J O U R N A L OF T H E AMERICAN CHEMICAL SOCIETY (Registered in U. S. Patent Ofice) VOLUME
(0Copyright, 1 9 3 , b y the American Chemical Society) JUNE 26, 1958
80
NUMBER 12
PHYSICAL AND INORGANIC CHEMISTRY [ COSTRIBUTION FROM DEPARTNEXT OF CHEMISTRY, UXIVERSITY
OF
XORTH CAROLINA]
The Stability of Metal-Tetraethylenepentamine Complexes B Y CHARLES N. ItELLLEY
AND
J. H. HOLLOWAY
RECEIVED DECEMBER 9, 1957 The stability constants for the reaction between a series of divalent and trivalent nietal ions with tetraethylenepentamine have been determined by potentiometric measurements. The complexes studied were those of H g + + (27.7), Cu+' (22.9), M g + + (F)!S r + + (-), B a + + X i + + (17.8), Z n + + (15.4), C d + + (14.0), P b + + (10-ll),M n + + (7.0), Bi+3 (>0), C a + + (-), Al+3 (-) and La+3 (T). The reported log K values were obtained a t 25.00 f 0.01 and a t an iunic strength of 0.10, A new method for determinmg the acidity constants of tetraethylenepentamine is described,
(o),
Introduction Experimental A. Reagents.-All chemicals except tetren used were AR The stability constants for some of the lower grade. The metal ion solutions were prepared from the members of the polyethyleneamine series with metal nitrate and distilled water, and were standardized metal ions have been compiled by B jerrum, Schwar- against standard EDTA solutions. The tetren pentahydrozenbach and Sillen.' Some recent work on the nitrate solution was prepared by dissolving the purified tetrapolyethyleneamines has been reported by Jonas- ethylenepentamine pentahydronitrate in distilled water. tetren (pentahydronitrate) sulution was standardized sen"3 and R e i l l e ~ . ~These ligands, containing only The against standard mercury(I1) and zinc solutions. All nitrogens as coordinating atoms, are of particular solutions were made 0.10 M in KNOl to maintain constant interest since they offer a basis for the selective ionic strength. B. Purification of Tetraethylenepentamh-The comtitration of metal ions such as cobalt, copper, nickel, zinc, cadmium and mercury which form mercial product, obtained from Union Carbon and Carbide Chemicals Co., was purified as the pentahydronitrate. One more stable bonds with nitrogen than with ~ x y g e n . ~hundred fifty grams of crude tetren was dissolved in 300 ml. These metal ions form stable ammine complexes in of 957, ethanol and cooled to 0". Four and one-half moles ammoniacal solution while certain other metal ions of HXOs is mixed with 3000ml. of water, 300 ml. of 95% ethanol, and also cooled to 0 . The HNOs-ethanol mixture (such as alkaline and rare earths, aluminum, bis- is added slowly to the tetren-ethanol mixture and the temmuth and scandium) either do not react or form perature maintained below 10'. This procedure i s potenhydrous oxide precipitates. tially hazardous and extreme care must be taken to keep the Previous work in this Laboratory had shown that temperature low. A yellow precipitate, which appears near end of the addition, was filtered by suction and recrystetraethylenepentamine (tetren) formed very stable the tallized five times from aqueous 5y0 H N 0 3 solution. The complexes with certain metal ions. The purpose resulting white precipitate is washed with acetone and absoof this investigation was to measure the chelate lute alcohol and dried a t 50". (A similar procedure has stability constants for a series of divalent and tri- been employed by Jonassen.2) valent metal ions with tetren. The data obtained C. Determination of Potential-pH Diagrams.have been utilized in the development of procedures The potential-pH data were collected a t 25.00 & for selective metal titrations6 0.01" by a procedure previously d e ~ c r i b e d . ~ Equilibrium (=t1 mv.) was reached within 1 minute (1) J. Bjerrum, G. Schwarzenbach and I,. G . Sillen, "Stability Conexcept with the nickel-tetren solution where the stants of Metal-ion Complexes," P a r t I, Special Publication No. G , T h e Chemical Society, London, 1957. time required to reach equilibrium was decidedly (2) H. B. Jonassen, F. W. Frey and A . Schaafsma, J . P h y s . Chcm., longer. Deaeration of the solutions was necessary 61, 504 (1957). in order to obtain accurate potentials in the more (3) H. B. Jonassen and I,. Westerman, THISJouawar., 79, 4275, 4279 (1957). negative potential regions where the presence of 73, 279 (4) C. N. Reilley and K LV. Schmid, J . EIisho Mitchell SOL., oxygen interferes ("mixed potentials"). (1957). (5) N. V . Sidgwick, J . Chem. Soc., 433 (1941). (0) C. N. Reilley and A . Vavoulis, unpublished results.
