Microtitration. The titration vas carried out in exactly the same manner with 5- t o 7-nig. samples, with the solutions in t h e sanie concentrations as in t h e niacrotitration. An ordinary flat silver electrode, niagnetic stirring with a flea, and a n Agla syringe buret (Burroughs Wellcome, Ltd., London) w r e used. Back-Titration. The solut'ion of t h e base was pipetted into a 25-nil. nieasuring flask, and 5 nil. of buffer solution and a measured volume of tetraphenylhorate-an excess over t h e base-\\-ere added. The volume was made u p to t h e mark, and t h e suspension was mixed well. ,411 )\-as filtered through a fine sintered-glass funnel without washing, and a n aliquot part of the filtrate \vas backtitrated viith Osaditon solution. The
infleetion point was taken as end point. Elimination of Halogen. An ion exchange column with a n inner dianieter of 10 inin. was filled with t h e resin to a height of about 80 inin. After washing well n-ith 2-Y nitric acid, water, 5-Y sodiuni hydroside, water, alcohol, chloroform, alcohol, watcr, nitric acid, water, and sodium hydrosick, it was conditioned with 2 N acetic acid and washed n-ith water to a neutral reacation. The solution to he titrated was passed through the column, washed out with 30 ml. of water, and tit,rated as described under "direct titration" or back-titration. " The column w i s regenerated by passing sodium hydroxide, water. acetic acid, and water through it.
ACKNOWLEDGMENT
The authors are indebted to Pharmacia, Ltd., for permission t o publish this paper and to Mats Carlsson for carrying out a number of the titrations. LITERATURE CITED
(1) Coopel', s. s., -LSAL. C H E J I . 29, 446 (1957). ( 2 ) Crane, F. E., Jr., I / ~ i d .28, ~ 124 (1966). ( 3 ) Crane, F. E.>Jr., . l n u / , C'him. Acta 16, 3T0 (1957). ( 4 ) Fakstorp, J., Pedersen, ,J. G. -1, Acta pharniacol. t o x i r d . 10, I (1954). (5) ItiidorfY, \i7.j Zsnnicr, H., Z. a d . Chen,. 137, 1 (1952).
RECEIVED for revie\\. Jariiiar?. 2 2 , 1957. -4ccepted September 1'3, 195T.
Potentiometric Titration with Controlled Current Application to Coulometric Titrations J. K. LEE1 and RALPH N. A D A M 1 Princeton Universify, Princeton,
N. J.
b The technique of potentiometric end point indication was invesiigated using controlled current as applied to microgram-level coulometric titrations. This method has not been previously applied to coulometric titrations. The accuracy of the results i s discussed in terms of the indicator cathode response and the indicator current levels employed. The method gives very good results for quantities of arsenic between 40 and 200 y. This technique should find further application in lowlevel coulometric titrations.
gram region, this amount of bromine is an appreciable fraction of the total and modifications must be made. The present study is concerned mainly with the low-level titrations. The titration procedure described is analogous to the sensitive end point method of Cooke, Reilley, and Furnian ( 3 ) . h small amount of bromine was generated prior to the sample addition. This is called the pregeneration time and the resulting indicator voltage is called the preset voltage throughout this discussion.
