Advances in Fingerprint Technology.pdf

ferric ions), it can be thought of as an electrochemical cell consisting of two
vessels connected by a salt bridge. One vessel contains the silver solution and
the other vessel contains the redox solution. In this case, the driving force of
the cell is the cell potential, E cell. It is also the driving force of the reaction.
This driving force is obtained from the Nernst equation. At room temperature
(25°C), this is given by:
where
E cell = E o cell – 59 log Q (millivolts, mV) (7.5)
E o cell = E o red + E o ox
E o red is the standard reduction potential for e – + Ag + Ag,
(7.6)
E o red = 799.6 mV (7.7)
E o ox is the standard oxidation potential for Fe 2+ + H 3Cit FeCit + 3H + + e – .
Therefore,
E o ox = –794.6 mV (7.8)
E o cell = E o red + E o ox = 5.0 mV (7.9)
E o ox is derived from the oxidation potential of Fe 2+ Fe 3+ + e – (–771 mV) and
the formation constant of Fe 3+ + H 3Cit FeCit + 3H + (K formation = 0.398).
The driving force, E cell, of the process and the thermodynamic Gibb’s free
energy, ∆G, are related by
∆G = – nFE cell
(7.10)
where n is the number of electrons involved in the electrochemical reaction
(n = 1 in our case), F is Faraday’s constant (96,487 coulombs/equivalent),
E cell is in volts (i.e., joules/coulomb), and ∆G is in joules. Consequently, a
positive E cell means a negative free energy and this means the process is
thermodynamically feasible and can proceed. For the Philips physical developer,
the cell potential is E cell = 90 mV. Using Equations 7.5 and 7.9, this
corresponds to Q = 0.03625. The reaction proceeds until equilibrium is
reached because at equilibrium, ∆G = 0 and thus E cell = 0 and Q = K eq. At