ANALYTICAL CHEMISTRY
1484 Jeanes, Allene, and Isbell, H. S., J . Research Natl. Bur. Standurds, 27, 125 (1941) (RP 1408). Keilin, D., and Hartree, E. F., Biochem. J . , 42, 221 (1948). Launer, H. F., and Tomimatsu, Y.,ANAL. CHEM.,25, 1767
(16)
Young, F. E., and Jones, F. T., U. S. Patent 2,588,449 (March
(17)
Young, F. E., Jones, F. T., and Lewis, H. J., J . Phys. Chem., 56,
1 1 , 1952). 738 (1952).
(1953). (12)
Launer, H. F., and Tomimatsu, Y., J . Am. Chem. SOC.,76, 2591 (1954).
F., Wilson, W. K., and Flynn, J. H., J . Research Natl. Bur. Standards, 51, 237 (1953) (RP 2456). (14) Sowden, J. C., and Schaffer, R., J . Am. Chem. Soc., 74, 499 (13)
Launer,”.
(1952).
(15) White, J. F., Taylor, M . C., and Vincent, G. P., Ind. Eng. Chem., 34,782 (1942).
RECEIVED for review December 21, 1953. Accepted June 7, 1954. Presented before the joint sessions of the Divisions of Analytical and Carbohydrate Chemistry. Symposium on Analytical Methods and Instrumentation Applied to Sugars and Other Carbohydrates at the 124th RIeeting of the AMERICAN CHEMICAL SOCIETY, Chicago, Ill. Mention of products by specific manufacturers does not imply that they are endorsed or recommended by the Department of Agriculture over others of a similar nature not mentioned.
Conductometric Standardization of Solutions of Common Divalent Metallic Ions Using Disodium Salt of Ethylenediaminetetraacetic Acid JAMES L. HALL, JOHN A. GIBSON, JR., PAUL R. WILKINSON, and HAROLD 0.PHILLIPS W e s t Virginia University, Morgantown,
W. Va.
An effort has been made to evaluate the use of conductometric methods for end-point determinations in the titration of solutions of the disodium salt of ethylenediaminetetraacetic acid and divalent metallic ions. Conductance methods may be used for accurate standardization of solutions of copper(II), zinc, lead, nicltel(11), cobalt, calcium, barium, strontium, magnesium, manganese, cadmium, iron(IL), and mercury(I1) in the concentration range from 0,001 to 0.5M, before dilution in the titration vessel.
R
ECENTLY the disodium salt of ethylenediaminetetraacetic acid (Versenate, Sequestrene, Complexone 111) has been proposed as a standard for establishing the concentrations of solutions of certain divalent cations ( 2 ) . The stability constants of the complexes are great enough t o make precise end-point determinations possible ( 1 7 , 18, 20). The stoichiometric relations for the reactions between the metallic ions and the reagent have already been determined by several methods with reported accuracies within 0.05 to 2.0%. Metal ion concentrations have been determined potentiometrically (1, 10, 11, 19),by use of indicators ( I ! 4 , 5 , 9 , 1 6 , 19), spectrophotometrically (12, I S , 22, 23), polarographically (6, 14,1.5,21),and by a specialized high-frequency technique ( 3 ) . The present work shows that conventional conductometric methods may be used for the standardization of solutions of several common cations. The accuracy compares favorably with the best previously described methods. REAGEVTS
Disodium Versenate. Standard solutions of the reagent were prepared from the analytical reagent (disodium Versenate dihydrate, manufactured by the Bersworth Chemical Co.) and from Versenate purified by the method of Blaedel and Knight (2). All solutions mere standardized with electrolytic copper, dissolved in a minimum amount of 6 S nitric acid. The end points were determined conductometrically as described below. Titration in either acidic or basic solution yielded the same molarity. Solutions O.lOOlM, 0.04724M, O.O1001M, and 0.001004.11 were prepnred. I n weighing the copper and disodium Versenate for these solutions, the weight of the I’ersenate was corrected for the difference in density between the Versenate and the brass weights. Rascd on a density of 1.8 for the Versenate, this correction was 0.06% relative to the metallic copper. Cation Solutions. Solutions of cupric nitrate, cupric per-
