Series editors' preface - Wood Tools

The two primary factors in choosing a buffer
are the desirable pH for the cleaning solution
and the rinse procedure needed to remove the
buffer. A range of materials that will act as
buffers are listed in Table 11.6. The buffer
chosen should have a pKa that is the same,
or close to, the desired pH of the cleaning
solution. The buffer should also be compatible
with the substrate, for example buffers
containing chlorides (including those in which
the pH is adjusted with hydrochloric acid)
should be avoided when cleaning metals.
Some buffers may be particularly appropriate
for specific applications, for example TRIS may
be appropriate if buffering an enzymatic cleaning
solution.
By definition, buffers are water-soluble
materials and residues can therefore be
removed using a neutral aqueous rinse.
Inorganic buffer compounds can only be
rinsed in water, whilst some organic buffers
are also soluble in solvents, offering an alternative
method of rinsing. Buffers are ionic
materials and the conductivity of the surface
can be measured and compared to the base
line conductivity of the surface before treatment
if there are concerns that residues may
be present.
pH buffers increase overall ionic strength of
a cleaning solution by increasing the concentration
of ions in solution. The ionic strength
of a buffered solution can be deliberately
raised by increasing the overall amount of a
buffer in solution, at or close to, its pKa
(Wolbers, et al., 1990).
11.5.3 Ionic concentration/conductivity
A surface with a weakly acidic character (e.g.
the presence of COO – groups) will attract
positively charged ions (ionic ‘dirt’). Divalent
metal ions such as Ca2+ , Mg2+ , Fe2+ etc., can
form an ionic bond to the acid group and act
as a bridge to attract more acidic material.
Salts, which are ionic materials, dissociate in
water to form ionic solutions. The presence of
salts in a cleaning solution can contribute to
the general effect of a cleaning solution by
creating ion exchange reactions to aid in the
removal of ionically bonded material. Thus it
may be advantageous to raise the ionic
concentration of a cleaning solution.
On some materials, excessive levels of
Principles of cleaning 533
conductivity may damage the substrate. It has
been observed that raising conductivity above
5000 μS will begin to damage an oil-painted
surface (Wolbers, 1992). Other types of
substrates may have similar limiting values, but
as yet these have not been quantified. With
the exception of archaeological materials that
may be contaminated by salts and therefore
have substantially higher base level conductivity,
a useful rule of thumb may be to limit
conductivity of a cleaning solution to about
ten times the conductivity of the surface
before cleaning.
Ion concentration can be measured in a
range of ways. One convenient method is to
measure the conductivity of the solution.
Conductivity is an overall measure of the total
number of ionic species in solution. The
standard international unit is the siemen (1
millisiemen (mS) = 1/1000 siemens; 1
microsiemen (μS) = 1/1 000 000 siemens).
Conductivity can be measured with a small
electrical cell where the electrodes are set a
fixed distance apart. A standard one millimolar
solution of sodium chloride (0.001 mmol)
has a conductivity of 146 μS. Deionized water
usually has a conductivity of less than 5 μS,
whilst tap water may have a conductivity of
around 300 μS, depending on the materials
dissolved in it. A 1% solution of triammonium
citrate has a conductivity of approximately
8000 μS.
The initial conductivity of a surface may be
measured by taking a large swab wetted with
deionized water and rolling it gently over the
surface for a few seconds. The swab can then
be pressed firmly into contact with the two
electrodes on the meter and a reading taken.
It has been observed, for example, that an oilpainted
surface may have an intrinsic conductivity
reading of 0–300 μS. As a general rule,
the conductivity of a cleaning solution applied
to such a surface should not exceed 3000 μS.
Many materials that may be incorporated
into an aqueous cleaning solution, such as
buffers, chelators, salts, acids and bases, may
contribute to the overall conductivity. Once all
the ionic materials have been added to a
cleaning solution (e.g. the solution has been
buffered, the pH set, chelators added etc.) the
overall conductivity of the solution can be
measured and adjusted. It may be reduced by
dilution or increased by the addition of salts