V O L U M E 26, NO. 12, D E C E M B E R 1 9 5 4 Residues and crude oils can be analyzed, although with more difficulty and less accuracy than with distillates. Solid or viscous materials can either be dissolved in a heavy oil, which does not produce hydrogen sulfide, or, alternatively, can be sampled as pellets dusted n i t h alumina powder (to enable weighing and handling without loss). The residuals which were analyzed were found to produce from 10% to somewhat more than 50y0 of the possible hydrogen sulfide calculated from the total sulfur vontent of the material. A few tests \!ere made to determine the applicabilitj- of the catalytic desulfurization procedure to three California crude oils. Two were fluid ( A arid B) and the third ( C ) was viscous. I n the tests, catalyst temperatures of 450” C. and, with the viscous crude, 490” C. n-ere used, purge times were increased, and, in some cases, white oil vias added to the reaction tube to act as a high temperature solvent. These tests, detailed in Table 111, show that the decomposition of the viscous crude to hydrogen sulfide is slow and that the decomposition increases a t the higher catalyst temperature and when iyhite oil is added to the reaction tube after the crude oil. The lighter crudes required less drastic reaction conditions. Examination of the catalyst after desulfurization of the viscous crude revealed that the upper section of the catalyst and the adjacent portion of the reaction tube were coated viith a tarry material. Thus it appears that the slow desulfurization results from the formation of a coating of high molecular weight material on the catalyst which slom s or prevents adequate contact b e b e e n the sample and the catalyst. The increased rate of hydrogen sulfide production found when white oil was added to the reaction tube after the viscous crude is attributed to the solvent action of the hot oil on the catalyst mating.
1933 LITERATURE CITED
Ball, J. S., U. S.Bur. of Mines, R e p t . Z n w s t . R I 3591 (Decemher 1941). Ball, J. S.,Rall. H. T., Waddington. G., and Smith, H. A l . , “Sulfur Compounds in Petroleum,” Division of Petroleiiin Chemistry, 119th Meeting of AMERICAN CHEMICAL SocIwrY. Cleveland. Ohio, April 1951. Blade, 0. C., Petroleum Refiner, 28, KO.3. 151 (1949). Borgstrom, P., Bost, R. W., and AIcIntire. J. C . , Ind. E n q . Chem., 22, 87 (1930). Faragher, W. F.. Morrell, J. C., and Coinay, S..Ibid., 20, 527 (1928). Faragher, 13‘. F., Alorrell, J. C., and Monroe, G. S.,Ibid., 19, 1281 (1927). Greensfelder, B. S.,Advances in Chem. Ser., KO.5, 11 (1951). Greensfelder, B. S., and Voge, H. H., Shell Development CO.. private communication. Hale, J. H., Thompson, C. J., Barker, hl. G., Smith, H. AI., and Ball, J. S., h . 4 ~ CHEM., . 23, 287 (1951). Hammar, C. G., Saensk Kern. Tidskr., 63, 135 (1951). Kolthoff, I. AI., and Furman, N. H., “Potentiometric Titrations,” 2nd ed., pp. 144-58, S e w York, John Wiley & Sons, 1931. Lykken, L., and Tuemmler, F. D., IND. ESG. CHEM.,A s . 4 ~ . ED., 14, 67 (1942). Peters, E. D., Rounds, G. C.. and Agazei, E. J., ANAL.CHEM., 24, 710 (1952). Smith, H. hl.. and Blade, 0. C . , Proc. Anr. Petroleum Inst., 27th Meeting, 111, 119 (1947). Tamele, AI. W., unpublished data. Tamele, AI. W.,and Ryland. L. B., IXD.ESG. CHEM.,A N ~ L . ED., 8, 18 (1936). Tunnicliff, D. D., Peters, I... D., Lykken. L., and Tuemmler, F. D., Ibzd., 18,710 (1946). RECEIVED for
review November 7, 1953. Accepted August 20, 1954 Presented before t h e Division of Petroleum Chemistry at the 126th Meeting of the . \ \ f E R l C A X C H E X I C A L SOCIETY, New York, s T.
