Series editors' preface - Wood Tools

546 Conservation of Furniture
buffer is usually required to maintain a stable
pH.
A conditional stability constant is a formation
constant that has been corrected for metal
ions which are complexed with hydroxyl ions
(OH – ), the chelating agent corrected for interaction
with hydrogen ions (H + ), and the
chelate-metal ion complex corrected for those
which include OH – and H + . It can be calculated
for a range of pH values at a given
temperature and concentration. For some
chelating agents, such as NTA and DTPA, the
effect of pH on the overall formation constant
is not very great. For some chelating agents,
such as EDTA, the affect of pH is marked.
With Zn 2+ , for example, the formation constant
increases up to a pH of 9.0 because competition
by H + for EDTA is decreasing. At a pH of
10.0, however, the strong attraction of Zn 2+ for
OH – becomes a factor, and there is an increasing
tendency for Zn 2+ to be in solution as an
OH – complex (the metal ion hydroxide precipitates
from solution). This decreases that
amount of Zn 2+ available for complexing to
EDTA, reduces the amount of [ZnEDTA] 2+
formed, which in turn decreases the conditional
stability constant overall. Conditional
stability constants, calculated for a single
chelating agent, for a range of pH, for different
metal ions and plotted against pH provide
curves that illustrate the effect of pH on
chelate-metal ion complex formation (Figure
11.22).
The conditional stability constant and the
overall formation constant are not interchangeable.
The overall formation constant
demonstrates the relative affinity of a particular
chelating agent for a particular ion without
taking into account the effect of pH.
Conditional stability constants are more
accurate for determining which metal ions will
be complexed by a particular complexing
agent at a particular pH. As can be seen in
Figure 11.22, a chelating agent’s affinity for a
variety of ions can significantly change with
pH. It is critical to know and monitor the pH
of the working chelating solution if one is
concerned about the ion preference of the
chelator.
The solubility of chelators varies with pH.
DTPA and EDTA, for example, have low
solubility in acidic conditions and pH must be
raised (e.g. with a 1M solution of sodium
Table 11.11 pKas of some chelating agents
Chelating agent pKa
Citric acid 3.13
4.76
6.4
NTA 1.9
2.5
9.8
DTPA 1.8
2.6
4.4
8.8
10.4
EDTA 2
2.7
6.2
10.3
HEDTA 2.4
5.4
9.9
Acids such as citric acid, NTA, DTPA, EDTA and HEDTA require
at least two of the COO – ions to be dissociated before they can
act as chelators. Although this can occur at a low pH, very
acidic conditions are unsuitable for most substrates.
Sources: Dean (1992); Dawson et al. (1986)
hydroxide or other suitable base) to bring
them into solution. For example, EDTA is
sparingly soluble at a pH of 2, but at a pH of
6.2, is soluble in water and capable of
complexing metals ions very strongly. Acids
such as citric acid, NTA, DTPA, EDTA and
HEDTA require at least two of the COO – ions
to be dissociated before they can act as chelators
(Table 11.11). In the case of citric acid,
for example, the pH must be above 4.76. If
there is ‘cleaning’ action below this pH it is
the action of the acid reacting with the
substrate itself and not a result of chelation.
The chelators discussed above are supplied
as free acid or salts in powder form. The free
acid is sparingly soluble in water and suitable
bases, such as sodium hydroxide, ammonium
hydroxide or tri- or di-ethanolamine, may be
added to the solution to bring the acid into
solution. Where the chelator is purchased in
the salt form, for example triammonium citrate
or disodium EDTA, this process has already
been carried out. The conservator has more
scope to formulate a chelating solution to a
pH suitable for a given substrate if the free
acid form of the chelator is used as the basis
for the cleaning solution. The same material
can be used to both bring the acid into