T
The source of the constant current employed in the generating circuit was identical with that described by Cooke and Furman ( 2 ) . The generating current 15-as measured by determining the I R drop across a standard resistance using a Le& R- S o r t h r u p student-type potcntiomrter. iill time measurements ~ ~ e niadc rc n i t h a synchronous motor clock (Dinico-Gray Co., 12IodP.l 201). The gcncrator anode v a s a pl~tinuniiridium foil, about 2 sq. em. in size. The cathode v a s a small pirce of platinmil wire in a shielded compartment. This cathode compartnient \vas filled nitli 1.Y hydrochloric acid which is the sanie acid concentration present in the titration cell. It is important that the cathode compartment does not leak; othernise the results of the titrations tend to be high. The indicating current was obtained froni a 1.5-volt d r y cell and a 1-megohm resistor (ea. 1.5 pa.) for the niicrograni range titrations. il 5-megohm resictor
APPARATUS AND REAGENTS
use of potentiometric end point detection with controlled current has been applied t o the volumetric titration of arsenic(II1) with bromate (1). For the macrot'itration, the end point break is the result of the differencein the polarographic voltage between the background reduction and the bromine wave which appears a t the end point. This same technique is applicable to milligram-level coulonietric titrations using fairly large generating currents. I n such a titration, the generation is carried out until a sudden change in indicator voltage occurs. The excess bromine needed t o bring about this change is small compared to tlie total bromine generated and does not eonst.itute a serious error. However, in the niicro1 Present address, Department of Chemistry, University of Kansas, Lawrence, Ka,n. HE
240
ANALYTICAL CHEMISTRY
decreased the current to about 0.3 p a . for the milligram-range titrations. Such l o x voltage sources do not maintain constant current, because of the changing voltage drop across the indicator electrodes. However, they are satisfactory for this titrat'ion. -4Lecds &- Northrup Model 7664 p H meter was used to measure the indicator voltage. The following Plectrodes !\-ere csaniined for selection of the most suitable indicating cathode: Elect'rode A, platinum-iridium foil. about 1 sq. mi., fused to a piece of platinuni wire. Electrode B, 26-gage platinum !vires 1 cni. long. Electrode C, 28-gage platinuni wire 3.5 cni. long coiled into a spiral. ,411 electrodes were sealed securely into soft glass tubing. Electrode C was found to be most desirable as an indicating cathode. Fiftymilliliter beakers were uscd as titration cells. The arrangement of elect'rodcs in tlie titration cells was siniilnr to t,hat described liy Cooke and Furnian ( 2 ) . The solutions w r e stirred vigorously with a niagnetic stirrer, but not so much as t,o cause cavit'ation. All polarogranis n-ere taken using the Fishfr Elecdropode. The scale of the Elecdropode was calibrated rvith a known current. The generation nicdiuni n.as 0.5 gram of potassium bromide dissolved in about 35 nil. of 1 S hydrochloric acid. This was prepared by weighing out reagent grade potnssiuni bromide and dissolving in t,he hydrochloric acid just hefore each E tibration. The st,andard arsenious solution was prepared by weighing out the approsimate quantity of Kational Bureau of
0.50
0.70
0.30 0.10 0 E VS S.C.E.
does not eliminate the second difficulty, the s l o ~end point change. This was overcome by pretitration, followed by sample addition and generation back to the preset voltage. K h e n a larger indicator current (1.8 Wa.) \vas used, the intersection with the hydrogen waves was as indicated by points X and Y in Figure 1. Figure 2 shows the variation of indicator voltage (using 1.8-pa. indicator current) as a function of bromine generation time. The generating current was 1.0 ma. Figure 2 includes only a short generation time which is of interest for end point detection. A suitable pregeneration time lies between 0.1 and 0.2 minute, for an indicator current of 1.8 pa. and a generator current of 1.0 ma. If this pregeneration is expressed in microequivalents rather than current, the proper pregeneration for any current value is easily calculated. If the pregeneration time a t 1.0 ma. is longer than 0.2 minute, the indicator current increases considerably. as indicated by Tablr I. Selection and Pretreatment of Indicating Electrode. Table I1 summarizes t h e response chaiacteristic of t h e various indicating electrodes.
0,lO
Figure 1 . Polarograms of the generation medium after various generation times at 1 ma. 1. 2. 3. 4. 5.
0.00 minute 0.20 minute 0.40 minute 0.60 minute 0.80 minute
Standards arsenious oxide. The oxide \vas dissoli ed according t o the procedure given by Killard and Furnian (8). Sufficient hydrochloric acid was added so that the final solutions were 1 S in hydrochloric acid. Two standard solutiom, 0.0901 and 0.0325.\-. maintained thvir strengths Tvithout appreciable c1i:rngc for more than a month. When a more dilute solution n a s needed. it n-as frcslily prepared by appropriate dilution of n dock qolution. EXPERIMENTAL A N D RESULTS
Choice of Indicator Current and Pregeneration Time. The choice of indicantor current level is dependent upon the currriit-voltage curves of the tJlrctroacti\-e systems involved in the titrntior,. For c w h application these currmt-voltagc C U I ves (polarograms) shciuld he detrrmined under conditions iling during t h r tit ration. The electrodrs, the titration cell containing the genrration medium. and the generating circuit ivere arranged as in an actual titration. The indicating cathode and t h r reference saturated calomel electrotlr were coniiccted to the “drop” aiid “pool” leads. respectively. of the Elecdropode. Polarograms ivere taken wheri no current had passed through the generating electrodes and after generating for 0.20, 0.40, 0.60. and 0.80 minute a t 1.0 ma. The rrsults are shown in Figure 1. It is shown in these polarograms that if a n indicator current of about 0.3 pa. is used (in the sense that the platinum indicator is a cathode), the voltage nil1 shift from about +0.2 volt t o +0.6G volt us. S.C.E. (from A t o B in Figure l’i as soon as a small excess of bromine appcurs. For the milligram scale titrations, this voltage break serves t o indicate the end point. The break is sharp enough to makv plotting unnecessary. A complete m:ithcm:itical ana11 sis of
the voltage change as a function of titrant added has been given by Gauguili and con-orkrrs ( 5 , 6) as li-ell as Delnhay
(4).
