Series editors' preface - Wood Tools

constant (k) (at standard temperature and
pressure) is defined by the following formula
(the [ ] brackets indicate concentration):
k = [C 2+ 2– a ] [CO3
]
[ CaCO
]
3
If the reaction is taken to move in the dissolution
direction only (i.e. CaCO3 → Ca2+ +
2– CO3 ) then the k from the initial reaction
becomes ksp, the solubility product (the
concentration of calcium and carbonate ions is
divided by 1 because the starting point is a
one molar solution):
ksp = [Ca2+ 2– ][ CO3 ]
1
The k sp number is often very small. For calcium
carbonate, for example, the k sp is 2.810 –9 . For
convenience, the negative log of the k sp is
often used (pK sp). For calcium carbonate, the
pK sp is 8.54. The practical importance of this
number is that the bigger the pK sp, the more
insoluble any given salt is in water. In other
words, the bigger the pK sp, the more stable a
salt is in water and the more difficult it will be
to remove. Sources such as Dean (1992)
provide tables that list solubility products for a
large number of salts. The solubility products
of some salts that may be encountered in furniture
conservation are listed in Table 11.9.
Table 11.9 The solubility products of some salts
that may be encountered in furniture conservation
Substance Formula pK sp
Barium sulphate BaSO 4 9.96
Cadmium sulphide CdS 26.1
Calcium carbonate CaCO 3 8.54
Calcium sulphate CaSO 4 5.04
Copper chloride CuCl 5.92
Copper carbonate CuCO 3 9.86
Copper hydroxide Cu(OH) 2 19.66
Copper sulphide CuS 35.2
Iron hydroxide Fe(OH) 2 15.1
Iron trihydroxide Fe(OH) 3 37.4
Lead carbonate PbCO 3 13.13
Lead hydroxide Pb(OH) 2 14.93
Lead sulphide PbS 27.9
Magnesium carbonate MgCO 3 7.46
Magnesium hydroxide Mg(OH) 2 10.74
Silver sulphide Ag 2S 49.2
Titanium trihydroxide Ti(OH) 3 40
Zinc carbonate ZnCO 3 10.84
Zinc hydroxide Zn(OH) 2 16.92
Source: Dean (1992)
Principles of cleaning 543
The principle of the formation constant can
also be extended to the formation of a bond
between chelating agents and metals ions. The
same sources that give tables of solubility
products also tabulate the formation constants
(log K f) for metal ions with chelating agents
(which may also be called organic ligands).
The log K f may also be referred to as log
K M'L'(ML)'. These numbers are useful in the
same way that solubility products are, in that
the larger the log K f, the more stable the
complex formed between chelator and metal
ion. The formation constants of some chelators
and metal ions are listed in Table 11.10.
To predict whether a salt will be dissolved
by a chelator, the log K f must be bigger than
the pK sp. If the log K f is one integer (number)
larger than the pK sp it is likely that the salt on
a substrate will dissociate and the metal
chelate complex will tend to form in solution.
For example, EDTA will bind Ca +2 with a log
K f of 11.0. If a solution of EDTA is brought to
a surface that had calcium carbonate on it, the
EDTA will bind calcium ions from the calcium
carbonate (which has a pK sp of 8.54).
However, Fe(OH) 3 iron trihydroxide has a pK sp
of 37.4, whilst Fe(III) and EDTA has a log K f
of 24.3. EDTA will not chelate Fe(III) from the
iron trihydroxide salt.
In the same way, the solubility product and
formation constants can be used to predict
whether a chelating agent will have an
unwanted effect on the substrate. The solubility
constants of materials in the substrate (e.g.
CaSO 4 in a gesso ground, or pigments such as
lead white Pb(OH) 2.PbCO 3) can be compared
with formation constants of these materials
with a given chelating agent.
Effects of pH and conditional stability
constants
The formation constant indicates the preference
for formation of the reaction product (or
complex) from the ionic species. This calculation
does not take into account the affect of
pH on the ionic species present. Chelation is
an equilibrium reaction and is influenced by
the presence of hydrogen and hydroxyl ions.
In aqueous cleaning solutions where acidic or
basic ligands are involved, chelation is pHdependent.
There is usually an upper pH limit
for the effectiveness of a chelator and metal
ions. Because chelators are pH-sensitive, a