Contin uous Co uIo met ric Analysis Using a Working Electrode of Predetermined Potential EDGAR L. ECKFELDT Leeds & Norfhrup
Co., Philadelphia 44, Pa.
b In a new method of electrochemical analysis a sample solution was made to flow through u cell in contact with a working electrode of large area and predetermined potential. The potential was chosen to cause an electrochemical reaction with a constituent in question and to exclude other reactions, in so far as possible. Certain operating conditions resulted in essentially 100% reaction efficiency and permitted calculation of the constituent concentration through a fundamental relationship using the measured variables of cell current and sample solution flow rate. Results obtained in determining iodide, iodine, and dissolved oxygen indicate that the method is suited for continuously measuring low concentrations of electro-oxidizable or reducible substances in flowing streams, but more work is needed to establish practical applications.
T
HE applicatioii of continuoub coulometric analysis to gas samples has been described by Shaffer, Briglio, and Brockman (8) and to liquid samples, by Eckfeldt, Proctor. and Perley (3, 4 ) . Commercial hstrunirmts using this principle have been on the market for wveral years (1, 7 ) . I n this technique A sample stream is made to flow continuously into an rlectrolytic reaction zone. where a secondary coulometric process causes a quantitative reaction n ith the constituent to be measured. The operation of such equipment is advantageously based on fundamental considerations including Faraday's law, and hence the calibration can be made dependent only 011 the sample solution flow rate and the magnitude of the electrolysis current, ,I' cYpresed by
control. 111 previous practice the end point condition is established, for example, by using a pair of auxiliary detecting electrodes. A potentiometric signal from these electrodes operates an automatic current controller, which in turn regulates the magnitude of the electrolyzing current. I n the technique described, auxiliary detecting electrodes are not needed. The desired coulometric reaction takes place as a primary coulometric process a t the surface of the working electrode, the potential of which is maintained a t a value to produce the desired reaction Because all of the bample constituent reacts before the solution leaves the cell, the current lewd is determined by the supply of the constituent in the ,mmple stream. This nlcrhod of operation doc>¬ require complicated electrical control equipment, because it is necessary only to maintain a constaiit potential a t the working electrode. Through the choice of the potential, the method offers considerable selectivity regarding the coulometric reaction Each electro-oxidizable or reducible substance that may
be present in a solution nil1 h a l e its own characteristic value of potential a t which it will commence to react a t an electrode. By suitably choosing the value, one or more of the substances can be made to react, while the reaction of substances of highez potential is excluded. I n a batch coulometric proccss a t controlled potential, the current starts a t some niaxinium value and falls continuously until the constituent is (lepleted and the current becomes Z C I O . Thc varying current is integrated 01 (Ir the entire period to give the coulombic measurement of the constituent. I n the present method, however, the constituent reacts in a short period of time, the maximum extent of which is determined by the transit time of the continuously flon ing solution through the cell. The area of the electrode and the other design characteristics of the electrode and cell are such as t o make 100% reaction possible. Under these conditions the instantaneous values of the current and the solution flow rate g i w the concentration value. as exprcssrd by Cqiwtion 1. I n practice it S O N i l d V POWER
Figure 1 , Experimental equipment assembly
2):
n;
=
-1
96,500 R
where S is the saiiipie solution normality, I is the electrolysis current in jrnperes, and R is the flon rate of the sample solution in liters per second. The present paper describes a new approach to continuous coulometric malysis, in which the working electrode not only causes the coulometric reaction hut also effects the neccssary current VOL. 31, NO. 9, SEPTEMBER 1959
a
1453
i 4
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4 Figure 2. construction
Cell
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E C'RICAL
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y,
7ESYINA~
n.tlsoL P L L S T ~ C C L S T I N G
ENLbRGEC CRG3P SEZTION O K T H I PUHEHINB3 IU T H E
GOLD E L E C T R O D E
CHAUIKL YUWiACt
METLL
i
b Figure 3. Working electrode construction
*,JNCUINGS
IN CHANNC~
SURFACE R E F E R E N C E ELECTRODE SOLUTION
50WT
IN i H b N N E L L A B ' R I N T Y OOI5'DEEP 0 2 5 0 WIDE
is expedient to keep the flon rate constant, thus making the concentration directly proportional to the current. The possibility of making measurements in this way has recently been suggested by Wherry and DeFord (IO). The present study was substantially completed before that paper appeared. The study was undertaken to investigate the feasibility of the method. and in particular, to establish nhether a coulometric cell could be made to operate a t efficiencies sufficiently high for quantitative analysis. EQUIPMENT AND SOLUTIONS