Potentiometric Titrations with Controlled Current Input RALPH N. ADAMS Frick Chemical Laboratory, Princeton University, Princeton,
The technique of current scanning polarography was extended to include an electrometric titration which is the analog of conventional amperometry. The method consists in applying a small, controlled current from a 1.5-volt dry cell and 1- to 5-megohm resistor to a polarographic electrode and a reference half cell. The resulting polarographic voltages are follow-ed with a continuous indicating pH meter or inexpensive V-TVRl as a function of volume of titrant. Zinc was titrated with ferrocyanide with satisfactory results. Using a platinum foil in stirred solution, arsenic(II1) w-as titrated with bromide in hydrochloric acid medium. Excellent results were obtained with considerably more ease than by the conventional amperometric method. In the titration of iron(I1) with cerium(IV), the new method is no more desirable than conventional potentiometry. The results are interpreted in terms of experimental polarograms and the fallacy of using theoretical polarograms is emphasized. The method was applied to the titration of cadmium w-ith ethylenediaminetetraacetic acid at the dropping‘ mercury electrode and found satisfactory.
T
HE recently reported technique of current scanning polarography ( 1 ) has been extended to include the analog of conventional amperomrtric titrations. The procedure consists in Ixissing a controlled current (ca. 0.1 to 2 p a . ) through a polaro-
N. J.
graphic. tvvurlring electrode and a reference half cell. The resulting polarographic volt,age is followed with a continuous indicating pH meter or vacuum-tube voltmeter, as a function of the volume of titrant. added. Titrat,ion curves are obtained which are not unlike those observed in ordinary potentiometric titrations. Such a technique was used in 1925 by Van Kame and Fentvick in their study of pdariacd bimetallic electrode systems ( 1 5 ) . I n recent years Gauguin and coworkers have developed a similar method which they have named “potentiometry at constant current” ( 2 , 5-7‘). The experimental techniques described herein are identical in most respects with those of the aforementioned wvorkcrs. A recent review by Furman covers the main points of their vork which leans heavily on the theoretical derivation of polarimtion curves ( 4 ) . Duyckaerts has also discussed the application of small currents to electrode syFtems (8). The study presented here, ne an extension of current scanning polarography, represents a purely experimental approach and derives only frqm actual current-voltage curves (polarograms). KO mathematical or theoretical treat,ment of these polarogtxms is necessary for titration purposes. In conventional amperomctry diffusion currents are measured a t some voltage-insensitive portion of the polarographic wavei.e., a t some fixed voltsge on the plateau. In the current input method, polarographic voltages w e measured a t a point onChe polarographic n-ave which is relatively current-inseneit,ive-i.e., near the foot of the xvave. The use of small currents gives selectivity of electrode reaction. Selectivity is lost when the
1934
ANALYTICAL CHEMISTRY
current level becomes highei than the diffusion current; this gives rise to the changing polarographic voltages and serves as the basis of the titration procedure. I t is usually unnecessary to plot end points in a titration a t controlled current input. Direct meter readings give the end-point volume with sufficient accuracy in most cases. With such one-shot response it is necessary t o titrate carefully in the region of the end point. As in conventional amperometry, applications of the current input method to new titrations require a preliminary examination of the polarograms of the background medium, the substance titrated, and the titrant An cvamination of such experimental polarograms and the input current level enables one to predict the variations in polarographic voltage during the titration. The fallacy of using theoretical polarograms for this purpose is illustrated later for the titration of iron(I1) with cerium(1V). The four determinations studied are fairly representative of the scope of conventional aniperometry. APPARATUS
Polarography. Most of the current-voltage curves in this study were made by the current-scan technique. The apparatus and experimental details of this method have been fully described (1). A few polarograms were made by conventional voltage scan, using the Fisher Elecdropode. A Leeds and Northrup Electrochemograph was used with the dropping mercury electrode. The two methods were used to indicate that polarograms taken by voltage or current scan are identical.
