D5281-98 – Standard Test Method Technical Guide

📒 Scope and Applicability

ASTM D5281-98 establishes a definitive procedure for the collection and measurement of hexavalent chromium [Cr(VI)] in the ambient atmosphere. The method employs an alkaline impinger buffer solution (0.02 N NaHCO₃, pH 8.2) to stabilize Cr(VI) during a wet impingement sampling technique. This stabilization is critical, as it prevents reduction to trivalent chromium [Cr(III)], which can occur in acidic media. The method is applicable over an atmospheric concentration range of 0.2 to 100 ng/m³, assuming a standard 20 m³ air sample, with the capability for upward range extension via dilution. All values are stated in SI units.

🌊 Sampling Configuration and Performance Parameters

Ambient air is drawn through three 500-mL glass impingers connected in series at a controlled rate of 15 L/min over a continuous 24-hour period to achieve a target volume of 20 m³. The alkaline impinger solution stabilizes the collected Cr(VI) and facilitates the dissolution of poorly water-soluble lead chromate [PbCrO₄] up to a concentration of 940 µg/L as Cr(VI). Each impinger solution must be analyzed individually to verify collection efficiency and assess potential breakthrough.

🧪 Analytical Detection Methodology

Following collection, a filtered aliquot of the impinger solution (typically 1 mL) is analyzed using ion chromatography. The sample is injected into an ammonium sulfate [(NH₄)₂SO₄] based eluent and separated on an anion exchange column. After separation, the Cr(VI) stream is reacted with an acidic solution of diphenylcarbohydrazide. The resulting colored complex is quantified using photometric detection, providing high specificity for hexavalent chromium. The working range of the method is directly tied to the 20 m³ sample volume but can be adjusted through dilution or concentration steps.