Charles N. Reilley, R. W. Schmid, and Fawzy S. Sadek University of North Carolina Chapel Hill
I
I
~helonApproach to Analysis (I) Survey of Theory and Application
Li
Table 1.
+ M(Hg)
Oxm+=
I1 Na
j
I
'
Elements Determinable by EDTA Titrations I-
I C
B
Ti
V
Cr
Mn
Fe
Co
Ru
Rh
Ni
Cu
s
i
Sr
Y
Zr
Nb
Mo
Tc
(1)
i
.,.
[ P
O
.- -8.
-
--.
[
F
S
Cl
Ba
Fr Ra *Lanthanides
*
Hf
Ta
W,
Re
Nd
Pm
-
0 s
Ir
Pt
I
Au
I
i
1
Ga
G e j As
Se
Br;
Cd
In
Sn
Te
I
- - --- - - - - Sb
1
Cs
:
I
Zn
I
Rb
N
1-
F Sc
+ Hg + M+-
fall into this category. The remaining elements in Table 1 have not vet been determined by a chelometric titration. Because of the present wide applicability of the chelon approach and because of its unusual future possibilities, this brief and introductory status report is given to acquaint chemists with this approach and to present some material suitable for use in analytical courses. A demonstration lecture on EDTA and complex formation by Johnston, Barnard, and Flaschka (9)serves as a suitable qualitative introduction to the quantitative approach presented here.
Mg Ca
Redb
which can then be titrated chelometrically. Elements such as silver ion, which by exchange reactions liberate directly titratable elements
Be
r---. IK
----.. / /
-
liberate an equivalent quantity of metal ion (M+"):
The introduction of chelonsl such as EDTA has virtually revolutionized the analytical approach to metal ion analysis. This approach commenced with the classical work of Schwarzenbach (31-38) on the volumetric determination of calcium and magnesium-a procedure which was rapidly and universally adopted for estimation of permanent water hardness. Since then the applicability of the EDTA titration has been extended to the determination of over 50 elements (3, 6, SO, 39), and its scope will undoubtedly be broadened even further. In Table 1, the elements enclosed in solid lines have been estimated by direct or back titrations with EDTA, and the elements (or their compounds) enclosed in dashed lines indirectly, i.e., via another element after precipitation. For example, sulfur in sulfate is determined indirectly through precipitation with excess barium ion and back titration of the unreacted barium ion with EDTA. Substances which may be reduced with the aid of liquid amalgams (such as reducible
_ _ -._,
Hg
TI
Pb
Bi
Po
At
I
;
!._-._.I
** La
Ce
Pr
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
Am
Cm
Bk
Cf
Es
Fm
Md
No
103
**Actinides A
Po
Np
(4, 12, IS, 28, 29)
organic compounds) will, under suitable conditions,
The Design of Chelons
B a e d in part on work supported by Air Force Office of Scientific Research, Air Research and Development Command, under contracts AF18(600)-1160 and AF49(638)-333. ' A generic term for a class of reagents ihcloding polyaminocarboxylic acids, polyamines, and related compounds which form stable, soluble, usually 1: 1 complexes with metal ions and may consequently be employed effectively as titrants for metal ions.