of Figure 2. The wave B represents the background oxidation wave for the 0.1N potassium chloride supporting electrolyte. The anodic oxidation wave of ferrocyanide is shown as F . Using a theoretical 1.5-fia. anodic current input to a rotating platinum electrode (1-megohm resistor and assuming zero cell voltage drop), Figure 2 shows that the polarographic voltage will be about +1.1 volts z's. S.C.E. when no ferrocyanide is present, With this ea. 1.1-volt drop across the electrodes opposing the source voltage, the actual current flowing through the polarographic cell is not 1.5 pa. but about 0 . 4 pa. The first excess of ferrocyanide over that required to precipitate the zinc completely should cawe the polarographic voltage to back off to about +0.2 volt us. S.C.E. Such RRS found to be the case. With the decrease in voltage drop across the electrodes, there occurs a corresponding increase in the electrolysis current to about 1.3 wa. (The current levels in Figure 2 are not drawn to scale and are intended to indicate approximate values only The change in current levels is indicated by the (dotted) line.) While it is preferable to maintain constant current in theoretical studies, the changes in current seem to be of little practical disadvantage for most titration purposes. Up to the end point, the current is fairly constant because the polarographic voltage changes very little in this interval.
Table I.
1-5 MEG.
I
End-Point Break, V./O.Ol 111. 3.32 0.10 3.34 0.16 3.34 0.12 Theory 3 . 3 3 for precipitate of composition KrZna [Fe(CN)6]
Fe(CN)S----, MI.
LOO
5.00
dx
q : 1 1" F4qx x4Yzl
WE
1'01.
Vol. Zn, M1. 5.00
aT iI I
Titration of 0.1M Zinc with 0.1M Ferrocyanide
x
m
REF
Figure 1. Apparatus for Potentiometric Titrations at Controlled Current Input W E . Polarographic working electrode X. Connections t o meter
Titrations. For the current input titrations the apparatus of Figure 1 was used. The voltage source is a 1.5-volt dry cell and the resistor a 1- to 5-megohm carbon resistor. Such a voltage source cannot provide continuously constant current when used in conjunction with a polarographic cell, owing to the voltage drop across the electrodes a h i r h in most cases is a very large fraction of the source voltage, and may either be bucking or additive to the source voltagc. However, because the drop across the electrodes-i.e., the polarographic voltage-remains fairly constant up to the end point of the titration, the current is steady during this period. It changes abruptly a t the end point when the shift in polarographic voltage occurs. I n any event, the circuit of Figure 1 was chosen for its simplicity and performs quite satisfactorily in the practical eense. Gauguin et al have discussed the use of a 6- or 12-volt battery as a current
Using 75 ml. of 0.1S potassium chloride supporting electrolyte, portions of 0.lM zinc solution were titrated with 0.1M potassium ferrocyanide. The results are given in Table I and are seen to be in good agreement with theory. As with most precipitation titrations, equilibrium near the end point was established slowly. However, this in no way impaired end-point reading which was taken a s the first fraction of a drop that produced the sudden voltage shift. The 30-second wait between additions near the end point could be speeded up by using a fixed electrode and stirred solution as in the next section. Titration of ArsenicQII) with Bromate. The polarograms of Figure 3 served as a basis for the titration of arsenic(II1) with potassium bromate. The wave marked S represents the back-
CURREM 0
8011rce ( 6 ) . Either a Leeds and Korthrup p H meter, Model 7664, or an inexpensive Heathkit VTVRI, hiodel V-6 (Heath Co., Benton Harbor,
Mich.) was used for the continuous voltage indication. Any VTF'PVI with moderately high input impedance can be used. A 5-ml. semimicroburet was used throughout. With the exception of the cadmium-disodium ethylenediaminetetraacetic acid determination, all titrations were done in open beakers,
I
1.2
l
l
1.0
Titration of Zinc with Ferrocyanide. The precipitimetric titration of zinc with ferrocyanide was based on the polarograms
1
Figure 2.