A diagram of the apparatus is sho\m in Figure 1. A regulated flow of a sample solution through the cell was obtained by using a constant-head sample reservoir to feed the solution through a capillary tube into the cell. The effluent from the ccll \vas discarded. The potential of the working electrode was regulated by iinprcssing a constant voltage across the cell terminals. This voltagc was obtained by appropriately tapping a current-carrying resistive circuit, powered by dry cells. The electrical current floning in the cell circuit was nicasured by a recorder. Cell Configuration. T h e cell configuration is of primary importance. T h e cell should be designed t o reduce t h e time factor needed for completion of the electrochemical reaction. This can be achieved by establishing conditions t h a t bring each part of the solution as close as possible to an element of t h e electrode surface. The geonietry of the ccll is thus iniportant. Tlic area of the, 1% orhiiig electrode should be a niaxinium and the thickness of the solution layer in contact with the electrode surfacv should be a minimum. Agitation of the solution will prevent stratification anti ensure 1454
ANALYTICAL CHEMISTRY
that the more remote parts of the solution approach the electrode more closely. These principles were applied in the cell used in the present study. A long flotv path of thin but wide cross section was established. The wide dimension made contact with a rather large total area of electrodr surface. The general arrangement of. the cell is shown in Figure 2. A working electrode of approximately circular shape abutted against a porous ceramic disk of about the same diameter. A chamber which contained a reference electrode and solution abutted against the other side of the porous disk. The design permitted the parts to be clamped firmly together. The sample solution passed through a rather wide, but shallow., channel cut as a labyrinth in the surface of the working electrode as shown in Figure 3. Some results were obtained Rith the channel surface smooth, as left from the machining operation, and others were made with the surface roughened by the application of a large number of punch marks. The electrode (Figure 3) was of pure gold and was embedded in a plastic backing material. It was coinposed of two l/l&xh gold plates placed side by side and electrically connected by a jumper soldered to the rear faces. This construction was used because a piece of 1/16-inchgold {vas available, narrower than the total desired n idth. The gold pieces were formed into an integral mass ~ i t the h plastic (Hj-sol 6000 Plt plastic) by a casting operation One minor trouble occurred in the casting operation because of warping of the plastic as it cured. However, sufficient thickness of gold nas available after machining the outer face flat t o permit a n end mill to be used to machine the labyrinth that formed the solution channel. As a final operation to ensure a good fit, the outer engaging surface of the electrode was lapped flat on a surface plate using fine emery
paper. This construction resulted in an electrode that stood up well in prolonged service. Without the punchings the effective area of the electrode was about 38 sq. em. and the volume of the solution chsnncl n-as about 1.5 ml. After a number of runs were completed, the broad surface of the channel was roughened by using a special tool resembling a center punch. The tool was inclined and was struck with a hamnier to create a conical depression and finlike elevation: as shown in the enlarged cross-section detail of Figure 3. Approximately 450 marks were made, all facing against the current of the sample stream which entered by the crnter port. The porous cell separator was :tn unglazed ceramic disk 31/a inches in diameter and inch thick (Selas Corp. of America, porosity designation 03). The disk was ground flat on both sides to ensure good fits with the other parts. The reference electrode ITS located inside a specially blown borosilicate glass chamber, as indicated in Figure 2. and consisted of a sheet of pure silver metal 1 x 3 inches. It was initially given a coating of silver chloride by anodizing in a solution of hydrochloric acid. Since a solution of 2147 potassium chloride was used in t,he reference electrode chamber, a silver-silver chloride electrode of constant potential was thus established. When the cell wits in operation, the very small crll currents did not significantly affect the reference electrode potential. Furthermore, the electrical resist,ance of the cell was low; measurements with an I A N 4960 bridge in R series of tests gave readings in the range of 5 to 10 ohms. Accordingly, the cell voltages may be and were used directly to express t'he potential of the working electrode. Solution-Handling System. Each reservoir bottle of the hlariotte
type had connected t o t h e tubulation at t h e bottom, a length of rubber tubing equipped with a pinch clamp. Flexible lengths of ’/s-inch polyethylene tubing connected t h e other parts of t h e solution system. T h e sample solution was led into and out of t h e cell by molded polyethylene connectors of t h e L & N 125057 type, fitted into t h e rear of t h e electrode construction. Solution flow rate vias regulated at a desired value by using a glass capillary tube in t h e solution Line leading from each bottle. A. three-rray stopcock enabled t h e solution flow t o he switched from one bottle to the other. The outlet from the cell was a length of */s-inch polyeth>lene tubing, the discharge end of which was positioned, in general, a t an elevation equal to about the midpoint of the cell, The elevation of the discharge point had a n effect on the seepage flow rate of the reference electrode solution through the porous cell separator Electrical Circuit. T h e details of the electrical circuit are shown in Figure 1. T h e rheostats, R1 and Rz, nere 100-ohm, wire-wound units of the radio typc, and each was ponered with a pair of No. 4 F H d r y cells. T h e resistor, R , in circuit with rheostat