chlorate, nickel nitrate, cobalt nitrate, lead nitrate, zinc sulfate, manganese sulfate, cadmium chloride, ferrous sulfate, magnesium sulfate, strontium nitrate, calcium chloride, barium nitrate, mercuric acetate, lanthanum nitrate, and cerium nitrate were prepared a t various concentrations from Baker’s analyzed or Mallinckrodt reagent grade chemicals. In addition, copper, zinc, and nickel nitrates were prepared by dissolving metal of known purity in 6n’ nitric acid. Konconductometric standardizations were made for most of the solutions; the purity of the calcium carbonate from which the calcium chloride solution was made, the strontium nitrate, and the barium nitrate was established by gravimetric analyses ( 2 4 ) . The normality of the manganese( 11), cadmium, lead, zinc, magnesium, and mercury(I1) salt solutions was determined with Versenate using the indicator method of Schwarzenbach ( 1 ). The solutions Ivere made in concentrations from 0.001 to 0.2;2f. Ammonia. Where ammonia was required, the C.P. product proved to be satisfactory for solutions of metal ion concentrations of 0.1.V or greater. At lower concentrations, errors introduced by impurities became appreciable and distilled ammonia was necessary. The ammonia was distilled into conductivity water to a concentration of 3M and was stored in polyethylene bottles. Water. Whenever the available distilled water was used, a correction equivalent to 0.4 ml. of 0.01M Versenate per 1000 ml. of a a t e r was required. Twice distilled water was preferable for all solutions 0.lM or less. Acid Buffer. Twenty-five grams of Baker’s analyzed sodium hydroxide and 65 ml. of glacial acetic acid were dissolved in water, mixed, and diluted to 250 ml. The p H of this buffer was 5.1. KO difference in end-point ratio was found for 0.01M copper(I1) solution titrated with and without this buffer. The use of U.S.P. sodium acetate for the buffer yielded a result 295 in error. APPARATUS
The most precise measurements were made a t 2000 cycles using a Leeds & Xorthrup Type 1553 ratio box and Type 4754 decade resistance with recommended oscillator and amplifier. A 50ppf. variable capacitor, and decade capacitors to provide a total caparitance up to 1 uf.,were connected in parallel with the known resistance. The null point was determined by observing the output wave on an oscillograph. Used in this way, the apparatus has a range of 0.01 to 10,000 ohms with a maximum error of 0.03yo a t 10 ohms or greater. .A dip-type conductivitv cell with platinized platinum electrodes and a cell constant of 0.0964 wa8 used. Titrations werr made a t room temperature. Additional conventional conductance measurements were made using a Model RCZI15 Serfase direct-reading conductance bridge. This instrument gave satisfactory results for work a t concentrations of 0.01.11 or less. Many of the determinations were also performed using two high-frequency instruments ( 7 , 8). Thefie instruments were satisfactory for routine work in the more dilute solutions but did not contribute any new or more useful results. Kumerical data are not included for these high-frequency determinations.
1485
V O L U M E 2 6 , NO. 9, S E P T E M B E R 1 9 5 4 Table I. Solution (Molarity) Cu(xOa)z (0,04990) Versenate (0.04724) Cu(N0a)P (0.1004) Versenate (0.04724)
Versenate (0.02197) a
Validity and Reliability of .Method Titrant (Molarity) Versenate (0,04724) Cu(N0a)i (0.04990) Versenate (0.0995) Ba(SOa)n (0.06003)
M1. Used 39.98 40.00 40.00 40,OO 4.99 4.99 40.00 40.01 25.00 15.00 5 00 100.00
Ba(N0s)z (0,05003)
Acidic solution, no buffer present; pH 10.1 t o 10.5.
