Advances in Fingerprint Technology.pdf

as that of print residue on porous surfaces and thus an anionic surfactant is
needed to give the residue a negative ionic character. Basically, the surfactant
molecule’s neutral, long side chain penetrates the lipid portion of print residue,
leaving the negative end exposed. The residue thus acquires a negative charge.
It attracts positively charged micelles to it and provides better “surfactant stripping.”
Apparently, its effectiveness in improving visualization was not that significant
because the current procedure does not call for this step; it only calls
for two prewashes: the distilled water wash and the acidic wash (with a nonchlorinated
acid) for basic paper. Furthermore, paper washed with a solution
containing an anionic surfactant would probably not yield any visible prints
when treated with a surfactant-free Ag-PD (see later discussion).
Mechanism of Silver Physical Development
Hereafter, we refer to the currently used silver physical developer as the UK-
PD. Again, this is the one based on what Morris 15 called the Philips Physical
Developer.
Electrochemical Considerations 1
The key chemical reaction in silver physical developer is the catalytic reduction
of silver ions by the reducing agent. For this case, where we use the UK-
PD, the key reactions are
Ag + + Fe 2+ Ag + Fe 3+ (7.1)
Fe 3+ + H 3 Cit FeCit + 3H + (7.2)
Ag + + Fe 2+ + H 3Cit Ag + FeCit + 3H + (7.3)
where H 3Cit is the (triprotic) citric acid and Cit 3– is the (trinegative) citrate
ion. Equation (7.1) is the reduction of the silver ions, Equation (7.2) is the
formation of the ferric citrate complex. The reaction quotient Q obtained
from Equation (7.3) is
Q = [FeCit][H + ] 3 /[Ag + ][Fe 2+ ][H 3Cit] (7.4)
This shows the strong dependence of Q on [H + ] and thus on pH. It shows
that the silver physical development process is highly pH dependent.
Having chosen the reversible ferrous/ferric redox couple for the reducing
agent gives the advantage that the process [Equation (7.3)] is reversible and
therefore treatable as an equilibrium system. Because the process is electrochemical
(silver gets reduced to silver metal as ferrous ions get oxidized to