Table Of ContentOTHER TITLES IN THE SERIES IN
ANALYTICAL CHEMISTRY
Vol. 1. WEISZ—Microanalysis by the Ring Oven Technique.
Vol. 2. CROUTHAMEL—Applied Gamma-ray Spectrometry.
Vol. 3. ViCKERY—Analytical Chemistry of the Rare Earths.
Vol. 4. HEADRIDGE—Photometric Titrations.
Vol. 5. BUSEV—The Analytical Chemistry of Indium.
Vol. 6. ELWELL and GIDLEY—Atomic Absoφtion Spectrophotometry.
Vol. 7. ERDEY—Basic Methods of Gravimetric Analysis Parts I-III.
Vol. 8. CRTTCHFIELD—Organic Functional Group Analysis.
Vol. 9. MOSES—Analytical Chemistry of the Actinide Elements.
Vol. 10. RYABCHIKOV and GOL'BRAIKH—Analytical Chemistry of Thorium.
Vol. 11. CALI—Trace Analysis of Semiconductor Materials.
Vol. 12. ZuMAN—Organic Polarographic Analysis.
Vol. 13. RECHNrrz—Controlled-potential Analysis.
Vol. 14. MILNER—Analysis of Petroleum for Trace Elements.
Vol. 15. ALIMARIN and PETRIKOVA—Inorganic Ultramicroanalysis.
Vol. 16. MosHiER—Analytical Chemistry of Niobium and Tantalimi.
Vol. 17. JEFFERY and KIPPING—Gas Analysis by Gas Chromatography.
Vol. 18. NIELSEN—Kinetics of Precipitation.
Vol. 19. CALEY—Analysis of Ancient Metals.
Vol. 20. MOSES—Nuclear Techniques in Analytical Chemistry.
Vol. 21. PUNGOR—Oscillometry and Conductometry.
Vol. 22. J. ZYKA—Newer Redox Titrants.
Vol. 23. MosHiER and SIEVERS—Gas Chromatography of Metal Chelates.
Vol. 24, BEAMISH—The Analytical Chemistry of the Noble Metals.
Vol. 25. YATSiMiRSKn—Kinetic Methods of Analysis.
Vol. 26. SZABADVΑRY—History of Analytical Chemistry.
Vol. 27. YOUNG—The Analytical Chemistry of Cobalt.
Vol. 28. LEWIS, OTT and SINE—The Analysis of Nickel.
Vol. 29. BRAUN and TΦLGYESSY—Radiometric Titrations.
Vol. 30. RuziφKA and STARY—Substoichiometry in Radiochemical Analysis.
Vol. 31. CROMPTON—The Analysis of Organoaluminium and Organozinc Compoimds.
Vol. 32. SCHILT—Analytical Applications of 1,10-Phenanthroline and Related Compounds.
Vol. 33. BARK and BARK—^Thermometric Titrimetry.
Vol. 34. GUILBAULT—Enzymatic Methods of Analysis.
Vol. 35. WAINERDI—Analytical Chemistry in Space.
Vol. 36. JEFFERY—Chemical Methods of Rock Analysis.
Vol. 37. WEISZ—Microanalysis by the Ring Oven Technique. (2nd Edition—large and revised.)
Vol. 38. RIEMAN and WALTON—Ion Exchange in Analytical Chemistry.
Vol. 39. GoRSucH—The Destruction of Organic Matter.
Vol. 40. MUKHERJI—Analytical Chemistry of Zirconium and Hafnium.
Vol. 41. ADAMS and DAMS—^Applied Gamma Ray Spectrometry 2nd Ed.
Vol. 42. BECKEY—Field Ionization Mass Spectrometry.
Vol. 43. LEWIS and Orr—Analytical Chemistry of Nickel.
Vol. 44. SILVERMAN—Determination of Impurities in Nuclear Grade Sodium Metal.
Vol. 45. KUHNERT-BRANDSTATTER—Thermomicroscopy in the Analysis of Pharmaceuticals.