I
l
I
0.6
0 8
E
e
EXPERIMEXTAL A N D RESULTS
,
VS.
I
0.4
,
1
-+ 0.2
8
1
0.0
S.C.E. (V.1
Polarograms for Titrations ofinc with Z Ferrocyanide B . Background oxidation
F.
Ferrocyanide oxidation
V O L U M E 2 6 , NO. 1 2 , D E C E M B E R 1 9 5 4 ground reduction which obtains up to the end point. This polarogram was taken on a typical titration mixture of 100 ml. of 1M hydrochl )Tic acid, containing ca. 1 gram of dissolved potassium bromide and 4 X l G - 3 S in arsenic(II1). I n Figure 3, with a cathodic cuIrrnt input of about 0.3 pa the polarographic voltage is about +0.3 volt us. S.C.E. With the appearance of the bromine reduction wave B , a t the end point, the polarographic voltage is shdted tq about +0.7 volt. I n this case, while the voltage drop across c!ie electrodes increases in magnitude a t the end point, the increase is additive t o the source voltage. The flow of electrolysis current is thus increaqed to ca. 0.4 pa. when free bromine appears. This sm:tll change is not represented on Figure 3. CURRENT
P
RED
E
vs.
0
S.C.E. (V.1
1935 was still about 0.36 volt per 0.01 ml. of bromate and the titrations were entirely satisfactory. The potential break by classical potentiometry (zero current conditions) for this same titration was only 0.04 volt per 0.01 ml. of bromate. Titrations of l O - 5 S arsenic(II1) with 0 . 1 S bromate were run, delivering about 0.04 ml. of bromate v i t h a commercial ultramicroburet. The end-point break was favorable even a t this level. By using small titration volumes, the indications were that the method could be satisfactorily used for such micro work. No further studies were made of these low level titrations Some work has been done on the use of this end-point detection method in microgram scale coulonietiic titrations with electrolytically generated bromine (11). Titration of Iron(I1) with Cerium(IV). On the basis of the formal potentials of the cerium( IT-), c.erium(III), and iron(III), iron(I1) systems in sulfuric acid medium, one viould predict considerable separation in the reduction potentials of cerium( IV) and iron(II1) a t a platinum electrode. Thus, a successful titration of iron(I1) with cerium(1V) would be expected using a small cathodic current a t a platinum electrode. i2ctually, the obseived potential break using cathodic curl ent is less than that obtained by classical potentiometr) . Larger breaks can be obtained with anodic current. The reasons for this are contained in the current-voltage curves of Figuie 4, which illustrate the fallacy of using other than experimental polarograms t o predict the behavior of a given system.
Figure 3. Polarograms for Titration of Arsenic(II1) with Bromine S. B.
E
Background oxidation Bromine reduction wave
14 CURRENT
Portions of arseaic(II1) solution were added to 100 mi. of 1-11hydrochloric acid containing 1 gram of potassium bromide and titrated with 0.1.V bromzte. A 1 sq. cm. of platinum foil was used :is the polarographic electrode and the solution was magnetically stirred. The cathodic input curreqt was made ca. 0.3 pa by using a 5-megohm resistor. The results are indicated in Table 11. All end points !isted were taken by direct reading as the volume a t which the indicated potential break occurred. This potential break was veiy large, a s seen in Table 11. Four similar conventional amperonietric titrations gave a mean value of 4.030 ml. with a precision of 1.8 parts per thousand. The agreement is very satisfactory and it appears that the current input method may be capable of slightly better precision in this application. Table I1 also indicates the titration conditions Khich were investigated-Le., duration of electrolysis, speed of titration, deaerztion, and method of voltage reading. These variables were without significant effect on the results. The electrode size was also found not to be critical, provided extremely small electrodes were not used. A few titrations of 4 X 10-.‘S arsenic(II1) were made in similar manner, titrating with 0.01.XV bromate. The end-point break
12 1
4
IO I
VS t
SCE. ( V I 0I 8
)
06 r
+(+) I
04 I
t
0, 2
(PA)
Fr IU +Fr
A
5 0 Y TITRATED
B
100 X
.s
(UII
TITRATED
-_ / -
Table 11.
