Series editors' preface - Wood Tools

542 Conservation of Furniture
(a)
(b)
HO .
O
C
O
.
HO C CH2 HO. C CH2 O
OH
CH 2
CH 2
O
C C
CH 2
Figure 11.21 Molecular structure of five common
chelating agents. The electron donating groups are
indicated (.): (a) citric acid; (b) NTA (nitrilotriacetic
acid); (c) EDTA (ethylenediaminetetraacetic acid); (d)
DTPA (diethylenetriaminepentaacetic acid); (e) HEDTA
(N-hydroxyethylenediaminetriacetic acid)
O
C
O
C
Citric Acid
N .
N .
.
.
OH
OH
.
CH 2 CH 2
(c) EDTA
(d)
(e)
HO .
.
O
C CH 2
HO C CH2 O
N .
CH 2 CH 2
N .
CH 2
O C
O.
H
DTPA
OH
HEDTA
N .
CH 2
CH 2 CH 2 N .
O
C
.
OH
CH2 C OH .
O
CH2 C OH
.
O
CH2 C OH .
CH 2
O
CH 2 C
O
.
HO CH2CH2 .
N CH2CH2 .
N
CH2 C
HO . CH 2 CH 2
NTA
CH 2
C
O
O
.OH
C
O
OH .
.
OH
OH .
acid (from which di- and tri-ammonium citrate
(DAC, TAC) are derived), and aminocarboxylic
acids such as nitrilotriacetic acid (NTA),
diethylenetriaminepentaacetic acid (DTPA),
ethylenediaminetetraacetic acid (EDTA) and N-
(2-hydroxyethyl)ethylenediaminetriacetic acid
(HEDTA). A third group derived from
polyphosphates (e.g. sodium tripolyphosphate
or STPP) is no longer used due to concerns
about environmental damage. The molecular
structure of five chelating agents is illustrated
in Figure 11.21.
A molecule must have two or more electron
donating groups (called ligands) in order to be
called a chelating agent. The number of
functional groups that participate in chelation is
referred to as the co-ordination number. These
functional groups can be ionizable groups (e.g.
acid or base groups) or neutral groups (such
as alcohols and ketones) that have unbonded
electrons which can exert weakly attractive
forces on metal ions. In order for a molecule
to function as a chelator, its structure must
allow the formation of a co-ordinated (bonded)
structure around a metal ion in solution. Other
molecules that have these functional groups but
cannot form a ring structure may attract
inorganic ions but cannot sequester them in
solution. As a general rule divalent metal ions
are co-ordinated very strongly to chelating
agents. Monovalent ions, such as sodium or
potassium, do not form strong co-ordinated
bonds with chelating agents and do not interfere
with the chelator co-ordinating with other
metal ions. This is the reason that sodium salts
of chelating agents are often commercially
available (e.g. sodium EDTA).
Formation constants
Both the formation of metal salts (e.g. corrosion
products on metals) and chelation are
equilibrium reactions. As such, it is possible to
assign an equilibrium constant (k) to these
reactions. In the case of the formation or
dissolution of a salt, the k value represents a
proportionality constant. That is, the k value
represents the proportion of the concentrations
of the dissolved species compared to the
starting salt concentration at a standard
temperature and pressure and at a one molar
starting concentration of metal salt.
For example, if the starting reaction is
CaCO 3 [ Ca 2+ + CO 3 2– , then the equilibrium