W. E. HARRIS University of Alberta, Edmonton, Alberta, Canada
THE
use of polarography in routine analysis and research is sufficiently extensive that today most courses in instrumental analysis include training in polarography. Numerous recording and manual instrnments have been described ( 1 ) for carrying out research, routine and instructional polarography. Simple polarographic instruments have been suggested by Mdler (2),Rulfs (5), and Mundy and Allen (4) for student use which allow the instructor to emphasize principles rather than experimental operations involving the dials of a closed box. In this paper a novel approach to instructional polarography is described which has been found to be effective in teaching introductory polarographic principles to senior students. Although polarographs conventionally include a battery and slide wire as the polarizing unit, it is sometimes convenient, when only one potential is required, to replace the battery and slide wire with a suitable reference half cell or working anode. Kolthoff and Harris (5)described the use of a mercury-iodide working anode in the amperometric titration of mercaptans. Ewing (6) has also suggested a number of other reference half cells which can be used to effect a similar simplification of apparatus for other amperometric titrations. In the present work the above concept has been extended, and a student polarographic device has been developed in which the usual battery and slide wire are replaced by a group of half cells, conveniently termed a half-cell bundle. The modification is schematically suggested by Figure 1in which the portion of the conventional polarograph circuit within the insert, D, E, F, G, is replaced by the half cells, H. Instead of the continuous curve usually produced by a polarograph with a recording unit, the device described provides a series of points because of the definite stepwise voltage variation. The instrument described is referred to as a chemical polarograph.
heating. One end of this copper wire was extended through the top of the half-cell bundle for contact with the rest of the circuit. The bundle was completed by adding one or two milliliters of mercury to each cavity, and filling with appropriate electrolytes selected with the aid of Table 1. Trapped air bubbles were removed by vacuum and the small cavity on top was half filled. To prevent subsequent adulteration of the bulk of the half-cell solution, a tight glass wool plug was inserted in this top cavity as shown. The potential of each half cell was measured potentiometrically. The reference half cells for use in polarography (1,6) generally involve either mercury or silver and their
APPARATUS
The equipment needed for a chemical polarograph is shown in Figure 2. The half-cell bundle was made from a block of polyethylene four inches in diameter and four inches long. Cavities for the half-ceU electrolytes, of the form shown in the diagram, were easily made with a drill press and appropriate drills. The bottom of each half cell was closed with a small piece of polyethylene softened with a Bunsen burner. Platinum wires, which were used to make contact with the mercury in the bottom of each half cell, were soldered to an open ring of copper wire and the assembly embedded in the bottom of the polyethylene block by 'Taken in part from a paper presented before the Division of Chemicd Education a t the 128th Meetine of the American Chemioal Society, Minneapolis, September, 1955 408
A , Current measuring device: B, bopping mercury electrode; C, salt bridge; D, saturated calomel electrode; E, potential meaaurins device: F. alide wire; G,battery; H,reference haU cells.
JOURNAL OF CHEMICAL EDUCATION
salts. I n this work a more complete series of additional mercury half cells has been developed for polarographic purposes. Compositions of electrolytes for twenty-nine half cells which cover the potential range from +0.4 to -1.0 volt versus S.C.E. are shown in Table 1. The electrode potential a t room temperature of a half cell involving the couple 2Hg-Hgz++ 2e is given by the following form of the Nernst equation
+
E
=
EDm-H,++
+ 0.03 log ox,++
(1)
It can be seen from equation (1) that the electrode potential can be fixed a t any value by controlling the mercury(1) ion activity. This has been done conveniently in the first eleven half cells of Table 1, by employing a series of saturated solutions of increasingly insoluble mercury(1) salts. I n these half cells the mercury(1) ion activity has in turn been fixed by the solubility product of the mercurous salt and the concentration of the anion. For instance, the electrode potentials of the mercurous chloride half cells are given by the following modification of (1). E = E9n.-E.,++ + 0 . 0 3 1 0 g K ~ ~ ~0.06loga~1~~,~ (2)
Thus potentials from +0.4 to -0.1 volt versus S.C.E.
TABLE 1 Composition of Electrolvtes for Mercun, Half Cells App~ozimate "8.