Titration of 4 X 10-3N Arsenic(II1) with 0.1N Bromate
End Point, 111.
4,036 4,040 4.010
Break, V./O. 01 MI. 0.30 0.38 0.40
4.040
0.38 0 38 0.39
4.040
0.42
4.040
0.45
4.010 4.030
Mean 4 03a.
Precision 0 . 8 p.p.0.
Titration Conditions Rapid, large increments added Slow, small increments Current aoolied 10 min. Drior t o beginning titration . Heathkit V T V M for voltage Heathkit V T V M for voltage pH Meter and V T V M for voltage pH Meter and VTVM for voltage Deaerated 20 min. with nitrogen prior to titration
c.
2 Y AFTER
EQUIVALENCE POINT
Figure 4. Polarograms Taken at Stages during Titration of Iron(I1) with Cerium(1V)
The polrtrogritms of Figure 4 xere taken a t various stages in the titration of a mixture of 4.00 ml. of 0.05Ol1.1 iron(I1) solution in a total volume of 50 ml. of 2J.1 sulfuric acid. The titrant waa 0.0520M cerium(1V) also in 2 N sulfuric acid. The polarograms were made by the current scanning technique, using two identical 1-cm. platinum wire electrodes. The solution W ~ magnetically E stirred a t a rate corresponding to normal titration conditions.
ANALYTICAL CHEMISTRY
1936 The potential of each electrode was measured against a saturated calomel electrode reference half cell a t each current setting. From these data the cathodic and anodic waves were plotted. Since the region close to zero current is of particular interest in this instance, Figure 4 shows only a limited portion of the curves. Figure 4, A indicates the behavior a t about 5OY0 titrated. The cathodic and anodic branches correspond to the reduction of iron( 111)and oxidation of iron( 11),respectively.
EMF (V.1
.1.0 .0.8
c
If
10.6
_ _ - - - - _----
P-
--
2 -0-
-a-A*
3.0
2.0
VOLUME
I 4.0
Cr(IV)
(MU
Figure 5. Anodic, Cathodic, and Derivative Voltages during Titration of Iron(I1) with Cerium(1V) A. B. C.
0 X
A
Anode us. saturated calomel electrode Cathode vs. saturated electrode Derivative polarographic voltage
The situation a t the equivalence point is pictured in Figure 4,
B. (This equivalence condition was approximated very well by titrating near to the end point and then adjusting the redox potential of the solution a s close as possible to the equivalence point potential. The latter was calculated from the formal potentials of the cerium(1V) to cerium(II1) and iron(II1) to iron(I1) systems in 2.M sulfuric acid. A few drops of dilute cerium(1V) and iron(I1) solutions >\ere used to make the final adjustments.) The cathodic wave is again that of iron(II1) reduction. However, with all iron(I1) oxidized, the anodic wave is now due t o oxidation of cerium(II1). I n Figure 4, C, uhich was taken a t about 2y0 after the equivalence point, no distinct cathodic wave for cerium(1V) reductinn is observed. The rerium(1V) wave is drawn out to a large degree and merges into the iron(II1) wave with no cleaily defined diffusion plateau. While the half-wave potential for iron(II1) reduction a t platinum electrodes has been found fairly reproducible by a number of workers, some disagreement exists for cerium(IV) reduction. Rogers reports a n Ei/2 of about +O.45 volt 6s. S.C.E. for cerium(1V) reduction in 0.1M sulfuric acid a t a stationary platinum electrode (14). Kolthoff and Lingane indicate a smeared-out wave in 2 N sulfuiic acid with no well-defined diffusion plateau (IO). More recently Kolthoff and Jordan find a n Elit of about +0.8 volt vb. S.C E. in ld!f sulfuric acid using a rotating electrode (9). The coalesced waves for mixtures of cerium(1V) and iron(II1) reported here have been observed previously in this laboratory in connection with coulometric generation studies. Well-defined waves with considerably more positive half-wave potentials have been obtained with solutions of cerium(1V) alone.