Vol. 46. CROMPTON—Chemical Analysis of Additives in Plastics.
Vol. 47. ELWELL and WOOD—Analytical Chemistry of Molybdenum and Tungsten.
Vol. 48. BEAAASH and VAN LOON—Recent Advances in the Analytical Chemistry of the Noble Metals.
Vol.49. TΦLGYESSY, BRAUN and KYRS—Isotope Dilution Analysis.
Vol. 50. MAJUMDAR—N-Benzoylphenylhydroxylamine and its Analogues.
Vol. 51. BISHOP—Indicators
Analytical Applications of
EDTA
and Related Compounds
Dr. R. Pnb٧
Czechoslovak Academy of Sciences
PERGAMON PRESS
OXFORD . NEW YORK
TORONTO . SYDNEY · BRAUNSCHWEIG
Pergamon Press Ltd., Headington Hill Hall, Oxford
Pergamon Press Inc., Maxwell House, Fairview Park, Elmsford,
New York 10523
Pergamon of Canada Ltd., 207 Queen's Quay West, Toronto 1
Pergamon Press (Aust.) Pty. Ltd., 19a Boundary Street,
Rushcutters Bay, N.S.W. 2011, Australia
Vieweg&Sohn GmbH, Burgplatz 1, Braunschweig
Copyright © 1972 R. Pribil
All Rights Reserved. No pari of this publication may be
reproduced^ stored in a retrieval system^ or transmitted, in any
form or by any means, electronic, mechanical, photocopying,
recording or otherwise, without the prior permission of
Pergamon Press Ltd.
First edition 1972
Library of Congress Catalog Card No. 72-153108
Printed in Germany
08 016363 7
PREFACE
PART I of this monograph deals with the uses of EDTA (ethylenediaminetetra-acetic acid)
and its derivatives in some fields of chemical analysis. Apart from Chapter 2, which
is essentially a translation of Professor Koryta's contribution to the 1957 Czech edition,
this monograph is a translation of an entirely new Czech manuscript, having little in
common with the Czech editions published in 1953 and 1957 as regards extent or concept
of the whole subject.
The monograph deals with the "passive" role of EDTA and some other substances, i.e.
their screening (masking) properties, which greatly improve the selectivity of the reactions
in common use, freshly selected here. I believe the "active" role of EDTA to be in its use
as a volumetric reagent in complexometric titrations; the latter is not included in this part
of the book, since it forms an independent sector of analytical chemistry.
Chapter 3 is dedicated to the reactions of "classical" gravimetric analysis, including
the precipitation reactions by means of organic reagents. The chapter on colorimetry
follows the same trend; attached to it is a section on "coloured complexing agents",
which can be used also in colorimetric determinations of some elements.
Some of the metallochromic indicators can also be regarded as complexones in the wider
meaning of the word; these contain one or two N(CH2COO'')2 groups per molecule, which
have high complexing capacity. The important substances amongst them are chiefly the
indicators derived from triphenylmethane dyes, such as Methyl Orange, Methylthymol Blue,
Thymolphthalexone, and others. These have dominated the field of complexometry in the
last few years as the most sensitive amongst the indicators. The experiments dealing with
their uses as colorimetric reagents are contained in Chapter 5.
The function of EDTA and its success is not exhausted by this account. It is also used
in other disciplines of analytical chemistry, such as polarography, chromatography and
electrophoresis, with good results, and also in electro-analytical, radiochemical and extrac
tive separation determinations, in flame photometry, qualitative analysis, etc.
I wish to thank particularly Professor Koryta for the theory section. Professor Belcher
and Dr. Townshend for contributing Chapter 1 and the note on "nomenclature". Dr. Chal
mers for reading the text and suggesting amendments, and various publishers for permission
to reproduce figures.
Prague R. P&IBIL
NOMENCLATURE
THE name "complexone" is claimed by the firm B. Siegfried (Switzeriand) to be their
own trade name; in the past they have often objected to the generic usage of this term.
