An EXPERIMENT
ILLUSTRATE the MECHANISM of the CONDUCTANCE PROCESS in IONIC SOLUTIONS to
LEWIS G. LONGSWORTH The Laboratories of The Rockefeller Institute for Medical Research, New York Cily
A simplified experiment for the accurate measurement of transference data by the method of mooing boundaries i s described. I n this method the conductance process i n the main body of the solution may be followed by obsernring the motion of the boundary while at the electrode the simul-
taneous growth of a visible diffusion layer illustrates the mechanism of electrical transport at a metal-solution interface. The experiment i s suitable for laboratory courses i n physical or electro-chemistry and i s capable of yielding precise results.
T
occurs a t the anode. Thus the solution for transference-number determination is placed in the tube A, Figure 1, in the bottom of which is sealed a cylinder of some metal which forms a soluble salt in combination with the anion of this solution. As an example, the solution in A may be 0.1 N potassium chloride and the metal cadmium. If now current is passed in the direction indicated by the arrow, cadmium chloride will form a t the electrode, and since the cadmium ion constituent is slower than the potassium ion c o n s t i t u e n t a boundary between the solutions of cadmium chloride and potassium chloride will leave the face of the electrode and move up the tube as indicated in Figure 2. Flcrrne 1 mcuan 2 Of course, as the boundary SrMpuaIeD METHOD p o n ~THG l ~ ~ BOUND. moves upward, the commonALRY ion constituent, which is the chloride ion in this example, moves downward the and eventually accumulates around the anode. Electrical neutrality
HE LABORATORY experiment usually employed for the illustration of electrical transference in ionic solutions and the phenomena occurring a t a solution-metal interface is based upon the classic researches of Hittorf and depends upon the analysis of those portions of the solution which surround the electrodes before and after the passage of a measured quantity of electricity. In this Hittorf or gravimetric method the electrolysis is seldom accompanied by visually observable changes in the solution and most of the time is spent in analytical work--operations which teach little concerning the mechanism of the conductance process. The laboratory experiment which is the subject of this paper overcomes some of these diEculties and requires only apparatus which is usually available for undergraduate laboratory instmction in electrochemistry. The experiment is a simplified form of the moving houndary method for the determination of transference numbers and possesses the following advantages over the gravimetric method.
1. The actual migration of an ion constituent may he observed. 2. The concentration change which occurs around one is preserved in the solution near the electrode due of the electrodes is visible and furnishes an accurate to the fact that only a fraction of the ions conception of how the electrode processes differ which are formed by the electrochemical oxidation of from those in the main body of the solution. the anode are necessary for the maintenance of the 3. The method is capable of yielding precise results in growing column of indicator solution at the adjusted a short time and it is possible to check the progress concentration and consequently only this fraction is of the experiment almost from the beginning. carried away from the anode by electrical mimation. A concentrated solution of cadmium chlorize thus DESCRIPTION O F THE EXPERIMENT forms just above the anode and slowly diffuses upward The experiment described below is a simplification, into the more dilute indicator solution. As the experidue to Franklin and Cady (1) and Cady and Longs- ment proceeds the thickness of this "diffusion layer" worth (Z),of the general moving-boundary method in gradually increases, the layer being quite visible bethat no special mechanism is required for the initial cause of ,+e variation of the refractive index in formation of the boundary by the superposition of two this region. The diffusion layer is indicated by the solutions. The "indicator" or following solution is shading above the electrode in Figure 2. If, at the formed as a result of the electrochemical process which conclusion of the experiment, the tube i s tip#ed, this 420
hemy solution of cadmium chloride may be seen falling d m the tube.