(7) R . W. Schmid and C. N. Reilley, Tins (1956).
2917
JOURNAI,
78, 5513
c. N. REILLEYBND J. H. H o ~ t o w . 4 ~
2918
VOl.
so
Calculations
A. Metal-Tetren Stability Constants.-The
potentiometric method employed for determining the stability constants of the metal-tetren complexes has been described earlier. 4,7 Essentially, the method consists of measuring the extent to which the exchange reaction HgZ++
+ Me+"
MeZ+*
+ Hg++
takes place by means of the mercury electrode. Log K values for various metal complexes were calculated a t 25" by substitution into equation l. [HgZ++] + 0.0296 log [Mn+n] [ Me2 *I K E ~ Z+
.EHg =
0.0296 log K x e z
(1)
The value for the standard potential of the mercury electrode was given by Latimer* as 0.612 volt us. the S. C. E. a t 25" ; the concentrations of the mercury chelate, metal ion and metal chelate were kept constant (0.001 M ) throughout each measurement. 125 I
I
400-
I:
300-
Y
w
200p
417
l
where [Z] * represents the stoichiometric conceiitration of tetren LYE=
14- Ks[H']
+ K ~ K I I H + ] +' K&&[H+I3 +
+
K5K4K3Kn[H+I1 KsK&aKzKi[H+16 ( 3 ) and Kg, Kd, KO, K?and K1 are the acidity (reciprocal dissociation) constants of tetren. With a knowledge of CYH, K H ~ was Z calculated from potential-pH measurements and the concentrations involved. For a solution containing equal amounts of free tetren and mercuric-tetren chelate, the experimentally determined potential-pH relationship is shown in Fig. 1 (curve I). Alternatively, K I I ~was Z determined from the potential of curve I a t high pH values (>11) where tetren exists only in the free base form and a13 = 1. In this way the stability constant of the mercury chelate can be determined without knowledge of the
tetren acidzty constants. N'ith a knowledge of K H ~ az ,log K scale was calculated from equation l and plotted a t the righthand side of Fig. 1. Next, a potential-PH diagram was constructed from experimental data for the various metal ions in such a way that, for definite concentrations of the chemicals involved in equation l , the log Ii' value for the metal chelate could then be read directly from the measured potential of the mercury electrode, in its PH-independent region (Fig. 1). X difference in log K of one unit corresponds to a difference in potential of 29.6 mv. Because curve I mas also obtained in the presence of Llgf*, Ca-+, Sr++, Ba++, L a f 3 and no stable tetren complex of these metal ions exist (Bi+O showed a slight tendency to form a stable tetren complex a t pH 8). The dotted lines in Fig. 1 represent the hydrolysis of the metal ions to forin hydrous oxide precipitates. This indicates the applicability of the electrode system Ilg 1 FIg-tetrrri
-, Me-tetren +",Mc+"
for the study of metal ion hydrolysis. As an example, the hydrolysis constant, K H = [M(OH)B]/ [M++][OHI2,was calculated for zinc (- 1.6 X 10'5) and cadmium (- 1.0 X 1013) from Fig. 1. Likewise, this electrode may be employed for determining the stability constants of the metal com-2ook plexes (e.g., zinc acetate) provided that the metal t 8 ion reacts more strongly with the complexing agent 2 4 6 0 IO than with tetren. P H. B. Dissociation Constants of TetraethylenepentFig. 1.--Potelitid-pH diagram for the determination of acidity constants of tetren were also tetren stability constants (25.00" in 0.10 Af KN03). Curve amine.-The determined from potential-fiH measurements on a I, 0.001 Af HgZ++ + 0.001 2 ; curve 11, calculated (W. M. solution containing equal amounts of free tetren Lstirner, "Osidation Potentials," Prenticc-Hall, Inc., New and mercuric-tetren chelate (curve I). Inspection I'ork, hT.Y., 1952, p. 179) potential for Hg + 2 0 H HgO HzO f 2 e - ; metal-tetren curves, 0.001 'If H g Z L C of equation 2 shows that, other terms being held f 0.001 31 51eZLL + 0.001 hf Me+"; curve 1 was also constant, the potential of the mercury electrode deobtained forMg++, C a + + , S r + + Ba++,La+3,Al+3and , Bi+3 pends linearly on CYH,or a t 25" 1
+
I
I
+
The stability constant for the mercury-tetren chelate, K H ~ zwas , also determined experimentally by potentiometric measurements. At 25", the potential of the mercury electrode in a solution containing free tetren and mercury-tetren chelate is given by the relationship (8) V \ hI. Latimer. "Oxidation Putentinls," 2nd ed., Prentice-Hall, N e w York, N. Y.,195'2,
+
+
+
+