The expected results with the current input method can be interpreted from the previous polarograms. Using either cathodic or anodic currents of 1 to 2 pa., the corresponding polarographic voltages would be about +0.4 to 0.5 volt bs. S.C.E. up to the equivalence point. This corresponds to the situation in Figure 4, A . -4t the equivalcnce point very little voltage change would be observed using a cathodic current. However, a sharp break is obtained on the anodic side. This shift in polarographic voltage is due to the disappearance of the anodic oxidation wave of iron(I1). Even 2% after the equivalence point, the potential break using cathodic current is somewhat less than that obtained on the anodic sidr, owing to the highly irreversible cerium( IV) reduction wave. These predictions were verified by running a titration identical with that represented by the polarograms of Figure 4. The same electrode system was used. The current source was a 45-volt B battery and a 22-megohm resistor, giving ca. 2 Fa. of curient through each of the platinum electrodes. I n this titration the potential of the anode and cathode was measured against saturated calomel electrode as a function of the volume of cerium(1V). The fact that the electrolysis current was not passed through the reference half cell in this instance does not materially change the conditions of the current input method. The titration curves are given in Figure 5 . Curve A shows the break obtained Rith anodic current. The poor break a i t h cathodic current is shown in curve B. Also, it is apparent that the indication is late with the cathodic current. Similar titration curves for this system have been given by Gauguin el al. (6). Since the present method is obviously closely allied to the derivative polarographic technique of Reilley, Cooke, and Furman ( I S ) , it is instructive to compare the results obtained by the two methods. The experimental setup in the previous paragraph cor responds to that used by the' above workers, and the derivative polarographic voltage WRS measured simultaneously with curves A and B by switching the pH meter leads across the t n o platinum electrodes. The derivative voltage is plotted as curve C. This curve is obviously the difference between the anodic and ea thodic polarographic voltages, curves A and B. The irreversibility of the cerium(1V) reduction wave is reflected in the higher derivative \ oltnge after the equivalence point. Thus, for this pat ticular titration, the cathodic current input method is unsatisfactory. While the end-point break is good using anodic current, equally good results can be obtained b) classical potentiometry, since the greatest shift in polarographic voltage is no larger than the difference in formal potentials of the two systems. The only apparent advantage of the current input method for the iron(I1)-cerium(1V) titration is that the potentials are established more quickly than in the case of a classical potentiometiic titration. [Since this manuscript n as submitted, Kolthoff h3s reported on some similar work regarding the utility of the current input method in the cerium(I\')iron( 11) titration ( 8 ) . ] The derivative polarographic method represents it much better end-point detection method for this pal ticular titiation. Titration of Cadmium with Disodium Ethylenediaminetetraacetic Acid. Since many conventional amperometric titrations employ the dropping mercury electrode, the application of the current input method to this electrode \\as examined. The compleximetric titration of cadmium++ m ith disodium ethylenediaminetetraacetic acid in acetate huffer of pH 4.2 was chosen foi this purpose. Cadmium ions are reduced in this medium giving a well-defined wave a t about -0 6 volt ZP. S.C.E. The CdY-- complex is reduced a t about -1.05 volt as shown in Figures 6 and 7. The conventional amperombtric titration of cadmium n ith disodium ethylenediaminetetraacetir acid was studied by Prtbil and llatyska, but no dptaik are given ( 1 2 ) . I n Figure 6, a small cathodic current input to the dropping mercury electrode should give i ise to a polarographic voltage of about
V O L U M E 2 6 , NO. 1 2 , D E C E M B E R 1 9 5 4
1937
i
INPUT CURRENT 1.5,JJA
50 % T I T R A T E D
E
0.4
VS.