NHa a t
Exptl.,
311. 42.23 42.25 37.83 37.82 5.03 5.03 37.82 37.81 23.58 14.15 4.69 43.86
Theor., RIl. 42.23 42.25 37.87 37.87 5.03 5.04 37.77 37.78 23.60 14.16 4.71 43.90
Difference, Versenate 0.00 0.00 1-0.04
+0.05 0.00 f0.01 -0.05 -0 03 f O 02 +0.01 +0.02 +0.04
all other titrations in presence of
Table 11. Molarities from Katio Determinations Source of Ion Pb(N03)z
Present Method 0.0998an b o.ii6ia. b 0.0972Qr b 0 . 0 9 9 ~b~ .
MnSOa
CdC0a
Zn
MgSOa CaC03
Ba(N0dz Sr(N0dz Hg(CnHj0z)z cu a
b C
Acidic buffer. Ammonia. N o buffer.
Other Methods
0 . ii59d 0.0975d 0.1004d n _ . innne _”_” 0 lOlld 0.09961 0.1OOOf 0.lOOOf 0.1142d
0.1008b 0.0997b 0 O996b 0.1002b 0.1139b
O.O1OOlat
h,
0.01001e
C
After Tenfold Dilution Present Other method methods 0.01001br C 0 OlOOOd
......
....
o.00996~
o.oioO3d
...... ...... ......
....
......
0.01002b ,..... 0.001005b
....
....
beaker. Ten milliliters of acid buffer or up to 20 ml. of 3 M ammonia, as required, were added and the solution was then diluted to 400 ml. and titrated with salt solution. Titration of the salt solution with the Versenate solution was usually satisfactory. Ordinarily conductance measurements were made for only a few milliliters on each side of the anticipated end point. The end point was taken from a graphical plot of the condurtance or resistance. RESULTS AND DISCUSSION
It was first demonstrated that the conductometric method gave stoichiometric ratios between Versenate and solutions of salts of copper and barium. For these determinations the Versenate was prepared by the method of Blaedel and Knight ( 2 ) . Resistance readings were taken after every 0.1-ml. addition of titrant for 2 ml. on each side of the end point. The cupric nitrate solution was made from electrolytic copper of 99.9% purity. The barium nitrate was recrystallized reagent grade salt, shown by a gravimetric sulfate determination to be 100.0% pure. For the Versenate titrations shown in Table I, the average error is 0.05%, which is within the precision of volumetric glassware for the volumes and apparatus used. 1.7541
,...
f ’ 4 1
J
.... . . I .
- 5.30
Schwarzenbach indicator method. From weight of metal used. f Gravimetric. d
‘
XI@
-5.25
- 5.20
EXPERIMEKTAL PROCEDURE
Ratios were determined between each of the metal ion solutions and a Versenate solution of approximately the same concentration. Concentrations are given before dilution in the titration vessel. Sufficient Versenate, either stock solution or purified powder, to require about 40 ml. of titrant was placed in a 600-ml.
111 -5.10
ML. VERSENATE ADDED
- 5.05
-5.15
43.00
-5.00
1.802 1.7981.794
-
1.790-
% 1.786X
u v 1.782-
0 0.4 0.8 L2 1.6 2.0 2.4 2.8 ML. ADD36.5 FOR CURVE 1.22.0 FOR II, 3 .0 FOR 111
Figure 2.
Effect of Ratio of Barium to Versenate on Curve Form I. 4 0 d . 11. 25 ml. 111. 5 ml. of 0.04724M Versenate Each titrated with 0.05000MBa(NOs)r
z
s
2 1.778 -
U
2 0 V
U 1.774 !e w V
% 1.7 70 -
Figure 1. Ratio of Copper(I1) to Versenate M1. of 0.04724M Versenate added to 40 ml. of 0.04990M Cu(N0s)z 11. M1. of 0.04990M Cu(N0a)i added to 40 ml. of 0.04724M Versenate I.
