An Update on the Use of the Sodium Rhodizonate Test for the Detection of Lead Originating from Firearm Discharges

Abstract

Abstract We have made a comprehensive investigation of the chemistry associated with the Sodium Rhodizonate Test for particulate lead deposited on surfaces as a consequence of a firearm discharge. This has been directed at addressing some of the problems that have hitherto compromised the value of this test to forensic science. In particular, we have found that aqueous solutions of sodium rhodizonate are considerably more stable if stored below pH 3. The rhodizonate dianion is then diprotonated, forming rhodizonic acid, and the half-life of the solution increases from about one hour to about ten hours. By ensuring that the area to be examined is pretreated with tartrate buffer so that its pH is adjusted to 2.8 prior to treatment with rhodizonic acid, the formation of a nondiagnostic purple complex, instead of the desired scarlet complex, is avoided. Whereas the scarlet complex changes to a blue-violet complex, upon secondary treatment with 5% HCl, which is diagnostic of the presence of lead, the purple complex decolorises completely under these conditions and thus its formation represents wastage of lead from within the test area and is associated with the fading problem that has previously plagued the test. The fading of the blue-violet complex can be eliminated by removing excess HCl, by means of a hair drier once the color has fully developed.