0.6 Figure 6.
S.C.E. ( V . ) I .o
0.8
I.2
Polarograms for Titration of Cadmium with Na2HZY
-0.6 volt as long as uncomplexed cadmium ions remain in solution. Figure 7 indicates the situation a t the end point, 1% here all cadmium should n3w be in the CdYcomplex and the voltage should shift out to about -0.9 volt. (.4 small wave due to free cadmium is obtained even a t the end point, due to dissociation of the complex.) Figures 6 and 7 also show the experimental current values for this type titration. The straight line marked “input current” is a recording of current against time for the 1.5-volt battery and 1-megohm resistor combination, without being attached to the dropping mercury and reference electrodes. This was recorded by placing the Electrochemogrnph in series, using a lox current ange. Thus, the input current corresponds to that current rhich would flow provided there were no voltage drop across the
electrodes. The actual current uhich f l o ~ sas , a comequence of the opposing voltage drop across the electrodes, is recorded as the series current in Figures 6 and 7 . The fluctuations, coincident with drop time, are due to the changing voltage across the electrodes during the life of the mop. Although the variations in the current, due to the changing voltage drop a c r o s the clectrodep, mere of no practical significance in the previously mentioned titrations, they must he considered Tvhen using the dropping mercury electrode. I n the cadmium-disodium ethylenedinminetetraacetic acid example, a3 the titration proceeds, the oppoqing voltage drop acrops the electrodes increases and the electiolysis curIent thus decreases. Near the end point, this curIent must be sufficiently large to tw above the residual cuirent level and yet qmall enough that it nil1
100 36 T I T R A T E D (THEORETICAL E. P.
INPUT CURRENT
.6 JJA
SERIES CURRENT 0 . 7
E I
0.4
I
1
0.6 Figure 7.
1
I
0.8
VS. S.C.E. I V . ) I
I
1
I .o
Polarograms for Titration of Cadmium with Na2H2Y
I
1.2
1
ANALYTICAL CHEMISTRY
1938 not skip over the uncomplexed cadmium wave before the end point. I n thib particular titration, the current level must also be above the small wave for cadmium which remains a t the end point; otherwise the voltage break nil1 be blocked completely or be quite small. The latter is the case shown in Figure 7 . The polarographic voltage a t the end point is blocked a t about -0.75 volt by the remaining cadmium wave and a poor break is obtained. By using a resistor of about 0.7 megohm, the current in the beginning of the titration n7as ca. 1.3 p a . due to the 0.6-volt drop across the cell. At the end point, where the cell-drop had increased to about 0.85 volt, the current had decreased to about 1.0 pa. This level was sufficient to give an appreciable voltage break without leading to an early end point. These difficulties can obviously be eliminated by using a higher source voltage and the necessary increased rehistance to limit the current. The voltage drop across the electrodes can then be made a very small fraction of the source voltige and mill have a negligible effect on the current (1, 6). The titration n a s carried out in an ordinary manner, deaerating u ith nitrogen after each addition. A 10-microfarad electrolytic condenser was placed across the p H meter leads (observing proper polarity) to smooth out the voltage fluctuation. Either the Leeds and Northrup p H meter or Heathkit VTVhiI was used for voltage measurement. Polarographic voltages were read at maximum drop age, which is the most stable point to observe. In contrast to the previous titrations discussed, end points had to be taken from a plot of polarographic voltage us. volume of titrant. However, no volume corrections need be applied to these plots. Figure 8 s h o w that the voltage shifts rapidly near the end point and remains essentially constant after the end point. Theoretically, the equivalence point should be a t the volume of disodium ethylenediaminetetraacetic acid where the polarographic voltage has shifted completely to the CdY-- wave. On this basis, the empirical straight-line extrapolation in Figure 8 appears to be justified. Table I11 compares the end-point volumes obtained with the current input method and conventional amperometry for the cadmium-disodium ethylenediaminetetraacetic acid titration. The amperometric titrations were run at an applied voltage of -0.75 volt us. S.C.E. While the results are more precise with conventional amperometry, the current input method has the advantage that changing capillary characteristics have little or no effect on the results. While it has not been thoroughly checked, it should be possible to use capillaries which have drop times too fast or irregular for precise diffusion current measurements.
ad!