(UO&
S.C.E.) +0.4
-0.05 -0.1 - 0 15 -0.2 -0.25 -0.3 -0.35 -0.4 -0.45 -0.5 -0.55 -0 6 -0 65 -0.7 -0.75 -0.8 -0.85 -0.9 -0.95 -1.0
Compositionc Saturated K,SO.. excess H d O . 5 g. K N O ~ ,i.8 HZPO,, g: KH~PO,,50 m ~ . H90, excess HgaP04 5 g. KNOs, 0.15 g. H3POl, 11 g. KHnP04, 50 ml. H20, excess HgaP04 5 g. KNOa, 6.5 g. KHnP04,1 g. KIHPOs 50 ml. H.0, excess Hg,P04 5 g. KNOs, 4 g. KH*PO,, 4 g. KIHPO&,50 ml. H20, excess HgaP04 10 g. KNOa,0.06 g. KCI, 50 ml. HsO, excess Hg,Cl, 10 g. KNOa, 0.4 g. KCI, 50 ml. H1O, excess HgA% 10 g. KNOJ, 2.6 g. KC!, 50 ml. H O , excess HgGh Saturated KCI. excess HerC1. 5 g. KCI, 1.1 g: KBr, 50;hl. HIO, excess Hg2Bh 5 g. KCl, 6 g. KBr, 50 ml. H20, excess Hg,Br3 1.8 g. HgBr,, 50 ml. saturated KBr 0 . 1 g. HgBh, 50 ml. saturated NeBr 5 g. KC1, 3.4 g. HgL, 5.0 g. KI, 50 ml. HnO 5 g. KCI, 10 g. HgIn, 15.5 g. KI, 50 ml. H.0 20 g. Hg4,38 g. KI,50 ml. H20 8 g. HgL, 55 g. KI, 50 ml. H20 0 . 9 z. HeL. 50 ml. saturated NaI 5 g. k ~ l ; 0 5 g. HgO, 6.9 g. KCN, 50 ml. HBO,10 g. ZnO, 2 g. Na808 5 g. KCI, 0.35 g. HgO, 0.8 KCN, 50 ml. H20, 2 g. NenS08 5 g. KC1, 1.3 g. HgO, 3 KCN, 50 ml. H20, 2 g. Na2SOs 5 g. KCl, 1.3 g. HgO, 5.5 g. KCN, 50 ml. H20,2 g.
g.
Z4
Na.Rn. A.w'--a
4 g. HgO, 15 g. KCN, 50 ml. H.0, 2 g. NenSO, 4 g. HgO, 25 g. KCN, 50 ml. H.0, 2 g. NerS01 0.15 g. HgO, 30 g. KCN, 50 ml. H20, 2 g. NenS01 15 g. KnHP04,5 g. NaaSOs, 50 ml. H90, 12 ml. saturated Na.8, excess HgS 50 ml. HsO, 5 g. KCI, 5 g. KOH, 2 ml. saturated NenS, 5 g. NenSO8, excess HgS 4.5 g. HgS, 5 g. KCI, 5 g. KOH, 50 ml. HzO, 15 ml. saturated N&, 5 g. Na2SOa 2 g. HgS, 5 g. KOH, 25 ml. HaO, 25 ml. saturated Na2S,5 g. NasS01
' To prepare these solutions or suspensions it is recommended that the chemicals be added in the order stated.
have been obtained by using phosphate, chloride, or bromide salts and controlline the concentration of the relevant anion. I n a similar way those half cells more negative than -0.1 volt and to -1.0 volt involve the couple HgHg++ 2 e. I n this case the electrode potentials were fixed a t various values by controllimg the mercury(I1) ion activity. This was done by employing insoluble salts or complexes of mercury(I1) and bromide, iodide, cyanide, or sulfide with appropriate amounts of a salt having a common anion. For example, the electrode ootential of a mercuric iodide half cell would be eiven by the following equation:
-
+
L
A, Microammeter, Weaton, Model 430, 30 microamperes. B, Electrolyt,o condenser (7). 2004 miorofarads, 12 volts. C. Eleotrolvsis cell. 3.5-am. test tube. ~ . ' ~ i si"ter+-&lsss ne salt bridge. 1 em.. Corning, Catalogue No. 30570. E, Potassium nityete salt bridge. lower end tipped with 1 om. of porous glass (8).upper end ~ i t h glass WOOL F. Half-cell bundle with 10 half-cell cavities. Portion removed to show cross aeotion of one half-cell and its electrical oanneotion and the oentrhl bottom hale for a supporting pin. =ire near the bottom oonneotine to 10 ha~f-oeua. G, partial oirole of
VOLUME 35, NO. 8, AUGUST, 1958
+
E = E'H.-H~++ 0.03 log KinstaSl4= 0.03 log UHSI,= -0.12 log ar-
+
(3'
The stabilities of these half cells for polarographic purposes have been tested with the following results. Normal polarographic currents may be allowed to flow without appreciable change of potential. A few of the half cells show concentration polarization with currents 409
of the order of 1000 microamperes, but correct potentials are restored by agitation. The electrical resistance of the half cells when installed in a half-cell bundle of the type shown in Figure 1 is 100 ohms or less. I n some cases the resistance has been kept low by addition of either potassium nitrate or chloride to the electrolyte. Their capacity is sufficient to allow many hundreds of polarographic measurements to be made without measurable change of potential. Stability toward air oxidation has been achieved by the addition of sodium sulfite to those half cells with the more negative potentials. I n view of their relative stability it has been found unnecessary to check the potentials of the half cells in a half-cell bundle oftener than every few months. A STUDENT EXPERIMENT