E H= ~ E H ~ ' 0.0296 log [ I Ks(H') KSK~(H+)~ L Y : K ~ K ~ ( H + )IJ;KIK~KZ(H+)* ~ KjK4KjK&'i(H +)"I (4)
+
+
From equation 4 i t can be seen that the value of any pH can be calculated from the corresponding poteiitial reading. In this way the value of OIH a t five different pH values can be obtained and substituted into equation 3. Solution of these CYHa t
June 20, 1958
STABILITY OF METAL-TETRAETHYLENEPENTAMING
2919
TABLE I1 simultaneous equations yields the desired acidity constants. STABILITY CONSTANTS FOR METAL-TETREN COMPLEXES For optimum accuracy in the calculations, the Stability constant Potentiometric p H values must be properly selected. For each Metal ion methoda Lit.) simultaneous equation, the p H chosen should cor- H g + + 2 7 . 7 f 0.1 .... respond to the point where successive terms on the c u + + 22.9 f .1 -24 right side of equation 3 have their maximum con- Ni++ 17.8 =!= .1 17.6 tribution. Zn++ 15.4z t .1 .... As an approximation, the five p H values corre- Cd+' 14.0 f .I .... sponding to the points a t which d(aa)/d(pH) is 1, Pb++ 10-1lC f 1 ,... 2, 3, 4 and 5 were selected. This was accom- M n + + 7.0 f 0 . 1 .... plished by drawing tangents to curve I correspond- Mg++, Ca++, Sr++, Ba++, ing to these preselected slopes. Al+3, Bif3, La+3 Kegligible .... Because of the highly acidic character exhibited a 25.00' in 0.10 M KNO3. 25.0". Reproducible reby K 1 and KP,no attempt was made to evaluate sults are difficult to obtain due t o the formation of hydrous their magnitudes by this procedure. Instead, oxide precipitate near the beginning of the constant potenthree simultaneous equations involving three un- tial-pH region. knowns were set up and solved at the selected pH tion of bivalent copper, nickel, mercury, zinc or values for Kg, Kd and K,. An approximate value cadmium. Also, a t a p H of approximately 4.5, of K z was then calculated by substitution into equa- copper may be titrated in the presence of nickel, tion 3 using CYHcorresponding to a p H (-3) where zinc, manganese, etc. the K z term is significant. The results of these calAnother interesting observation is that metalculations, together with the values reported in the tetren chelates are usually more stable (by a difliterature,2are listed in Table I. ference of about 3 in the log K) than the corresponding metal-trien complexes. Thus many indiTABLE I cators, forming metal-indicator complexes too DISSOCIATION CONSTANTS O F TETRAETHYLENEPENTAMINE stable to be used for metal-trien titrations, are Potentiometric methodD Lit.b now applicable for the titrations involving tetren. The enhanced stability of the metal-tetren comPKI ....... 2.65 plexes such as copper may be attributed to a tendPK2 4 . 1 i 0.1 4.25 ency toward octahedral structure so that the extra PKa 8 . 2 =!= . 1 7.87 ligand would play an important role in addition to PK4 9 . 2 ==! .1 9.08 a statistical increase in the probability of chelate 10.0 i . 1 9.92 pK6 formation. Also, the basic character of the nitro25.00' in 0.10 &fKNO3. ' 25.0". gen atoms on the tetren molecule is greater than Results and Discussions on the trien molecule. The difference in basicity The stability constants for the complexes formed of the four more basic nitrogens may be designated 3 4 between the various metal ions and tetraethylenepKa trien, which equals 2 pentamine are listed in Table 11, together with the as, C pKa tetren 2 1 values reported in the 1iteratu1-e.~ The interpretation of potential-pH diagrams for units. Similar studies on metal-pentaethylenehexamine the formation of metal chelate complexes has been adequately discussed by previous investiga- complexes are now in progress. Acknowledgment.-The authors wish to thank t o r ~ only ~ ~a few ~ ~of ~ the~many ~ ~interesting ; results obtainable from Fig. 1 will be discussed. Mr. B. D. Armes for his assistance in the preparaFor example, using tetren as the chelating agent, tion and analysis of the metal ion solutions. This research was supported by the United States metal ions such as calcium, barium, magnesium, strontium, aluminum, lanthanum (and possibly Air Force Office of Scientific Research of the Air other rare earths) would not interfere in the titra- Research and Development Command under contract No. AF18(600)-1160. (9) C. N. Reilley and R. W. Schmid. A n d . Chon., in press. (10) R. W . Schmid and C. N. Reilley, ibid., 29, 2G4 (1987).
CHAPELHILL, n'. C.
