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Advances in Fingerprint Technology.pdf

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Figure 7.2 (cont<strong>in</strong>ued)<br />

adherence on residue that is on porous or nonporous surfaces. However, on<br />

nonporous surfaces, the colloidal gold treatment can take up to 120 m<strong>in</strong> to<br />

have a sufficient amount of particles adhere to latent pr<strong>in</strong>t residue.<br />

Alkal<strong>in</strong>e Paper<br />

Alkal<strong>in</strong>e paper has a pH > 7, as opposed to acid paper which has a pH < 7.<br />

Paper with calcium carbonate filler is alkal<strong>in</strong>e. The alkal<strong>in</strong>ity results from<br />

calcium carbonate be<strong>in</strong>g a salt of a weak acid (carbonic acid) and a strong<br />

base (calcium hydroxide). Although it is practically <strong>in</strong>soluble <strong>in</strong> water, it<br />

dissolves, then hydrolyzes and produces a basic solution. If alkal<strong>in</strong>e paper is<br />

treated with an Ag-PD, it turns black, which upon dry<strong>in</strong>g sometimes turns<br />

brownish-black; the most likely reason for this is that the silver physical<br />

developer becomes destabilized by the paper — and only <strong>in</strong> the vic<strong>in</strong>ity of<br />

the paper — caus<strong>in</strong>g the developer to deposit silver, silver oxide, and/or ferric<br />

oxide on the paper. To elaborate on this, the alkal<strong>in</strong>ity of the paper causes a<br />

local change <strong>in</strong> pH, which causes destabilization. The destabilization causes<br />

a premature reduction of silver on the paper. The hydroxide ions locally<br />

formed <strong>in</strong> the paper react with the developer’s ferric ions and silver ions to<br />

produce <strong>in</strong>soluble ferric hydroxide and silver hydroxide, respectively, on the

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