The above technique \%-asfound unsatisfactory for microgram-level w x k for two reasons. The ~ K C P S Sbromine necessary for the voltage change represents a positive titration error of some considerable size; and with the small generating currents used (0.5 t o 1 ma.), the voltage break is not sharp because of the slow rate of addition of bromine. The first difficulty might be eliminated by using a lower indicating current, effectively increasing the sensitivity of t h e electrode toward bromine. This is not practical, as very low indicator currents intersect residual current wives a t potentials more positive than the bromine wave. This residual curront is probably due mainly t o the reduction of t h e oxide film on the electrode surface. The indicator electrode potential remains a t these more positive potentials and fails to “recognize” a n y bromine. d satisfactory solution was found in deliberately fixing the indicator current level well above the plateau level of the minimum ewess bromine polarogram. The response is then due to the background wave (hydrogen). Apparent shifts in the foot of the hydrogen wave were then measured. The modification Table I.
I
-
I
;I
tOl5-
+OIC W 0
cn a’
I
I
>+OB
W
-
I/
If
14
Figure 2 . Polarographic voltage as a function of .generation time at 1 .O ma.
Change of Indicating Current as a Function of Bromine Concentration“
Timeb, RIin. 0.00
0.20 0.40
B w , Meq. 0
Polarographic Voltaged -0 056
12.4 10-5 +O 064 +o 545 24.9 10-6 + O 6iD 3 7 . 3 10-6 0.60 Electrodes immersed in generating medium onl)., 110 arsenic present. b At 1 ma. c In 40 ml. d L’S. S.C. E.
Indicating Current, pa. 1 8 1 8
2 2 2 4
Q
VOL. 30, NO. 2, FEBRUARY 1958
241
Table II.
Initial. Potential
+ O . 254 + O . 149
$0.135 $0.135
indicating Cathode Response as a Function of Electrode Size
Time of Equilibrium, Potential Min. b0.2 min. of bromine generation Electrode A $0.594 0.7 + O . 559 1.5 f-0.551 1.5 + O . 546 1.5
3 Min. of Generation Potential
+ O . 704
$0.742 $0.734 f0.730
Electrode B $0.344 $0.079 + O . 014
+ O . 364
$0.683 $0.703 $0.689
0 0 0
f-0.104 i-0.024
Electrode C 0.264 0.194 0.044 0.034 0.036 -0.065 -0.106 -0.045 -0.054
0.484 0.154 0.119 0.094 0.094
where I is the generating current in milliamperes. The amount of bromine required in the solution a t the end of the titration is:
+$0.734 0.724
0.1
0.1
+O. 726
0.1 0.1 0.1
$0.734 f0.734
After soaking in generating medium for more than 24 hours0 0.045 0.1 -0.011
0.042 0.046 0.101 0.064
-0.055 -0.056 -0.055 0.050 0 Vs. S.C.E. b A t 1 ma. e Readings taken over a period of a month.
mine a t the indicating electrode caused a decrease of the potential. B shows the observed drop of potential as a function of time. If this factor is taken into account, A should be linear to larger volumes. Assuming that the potential is a direct measurement of the concentration of bromine in the solution, a simple mathematical treatment can be derived as follows: The milliequivalents of bromine after the pregeneration time of T p seconds is given by the equation:
0.1 0.1 0.1 0.1 0.1 0.1
+0.735 i 0.005
where Vi is the initial volume of the solution in the titration cell. V I is the final volume after the sample was added. The error in milliequivalents of bromine caused by the addition of the sample volume is:
Vi
X V,
T X I
-P 96,500
= meq.