R2 had a value of 100 ohms. Rheostat R3 was a 200-ohm 10-turn Helipot. These rheostats provided means for adjusting the cell voltage to the desired value. A p H meter of the L&N 7664 type was placed across the cell terminals to measure the applied cell voltages and had a negligible eEect on the circuit. The electrolysis current flowing in the cell circuit as measured by measuring the voltage drop across the calibrated resistor, R,, using an L&K Speedomax recorder. R , was adjustable and comprised a n L&K 4750 resistance box. For most of the work, a full-scale recorder range of 5 mv. was used, and the recorder zwo as adjusted to be 10% up-scale so that small currents and reversals of current could be measured. The reversing switch enablcd a rapid change-over to be made in measuring the anodic and cathodic cell currents. The current rneasurement introduced a negligible effect in the circuit, since resistances set in the 4760 resistance boy were usually kept within the range of 1, to 50 ohms. Automatic adjustment of the Helipot rheostat, RB. was provided by connecting the rheostat shaft to a reversible clock motor through a n appropriate gear train. The time for complete travel of the contact from one end of the slidenire to the other \vas 30 minutes. Rheostat R1 provided means to adjust the total voltage span across RJ. 122 provided zero adjustment of the voltage span of Ra. Thus, the voltage across the cell could not only be reversed in polarity b u t also made to pass through zero. When manual adjustment of voltage was desired, the contact of R3 was placed in the full-voltage position, H . R1 and Rz then gave simple manual ad-
SOLUTION
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O.D., 2 5 T U R N S ,
26 AW6
Figure 4. lodinegenerating equipment
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BEAKER
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justment of the voltage across the cell from about - 1.5 to $3 volts.
Iodine Generator. A special iodinegenerating system was constructed for determining iodine. This system allowed a known amount of iodine to be electrolytically produced in t h e incoming solution t o t h e cell, t h e amount produced being known as accurately as t h e generating current could be measured. This method of establishing knorvn iodine solutions was considered easier and more reliable than making u p and storing very dilute iodine standard solutions The details of the iodine-generating system are shown in Figure 4. A stream of potassium iodide solution entered the tube a t the top of the apparatus, flowed past the anode in the left-hand horizontal tube, then passed to the cell through a connecting polyethylene tube, not shown. The cathode of the cell was isolated from the anode and the flowing stream by a n agar-agar plug. The electrical circuit connected to the iodine-generating cell produced a constant current of small magnitude. When the w i t c h was in position 1, the 22.5-volt battery delivered current through a series circuit including the iodine-generating cell, a current-limiting resistor, R L (of the order of 0.1 to 0.5 megohm), and a 20-ohm N.B.S. resistor, Rs. The value of R L was chosen in each case to give the desired value of current. The current flowing in the circuit was checked from time to time by making connections (D and F ) from the K.B.S. resistor to the recorder used for measuring the current in the analysis cell. Doing this interrupted the principal measurement briefly, but enabled a direct comparison t o be made betn-een the iodine generation and measurement currents.. Generation of iodine wis stopped when the sn-itch was turned to position 2. Means for Solution Agitation. Three different means were used t o study t h e effect of solution agitation, because i t was expected t h a t increased motion of t h e solution with respect t o t h e electrode might increase t h e
-POROUS
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AGAR
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SEPARATOR
I w WORUINC ELECTRODE
I
/’
SOLUTION
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7 SOLUTION
CHANNEL
, / ,r COYYUNICATINO I
PUMP
/
PASSAGE
CHAMBER
Figure 5. Magnetic solution agitator electrode efficiency. T h e first permitted a pulsating air pressure t o be applied t o t h e sample solution as it entered t h e cell. A second means of agitation involved the polyethylene outlet tube that carried the discharge from the cell. This tube was about 12 inches long and had its outer end held firmly in place by a clamp. A laboratory stirrer was equipped with a 1/4-inch diameter steel shaft about 8 inches long. The outer end of this shaft ran out of line, describing a circle of about 1/2-inch diameter. The stirrer motor was mounted with respect to the polyethylene outlet tube in such a way that the outer portion of the shaft in rotating struck against the polyethylene tube at approximately point X in Figure 1. When the stirrer operated at speeds of the order of several hundred revolutions per minute, the outlet tube was given a brisk whipping motion that caused the solution inside the tube and cell to undergo a longitudinal oscillation. After most of the tests were conipleted, the cell was modified to agitate the sample solution by using the oscillatory motion of a n armature moving in an alternating current magnetic field. This arrangement is shon-n in Figure 5. -4 hole. drilled through the electrode at a point about midway VOL. 31, NO. 9, SEPTEMBER 1959
1455
4 Figure 6. Currentvoltage plot of distilled water
w Figure 7. Currentv o l t a g e p l o t of iodide solution
-0.2
0
+o?. CELL
+0.4