Accordingly, Professor Fritz Feigl suggested "complexan" {complex anion) and this has
found occasional usage. During the time Professor R. Belcher was President of the Analyt
ical Chemistry Division of lUPAC (1957-61), he made several attempts to persuade this
firm to abandon their claim, but they remained obdurate; they even objected to the new
term "complexan".
As far as can be established. Professor G. Schwarzenbach first used this term, hence the
claims of B. Siegfried are probably without foundation. It is certainly the opinion ol
various patent authorities that this name can be used generically without fear of legal
repercussions. Unfortunately, in trying to avoid the use of Schwarzenbach's original name,
a new and bewildering progeny has developed: chelons, versenes, trilons, etc. It remains
for some fearless soul to bell the cat and to use the original term profusely. Perhaps some
of the preceding monstrosities will then disappear to the oblivion they deserve. I have given
preference to the abbreviations in common use, EDTA, DCTA, NTA, EGTA, etc.; these
are probably the most innocuous names in use, although with suitable juxtaposition ol
vowels some of the names are suggestive of characters from the works of Tolkien—or even
the Brothers Grimm. Whenever in this text such abbreviations are used, the substance is
always a sodium or ammonium salt of the particular acid.
XXI
CHAPTER 1
THE DEVELOPMENT OF EDTA
AS AN ANALYTICAL REAGENT
Ethylenediaminetetra-acetic acid (EDTA) is now a well-known and widely used analytical
reagent. It forms water-soluble complexes with most metal ions and finds extensive use as
a titrant for metal ions, as a masking agent and in other, less important ways.
EDTA, and the related compound nitrilotriacetic acid (NTA) were first produced by
1. G. Farbenindustrie in the mid-1930s. They were named Trilon Β and Trilon A, respec
tively [1]. The compounds formed stable, water-soluble complexes even with calcium and
magnesium, and thus were suggested for use as water softeners as well as dyeing assistants
(heavy metal complexes).
Their water-softening action is the first record of the masking properties of EDTA
that are now so useful.
.CH.COOH -OOCCH. ^ ^ CH.COQ-
HN;—CHXOO- j)HN—CH, — C H , — N H .^
"^CHXGOH HOOCCH. CHXOOH
NTA EDTA
In the early 1940s, many metal complexes of EDTA and NTA were prepared and
studied [2-7]. These investigations revealed that EDTA always formed 1: 1 complexes,
an ideal situation for complexometric titrimetry, because problems arising from the
stepwise formation of complexes are avoided [8]. All the complexes were water-soluble
and were also colourless, unless the metal ion itself was coloured. Such complexes would be
ideal for masking metal ions, and thus preventing them from interfering in particular
analytical procedures. Bjerrum [9] and Leden, in the early 1940s had appreciably improved
the mathematical treatment of complex formation in solution, and a little later, Schwar
zenbach pubhshed a series of papers [10-15] on the measurement of the acid dissociation
consents of some complexones, especially EDTA [14], NTA [10] and uramil-jV,iV-diacetic
acid [12], and of the stability constants of many of their metal ion complexes. These stu
dies gave a firm theoretical background, both to the titration procedures Schwarzenbach
was developing and, later, to the use of the compounds as masking agents.