or The largest electrode, electrode A , required a long time to reach a stable potential. The smallest, electrode B , responded very quickly. The potential change, however, was rather small. Electrode C came to equilibrium fairly quickly and yet gave a suitable change in potential when bromine was generated into the solution. Thus this electrode was selected to be the indicating electrode for all titrations. I n the milligram-range titration, no pretreatment or special care is necessary for the indicating cathode. For the microgram range titration, the following pretreatment is recommended: The indicating electrode is soaked in the generating medium. Bromine is then generated (0.20 minute a t 1 ma.). The cell is drained and the treatment repeated until relatively reproducible initial potentials are obtained. The electrode is then ready to be used as the indicating cathode. No further treatment is necessary if the electrode is kept in the generating medium. Effect of Volume Change Due to Sample Addition. Addition of arsenic(111) sample in the form of solution will change the total volume of the solution in the titration cell. If the preset potential method is used, the concentration of bromine before the addition of the sample and a t the end point of the titration must be the same. However, as the volume is increased n-hen the sample is added, the absolute amount of bromine generated must also increase. This should give high results in the titration. To investigate this effect the apparatus was set up as in the actual titratiqn. 242
ANALYTICAL CHEMISTRY
b
The amount of bromine used in the titration after a titration time of T ; and pregeneration time T,, is:
0.10. A
or
.".eL 0
510 ML.
Figure 3. A.
B.
0
1 2
MINUTE
Change of voltage
As a function of volume added As a function of time
A current of 1 ma. was passed through the generating electrodes for 0.2 minute. The indicator electrode voltage mas recorded. Increments of 2.00 ml. of 1N hydrochloric acid were added to the titration cell. The decrease in potential was recorded in each instance. The average time for adding a total volume of 10.00 ml. was 2 minutes. Curl-e A in Figure 3 is a typical plot of the change of potential as a function of the volume of hydrochloric acid added. The decrease of the polarographic voltage is a fairly linear function of the volume added when the volume is small. Further investigation revealed that evaporation and reduction of bro-
The amount of arsenic(II1) in grams in the sample is then,
0.03745
=
grams of arsenic (1)
Titrations of Arsenic(II1). A. TIMILLIGRAM RANGE.About 35 ml. of 1N hydrochloric acid and 0.50 gram of solid reagent grade potassium bromide were placed in the titration cell. The solution was stirred until all solid was dissolved and the air bubbles removed. Arsenic(II1) sample was added to the titration cell. A current of 0.3 pa. was passed through the indicating electrode system for a few minutes until a stable initial potential was obtained. Bromine generation was then initiated. The sudden and continuous rise of potential indicated the end point of the titrations. TRATIONS IN
The results of the determinations are reported in Table 111. The accuracy of the titration decreased as the amount of sample used decreased. If an accuracy to 0.1% is desired, the sample should not be smaller than 10 mg. The titra-
Table 111.
Titration as Arsenic(ll1) in Milligram Range
As
Taken 9.73 7.29 4.87 3.64 2.43
As
Error,
Found 9.74 7.28 4.88 3.66 2.44
$0.11 -0.14 $0.20 +O. 33 $0.50
%
tions were carried out by three different operators and reasonably good results were obtained in all cases. This fact indicates that the end point is so sharp that even an inexperienced operator can recognize it without difficulties.
B. TITRATIONIN MICROGRAM RANGE. The generating medium was prepared as described in A. A current of 1.8 pa. was passed through the indicating electrode system for 5 minutes prior to the beginning of the titration. A small quantity of bromine was generated and the corresponding indicator electrode voltage recorded. For generating currents of about 1 ma., 0.20 minute was used. A proportional pregeneration time mas used for other generating currents. I n any case, the preset potential should never be more than 0.2volt from the initial potential. The sample was then introduced before any appreciable amount of bromine was lost. To obtain accurate results, the volume of the sample added should not be larger than 10 ml. Bromine generation was initiated until the preset voltage was reached. The amount of arsenic in the sample was calculated, using Equation 1. The stirring rate should be held relatively constant during the titration. Air bubbles picked up by
Table IV.