along the solution pathway through the cell, formed a communicating passage with a puniping chamber niachined in the plastic backing behind the electrode. A diaphragm of flexible Teflon material 1 inch in diameter and 10 mils thick formed the back wall of the chamber. The diaphragm was given a vibratory motion by connecting it to the working mechanism of a laboratory marking device, called “electric marker.” This device (Ideal Commutator Dresser Co., Sycamore, Ill.) has n vibrating, hardened point and is intended to engrave glass and other hard surfaces. For the present purposes the hardened point was removed, and a special chuck was substituted to prasp the rod that made connection with the diaphragm. When plugged into the 60-cycle line, this device produced a vibratory motion of the diaphragm of 120 cyclps pel w o n d , with an amplitude of about 20 or 30 mils. Before use, air was removed from the pumping chamber by temporarily loosening the O-ring seal of the diaphragm and allowing the solution to fill the chamber completely. Solutions. For the iodide deteiminations, solutions of known iodide concentration were made u p voiumetrically as needed from a stock solution of 0.01.V potassium iodide. This standard solution was prepared by weighing exactly 1.660 grams of reagent grade potassium iodide and adding it t o distilled n a t e r t o make 1 liter. of solution. I n most instances the iodide sample solutions tiere made u p in a substrate of 0.2M sulfuric acid solution. Earlier work had shown that iodide was determined somewhat more efficiently when a sulfuric acid substrate of about this strength was used. Hence, iodide sample solutions 0.5 X 1.0 X 10-4, and 2.0 X 10F4N were made with a 0.2M s ilfuric acid substrate solution. One run n a s carried out on a n iodide solution which had been made u p simply with distilled water as substrate. The solution supplied to the iodine generator in the iodine runs was an approximately 1% potassium iodide 1456
+06
+08 PREFERRED
VOLTAGE
ANALYTICAL CHEMISTRY
OPERbTlNG
I
I
3 1
0 4
-
VOUAGE
I 0 5
G 6
0 7
0 8
CELL VOLTAGE
solution The 2iV potassiuni chloride solution used in the ‘reference electrode chamber was made by adding 149 grams of reagent grade potassium chloride to distilled water to make 1 liter of solution. The solutions used for the oxygen determinations were brought into equilibrium with the air by simply allowing them to stand in contact with the air The O.lX potasdum chloride solution used as a substrate in some of the oxygen runs was made by diluting the 2 5 potassium chloride solution used in the reference electrode chamber. The concentration of oxygen in the sample solutions wa3 calcrilated from published data (6 9). .It the operating temperature of the equipment, about 28” C., the oxygen concentration was 2.4 X lop4 mole per liter. or 9.6 X 10d4N assuming a 4-electron -hange for the reduction of the oxygpn. PROCEDURE
The variables studied 111 Jppraising the method included the currentvoltage relationships oi the -ell the residual current characteristics, the measuring efficiency of the cell including the effects of agitation, and the speed of response of the cell to a concentration change. Because the cell was sensitive to small quantities of iinpurity, precautions were taken in preparing it for operation Cell Preparation Procedure. Before the cell was assembled for making measurements, the poioue disk was cleaned thoroughly by soaking i t for about 0.5 hour in a n alcoholic potassium hydroxide solution I n the iodine runs, a slight deposit, presumably of silver iodide, appeared on t h e porous disk, and was remo.ied with a sodium thiosulfate solution. Other reagents such as hydrochloric acid solution and aqua regia mere used from time to time in attempts to produce utmost cleanliness of the disk F o h n ing
all these chemical treatments, the disk was allowed to soak in a number of changes of distilled water before being used. The working electrode was given a treatment in alcoholic potassium hydroxide solution. followed by a brief etching treatment in aqua regia solution. If there mere any stains in evidence on the electrode following the treatment with alcoholic potassium hydroxide, a 1 to 1 solution of nitric acid was used prior to the aqua regia treatment. The electrode was then allowed to stand in several changes of distilled water before being assembled In the cell. I n the cleaning treatment of the electrode and the porous disk, the alcoholic potassium hydroxide solution removed the silicone grease that had been applied in the previous assembling of the cell. I n assembling the cell, silicone grease was applied to the outer edges of the working electrode and to the engaging surface of the reference electrode chamber. This procedure helped to avoio leaks in the awmbled cell. Xevertheless, a small amount of solution seepage occurred through the unglazed edge of the poroiib disk but had no detrimental effect. Even n ith the above precautions ot cleanliness, large residual currents were usually observed initially and were indicative of impurities. Two techniques were used in general to decrease the residual current. The seepage of the reference electrode solution through the porous disk mas increased by positioning the end of the outlet tube at a lower level for several hours, thereby increasing the hydrostatic pressure differential. The temporary increase in flow of solution through the porous disk definitely improved the functioning of the cell. The other technique used to condition the cell was to impress extremes of voltage on the cell, both positive and negative. The electrode behaved as if it had substances adsorbed on it. which were removed more quickly if the electrode v, ere subjected t o the strong
4
0 20
Figure 8. Current-voltage plot of on iodine solution
0 80
-J 3
U
r! ( I
J w
a
b
010
,
Figure 9.