I
E
vs. S.C.E. O,s/ ‘v”
ii
I I
I
I
1.0
2.0
3.0
VOLUME NA,H,Y
Figure 8.
1 1
L
4.0
(MLJ
T i t r a t i o n of Cadmium with YazHnY
The technique was applied to the dropping mercury electi ode and satisfactory results were obtained in the titration of cadmium with disodium ethylenediaminetetraacetic acid. When applying the simple apparatus described herein to the dropping mercury electrode, account must be taken of the decreasing current level during the course of the titration. This difficulty is caused by the increasing potential drop across the polarographic cell and can be readily eliminated by using a higher voltage source. Some experimental comparisons of the present method with the derivative polarographic technique were made. The latter employs a small ronstant current passed through two identical platinum indicator electrodes and gives an excellent differential response when both polarographic systems involved in the titration are reversible. I n such a case, the best end-point breaks obtained with the current input method are generally no larger than those of classical potentiometry and the method has no particular recommendation. I n general, the current input method gives larger breaks than classical potentiometry only when one of the polarographic systems involved is the background medium. I n the titration of an irreversible system with a reversible titrant or vice versa, both the present mqthod and the derivative polarographic should give good results. An advantage of the current input method is the option of using cathodic or anodic current. This presenta the possibility of a number of new variations on conventional amperometric themes which may be particularly useful in specific situations. LITERATURE CITED
Table 111. T i t r a t i o n of 4 X 10-4M Cadmium(l1) w i t h 0.01M Na2FI2Y Current-Input Method,
Conventional Bmperometric,
4.00
3.95 4.00 3.95
M1.
M1.
3.95 4.14 4.00
Mean 4.01 Precision, 12 p.p.t.
3.97 5.7
CONCLUSIONS
The method of potentiometric titration a t controlled current input has been shown to compare favorably with conventional amperometry in several applications. I n precipitimetric titrations, stirred solutions can be used to speed up the slow reaction near the end point. I n the titration of arsenic(II1) with bromate very good results were obtained and the indications were that this determination can be used a t very low concentration levels. The fact that diffusion currents are not measured in the present method is an attractive advantage in solid electrode work.
(1) Adams, R. N., Reilley, C. N., and Furman, N. H., ANAL.CHEY., . 25, 1160 (1953). (2) Bertin, C., Anal. Chim.Acta, 5 , 1 (1951). (3) Duyckaerts, G., Ibid., 5 , 233 (1951); 8, 57 (1953). (4) Furman, N. H., AXAL.CHEM.,26, 84 (1954). (5) Gauguin, R., Anal. Chim. Acta, 5 , 200 (1951). (6) Gauguin, R., Charlot, G., Bertin, C., and Badoa, J., Ihid.. 7, 360 (1952). (7) Gauguin, R., Charlot, G., and Coursier, J., Ibid., 7, 172 (1952). (8) Kolthoff, I. hl., ANAL. CHEM.,26, 1684 (1954). (9) Kolthoff, I. hl., and Jordan, J.. Ibid., 25, 1833 (1953). (10) Kolthoff, I. hI., and Lingane, J. J., “Polarography,” Vol. 1, p. 420, Kew York, Interscience Publishers, 1952. (11) Lee, J. K., unpublished data. (12) PIibil, R., and lIatyska, B., Collection Czechoslav. Chem. Commum., 16, 139 (1951). (13) Reilley, C. N., Cooke, W. D., and Furman, N. H., Asar.. CHEM.,23, 1223 (1951). (14) Rogers, L. B., U. S. Atomic Energy MDDC-1015 (1947). (15) Van Name, R. G., and Fenwick, F., J . Am. Chem. Soc., 47, 19 (1925). RECEIVED for review March 31, 1954. Accepted October 20, 1954. Presented before t h e Division of Analytical Chemistry a t Meeting-in-Miniature of the A M E R I C A N C H E J r X C h L SOCIETY, Xew York, N. Y., 1954.
