ROBERT L. PECSOK University of California, Los Angeles, California E T ~ ~ E N E D I A M I N E T E T R A A C Eacid I I C Was
first made available by the I. G. Farbenindustrie (1) in 1936 and has since found increasing use as a complexing agent for metallic ions. Its sodium salts are referred to in theliterature by several trade names: Versene, Sequestrene, Trilon B, Complexone 11,111, or IV, and will hereafter be called "versene." Inasmuch as an excellent bibliography is available (2), this paper is not intended as a review, but rather indicates some of the analytical applications with particular reference to polarography. Some of the polarographic characteristics of a number of versene complexes, as determined by the present author, are reported, and its usefulness as a supporting electrolyte is discussed. The acid itself
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This system of rings is further stabilized by the ethylenic linkage between the two nitrogen atoms. The formation of more than four such rings is preHOOCCHZ vented by steric effects, leaving at least one water /CHzCOOH \ or H,Y N-CHpCHa-N~ molecule in the coordination sohere which is reolace\ / able by such groups as: OH-; N H , SCN-, ete. (4). CHICOOH HOOCCHz The central metal ion is thus more or less surrounded by is sparingly soluble in water and therefore of little ring stmctures so that the characteristic reactions of the interest; however its sodium salts are soluble to the metal are repressed, and the reduction potentials extent of approximately 0.3 F. Solutions appear t o be altered. stable over long periods of time and are attacked by The stability constants of a number of these complex only the strongest oxidizing agents. The four re- ions have been determined by Schwarzenbach and his placeable hydrogen atoms, with pK values of 2.00, co-workers (3, 5, 6), largely from pH measurements of 2.67, 6.16, and 10.26 (3), allow two practicable titra- the liberated hydrogen ion. The constants are listed tions to end points at pH 4.5 and 8.2. Figure 1 has been in the table as log K for the reaction: prepared to show the relative amounts of the five posM + + + Y4- = MY-sible species present as a function of pH. While this type of graph is not new, it is all too seldom seen and Stability Constants of Versene Complexes used, and can be a great convenience in interpreting phenomena occurring in solutions of polyprotic acids. Metal log K Metal k!K serves to emphasize the necessity for careful control Nit+ 18.45 Fet+ 14.22 of pH. Cu++ 18.38 Mn++ 13.47 The peculiar ability of versene to form very stable Pbc+ 18.2 Ca++ 3.51 Cd++ 16.48 Sr++ 2.30 complexes results from the availability of four carboxyl Zn++ 16.15 Mg++ 2.28 and two tertiary amine groups t o enter into fiveCo++ 16.10 Ba++ 2 07 membered rings, each ring including the metal ion. La+++ 15.40
1 Presented at the Southwest Regional Meeting, American Chemical Sooiety, San Diego, California. May 10. 1952.
A number of techniques are useful io its analytical applications: (1) Gravimetric separations and determinations in which versene complexes a large number of elements, while others are precipitated by oxine, ammonia, etc. (7-1 1). (2)' Titration of a metal with standard versene solution using a specifir indicator (IZ), e. q., water hardness test with indicator eriochrome-schwarz-T.
598
JOURNAL OF CHEMICAL EDUCATION
in nonferrous alloys. Pfeiffer and Offermann (29) apparently did a few polarographic experiments with the copper-versene complex, but did not give any data. Furness, et al. (SO), have used the copper complex in order to analyze commercial versene products. Koryta and Kossler (31) described a rather thorough investigation of the cadmium nitrilotriacetic acid (Trilon A) complex, and in the same paper stated that the polarographic waves of the versene complexes mere not suitable for determining stability constants. In a very interesting paper concerning the kinetics of versene complex formation, Ackermann and Schwarzenbach (32) were able to follow the rate of appearance of the copper-versene complex polarographically. An excellent discussion of the iron-versene system has been reported by Kolthoff and Auerbach ($3). In view of the incomplete data available, it was of interest to investigate critically the polarographic behavior of these complexes. The results of a .preliminary survey are described below. Polarograms were recol.ded with a Sargent Model XXI Polarograph, previously calibrated ~vitha potentiometer. The H-cell with external saturated calomel electrode was maintained in a mater thermostat a t 25.0°C. All potentials were measured and are leported