unreliable results. The redox electrodes are connected through a high-
impedance voltmeter or pH meter to a reference electrode (such as a RE-5C cop-
per/copper sulfate reference from McMiller Co., which is specifically designed
for field use) and readings taken immediately, discarding all results showing
strong drift. The necessary impedance of the meter depends on the surface area
of the electrode, the required precision of the measurement and the exchange
current density of the system studied. The currents involved in the establishment
of Eh in the soil are very small, so the potential can easily change (drift) if the
current drawn through the measuring circuit is too large. The widespread prac-
tice of waiting for a drifting signal to become “stable” before taking the reading
is therefore considered a dangerous approach – it cannot be excluded that the
drift in some cases is simply due to the measurement itself, changing the poten-
tial towards a new equilibrium that has nothing to do with the equilibrium exist-
ing in the soil when the redox electrode is not connected! Simple field tests
showed that the use of a voltmeter with too low impedance for just 1 min can
change the potential of a platinum electrode by several 100 mV, and that it takes
several hours before reliable measurements with a high-impedance meter can be
made on that electrode again. This short-term drift, caused by the measurement
itself, should not be confused with the long-term drift of permanently installed
electrodes; the latter is (among other things) probably due to a slow oxygen des-
orption from the electrode surface and means that it can take several days after
installation before the electrode is in equilibrium with its surroundings.
This means that spot measurements, e.g.in dipwells or directly in the soil,
cannot be compared with results from permanently installed electrodes and that
the absolute values from such spot measurements are probably of little use.
Experience has shown that redox electrodes can be left in the soil for at least 5
years without major problems.
For most applications platinum has become the preferred material for redox
electrodes. However, there is no evidence to suggest that it should actually give
better results than gold when measuring Eh in soil with permanently installed
electrodes. It is therefore considered that gold electrodes are a good alterna-
tive to platinum due to their lower price and lower tendency to form oxides
and possibly also sulfides.
2.4 pH
pH is one of the most frequently measured soil parameters, and probably the
most indicative single factor for soil functioning and processes. In an archaeo-
logical context, pH is a key parameter for the preservation of archaeological
remains of several different materials, e.g.bone, shells, iron, bronze and even
flint. pH measurements are therefore normally included in monitoring pro-
grammes at archaeological sites preserved in situand in reburial experiments.
314 Chapter 12