Figures 1 and 2 show samples of curves from which the end points used for Table I were obtained. I n general these curves consist of two straight-line branches. Curvature some distance from the end point, or continuous curvature as in curve 2 of Figure I, is attributed, in part, to incomplete buffer action. This curve is typical of cation solutions containing a slight excess of acid, as for the copper nitrate solution used in this titration. “Rounding” in the vicinity of the end point as for curve 1 of Figure 2 is attributed to partial dissociation of the complex in the vicinity of the end point. The rounding which occurs for the curves for barium-Versenate titrations is typical of cations forming complexes with low stability. The rounding decreases with decreasing concentration as shown by curves 1 to 3 in Figure 2. This is in agreement with results reported by Schwarzenbach and Ackermann (I@,who showed that the stability of the complexes increases with decressing ionic strength.
1486
ANALYTICAL CHEMISTRY Equations 1 and 2 show the dependence of the ionization of Versenate on pH. When the disodium Versenate is dissolved in water, the dihydrogen Versenate ion further ionizes to some extent, as shown in Equation 1. If this solution is added to a solution of low pH, the pH rises owing to the reversal of Equation 1 and the action of Equation 2. The Versenate-metal ion complexes form strong acids, as demonstrated by the action of cobalt on the Versenate ions as shown by Equations 3 and 4. The effect of these reactions on the conductance curves is shown by curves 1 and 2 in Figure 3. As Versenate ion is added to cobalt nitrate solution (curve I), complexing occurs causing liberation of hydrogen ions. This increases the conductance and decreases the pH, so that partial dissociation of the complex results a t the equivalence point. After the equivalence point, however, addition of excess Versenate causes the pH t o rise and the dissociation of the complex is reduced. For this curve, extrapolation of the straight-line portions to the point of intersection does not give the correct equivalence point. The addition of cobalt nitrate solution to Versenate as in curve 2 decreases the pH, but in this case the excess of Versenate present, decreasing to zero a t the end point, acts as a buffer, causing continuous curvature. The same titration with a suitable buffer present is shown in curve 3. A sharp and reproducible end point results. This curve is typical of those for cation solutions titrated in this pH range.
""4 2 0 ML.
Figure 3.
22
24
20
26
30
Effect of Buffer on Cobalt(I1)-Versenate Titration
CONCLUSION
I. 26 m l . 0.0995M Co(N0s)r w i t h 0.1000M Versenate, no buffer 11. 25 ml. 0.1000.W Versenate w i t h 0.0955M Co(N0s)i no buffer 111. S a m e as 11, b u t w i t h a c i d buffer
As illustrated by Table 11, the method may be used for the standardization of a number of divalent ions. For the titrations upon which these results were based, conductance measurements were made using greater volume increments in the vicinity of the end point than for the work shown in Table I. This increases the rapidity of titration with little loss in accuracy. For the work included in Table 11, a maximum error of 0.2% with an average error of only 0.08% occurred between duplicate determinations. The molarity of the metal ion solution as determined by the conductometric method lies, in most cases, about 0.3% below the molarity as determined by the indicator method. This difference represents about 0.1 ml. of solution and possibly is due to partial dissociation of the complex a t the end point, giving rise to a color change before the actual equivalence point is reached. In addition to the ions shown in