versus the saturated calomel electrode. The capillary had a rate of flow of 1.811 mg. per see., ard a drop time of 4.65 sec. at -0.5 volt. No maximum suppressors were required. The pH of solutions was adjusted with sodium hydroxide or nitric acid and measured v.ith a Beckman Model G pH meter. Reagent grade disodium ethylenediaminetetraacetate was purified by recrystallization. All other cherxicals mere of reagent grade and were not further purified. Solutions mere standardized by accepted procedures. A number of metals were polarographed in 0.25 F versene at small intervals of pH in the range from 3 to 12. The following ions gave no waves before the final current rise, and therefore it must be assumed either that their versene complexes are extremely stable or that their reduction requires considerable overvoltage: stannous, nickel, cobalt, lead, zinc, cadmium, manganous, aluminum, the alkaline earths, and the alkali metals. In analyzing amixture, the above elementswill cause no interference m d need not be separated prior to polarographing. Ceric ion yielded a gradually increasing current between zero and one volt, with no well-defined wave at any pH. The data for a number of metals are presented in Figure 2, in which the half-wave potential for 0.4 and these do not include complete data over wide limits millimolar metal in 0.25 F versene is plotted versus the of pH and concentrations. Souchay and Faucherre pH of the solution. The diffusion current is directly (27) list half-wave potentials for copper, bismuth, proportional to the concentration for these waves, and titanium, and uranyl ion in 0.1 M versene with (a) therefore they are of potential analytical use. All are 1 M potassium carbonate, pH 9.5, and (b) 2 M sodium well formed and accurately measurable. For the copper acetate, unstated pH. In a second paper (28), the wave, this proportionality is within +-2 per cent from same authors used a mixed versene-citrate buffer, pH 9, 10" to 0.015 M copper. t,o determine iron, copper and bismuth simultaneously Provided reversible waves are obtained, the variation 13) Addition of excess versene to the metal in solution, followed by determination of the excess versene by titration with standard magnesium solution ( I S ) . (4) Addition of excess versele to a neutral solution of the metal, followed by titration of the liberated hydrogen ion to the pH of the reagent (12-14). (5) Titration of an unbuffered, nearly neutral solution of the metal with tetrasodium versene, noting a large increase in pH at the equivalence point (14). (6) Potentiometric determinations a t a platinum electrode, particularly for iron (15, 16). (7) Specific applications to redox titration involving otherwise unstable oxidation states (17-20). (8) Colorimetric procedures: (a) the versene complexes are more intensely colored than the corresponding aqno ions (91-241, and ( b ) versene can eliminate interferences by sequestering unwanted ions in the aqueous phase while the desired element is extracted into an organic solvent as another colored complex (25). (9) Amperometric titratiorsat thedropping mercury electrode to the disappearance of the diffusion current of the metal in question (26). The potential of the electrode is maintained a t a value yielding a diffusion curreqt for the uncomplexed metal but not sufficiently negative to cause reduction of the complex formed. (10) Direct polarographic determinations. Applications are indicated below. The powerful complexing property of versene suggests its use as a supporting electrolyte for polarographic determinations. Only a few papers concerning the polarography of versene complexes are available,
DECEMBER, 1952
of half-wave potential with pH and concentration of versene can be used to calculate complex formulas and stability constants. Since a number of complexes are possible, e. g., MY-, M H Y , MYOH--, and reduction may proceed either t o the metal or t o a lower oxidation state, no genelal equation is given. For a reduction of the type: MY--
+ 2H+ + 2e = 'M f Hay--
LITERATURE CITED
it follows that: (El/,).
- (Ex),),
0.059 2
Kc KsKd
0.059 2
= -log - - p -log
tion of iodide ion which precipitates cuprous iodide. Versene complexes offer the polarographer another set of half-wave potentials for metal analysis. In addition, all metals except beryllium can be retained in alkaline solutions. Whether or not versene is superior to other supporting electrolytes will depend on the particular combination of metals involved.
(H,Y--)
+
(1) I. G. Farbenindustrie, German Patent 638,071 (1936). (21 Alrose Chemical Co., Providence 1, R. I . , Technirel Bulletins. Mav and October. 1051.