Titration of Arsenic(ll1) in Microgram Range
As Taken, y 243.30
84. 26
60.82
AS
Found, y 243.18 243.46 244
84.36 84.19 84.30 84.19 84.21 60.81 60.95 61.01 60.64 60.68
Error, m /o
-0.09 $0.06 +0.32 -0.64 +o. 02 $0. 02 -0.04 -0.02 $0.12 -0.08 $0.05 -0.08 -0.06 -0.17 $0,22 $ 0 . 32 -0.30 -0.23 $0.68 $0.06 +o .72 +0.45 $0 .39
the indicating electrode during titration will give rise to erroneous results. The results of typical titrations are tabulated in Table IV. DISCUSSION
The present end point detection method offers the following advantages: A very small amount of hydrogen is generated a t the indicating electrode before the titration. This should reduce traces of oxidizing substances (oxide) that might be on the indicating electrode. The pregeneration of bro-
mine will eliminate reducing substances in the titration cell before the addition of the sample. The above practice also pretreats the indicating electrode according to the method prescribed by Mj7er and Swift before each titration ( 7 ) . The nonreproducibility of the indicating electrode from titration to titration does not affect the results. While the present work deals only with the titration of arsenic(II1) with bromine, it is believed that the present technique can be applied to other low level coulometric titrations. ACKNOWLEDGMENT
The invaluable assistance of Jean dnderson in the preparation of the manuscript is gratefully acknowledged. LITERATURE CITED
Adams, R. N., ANAL. CHEM.26, 1933 (1954). Cooke, W. D., Furman, K. H., Zbid., 22, 869 (1950). Cooke, W. D., Reilley, C. Ti,, Furman, N. H., Zbid., 23, 1662 (1951). Delahay, P., ‘ Y e w Instrumental Methods in Electrochemistry,” pp. 251-4. Interscience, Kew York. 1954. Gauguin, R., Charlot, G., Bertin, C., Bandoz, J.. Anal. Chim. Acta 7. 360 (1952$.’ Gauguin, R., Charlot, G., Coursier, J., Zbid., 7, 172 (1952). Myer, R. J., Swift, E. H., J. Bm. Chem. SOC.70, 1047 (1948). Willard. H. H.. Furman. N. H.. “Elementary Quantitative Analysis,” p. 273, 3rd Ed., Van Nostrand, New York, 1940. RECEIVED for review October 10, 1955. Accepted August 24, 1957.
CompIexometric Titration of Copper and Other Metals in Mixture 1-(2-Pyridylazo)-2-naphthol (Dye) as Indicator K. L. CHENG Ufica Metals Division, Kelsey-Hayes Co., Ufica 4,
b The total amount of copper and zinc, cadmium, or nickel in a solution can be titrated by (ethylenedinitril0)tetraacetic acid (ethylenediarninetetraacetic acid, EDTA) with 142pyridylazo)-2-naphthol as the indicator at pH 2.5 to 10. In the absence of copper, zinc or cadmium can be titrated at pH above 5 by EDTA with 1-(2pyridylazo)-2-naphthoI as the indicator. Zinc, cadmium, or nickel can be titrated directly b y EDTA in the presence of copper which is masked by the
N. Y.
addition of a slight excess of thiosulfate. The amount of copper in the mixture can be calculated by difference. The difficulty of obtaining a sharp end point in the complexometric titration of copper with 1 -(2-pyridylazo)-2-naphthol as the indicator may be eliminated by addition of alcohol, dioxane, or other organic solvent.
T
stability constants (log K ) for complexes of (ethylenedinitrilo) tetraacetic acid (ethylenediamineHE
tetraacetic acid, EDTA) with copper(I1) and zinc(I1) are reported to be 18.3 and 16.1, respectively (6). Copper(I1) forms a more stable complex than zinc with 1-(2-pyridylazo)-2-napthol (a dye); a t p H 2.0 copper(I1) forms a complex with the dye but zinc does not. However, when a mixture of copper and zinc was titrated by EDTA with the dye as the indicator, both copper and zinc were titrated instead of copper alone (1). Zinc can be titrated with EDTA by VOL. 30, NO. 2, FEBRUARY 1958
243