Tables I and 11, the present method has been found to give sharp and reproducible end points for cobalt, iron(II), nickel, lanthanum, and cerium(II1). Under the conditions used here, the method was not successful for determining chromium(III), tin(II), silver, beryllium, and aluminum, even though there was evidence that complexes of these ions were formed with the Versenate ion. For ions which form stable complexes in the range pH 5 to pH 6, the conductometric method is most satisfactory in this range of pH. For ions which do not form stable complexes in this pH range, such as the alkaline earths, titrations are performed in the presence of an excess of ammonia. At the higher values of pH there is less difference in slope of the conductance curves on the two sides of the end point. In order to attain satisfactory end-point determinations, it is necessary to buffer the solutions within a narrow range of pH. The necessity for a buffer may be shown with the aid of the equations: HoY-- F? H +
+ HY---
HzY-- + H+@ HsYHZY-- + CO++F! COY--+ 2H+ HY--- + Co++ # COY-- H +
+
(1)
(2)
(3) (4)
Low-frequency conductometric methods offer a rapid and accurate method for the determination of metal ion-Versenate ratios over a wide concentration range by use of commercially available apparatus. The instrumental accuracy is not limited by the conductance apparatus but by the uncertainty of measurements in volumetric glassware. The combination of pH and conductometric studies of these titrations should aid in the selection of the best conditions for nonconductometric titrations. ACKNOWLEDGMENT
This work was in part a joint undertaking of the department of chemistry of West Virginia University and the Office of Ordnance Research, U. S. Army. Appreciation is expressed to the Bersworth Chemical Co., suppliers of reagent grade Versenate. LITERATURE C I T E D
(1) Biedermann, W., and Schwareenbach, G., Chimia (Switz.), 2, 56 (1948). . 46, 741 (1954). (2) Blaedel, W. J., and Knight, H. T., I s a ~CHEM., (3) Ibid., p. 743. (4) Cheng, K. L., Kurtz, T., and Bray, R. H , Ibzd., 24, 1640 (1952). ( 5 ) Flaschka, H., Mikrochemie uer. Mikrocham. Acta, 39, 315 (1952). (6) Furness, W., Crawshaw, P., and Davies, W. C., AnaZyst, 74, 629 (1949). (7) Hall, J . L., and Gibson, J . A., Jr., -4x.4~.CHEM.,23, 966 (1951). (8) Hall, J . L., Gibson, J. A , , Jr., Phillips, H. O., and Critchfield, F. E., J. Chem. Educ., 31, 54 (1954). (9) Hernandez, H. R., Biermacher, IT.,and Mattocks, A. AI., Bull. NatZ. h'ormulary Comm., 18, 145 (1950). (10) Martell, A. E., J . Chem. Educ., 29, 270 (1952). (11) Martell, A. E., and Bersworth, F. C., Proc. Sci. Sect. Toilet Goods Assoc., 10, 26 (1948). (12) Plumb, R. C., Martell, A. E., and Bersworth, F. C., J . Phys. & CoFoid. Chem., 54, 1208 (1950). (13) Pribil, K., and Hornychova, E., Chem. L i s t y , 44, 101 (1950). (14) Pribil, R., Koudela, Z.. and RIatyjka, B., Collectton Czechoslov. Chem. Communs., 16, 80 (1951). (15) Pribd, R., and bfatyska, B., Chem. Listy, 44, 305 (1950). (16) Schwareenbach, G., Hela. Chim. Acta, 29, 1338 (1946). (17) Ibid., 30, 1798 (1947). (18) Schwarzenbach, G., and Ackermann, H., Ibid., 31, 1029 (1948). (19) Schwarzenbarh, G., Biedermann, W., and Bangerter, F., Ibid., 29, 811 (1946). (20) Schwareenbach..G.. and Freitag, E., I b i d . , 34, 1503 (1951). (21) Souchay, P., and Fancherre, J . , - A d . Chem. Acta, 3, 252 (1949). (22) Sweetser, P. B., and Bricker, C. E., ANAL. CHEM.,25, 253 (1953). (23) Ibid., 26, 195 (1954). (24) Wilkinson, P. R., and Gibson, J. A., Jr., and Headlee, A. J. W., Ibid., 26, 767 (1954). RECEIVED for review October 23, 1953. Accepted June 5, 1954. Presented before the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, March 1 t o 5, 1954.