The classical titration of acid with strong base is well known H + + OHH1O log K = 14 (3)
-
It is obviously a practical analytical titration in view Of the large value of log K . Similarly the titration of acid with weaker bases such as ammonia or cyanide Volume 36, Number 1 1 , November 1959
/
555
ion is also applicable. G. N. Lewis (11) considered complex formation as a type of acid-base reaction. Thus the reaction Cu++
+ 4NHs
-
Cu(NH&++
log K = 12.6
(4)
might constitute a suitable means for titrating Cu++ ions, copper in this case being the acid (electrophilic) and ammonia the base (nucleophilic). Similarly one might consider the titration of cadmium with cyanide ion, according to Cd++
+ 4CN-
-
Cd(CN),--
log K = 18.8
(5)
I n view of the large equilibrium constants for these complex formation reactions, such titrations might appear feasible since they are somewhat analogous to the titration of acid with base. However, the resulting titration curves (IS) illustrate that, although the titration of hydrogen ions with base yields adequately sharp end points, the titration of metal ions with a simple ligand (such as ammonia or cyanide) usually yields drawn out and completely unusable end point brealm. This disappointment arises not from the impressive values of log K for the reactions written above, but rather from the stepwise formation of the metal complexes. Consider the titration of copper with ammonia proceeding by two eztreme paths. After 4 ammonia molecules have been introduced, the solution may contain: Path 1 Cu(NH8)'+ Cu(NHa)++Cu(NHdC+Cu(NH$)++Cut+ Path d Cu(NH&++ Cu++ Cut+ Cut+. .
.
. ..
Had path 2 been followed, a suitable sharp end point would have been obtained in the titration of fairly concentrated solutions and such methods would already have found wide use. However, tho prevalent reaction initially is more nearly path 1. This mode of complexation is attributed t o the fact that the Cu(NH8)++is effectively less acidic than the "free" (hydrated) copper ions, Cu++. Thus, as ammonia is added to a solution containing Cu++ as well as Cu(NHa)++,the ammonia reacts in preference with the free Cu++ ions. The relative Lewis acidities of the hydrated copper ions and the various copper ammines is shown by the values for the formation constants
+ + + +
Cu++ NHa F? Cu(NH.)++ Cu(NH3)++ NHx F? Cu(NH&++ Cu(NH&++ NH1 $ Cu(NH&++ Cu(NH&'+ NHs F? Cu(NHa)r++
lag KI = 4.1 log KS = 3.5 log Ka = 2.9 lag K4 = 2.1
/
Journal of Chemical Education
I n order to get this exclusive formation of the coppertetrammine and thus a reaction by path 2 rather than path 1, the following imaginary approach might be considered. If copper were added to a very concentrated solution of ammonia, the copper tetrammine would be the prevalent form of complex ion in solution. Hence if such a solution were packaged in an ammoniaimpermeable membrane such that each package contains four ammonia molecules, then these little packages of concentrated ammonia could be added to a copper ion ~olutionand a very effective titration obtained. An easier and certainly a more practical approach to packaging the nitrogen ligands into a small volume is to join each of the four basic nitrogens by an ethylene linkage as shown in (11). In this way triethylenetetramine may be considered as an extremely small droplet of a concentrated ammonia solution. Furthermore, the chelon titrant is now a single molecule and, because the reaction is 1: 1 (one copper to one triethylenetetramine), the dissociation of the resulting complex does not increase so rapidly upon dilution. It is actually the 1:1 complex and the preconcentration of the ligand atoms, rather than the formation of "five-membered rings" in the resulting complex, which give rise to the well-known chelate effect. The uitrogens could have been connected by linkages such as -CH&HdXbut the nitrogens would then be even further removed from one another and the "droplet" would be less concentrated in basic nitrogen. This is less desirable since weaker chelates would be formed. Ring strain in the resulting structure also needs consideration. The nitrogens might be joined by ethylene linkages in a different way as shown in (111)
(6)
The stepwise formation of the copper ammine complexes causes the free copper ion concentration to decrease more rapidly during the course of the titration than it would have by path 2. Consequently the value of pCu (-log TCu++]) increases rather rapidly upon the addition of ammonia and no strong inflection is obtained a t the equivalence point. Even had path 2 been followed, the binding of copper t o four ammonia molecules would decrease rapidly upon dilution and thus dilute solutions of copper could not be titrated. This results from the fourth power dependence on ammonia in the equilibrium constant expression. Both these effects can be overcome by the chelon approach. Compare complexes (I) and (11). 556
J (I) Ammonia (11) 1 ethylenetetramine ("tried')
/CHZ-CH2-NH2 H2N-CH-CHP-N
\CH~CH,NH, (111) Triaminotriethylamine (%en")
The nitrogens in (111) are concentrated approximately as effectively as in triethylenetetramine. However, bonding of (111) with metal ions of square planar configuration (e.g., Cu++) is not nearly so effective as that of the linear triethylenetetrarnine, (11). This is attributed to the fact that all the nitrogens in (111) cannot be used for coordination with copper without a considerable strain in the resultine: .. complex. On the other hand, the spider-like configuration of (111) is well suited for complexation of metal ions which bond in a tetrahedral mode, such. as zinc. This
structural factor is borne out by the following stability constants (1).