The first EDTA titration to be described in detail by Schwarzenbach was that of water
hardness (calcium in the presence of magnesium) [16] although prehminary reports of
other possible titrations had been made previously [17]. In his early papers, he favoured
titration of the hydrogen ions released by the formation of the complex, using conventional
4 THEORETICAL INTRODUCTION
acid-base indicators as a means of establishing the metal ion concentration [16, 18]. His
other great contribution to compleximetric titrations was the development of metallo-
chromic indicators. These were organic complexing agents that changed colour on com-
plexing with a metal, so if the metal ion could be removed from the indicator complex by
the complexing titrant there would be a consequent colour change. The first indicator, for
calcium, was murexide [16]; this was followed by Eriochrome Black Τ [19] for calcium and
magnesium, and other known chromogenic complexing agents. He also synthesized a
novel series of metallochromic indicators by introducing the iminodiacetic acid grouping
into established acid-base indicators, the first of these being metalphthalein [20], the imino
diacetic acid derivative of o-cresolsulphophthalein. Such indicators functioned with many
metals, and led to the synthesis by Pribil and Körbl of the now widely used Xylenol Orange
[21] and Methylthymol Blue [22]. It was the appearance of such indicators that finally
established compleximetric titrimetry as a universal method for the titration of metal ions.
In recognition of these important contributions, Schwarzenbach was awarded the Talanta
Medal in 1963.
As is often the case, in the years before Schwarzenbach's first report of the use of
EDTA as a titrant, many other workers had had the opportunity to use EDTA for analytical
purposes. The inorganic chemists [2-7] did not appear to have realized the analytical
potential of the complexans, but Beck utilized NTA as a masking agent for the separation
of cerium and the lanthanides [23] in 1946. This is the first report of a non-titrimetric use
of a complexan. He also published a method for the titration of scandium with NTA,
using murexide as indicator, one year later [24]. Diehl [25] recounts, in his Anachem
address, his early experiments with EDTA. He recalls how a Mr. Bersworth, who supplied
him with a sample of EDTA in 1941, had essentially been determining water hardness as
early as 1938 by an EDTA titration, using soap as an indicator. Diehl apparently refused
to believe that EDTA formed strong complexes with calcium and magnesium, and did
nothing further with the compound other than prepare its cobalt complex. After six years
had elapsed, however, further experiments allowed him to develop, independently, an
EDTA water hardness titration, using soap, and later, calcium oxalate, as indicator. Even
the colour of the calcium-murexide complex, which Schwarzenbach accidentally redis
covered [26] and which inspired the concept of metallochromic indicators, had been known
for nearly a century [27]. It had never been used previously as a reagent for calcium,
however.
The story of Schwarzenbach's rediscovery of the calcium-murexide colour is interesting
in that it is another example of how valuable the chance observation of the imexpected
can be. Apparently, some samples of uramil-iVr,iV-diacetic acid, which is prepared from
aminobarbituric acid, were contaminated with murexide, an oxidation product of amino-
barbituric acid, and so had a pink tinge. Introduction of hard water (containing calcium)
into vessels that had contained the contaminated complexan gave the now well-known
colour of the calcium complex. This colour change not only indicated the possibility of
using murexide as a calcium indicator, but gave Schwarzenbach the general idea of^jpe-
tallochromic indicators, so that indicators for other metal ions were soon found.
Since the early days, in which Schwarzenbach, Pribil and Flaschka were the outstanding
pioneers, compleximetric titrations have developed apace. Better indicators have appeared,
improved masking agents have been used, and new complexans have been developed.
West [8] describes 78 complexans or similar compounds, those with most analytical signi
ficance being NTA, EDTA, 1,2-diaminocyclohexanetetra-acetic acid (DCTA), ethylene-
glycol-bis-(2-aminoethylether)tetra-acetic acid (EGTA), diethylenetriaminepenta-acetic
THE DEVELOPMENT OF EDTA AS AN ANALYTICAL REAGENT
acid (DTPA) and triethylenetetraminehexa-acetic acid (TTHA).
+ .CH2C00-
CH,^ ^CH^ ^CH^COOH
CH
X y \'h ^CHjCOO-
CH2 NH(^
CH.COOH
DCTA
"OOCCHJX + + ^CHjCOO-
^NH — (CH.).—O—(CH2)2—O—(CHO.—NHC;
HOOCCH/ ' " ^CHjCOOH
EGTA
-OOCCH2 + ^ + .CH2COO'
)NH—CH2—CH,—NH —CH,—CH,—NHC.
