A Laboratory Experiment CHARLES N. REILLEY University of North Carolina, Chapel Hill, North Carolina
C~~L~M methods E TofR analysis I C are made possible by Faraday's fundamental researches upon electrochemical equivalence. Quantitative experiments using the coulometric principle have long been included in physical chemistry courses, but only recently has there existed much stimulation for incorporating this principlein analytical determinations. Electrodepositions based on the works of Claasen, Fischer, Luckow, Land, and Treadwell have been the basis for many analytical separations and determinations but these processes do not rely upon the measurement of the amount of current nor upon operation a t 100 per cent current efficiency and therefore are not considered as coulometric procedures. Coulometric procedures may be divided into two general classes: those carried out a t controlled electrode potential and those a t constant current. I n the controlled potential method, the working electrode is kept at a fixed potential such that only the desired reaction can take place. The current decreases as the process proceeds, so that the total amount of current (coulombs) must be determined by means of an external coulometer (I, 9, 10, 11) or by experimentally fitting a mathematical expression for the current-time relationship (IS). The controlled potential method, though possibly more versatile than the constant current procedure, requires more time and involves the additional step for calculating the total number of coulombs. Constant current procedures require only a knowledge of the constant current and elapsed time to determine the number of coulombs. These procedures have been applied in two ways: electrolytic stripping of plated deposits and coulometric titrations. The stripping of plated deposits has been used to measure the thickness of metal and metal oxide films (d,6,13) as well as to determine the quantity of the plated metals in chronopotentiometry. Coulometric titrations (3, 5, 7, 8, 14) have been so named because a definite amount of reagent is generated in the solution electrically, this amount being directly determined by the number of coulombs. The eqnivalence point may be detected in any of the familiar ways (amperometric, potentiometric, photometric, with indicators). Some typical coulometric titrations are summarized in the table. Coulometric titrimetry has several unique advantages: 54
( 1 ) N o Standard Solutions Repired. In coulometric titrations the primary standard is the voltage of the standard cell (employed to determine the value of the current) and tbefregueniy of the alternating current used to operate the timer. Thus this single composite standard can replace a host of different chemical standards employed in the various possible titrations. Standard solutions can be prepared conlometrically by generating the desired chemical in a solution and diluting the resulting reagent to a known volume (3). In short, the electron is a most versatile reagent. (2) Utilization of Unstable Reugents. Unstable reagents may be generated and used on the spot so that time for them to decompose or evaporate will he considerably decreased. An excellent example of thisis the generation of chlorine (5) as a reagent. The preparation, storage, and precise transfer of a standard solution of chlorine by the usual volumetric means would be an extremely difficulttask. Yet chlorine may he easily prepared in known quantity by electrolysis of a sodium chloride solution. (3) Pretitration of Impurities. By pretitration of the generating solution before addition of sample, Coulometric Titrants
Titrant generated
HsO+ Clz Br*
Substance fm generation
OHFef+
Ha0 Fe+"
Cu+
Cu++(in C1-or Br- media) Ti+' CuGSn+' (in C1media)
Ti +a Cu Sn (+ SnCb)
Subslanee titmted
Anodic ometation Ha0 Banes As+'. SO1-C1Br AS+': Sbf'; I; H& SOz TI+; UOt+; Cu+ CNS-; N2H4; NH,OH Thiodiglycol; mustard gas; 8-hydroxyquinoline; aniline: acrolein. resor-
Cr04--; W - ; Br* Fe +a Fe'" C u t + Fe+a
544
JOURNAL OF CHEMICAL EDUCATION
much more accurate results can be obtained. Further, since the end-point indicator corrections are automatically cancelled and the effect of impurities in the generating solution is minimized, the method can he applied to solutions of much greater dilution. The importance of this characteristic is too often underemphasized. (4) Ease of Micro-addition of Reagent. Currents of the order of 60 electrons per second can be measured using modern electrometer tubes (4). Hence it seems that there is no practical minimum to the magnitude of currents that can be used to generate reagents electrically-the difficulty always lying in the detection of the equivalence points. The amperage of the current (analogous to the strength of a volumetric reagent) can he set accurately to almost any level, and extremely small quantities can easily be added near the equivalence point.