the tube, g, in which the boundary is observed. The upper end of this tube is sealed to the cathode chamber while a cylinder of cadmium, h, to the base of which a ELEMENTARY THEORY connecting wire, k, is soldered, is sealed into the lower In accordance with the definition of the transference end of the tube by means of the piscein plug, i. The number, the passage of 1 Faraday, or 96,500 coulombs, tube, 1, leads the anode connection out of the water of electricity through a solution of 0.1 N potassium bath, care being taken that the rubber connection, j, chloride, for example, causes T+ equivalents of potas- does not obscure the solution-metal interface where the sium ion to pass a fixed point in the solution. T+ is the boundary is first detected and where the diffusionlayer cation transference number and this number of equiva- is observed to form. This interface should be a few lents is contained in a volume of solution, V, in ml., mm. below the first graduation on the tube. The introduction of the 0.1 N potassium chloride into equal to 'OoO '+, where C is the concentration in the cell, which has been preC viously cleaned and thoroughly equivalents per liter of solution. Of course, the h dried, may be accomplished as quantity of electricity, f , necessary to cause the boundfollows. With the cathode and ary to move through a smaller volume, v, will be pro; inner tube c removed, a few ml. portionately smaller than F, so that of solution are poured into the cell. A solid glass rod, with a flat end and a diameter Since the amount of electricity involved in a deslightly less than that of the e termination is less than can be measured accurately graduated tube, is used as a piswith a conlometer it is necessary to determine the ton in the latter until it is ennumber of coulombs by passing a constant current, i, f tirely filled w i t h solution. through the tube and multiplying this by the t i e , t , d Care should be taken that the in seconds, required for the boundary to move through bubble of air wbich clings tethe given volume, v. Thus f becomes equal to it and naciously to the face of the equation (1) may be written: g anode is forced out of the tube. After the graduated tube has been filled with solution the inner tube c is placed in position the equation which is used in computing the transferand the, cathode chamber filled. ence number from the experimental data. The silver-silver chloride cathode which is finally inserted EXPERIMENTAL DETAILS 1 is of the form described by j Smith and MacInnes (4). I t The Moving Boundary Cell. An easily constructed k . moving boundary cell which is suitable for measureis prepared by heavily plating I platinum gauze with silver and ments that can be made by the simplified method outlined above is shown in Figure 3. In this method the FIGUKE 3 . - ~ n e MOT- then thoroughly chloridizing 'NG BOUNDARY CELL the silver deposit by electrolyzcathode must be sufficiently far removed from the tube ing as anode in a solution of in which the boundary is observed to prevent the concentration changes which occur there from pene- hydrogen chloride. Since this electrode must carry trating, either by diffusion or convection, into the considerable quantities of electricity with no evolution neighborhood of the boundary. The cathode chamber of gas the area of the electrode should be a t least one e, Figure 3, is consequently of the type described by square centimeter. It is necessary to immerse the tube in which the MacInnes and Brighton (3), slightly modified, since this arrangement satisfies the preceding requirement boundary moves in a water bath ( T , Figure 4) in order and is quite compact. The inner tube c is closed a t to dissipate the heat which is developed in the tube by the lower end, thus retaining the heavy solution of the passage of the electric current. Otherwise convecpotassium chloride which is developed around the tion currents in the tube will distort and even destroy silver-silver chloride cathode f , and is supported by the the boundary. The author has used a cylindrical glass projections d. This tube may be slipped out of the cell jar, 9 an. in diameter and 24 em. high, for the water when the rubber stopper b, wbich carries the cathode bath. Only rough temperature control is required, and connection and the vent a, is not in place and it is with it is unnecessary to stir the water in the bath. The Electrical Circuit. The measurement of the this inner tube removed that the cell may be most current and the maintenance of this current a t a conconveniently filled. The author is indebted to Dr. D. A. MacInnes for the stant value may be accomplished as follows. The very practical suggestion that a 1-ml. graduated pipet, fixed resistance R, Figure 4, is placed in series with the on which the graduations do not extend to the tip and moving boundary cell and the current through the from which the tip has been removed, may be used for latter is determined by measuring, on the potenti-
ometer P, the potential drop across this accurately calibrated resistance. A portion of the battery, B, is shunted across the rheostat R' which has a sliding contact F and by adjustment of this contact a constant current through the cell may be maintained. With the potentiometer set a t a given value the observer continually adjusts the contact F so as to keep the needle of the galvanometer G near its zero position. As the boundary ascends the tube the length of the column of indicator electrolyte increases correspondingly and,
of the two solutions. Although a trained observer can locate the average boundary unaided, the simple optical system shown in Figure 4 greatly helps in the detection of the boundary. A source of diffused light, CD, should be placed behind the graduated tube. An opaque screen, S, such as a rectangular piece of cardboard, is held in one hand and placed between the light source and the tube while the boundary is viewed with a simple reading lens, L, held in the other hand. When the upper edge of the opaque screen is nearly in line with the boundary, the lens, and the eye, this edge appears to be distorted. Keeping the lens and the eye approximately on a horizontal plane with the upper edge of the opaque screen, and in line with the tube, these are moved slowly up or down until a distortion of the edge of the screen indicates the position of the boundary. As the boundary leaves the face of the anode immediately after the electrical circuit is closed it is quite easily located and the observer should familiarize himself with its detection in this interval before it has reached the first graduation on the tube.* RESULTS OF A TYPICAL EXPERIMENT
The results of an experiment which was performed in the manner outlined are given in Table 1. In this table, which also illustrates a method of recording the data, the time recorded in the second column is that a t which the boundary passed the graduation corresponding to the volume recorded in the first column. The differences between successive time observations, reduced to seconds, are recorded in the third column and it is the approximate constancy of these values which indicates that the boundary has moved at a uniform rate.