HOOCCHi^ " I " CH2COOH
CH2COO-
DTPA
-QOCCH,^ + + + + .CH,COO-
~ >NH—CH,— CH,—NH—CH2—CH,—NH—CH,— CH2—NH^ "
HOOCCH,^ - I " 1 ' ^CH,COOH
CH,COO- CH2COO-
TTHA
The rapid developments in compleximetric titrimetry have tended to overshadow the
other analytical applications of complexans. Nevertheless, such applications have also
developed rapidly and are undoubtedly as important as those in the titration of metal ions.
It is the purpose of this book to consider these applications in detail, and not deal with the
titrimetric aspects, which have been adequately dealt with elsewhere [8, 26].
Thus, the differing stabilities of various metal-complexan chelates, and the resultant
differences in their behaviour toward inorganic and organic reagents, have been made the
basis of a number of highly selective gravimetric, titrimetric and colorimetric procedures,
many of them satisfying sorely felt analytical needs. The increased selectivity achieved by
the use of the complexans makes it possible to dispense with certain particularly ex
pensive organic reagents, and makes the reactions of other such reagents highly selective
or even specific.
The new indicators containing an iminodiacetic acid grouping, such as Xylenol Orange,
Methylthymol Blue and Alizarin Fluorine Blue [28] have been found to be excellent spectro-
photometric reagents for many metal ions or anions. The complexans themselves also
formed coloured complexes with certain metal ions. These colour reactions have found use
not only in quahtative tests, but also in colorimetric analysis. The reactions of cations with
the complexans are attended by marked shifts of their Polarographie half-wave potentials,
and by considerable changes in redox potentials; this again opens the way for a number
of new Polarographie and Potentiometrie procedures.
The electrochemical nature of the metal complexes (which as a rule are negatively
charged) is, of course, radically different from that of the parent cations; and this fact
again is of importance in connection with electrophoretic, chromatographic, and ion-
exchange methods of analysis. Finally, the introduction of the iminodiacetic acid group
into an ion-exchange resin initiated the study and synthesis of chelating ion-exchangers.
6 THEORETICAL INTRODUCTION
Chapter 2 is intended chiefly for analysts and theoretical considerations are restricted
to the essentials. The reader wishing to obtain more detailed information about the com
plexes used in analytical chemistry is recommended to read the excellent book by Ring-
bom [29].
REFERENCES
1. PICK and ULRICH, Ger. Pat. 638071, Nov. 9, 1936.
2. PFEIFFER, P., and OFFERMAN, W., Ber, 75 B, 1 (1942).
3. BRINTZINGER, H., and HESSE, G., Z. anorg. Chem. 249, 113 (1942).
4. KLEMM, W., and RADDATZ, K.-H., Z. anorg, Chem, 250, 204 (1942).
5. BRINTZINGER, H., THIELE, H., and MÜLLER, U., Z. anorg. Chem. 251, 285 (1943).
6. PFEIFFER, P., and SIMONS, H., Ber. 76 B, 847 (1943).
7. KLEMM, W., Z. anorg. Chem. 252, 225 (1944).
8. WEST, T. S., Complexometry with EDTA and Related Reagents, British Drug Houses, Poole, England,
1969.
9. See footnote in SCHWARZENBACH, G., and SULZBERGER, R., Helv. chim. acta 26, 455 (1943); and the
dedication in reference 26.