TITRATION
The principle of coulometric titrimetry has been successfully demonstrated in our instrumental laboratory course, using the instrument shown in Figure 1. In the analysis, students are given liquid samples of the stock arsenite solution accurately measured from a buret. After dilution to exactly 100 ml., 10-ml. aliquots are titrated by electrically generated iodine. Starch is used to indicate the end point. The potassium iodide solution is pretitrated until the first permanent blue-violet color before the first aliquot is added. This blank correction is necessary since a small quantity of arsenite is placed in the KI stock solution to insure that no iodine will be present in the initial solution from air oxidatiou of iodide. Practice runs may be made with the stock arsenite to check reproducibility of procedure and to standardize the arsenite. With care, students will obtain precision and accuracy within two parts in a thousand. Although other end-point techniques could be used, the visual end point helps to emphasize that the electrodes in the solution are for generating reagent and not for indicating an end point. The use of indicator electrodes t,ends to be especially confusing to students since potentiometric, conductometric, and amperometric refer to several different ways of determining an end point whereas coulometric, like volumetric, refers to the method of measuring reagent quantity. The necessary stock solutions consist of 12 g. AS& per 4 liters of solution (slightly acid for stability); a 1 molar solution of iYa?SOa for use in the separated cathode compartment; and a buffered KI solution (240 g. XI, 41 g. NaHCOZ,water sufficient for 4 liters of solution, 1ml. AszOasolution). C O U L O ~ I CCELL
(5) No Dilution. Since the titrant is actually the electrons themselves, addition of titrant does not cause dilution of t,he sample solution. This is of particular importance in the titration of microgram quantities of materials because dilution can alter the pretitrated reference state of the indicator in such a manner that the procedure described under advantage (3) will no longer be corrective. For this reason the internal generation procedure (7) is much preferred over the external generation procedure (3) in estimating microgram quantities coulometrically. (6) Remote Operation. Because the amount of tit,raut added and the detection of the equivalence point may both be electrical in nature, it is relatively simple to operate a t more remote distances than would be feasible with regular volumetric procedures. This is of particular significance in titrations of radioactive or dangerous materials. (7) Automatic Procedures. Because of the large amount of research in the development of electronic control circuitry during the past decade, automatic titrimetry is made especially simple when all controlled quantities can be made electrical in nature, as is the case with coulometric titrimetry. ~. ~~
The anode is a 2 X 3-cm. platinum-foil electrode made by fusing a platinum wire into soft-glass tubing and subsequently spot-welding the foil to the wire (heating the wire and foil with a burner and tapping a t the same time with a small hammer will accomplish the same result,). The cathode must be isolated from the body of the solution so that the electrically generated iodine formed at the anode will not be stirred into direct contact with t,he cathode and thus be reduced. For this purpose a fine fritted disc filter stick is filled with an inert electrolyte (1 M K.SOa), and a platinum wire is then immersed in this tube for cathodic contact. These two electrodes are immersed in a 150-ml. beaker containing 50 ml. of buffered KI solution. Rapid stirring is accomplished by means of a magnetic stirrer. When the current passes through the cell, the anode reaction is 21- - 2e Iz
-
-
and the cathode reaction is 2H,O
+ 2e
HI
+ 20R-
The products of the oathode reaction will only slowly diffuse into the solution proper, and will in any case have relatively small effect.
OCTOBER, 1954
545
The cathode electrolyte was chosen to be K?SO, since acid electrolytes have a tendency to generate hydrogen peroxide from oxygen of air.
