e
A Cornparlson of Endpoint Breaks In Various Potentlornetrtc Tltratlons Ion determined
F-
pa4=-
SO,2--
TiDant
Medium
Sensing electrcde
La3+
aluminum rad F- ISE aluminum wire cobalt wire pH glass steinless steel rod aluminum rcd beer can aluminum wire aluminum wire aluminum wire Pb ISE cadmium wire cobalt wire Pb ISE
La3+
60% methanol 60% meman01 60% methanol 60% methanol 60% methanol 60% methanol 60% methanol 60% memanol aqueous
Ce3+
aqws
PbZ+
aqueous aqueous 80% melhanol 80% methanol 805 methanol
Pb2+
Mean endpoint break, mVa 190 160 150
100
95 95
160 130
copper rcd "reference" copper rod "reference"
4OOa
300b >500b loo 150
125 105
M a n y t i h r ISE's and metal rods yielded smaller breaks. Weak3 O c m in W i v e direction.
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a s sensor and titration vessel.'l'hisset-up is shown in Figure 1. The aluminum can was insulated tiom thestirrer hv means of a plastic pad. Obviously, nonalcoholic beverage Eans are recommended for secondary school students. A comparison of t h e fluoride ISE with aluminum sensors is given in Figure 2. Several points are noteworthy:
1) The decrease in emf after a maximum when using aluminum ( ~ i2~~ . C) indicates a transient response:if suffi. cient time is allowed after each addition of titrant the emf will approach the initial level. 2) With an aluminum can as sensor (Fig. 2C) the inital emf was beyond the limit of our detector (-1200 mV), hence we reversed the polarity of the sensing.couple. 3) As shown in Figure 2D,the standard reference elec~rtndeneed nor be used and may k replaced by a mctallic sensor. M"nilethis ~
432
Journal of Chemical Education
rurve was obtained uamg two types of aluminum, n bimetallic
sensor ic.~..coppw versus aluminum) was also found wrrkable.
A stainless steel rod ranked fairly high in the magnitude of the endpoint break obtained in this titration. One can, therefore, use a stainless steel knife or fork as sensing electrode and can obtain usable titration curves with a knifelfork combination. Obviously many more combinations are possible; this should stimulate the imagination of t h e student experimenter. When a silver electrode or a silver/silver sulfide ISE is used t o monitor the polentiometric titration of halides, the magnitude of the endpoint breaks is inversely relnted to the sol~
~
ubility product of the precipitated species (8).Addition of a nonaqueous solvent to the titration medium will reduce the K.? of the inorganic precipitate and thus enhance the endpomt breaks. When, however, an aluminum sensor is used in the titration of halides versus silver, the breaks were almost alike for iodide. bromide. and chloride. exceedinn that for iodide with the silverlsulfide ISE. This shows t h i t the response of aluminum sensors in this titration is independent of K A ~ xRepresentative . titration curves for bromide versus silver are shown in Figure 2 of reference (2);the corresponding data are in the table. While a partially nonaqueous medium did not enhance the endooint breaks in the titration of halides, when using an aluminum sensor, in the titration of low levels of fluoride, the presence of a nonaqueous solvent is mandatory for good endpoint breaks. We have not done any experiments on this titration using any other sensors; there is room here for experimentation. Orthophosphate Titrations
Orthophosphate forms very insoluble compounds with cations such as silver and lead(I1); these cations have, therefore, been used as titrants for phosphate (9,lO).For the titration with lead(II), the optimum pH is between 8.25 and 8.75; this is obtained using an ammonium acetate/ammonia buffer (9).For the titration with silver. a borate buffer in 6&70%'methanolic solution was previou$ly found most suitable (10).These titrations. as well as orthoohosohate . . versus lanthanum(lll~and rerium(111),were recently revien,ed by the writer (41. Some sensors virldine. sharr) endooint breaks in this titration are listed in the tabre. ~ l i m i n u hwires surpassed all other sensors. With aluminum sensors, the endmint breaks were in the negative direction, opposite to thatcornmonly found in the titration of anions versus cations. Twical .. titration curvcs werr prcwL~uslypresented (Figs. 1,3,4, and 5 in ref. 141). No improvement in endpoint hreaks was noted with partially nonaqueous media. Sulfate Titrations
This topic was recently discussed by the writer (3).The titrants were lead and barium nitrates; the titration media were 80% methanol/20% water. Unlike the previously dis-
cussed titrations, aluminum sensors yielded smaller breaks than the conventional sensors. As shown in the table, cobalt and cadmium wires, of high purity, yieldedlarger breaks than previously obtained in this titration. Surface treatment and purity of the wires or rods seems to be of great significance and needs further investigation. Concludlng Remarks
Most student laboratories are equipped with a pH meter and possess burets. Besides some common items, this equipment is all that is required for carrying out successfd-though perhaps not thermodynamically useful-potentiometric titrations. While a reference electrode is nice to have, it is not absolutely required for carrying out successful potentiometric titrations. I have shown that aluminum sensors vield endooint breaks that surpass thuseofronventional IS& 6 the titriitiun of halides versus silver ions, fluuride versus lant hanum(ll1) and orthophosphate versus IanthanumUII), cerium(III), or lead(I1). Stainless steel rods, including forks and knives, yield quite adequate endpoint breaks in many titrations. Ion-selective electrodes are not as "selective" as the name implies: If you have access to only a single one, i t is quite likely that it can be used for sensing many unexpected potentiometric titrations. Let your students start collecting their soft-drink cans to use as titration vessels cum sensors in potentiometric titrations. Acknowledgment
This work was performed under the auspices of the U.S. Department of Energy a t Lawrence Livermore National Laboratory under Contract W-7405-ENG-48. Literature Cited (1) Selig, W . S.,J. CHEM. E~uc.. 61.80 (1981). (2) Selig, W. S.,Z.Anal.Chem.,311.665 (1964). (3) Selig, W.S.,Ind.En#.Chem. Produd R&D,23,140(1964). (4) Selig.W.S.,ArnLob.,April 1964,36. ( 5 ) Taulli,T.A.,sndIrani,R. R,Anol.Chem.,35,10M1(1963). (6) Lingane, J. J., '"Electrosnalytical Chemistry,"2nd ed.. Interscience, New York, 1958, " *. 0, (7) Selig, W.S., Mierochom. J . , 28,489l1983) (8) Lingane, J. J.,ref. (61.p.116, (9) Sdig, W..Mikrochim.Act% 1970,564. (10) Selig, W.,Mikrmhim.Arra, 197611,9.
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Number 5
May 1985
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