10. SCHWARZENBACH, G., KAMPITSCH, E., and STEINER, R., Helv. chim. acta 28, 828 (1945).
11. SCHWARZENBACH, G., KAMPFTSCH, E., and STEINER, R., Helv. chim. acta 28, 1133 (1945).
12. SCHWARZENBACH, G., KAMPIFSCH, E., and STEINER, R., Helv. chim. acta 29, 364 (1946).
13. SCHWARZENBACH, G., WILLI, Α., and BACH, R. O., Helv. chim. acta 30, 1303 (1947).
14. SCHWARZENBACH, G., and ACKERMANN, H., Helv. chim. acta 30, 1798 (1947).
15. SCHWARZENBACH, G., and ACKERMANN, H., Helv. chim. acta 31, 1029 (1948) and later papers.
16. SCHWARZENBACH, G., BIEDERMANN, W., and BENGERTER, P., Helv. chim. acta 29, 811 (1946).
17. SCHWARZENBACH, G., Hauptvortrag, Wintersamnilung der Schweiz. Chem. Gesellschaft, Bern, Peb.
1945; see also Helv. chim. acta 29, 1338 (1946).
18. SCHWARZENBACH, G., and BIEDERMANN, W., Helv. chim. acta 31, 331, 456, 459 (1948).
19. BIEDERMANN, W., and SCHWARZENBACH, G., Chimia {Switz.) 2, 1 (1948).
20. ANDEREGG, G., FLASCHKA, H., SALLMANN, R., and SCHWARZENBACH, G., Helv. chim. acta 37,113 (1954).
21. KÖRBL, J., and PtoiL, R., Chemist Analyst 45, 102 (1956).
22. KÖRBL, J., Coll. Czech. Chem. Common. 22, 1789 (1957).
23. BECK, G., Helv. chim. acta 29, 357 (1946).
24. BECK, G., Anal. chim. acta 1, 69 (1947).
25. DIEHL, H., Anal. Chem. 39 (March), 37 A (1967).
26. SCHWARZENBACH, G., and FLASCHKA, H., Die komplexometrische Titration, P. Enke Verlag, Stuttgart,
5th Ed., 1965.
27. BEILSTEIN, F., Annalen 107, 186 (1858).
28. BELCHER, R., LEONARD, M. Α., and WEST, T. S., Talanta 2, 92 (1959); J. Chem. Soc. 1959, 3577.
29. RINGBOM, Α., Complexation in Analytical Chemistry, hiterscience Publishers, New York, 1963.
CHAPTER 2
THE NATURE
OF EQUILIBRIA OF COMPLEXES
A N D METHODS OF STUDY
An important group of effective chelating agents are the amino-acids which are bound to
the central metal atom by both the amino and the carboxylate group. A particularly good
agent is glycine in which both groups are present in an arrangement which will give a
five-membered chelate ring during complexing. The properties of glycine are further en
hanced in the so-called complexones [1]. To these belongs the large group of aminopoly-
carboxylic acids in which several carboxyalkyl groups are bound to the nitrogen atom.
These are usually present in solution as betaines:
/CH2COOH Η Η
HN--^H,COOH ÍHOOCCH2\| I /CHiCOOH
\^2C00- -00CCH2^ ^CH2C00-
The dissociation of the protons from the carboxyl groups takes place easily, so that these
complexing agents are fairly strong acids. The dissociation of a single carboxyl group
increases the acidity of another carboxyl group, so that two protons dissociate in a single
step [2]. The two dissociation constants involved here have similar values. The betaine
proton only dissociates in the alkaline pH range.
A so-called normal complex will form when a metal ion reacts with a completely
dissociated anion of the complexing agent, e.g. the complex will form in the case of ni
trilotriacetic acid (NTA) (symbolized here by H3X) according to the equation:
M"+ + ^ MX<«-3>+ (A)
The stability constant of such a normal complex is given by:
Kux = [MX]/[M] [X] (2.1)
(The ionic charges are left out from the concentration symbols.) A higher complex, MX^S"^^"^,
sometimes forms in an excess of the complexing agent; its stability constant is given by:
KMX, = [MX2]/[M] \ΧΫ (2.2)
Protons are accepted in an acid medium by one or more carboxyl groups, so that
a hydrogen (or protonated) 'complex' MHX^''-^^+ is produced. The appropriate equilibrium
constant is given by:
= [HMX]/[MX] [H] (2.3)
One speaks of a complex having a slightly basic character because it is capable of
accepting a proton.
7