I T
TIMER
COULOMETRIC SUPPLY
This supply was designed specifically for student use in this particular experiment, but could also be used in other experiments such as the determination of the electrochemical equivalence of silver or in determining transference numbers by the Hittorf method. To operate (see Figure 2), the power switch, S,, is turned on and the set is allowed to warm up for five minutes. The current (a~~roximatelv 50 ma.) is . standardized versus an mercury battery (Mallory RM-12 used as a standard ceu) by -
F ~ W - a. schom.tic
DI.W.~
fop ~
~
~
t
~ h p~p l yt
-
A
nnnhinv in..t,hn rivht,ha,nrl -. .-.- t,hn ....hnt,t,nn . - ... ... . -. .nnnnr - . - . .--.- - ...... rnrnnr ..- ...- mrl ......
turning the "calibratev knob until the meter reads zero. The instmment is now ready for use. When the output switch, &, is thrown to "on" (down position in Figure 2), the timer commences to operate and simultaneonsly a constant current flows through the cell. This outputswitch, therefore, is analogous to the stop On a buret. of reagent can be added by a quick flip of the switch on and off. A convenient t i e f o r a ticration is around 500 seconds. This time is a compromise between accuracy of timing and boredom to the o~erator. The instrumen; is readily assembled and has required very little attention. In fact, a graduate student with no prior knowledge of electronics duplicated the original model in less than one day.
B, EM-12 Mallow mercury battew (see text). C, 10 mid. 450.7. eleotrolytic condenser. Oh, Staneor ClW2 filter choke, 10.5 Bye, 110 ma. E, E I ~ (cou~ometrie) ~ ~ 0~11 ~ csee~ I M~, d . .~~ . ~ 15-~-15 ~ ma. A more sensitive meter can be used if resistor is olaced in series or osrallel. R L 30-ohm, 10-watt wire-wound resistor. Rz, 20,M)O-ohm, lo-watt wirewound resistor. Rt, Resistance value varied but waa approximately 2300 .hm. d , by d n g2 0 a 0 0 - 0 h ~ ,2 5 - ~ . % resistor tt and a resistor (e. c.300 ohma, 5 wattd in series. R., 250-ohm, 2Cwatt variable mistor (Ohmite 0154). &. SPST toggle switch. 62,DPDT toggle switch. Sa, Tap key from war surplus (Mdlory 2001 should be auitsble replacement). T, st.ncor ~ ~ 8 4 power 0 9 transformer (700 VCT. 90 ma., 5-v., za.; 6.a-~.. 3 a.). Winding with leads, zr.goes to filament (not shown) of 6 V 6. Timer. GraLab Micro-Timer type 202, mfd. by Gray Laboratory and Mfg. Ca.. Dayton, Ohio. Cabinet. sloping front osbinet from Par-Metal Products, ~ o n Island g City. N. Y. (14 x 8 x 8).
( 5 ) FARRINOTOS, P. S., oon
AND
E. H. SWIFT,Anal. Chc t r . , 2 2 ,
,."X",
O O J {'"d",.
(6) FRANCIS, H. T., J. Eleclrochem. Sac., 93, 79 ( 1 9 s ) .
(7) FURMAN, N. H., W. D. COOKE,AND C. N. REII,I,EY,Anal. Chen., 23, 915 (1951). (8) LINGANE, J. J., "Electro~nslyt.ic&lChemistry," Interscience Publishers, Inc.. New York. 1953. (9) LINGAXE, J.'J., .J.'Am. Chem.'~oc.,67, 1916 (1915). LITERATURE CITED (10) LINOANE, J. J., A N D S. L. JONES, Anal. Chem., 22, 1220 (1) BOGAN, S., L. MEITES, E. PETERS,A N D J. M. STURTEVANT, (1950). (11) LINOANE,J . J., A N D L. A. SMALL,Anal. Chem., 21, 1119 J. Am. Chem. Sac., 73, 1584 (1951). (2) CMFBELL,W. E., AND U. B. THOMAS, Tmm. Electroehem. (1919). , T., ASD A. It. WILLEY,J. Electroehem. Soc., 99, (12) I i u x z ~ C. Sac., 76,303 (1939). (3) DEFOBD,D. D., 3. N. PITTS, AND C. J. JOHNS. Anal. Chem., 354 (1952). 23, 938 (1951). (13) MACNEVIN,W. M., A N D B. B. BAKER,Anal. Chex., 24, (4) ELMORE, W., A N D M. SANDS,"Ele~tronics: Experimental 986 (1952). Teohniques," MeGrew-Hill Book Co., Ine., New York, (14) SEASE,J. W.,C. NIEIIANN, .AND E. H. SWIFT,A m l . Chem., 1949.
19, 197 (IW7).
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