I
of iodide solutions
f
Meosurernents
VI L
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2
006
0
0
0
-03
-92
-31
0 +01 C E L L VOLTAOE
+OP
oxidizing and reducing conditions induced electrolytically by the extremes of voltage. After a cell had been conditioned, it continued to perform satisfactorily for extended periods of time. Current Voltage Relationships. The information needed to establish the current-voltage relationships was obtained by maintaining a constant flow rate of the sample solution through t h e cell, setting the voltage across t h e cell a t a particular value, and waiting for tho current to become constant. Fifteen minutes or more were required, in general, to achieve reasonable constancy. The voltage was then set a t another value, and the waiting repeated until a satisfactory constant current was observed. Observations of this kind were made on the sample solutions, and also on the respective substrate solutions to establish the residual current curves.
-
Effect of Flow Rate. T h e solution flow rate influenced the cell efficiency. Accordingly, current a t a given cell voltage was measured a t different flow rates. T h e flow rate was usually changed b y changing the flow-restricting capillary t u b e leading into t h e cell. Flow readings were obtained in t h e range from about 0.3 to 14 ml. per minute. The sample solution floiv rates were measured by catching the discharge from the cell in a 10-ml. or larger graduated cylinder and noting the time to collect a known volume. The flow of solution from thr secondary electrode chamber and into the sample stream was kept negligible during a run. The limit
+
of error oi the flow-rate measurement was of the order of 17'. Solution Agitation. The effect of solution agitation was studied by comparing the cell currents with and n-ithout the agitating means in operation The different types of agitators were tested on the several sample solutions: air pulsations, on oxygen solutions: vibration of the outlet tube, on iodine solutions; and magnetic agitation on iodine solutions. Speed of Response. The speed of response of t h e cell was measured by using an iodide sample solution in one reservoir and t h e substrate solution containing no iodide in a second reservoir. T h e flow rates of t h e two solutions were adjusted initially to about the same value. The sample stream to the cell was then switched back and forth betneen the t n o solutions using the three-way stopcock in the tubing lines. A fast recorder chart speed of 1 inch per minute was used in studying the speed of responsr. RESULTS
Residual Current. T h e results obtained on distilled water (Figure 6) shorn t h e very small residual currents t h a t can be obtained. I n t h e region from about 0.10 t o 0.46 volt, the residual current is less than 1 pa. The residual currents in the measurements of iodide and iodine are shown as dashed curves in Figures 7 and 8, respectively. .4t the preferred op-
I
2 3 4 S A M P L E F L O W - R b T E IN M L D E R M I N
5
erating voltages, the residual currents are satisfactorily small in both cases. I n Figures 6, 7 , and 8, the abscissa represents the voltage impressed across the cell, the sign indicating the actual polarity of the working electrode. The potential of the working electrode is considered to be numerically equal to the cell voltage, and thus the values are referred to the 2X silver chloride electrode. A closer look a t the distilled water residual current was obtained one morning after the cell had been operating overnight under steady conditions at SO.116 volt. The residual current was so Ionthat a 100-ohm measuring resistor and the lowest millivoltage range of the recorder (I niv. full scale) were used to measure it. Under these conditions the residual current was found to be only 0.02 pa., anodic. After solution n a s added to replace the small loss of solution from the reference electrode chamber, the current changed to 0.04 la., cathodic. Iodide Determination. The eurrent-voltage characteristics of an iodide sample solution are shown in Figure 7. S t low voltages, no significant current is observed for either t h e iodide or the substrate solution. At about 0.5 volt, the iodide h-ave appears as a sharp rise in the current. At voltages betneen about 0.6 and 0.7 the curve flattens. This plateau is t h e working range for determining iodide, a n optimum voltage setting for the purpose being about 0 67 volt. A steep rise in the curve appears above VOL. 31, NO. 9, SEPTEMBER 1959
0
1457
0 E H E R A T ING C U R REH TS Q A N D O 0 0 6 3 MILLIAMPERES
1
I
S A M P L E FLOW-RATE
I
I
I
I
IN H L P E R M I H
Figure 10. Measurements of iodine solutions
0.7 volt, possibly caused by the reaction of the gold electrode itself. Previous experimentation with a graphite working electrode showed the iodide plateau extended to somewhat more positive voltages. The residual current curve also shons a sharp rise in current a t more positive cell voltages, paralleling the iodide wave. The effectiveness of the cell in determining iodide is shown in Figure 9, where the cell current as ordinate is plotted against the solution flow rate as abscissa. The straight lines fanning out from the origin are the theoretical relationships of Equation 1 for the iodide concentrations tested. The cell current values tend to approach the theoretical lines at the lower flow-rate values. Figure 9 shows a marked difference in the performance of the working electrode in the punched and unpunched conditions, the difference being more noticeable at the higher flow rates where the deviations are larger. Figure 9 also gives limited information on the effect of the substrate solution. The efficiency of the electrode was somewhat lower in the run where the iodide was simply in distilled water, rather than in a sulfuric acid substrate as in the other runs. The efficiency of a cell under a given set of operating conditions can be expressed by the ratio of the observed to the theoretical currents. The iodidemeasuring efficiency of the cell in terms of per cent is shown in Table I. The percentage figures 1% ere obtained by taking values of current from Figure 9 a t flow rates of 1.5 and 3.0 ml. per minute. With the smooth electrode the deviations from 1 0 0 ~ oefficiency were about twice those with the punched electrode. With the punched electrode and a t a flow rate of 1.5 mi. per minute, the efficiency mas 90% or better in all cases, averaging 93y0. The efficiency fell off at the higher flow rate. Iodine Determination. T h e current-voltage relationship of an iodine 1458
ANALYTICAL CHEMISTRY
IP I
Figure 1 1.