In the author's classes in instrumental analysis, the preparation of the reagent and instruments for each of the experiments is assigned to groups of two or more students. In this way the preparatory work for an individual student is confined to only one experiment. The group assigned to the polarographic experiment is expected to fill the half-cell bundle, measure and post the half-cell potentials, and assemble and test the equipment. Thallium sulfate diluted with potassium sulfate is a convenient mixture for a student polarography experiment since oxygen removal can be carried out with sodium sulfite rather than with nitrogen. To illustrate the characteristics of a thallium current voltage curve it is recompended that half cells with potentials of about -0.2.; -0.3; -0.4; -0.43; -0.45; -0.47; -0.5; -0.6; -0.8, and -1.0 volt versus S.C.E. be installed in the half-cell bundle. Experimental data for current voltage curves are obtained by reading the current on the microammeter with each of the ten half cells in turn. The condenser is helpful if the current is more than one or two microamperes, but is unnecessary when the current is less than this value. Inexperienced students obtain enough data in a three hour laboratory period to enable them to report the following: (1) The current voltage curve for thallium. (2) The residual current. (3) The curve for log (id-i)/i versus potential and the value on n. (4) The half-wave potential. (5) The percentage of thallium in the unknown. The students calculate the percentage by the absolute method using the Ilkovic equation with D = 2.00 X ~m.~/sec., and using their experimental values for diffusion current and the capillary constant. The author has used various modifications of the chemical polarograph in instrumental analysis classes for several years and has found the apparatus described to be very suitable for instructional polarography a t the senior student level. It is simple, inexpensive, rugged, and portable. Since there is no sliding contact a t any point, students are not plagued by imperfect connections caused by corrosion if the instrumental analysis laboratory is located in a part of the building with a corrosive atmosphere. Students are able t o obtain enough data in one afternoon t o get considerable insight into the elementary principles of polarography. An example of a current voltage curve typical of those
"
0
-0 9
Potential. Volt. Fiwr. 3.
c u m n t vo1t.g.
vs.
S. C. E.
c u m 0btlin.d with I Chernisd Pol-. w=~h
A. Residual current with 0.05 M ammonium sulfate, 0.1 M ammonium hydroxide, 0.03 M sodium sulfite and with one drop of saturated thymo1 Der 50 m1. of 8olution. B, Above with 8.62 X 10.. M thallium sulfate.
reported by students is shown in Figure 3. It can be seen that enough experimental data are obtained so that the student is left in no doubt as to the characteristics of a polarographic current-voltage curve. A summary of other student results is shown in Table 2. Unfortunately, the author did not retain for comparative purposes the data reported by classes several years ago using a battery and slide wire type of polarograph. However, generally unsatisfactory results were obtained by the students with the amount of laboratory assistance available, and this led the author to seek other means of carrying out the experimental work. TABLE 2 Summary of Student Performance for Thallium Analysis Using the Chemical Polamgraph Item reported AE A log ( i d - i ) / i El/,
Per cent th.dlium
Cowed
ualue(s)
0.059 -0.47 volt from 4% to 28%
....
Time taken far experimental work" a
Median of student resdts
P~obable deviation
0.060 0.010 -0.48 volt 0.01volt . .. . 0.96% (From correct values) 2haurs 0.6hour
Ineludina weighing of unknown and dilution to volume
GENERAL
The half cells described in this paper can also be useful for a variety of amperometric titrations and for routine quantitative polarography where diffusion currents need t o be measured at only a few potentials (6). Since the list in Table 1 is more complete than previous compilations of half cells, a wider variety of choices is now available to the analytical chemist setting up routine methods of analysis. The range of potentials can be conveniently extended t o cover the interval from +1.4 volts t o -2.0 volts versus S.C.E. by also including a low resistance Weston cell in the polarographic or amperometric titration circuit. ACKNOWLEDGMENT
The author acknowledges the contribution made by many students in instrumental analysis: Miss M. Hayes whose data are reported in Figure 3, those JOURNAL OF CHEMICAL EDUCATION
whose results are summarized in Table 2, and those who tested the preliminary versions of the instrument described. LITERATURE CITED (1) KOLTHOFF, I. M., AND J. J. LINGANE, "Pola~ography,"2nd ed., Interscience Publishers, h e . , New York, 1952, Chaps. 16 and 17. ( 2 ) MULLER, 0. H., J. CHEM.EDUC., 18, 111 (1941). (2) RULFS,C. L., J. CHEM. EDUC.,25, 224 (1948).
VOLUME 35, NO. 8, AUGUST, 1958
(4) MUNDY,B. W.,AND N. W. ALLEN,J. CHEM.EDUC.,30, 143 (1953). (5) KOLTROFF, I. M.,AND W. E. HARRIS,Ind. Eng. Chem. Anal. 18, 161 (1946). (6) EWING, G. W., "Instrumental Methods of Chemical Analysis," MoGraw-Hill Book Co., Ine., New York, 1954, p. 77. A,, 2.Anal. Chem., 116, l(1939). (7) NEWERGER, (8) CARSON, W.H.,C. E. MICHELSON, AND K. KOYAMA, Anal. Chem., 27, 472 (1955).