0.059 pH
where (EL,,), and (E,,,), are the half-wave potentials of the versene complex and the aquo ions, respectively; ,A"A",. K , is the dissociation constant of the complex MY--; (6) SCHWARZENBACH, G., AND E. FREITAC~, ibid., 34,1492 (1951). K1 and Kpare the third and fourth ionization constants (7) PRIBIL,R., Chimia (Swilz.),4, 160 (1950). (8) PRIBIL,R., AND J. KUCHARSKY, Colleelion C ech. Chem. of ethylenediaminetetraacetic acid; and p is the number Commun., 15, 132 (1950). of versene molecules ner metal ion in the comnlex fw = 1) .. , (9) PRIBIL,R., AND V. SEDLAR, ibid., 16, 69 (1951). Similar equations can be derived for other types of re- (10) PRIBIL, R., A N I ) P. SCHNEIDER, Cham. Lislg, 45, 7 (1951). (11) PRIBII.,R., ibid., 45, 85 (1951). ductions. G., W.BIEDERMANN, AND F. BANGERTER, No evidence has yet been presented that any com- (12) SCHWARZENBACH, Chim. Ada, 29, 811 (1946). plexes exist with a versene to metal ratio greater (13) B ~Hela. ED~RMA W.. X AND X . G. SCH~'ARZENBACFL. Chimia (Switr.). than 1: 1. This is not surprising in view of the ex2, 56 (1948). tensive ring formations involved. (14) SCHWARZENBACH, G., AND IV. BIEDERMANK, Helu. Chim. Acta., 31., 459 (1948). All of the complexes herein reported,except antimony, . , AND B. MATYSKA, Colleclion Czech. yield reversible or nearly reversible waves in the region (15) PRIBIL,It., 2. KOUDELA, Chem. Commun., 16, 80 (1951). of pH 3-9, becoming irreversible at higher pH. (16) SCHWA~ZENBACH, G., -AND J. HELLER,Helc. Chin,. Aeta, The antimony complex yields a well defined, easily 34, 576 (1951). measurable wave; however, the experimentally de- (17) PRIBIL,R., AND V. MALICKY,Collection Czech. Chem. Commun., 14, 413 (1919). termined equation of the wave indicates a one-electron (18) PRIBIL, AND V. SIMON, ibid., 14, 454 (1919). rather than a three-electron reduction. The half-wave (19) PRIBIL,R., R.. AND J. HORACEK, ibid., 14, 626 (1949). potentials plotted in Figure 2 for the chromium and (20) PRIBIL,R., A N D L. SYESTKA, ibid., 15, 31 (1950). vandium systems are for the +3 to + 2 and + 2 to +3 (21) PRIBIL, R., AND J. KLWBALOYA, ibid., 15, 42 (1950). Chem. Lisly, 44, 101 couples, respectively. It should be noted that the large (22) PRIBIL,R., AND E. HORNYCHOVA, 1104n> > - . - - ,. negative potential, about -1.3 volts, represents an ex(23) PLUMB, R. C., A. E. MARTELL, AND F. C. BERSWORTH, J. tremely powerful reducing agent. Some difficulty was Phys. and Colloid Chem., 54, 1208 (1950). encountered with the chromous complex; measure- (24) MOELLER, T., AND J. C. BRANTLEY, J . A m . Chem. Sac., 72, 5447 (1950). ments must be made immediately after chromous ion V., AND V. V ~ s ~ ~ , ' C o l l eCzech. ~ t i n Chem. Camis added to the air-free versene solution. Oxidation by (25) SEDIYEC, mun., 15,260 (1950). water is complete within an hour. The complete in- (26) PRIBIL, R., A N D B. MATYSKA, Chem. hisly, 44, 305 (1950). vestigations of the copper and vanadium systems will (27) SOUCHAY, P., A N D J. FAUCHERRE, Anal. Chim. Ada, 3, 252 11949) be reported elsewhere. ,.".-,. J., AND P. SOUCHAY, Bull. m e . ehim. (France), In conclusion, versene appears t o be most useful as a (28) FAUCHERRE, 722 (1949). supporting electrolyte for the determination of small (29) PPEIFFER, P., AND W. OFFERMANN, Ber., 75B, 1 (1942). amounts of iron, copper, bismuth, titanium, antimony, (30) FURNESS,W., P. CRAWSHAW, AND W. C. DAYIES, Analyst, chromium, and vanadium in the presence of large 74, 629 (1949). amounts of elements which do not yield waves. If a (31) . . KORYTA.J.. AND I. KOSSLER.Colleetion Czech. Chem. ~ o m m k . , ' l 5 , 2 4 1(1950). large excess of either iron or copper is present, preventH., A N D G. SCHWARZENBACH, Helv. Chim. ing accurate measurement of following waves, the re- (32) ACKERMANN, A d o , 35, 485 (1952). duction of iron can be prevented by the addition of (33) KOLTHOFF, I. M., AND C. AUERBACH, J . A m . Chem. Soc.. fluoride, and/or copper can be removed by the addi74, 1452 ('952).
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