solution is shown in Figure 8. I n general, t h e cathodic current increases in going t o t h e more negative cell voltages. A steep increase in current appears a t about +0.23 voIt and represents t h e beginning of the iodine wave. A plateau extending from about +0.1 to -0.05 volt is t h e working region for determining iodine. A steep rise in t h e current on t h e far left-hand side for voltages more negative than about -0.2 volt is presumably caused by the discharge of oxygen at the electrode. The residual current curve closely parallels in appearance the iodine curve but is displaced towards lower currents, with its horizontal portion being close to zero current. The preferred operating voltage for determining iodine is about +0.05 volt. The performance of the cell in determining iodine is shown in Figure 10. I n this figure the data are plotted in a manner different from that used in Figure 9. Because, with electrolytic generation, the iodine enters the cell a t a constant mass rate regardless of flow rate of solution, the m e a s u r d current can be compared directly with the generating current. Three significantly different generating currents were used, as indicated in the figure. The manner of calculating the ordinate permits results at different generating current levels to be plotted on the same coordinates. The different points along the upper and the lower curves in general correspond to solutions of very different concentration. Without agitation, as shown in the loner curve of Figure 10. the cell efficiency falls off with increasing flow ratt in a general way paralleling the resultp with Iodide At a flow rate of about 1 . 5 ml. per minute. the efhciencv IS about
l
l
,
~
Measurements of oxygen solutions
90%, while a t 3.0 ml. per minute it has dropped to about 80%. Furthermore, this curve indicates that the cell efficiency is substantially independent of the solution concentration. The same can be said for the iodide results of Table I. Some scattering of the points, especially a t the higher flow rates, shows that the cell efficiency is not a strictly reproducible characteristic. Oxygen Determination. Currentvoltage studies of air-saturated solutions have shown t h a t a plateau exists in the voltage range frbm -0.6 t o -1.1 volts. Accordingly, measurements of t h e oxygen concentration of air-saturated solutions were carried out at a cell voltage of -0.82 volt (Figure 11). The theoretical line in the graph was constructed using Equation l and a normality figure of 9.6 X 10-4. The results mere all obtained with the gold electrode in the smooth condition. Although no extensive study was made of the residual current because of the difficulties of excluding the last traces of oxygen from the simple apparatus uwd. indications were that the residual currents in the oxygen determination v, ere also small The results for oxygen shown in Figure 11 and the estimated cell efficiencies closely parallel those for indid? previously given . Effect of Agitation. When properly carried out, solution agitation caused a marked increase in t h e electrode efficiency. T h e effect of vibrating the outlet tube t o induce agitation is shown in the upper curve of Figurc 10, At flow rates u p t o about 4 m! per minute the electrode efficiency with agitation ranged from 98 tc 1007(,. The marked improvement causcd b j agitation is s h m . for p-r-
ample, by comparing results at a flow rate of 3 ml. per minute, nhere the efficiency was 78y0 with no agitation arid 96% with agitation. Thc improvements in efficiency A ith the magnetic agitator were even better (Table 11). Over the range of flow rate covercd by the table, the efficiency with agit:ttion in every instance approached 1 0 0 ~ o . A t the highest flon rnte, appro~imately 8.0 ml. per minute, the striking effect of the agitator \vas to raise the efficiency from about 69% t o 97%. The agitator caused small changes to take p l : in~ the f l o r rate, as the figures of the table show, probably as a result of turbulent conditions induced in the solution. The magnetic agitator introduced another, somewhat more objection:ible effect; the current record became jumpy when the niagnetic agitator was turned on. Hoa ever, the indicated improvement in elect rode efficiency was easily observed. Speed of Response. The speed of response of t h e cell is s h o n n in Figure 12, which was constructed by tracing over t h e pen line of the original recorder chart. At t h e points marked A in t h e record, t h e stopcock was changed t o introduce iodide solution into t h e cell. A t t,he points marked B, t h e stopcock was adjusted t o introduce iodide-free substrate solution into the cell. T h e short delay periods following the indicated points represent the amount of time taken for the solution to flow from the stopcock to the cell. The slight step in the record following a point B was caused by the slightly different solution flow rate in going from one solution t o the other. The transition in going in either direction was substantially completed in 1 minute. Experience in general with the cell has shown that a faster solution flow rate gives a faster response. Although not actually tested, the cell volume would appear to be a factor: the less the solution holdup in the cell, the faster the response. KO attempt was made in the present work to maximize the speed of response. DISCUSSION