+ trim Cu(trien)++ tren + Cu(tren)++ trien * Zn(trien)++ + tren Zn(tren)++
Cu++ Cu++ f Zn++ f Zn++
~3
~3
log K log K lag K log K
= 20.4 = 18.8 = 12.1 = 14.7
(7)
Another feature to he considered in the design of chelons is that certain metal ions (such as Co, Ni, Cu, Zn, Cd, and Hg) form more stable coordination bonds with basic nitrogen than with oxygen (5'4); in ammoniacal solution such metal ions form stable ammine complexes. I n contrast, other metal ions (such as alkaline and rare earths, Al, Bi, Pb, and Sc) either do not react or form hydrous oxide precipitates. Thus the allnitrogen type chelons, such as (11) or (111), complex with a rather restricted group of metal ions. By incorporating both oxygen and nitrogen atoms as ligands in a chelon, a more general complexing reactivity is accomplished. The structure of ethylenediaminetetraacetate (abbreviated EDTA) -OOCH,C
\
Various steric effects have also been accomplished by modifying the EDTA molecule through substituting methyl group(.$ on the ethylene linkage and by replacing ethylenediamine with cgclized compounds such as 1,Bcyclohexane diamine. Occasionally some what greater stability was found and may be attributed to a slight preorientation of the ligands and increased basicity of the nitrogen ligands. Diethylenetriaminepentaacetic acid (DTPA) is closely related to EDTA in structure but has more ligand groups and forms definitely more stable complexes. Other possibilities consist of changing the nitrogen ligands for phosphorus and the acetate ligands for hydroxyethyl, thiol, or phosphonate groups. The greater success has arisen from the introduction of a hydroxyethyl group in place of one of the acetate groups. The resulting compound forms an exceedingly strong complex with iron(II1) although its complexing tendency with many other metal ions is weaker.
CH*COO-
/
Available Chelons and Their Properties
contains two nitrogen (ammonia-type) ligands and four oxygen (acetate-typk) ligands and each molecule behaves as though it were a highly concentrated droplet of ammonia and acetate ions-but forms a 1:I complex! Figure 1 illustrates its general complexing ability in titrations.
Figure 1.
The dependence of log a on pH for Trien, Tetren, EEDTA, and
EGTA.
It is easily seen that EDTA represents a structure containing about the highest obtainable pre-concentration of nitrogen and oxygen ligands and thus is almost ideal for general complexing of metal ions. Homologous reagents containing 1, 3-diaminopropane and propionate arms (in various combinations) have been synthesized and studied. I n these cases less reactivity has been found in accord with the expected results.