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FIGURE 4.-THE COMPLETEASSEMBLY Electrical Apparatus Used in a Typical Determination. P-Students' type potentiometer &Portable type galvanometer R-3M)A resistancefrom a dial decade box R'-7000n adjustable rheostat A-90 volts "B" battery B-150 volts "B" battery
since this solution is a poorer conductor than the one which it replaces, the total resistance of the cell continuously increases. The potential applied to the cell must therefore be continually increased in order to maintain a constant current. The time, to the nearest second, a t which the boundary crosses successive graduations on the tube is obtained with the aid of a watch. When the current is constant the time intervals necessary for the boundary to sweep through successive 0.1-ml. portions of the tube should be identical and the constancy of these intervals furnishes an important check upon the progress of the experiment. Obsemtion o f the Boundarv. The boundarv is visible because of the differencein the refractive indices
TABLE 1 Dsmsm~rroaOP
CATSONTEANSPBBBNCB NOXB&R or 0.1 N KC1 Repistmce (R,Fig. 4) = 3003 TBB
-
Potentiometer setting Current
Vdumc, d.
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1.2188 volts 0.004082 ampere
Time
Al, rrconds
Total 4885
DISCUSSION OF
TBE RESULTS
In the computation of T+ from the data of Table 1 by means of equation (2) it is evident that u = 1 ml., C = 0.1, F = 96,500, i = 0.004062 and t = 4885. The value of T+ computed in this manner, 0.4868, is subject to a correction of 4-0.2 per cent. for volume changes and thus becomes 0.487~. This relatively small correction unfortunately involves some rather d i c u l t computa-
* Since it is necessary to adjust the potential across the cell very frequently in order to maintain a constant current, it may he desirable to have two observers. one followine - the boundam while the other is regulating the potential.
tions but is completely treated in reference (5). page 203. The value, 0.4878, is 0.5 per cent lower than the value 0.4898, which has been obtained in the most recent determinations (6) and it was observed later that this discrepancy between the two values was almost entirely due to the manufacturer's calibration of the 1ml. pipet which was used in the construction of the moving boundary cell. An independent calibration of this volume gave the value, 1.0046 ml. Using this figure for the volume, the data of Table 1 give a corrected transference number of 0.4895, a value which agrees well with the value previously mentioned. This value, which has been obtained by the moving boundary method, is in excellent agreement with the most recent determinations of this constant by the Hittorf method (7). It is probable that the l-ml. pipet was calibrated by the maker "to deliver" instead of "to contain." If the measurements of time and current are of s&cient accuracy to warrant it, an independent calibration of the tube volume should be made. The particular tube which was used in the preceding determination was graduated for a length of 16 cm. Since this is somewhat longer than the tubes which are ordimarily used in precise measurements by the moving boundary method, a large battery is required. Thus in the experiment described above a total of about 240 volts in the form of "B" batteries was required. The battery requirements may be decreased by either shortening the tube or using a lower current or both. The latter expedient is practical only within certain limits, however, since the boundary is diicult to locate
if the current density is too low. It is also practicable to decrease the drain on the battery, B, Figure 4, by regulating the current with a variable resistance in series with this battery instead of the shunted resistance, R'. In this brief paper it is not possible to discuss such subjects as the persistence and visibility of the boundary and the concentration relation between the leading and indicator solutions. The entire subject of the moving boundary method, however, is fully treated in a recent review by MacInnes and Longsworth (5), a paper which also contains a complete bibliography of the subject up to 1932. The author wishes to express his thanks to Dr. D. A. MacInnes for his kind suggestions in connection with this work. LITERATURE CITED
(1)
FRANKLIN AND CADY,"Ionic velocities in liquid ammonia."
3. Am. Chcrn. Soc., 26,499-530 (1904). (2) CADYAND LoNGsWoRTn. "A modification of the m o v i n ~ boundary method for the determination of transference numbers," ibid., 51, 1656-64 (1929). MACINNES AND BRIGHTON, "Moving boundary method for the detumination of transference numbers. 111. A novel form of apparatus," ibid., 47, 994-9 (1925). S m ~ nAND MACINNES, "The moving boundary method of determining transference numbers. 11." ;bid.. 46, 1398403 (1924).
AND LONGSWORTH, "Transference numbers by the method of moving bbundaries." Chem. Rm.,11, 171-230
MACI~NES
1,----,. 1 9.17)
LONGSWORTH, "An application of moving boundaries to a
study of aqueous miAures of hydrogen chloride and potassium chloride," I. Am. Chern. Soc., 52, 1897-910 (1930). MACINNES AND DOLE, !'The transference numbers of patassium chloride," ibid., 53,1357-64 (1931).