Need for Constancy of Working Electrode Potential. T h e electrical circuit was designed for automatic and continuous scanning of t h e cell voltage variable. I n t h e early work using automatic scanning, very large residual currents were obtained a n d could not be eliminated regardless of t h e pretreatment given t h e cell components. It became apparent after continued work t h a t t h e residual current was csused primarily by the changing voltage applied to the cell. The cell behaved as a n electrical capacitor having a very large value of capaci-
ELECTRODE FLOW R d T E
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\aim o ~ O V O L T
0
I
2
'
I A - IODIDE
3 0 L U T 1 3 U TURHED
I 8-SUOSTRATE
3 4 5 T I M E ri M l h U T E S l A 9 0 1 T R A R Y Z E R O 1
OM
SOLUTION TURHED ON
e
7
6
9
Figure 12. Speed of response of cell ~~
Table I.
~~
Iodide-Measuring Efficiency of Cell (Per cent) Iodide Normality 1 . 0 X lo-' 2 . 0 X Flow rate 1.5 ml./min. 81 86 95 90 Flow rate 3.0 ml./min. 64 72 86 84
dv.
Efficiency,
Dev. from 100% Efliciency,
Electrode
0 . 5 X 10-4
'7,
70
Smooth Punched
84 94
84 93
16 7
Smooth Punched
70 84
69
31 15
85
Table II. Effect of Magnetic Agitator in Determining Iodine Using Punched Electrode Sample Cell Flow Rate, Current, Ma. Efficiency, Agitator Ml./Min. Obsd. Theoret. 70 Off 1.42 0.118 0.127 93 On 1.33 0.123 0.127 97 Off 1.56 0,109 0.127 86 On 1.35 0.127 0.127 LOO Off 5.48 0.082 0.131 63 On 4.80 0.128 0.131 98 Off 8.18 0.091 0.131 69 On 7.50 0.127 0.131 97
tance. A changing voltage thus caused the flow of considerable current. After this was recognized, residual current and the solution concentrations themselves were measured using constant applied voltages. When a constant voltage is applied, the residual current can be reduced to very low proportions. The need for constant cell voltage presents no problem when the method is to be applied to monitoring the concentration of a constituent in a sample stream. When i t is desired to make a qualitative examination of a sample solution over a range of electrode potential, however, the use of a continuously changing cell voltage should be avoided. Rather it is advisable to obtain the information in a stepwise fashion, using in turn a number of constant cell voltages of progressively different value, as was done in the present york.
Selectivity of Method. T h e current-voltage curves for iodide a n d iodine show t h a t t h e electrode potential in t h e present work was effectively discriminating among t h e possible electrode reactions. T h e half-wave potential from t h e current-voltage curve for iodide was about $0.52 volt, and t h a t for iodine, about +0.25 volt, both referred t o t h e 2 N silver chloride electrode. As the 2 N silver chloride electrode is about 0.025 volt more negative than the saturated calomel electrode, these half-wave potentials have the values of about +0.50 and +0.23 volt relative to the saturated calomel electrode. These potentials are in agreement with the results of Kolthoff and Jordan (61, although conditions are different in the present work. Cell Efficiency. T h e higher efficiency of t h e electrode in t h e punched VOL. 31, NO. 9, SEPTEMBER 1959
1459
condition was probably caused in part by the increase in electrode area introduced by the punching. Also, the roughness of the surface probably caused more turbulence and hence better interchange of the solution at the electrode surface as the solution flowed through the cell. At the lower flow rates where the cell efficiency approaches loo%, the points appear to lie rather reproducibly on the curves of Figures 9, 10, and 11. At higher flow rates, however, appreciable scatter can be noted, indicating a variable cell efficiency. The reasons for this scattering were not completely ascertained. During the course of the work air bubbles occasionally became lodged in the solution channJ inside the cell. The bubbles were removed by sucking the solution rapidly through the solution channel for a brief time. When air was thus removed from the cell, the cell efficiency usually increased. It should not be assumed, however, that air bubbles were the only cause of variable cell efficiency. It is desirable to operate the cell a t an efficiency close to 1 0 0 ~ o as , under this condition the uncertainties caused by scattering are avoided and the calibration can be made in accordance with Equation 1. The present work shows that operating conditions to give a high efficiency can be established b y minimizing the solution flow rate, increasing the area and roughness of the working electrode, and agitating the solution that is in contact with the working electrode.