A great number of chelons have been synthesized and a few of them are now commercially available. Tables 2 and 3 summarize the properties and sources of some of these. By comparing the different stability constants for calcium and magnesium, we find that EGTA forms a much more stable complex with calcium (log K = 11.0) than with magnesium (log Ii = 5.4). This difference permits the selective titration of calcium in the presence of magnesium ($6, $7) a t pH 9, whereas, with EDTA, calcium (log K = 10.7) and magnesium (log K = 8.7) are cotitrated a t this pH value. The all-nitrogen type chelons, such as triethylenetetramine (trien) and tetraethylenepentamine (tetren), form stable complexes with a fairly restricted set of metal ions and thus may be used for the selective titrations of these ions in the presence of the others (15, 16,21). Also copper can be readily titrated in the presence of nickel, zinc, and cadmium. As discussed more fully later, extreme care must be taken when comparing the values of the absolute stability constants for a given metal with various chelons. From Table 3, copper would seem to form more stable complexes with trien than with EDTA; while this is true a t high pH, the reverse is true at low pH due to the "pH effect." In contrast, comparison of the ratio of the absolute stability constants for a given chelon with two metal ions is generally in agreement with their effective stability. For most accurate comparisons, account must be taken of the formation of metal chelonate derivatives ("mixed" complexes) such as MHY-, M(0H)Y-a, MY (NHa)-=,MY (halides) because their formation will increase the net stability of the metal complex. Not to be overlooked in the choice of the chelon is the purity of the commercially available material. For example, the technical grades of trien and tetren are most unsuited for the titration purposes because of contamination with lower aminee. These lower amines react with metal ions only slightly less efficiently than the present chelon and cause the end points to be drawn out and barely detectable. Hence old solutions Volume 36, Number I I , November 1959
/
557
Table 2. Chelon
Chelons Most Freauentlv Used in Metal Titrations
Abbrev.
Formula
Triethylenetetramineol c, h
Trien
,CHn-NH-CHI-C&-NH.
Nitrilotriaoetio Aoid o - d s I Ammoniatriacetic Acid
NT.4
-0OCCHz-N
Ethylenediamioetetraacetic acid o - 0
EDTA
N-Hydroryethylenediamiiettisoetio ao8dd.I
HEDTA
CHEOO-
9.73 2.49
.
OOCHIC
EEDTA
Ethyletherdiaminetetra~cetie aoidd.1
Y
10.26 6.16 2.76 2.0
\CH,COO -
CHx-CH-N
-
-0OCHzC
N-H~CH~C-N-CH~-CH-N/
10.55 8.60 4.27 2.41 2.08
\CBCOO-
11.70 6.12 3.52 2.40
"--".s,
... ...
-OOCH,C
--"."."". ""., .......
...
...
...
9.49 8.82 2.67 1.90
>N-CH-c&-0-CH-C&N
\.--. ,.
(26) SADEK, F. S., S c ~ mR. , W., AND REILLEY,C. N., Talonta. 2, 38(1959). (27) SCHMID, R. W., AND REILLEY,C. N., Anal. Chem., 29, 264 (1957). (28) SCHW.~RZENBACH, G., Anal. Chim. Ada, 7,141 (1952). , 80, 713 (1955). (29) S C H W ~ E N B AGC ,HAnalyst, (30) SCHWARZENBACH, G., "Complexometric Titrations," H.
Irving, translator, Interscience Publishers, Inc., New Yark, 1957. (31) SCEWARZENBACH, G., AND BIEDERMANN, W., Chimia, 2, 56 (1948). (32) SCHWARZENBACH, G., A N D BIEDERMANN, W., Helu. Chim. Acta, 31,678 (1948). (33) S C ~ ~ E N B A GC ,HBIEDERMANN, , W., A N D BANGERTER, F., Helv. Chim. Ada, 29,811 (1946). (34) SIDGWICK, N. V., J. Chem. Soe., 433 (1941). (35) SIGGIA,S., EICHLIN,D. W., AND REINHART, R. C., Ana C h m , 27, 1745 (1955). (36) SWEETSER, P. B., AND BRICKER,C. E., Anal. Chem., 25, 253 (1953). (37) Ibid., 26,195 (1954). (38) UNDERWOOD, A. L., J. CHEM.EDUC.,31,394 (1954). (39) WELCHER, F. J., "The Analytical Uses of Ethylenediamine
Tetraacetie Acid," D. Van Nostrand Company, Inc., Princeton, 1958..