Reducing the solution flow rate below
a certain point is undesirable, because the speed of response of the cell would then become unsatisfactorily slow. Also, a t very low flow rates, metering becomes more difficult, and the magnitude of the output signal decreases. Increasing the area and degree of roughness of the working electrode is important. Going to very large electrode areas, however, might cause larger cell holdup volumes, which would be undesirable from the standpoint of speed of response. Perhaps the greatest opportunity to improve cell efficiency is through the forced agitation of the solution. For the same high efficiency, the magnetic agitator allowed the flow rate to be increased about tenfold over that allowable without agitation. SUMMARY
It is feasible to carry out a quantitative electrochemical process on a flowing sample stream using an electrode of predetermined potential. Operating conditions and design factors have been established in a cell of reasonable size and construction to give a substantially 100% electrode efficiency. I n measurements on sample solutions of iodide, iodine, and oxygen, the electrode efficiency varied with the solution flow rate, area and roughness of the working electrode, and solution agitation. but was largely independent of the particular constituent and its concentration. Although the present work
is encouraging, further work is needed to show the extent of the utility of the method in practical applications. ACKNOWLEDGMENT
Assistance was given in the preliminary stages of this study by Donald E. Walker. LITERATURE CITED
(1) Consolidated Engineering Corp., Pasa-
dena, Calif. (now Consolidated Electrodynamics Corq.),, Biill. CEC-1810, Model 26-102 Titrilog. (2) Eckfeldt, E. L., U. S. Patent. 2,621,671 IDec. 16. 1952). (3) Ibid., 2;758,079 (Aug. 7, 1956). ( 4 ) Eckfeldt, E. L., Proctor, \I7, E., Jr., Perley, G. A., “Electrolysis Instrument for Continuously Recording the Concentration of Dilute Chlorine Water,” 50th Anniversary Meeting, Electrochemical Society, Philadelphia, hlay 7,
1952. (5) “International Critical Tables,” Vol 111, p. 257, McGraw-Hill, New York, 1928. (6) Kolthoff, I. M., Jordan, J., J . A m Chem. Sac. 75, 1571 (1953). (7) Leeds & Northrup Go., Philadelphia 44,Pa., Data Sheet ND46-94(I), Model No. 59175-’41 residual chlorine an-
alyzer.
(8) Shaffer, P. A., Jr., Briglio, A., Jr., Brockman, J. A , . Jr.. ANAL. CHEM 20, 1008 (1948). (9) Speller, F. K., J Franklin Inst 193. 515 (1922). (10) iVherry, T: C., DeFord, D. D., Control Eng. 5, No. 3, 115 (1958).
RECEIVED for review December 17, 1958 Accepted April 29, 1959. Presented in part before the Division of Analytical Chemistry, 135th lleetinp, ACS, Boston. Mass., April 9. 19551
Coulometric Titration of Manganese with Electrogenerated Vanadyl Ion DONALD
G. DAVIS
Georgia Institute o f Technology, Atlanta
b Permanganate ion has been titrated in 0.5F sulfuric acid with electrogener ated vanadyl ion. Because vanadyl ion is a rather weak reducing agent, the titration is specific. Chromium was the only interfering substance of importance and its interference was slight under optimum conditions. The end point of the titration was detected amperometrically, using two similar platinum electrodes. Argentic oxide was used to convert manganous ion to permanganate prior to the titration.
A
vanadpl solutions havc been used for various titrations ( 4 ) including that oi permanganbte LTHOUGH
1460
0
ANALYTICAL CHEMISTPY
13, Ga. ( I S ), electrogenerated vanadyl ions have not been used as a coulometric titrant. The vanadium(1V)-vanadium(V) couple has been used electrbchemically in a chemical coulometer (1%’) based on the oxidation of vanadyl to vanadium(V). and electrogenerated v& nadium(V) has been suggested as a coulometric titrant (9). The E o of the yanadium(1V)-vanadium(T’) couple is reported as -1.000 volt in acid solution (?‘). Thus vanadyl ion is a n extremely weak reducing agent This fact indicates that titrations of oxidizing substances n ould be possible only if the titrate %ere of great oxidizing powcv On thiF h+ it T R F n o v d that N fairh- qxvifil titrstisn iif
permanganate would be possible. A titration of this sort would be especiall: useful for the analysis of complex alloys, especially those containing both manganese and vanadium. The presence of vanadium would actually tic advantageous, hevause the cnnccn tration of vanadium(V) necessary for the generation of vanadium(1V) ivould be increased, reducing the possihilit) of the occurrence of undesirable rlectrode reactions. Methods for thc dt. termination of manganese in the presenc~ oi vanadium arc generally coniplp,
