Analytical Methods for Determining the Purity of Lewisite

Problem/Opportunity

The Chemical Agent Standard Reference Material (CASARM) Group of the U.S. Army is responsible for supplying traceable analytical standards to all analytical laboratories that perform chemical agent analysis. The analytical methods used to determine the purity of CASARM must be traceable.

Argonne was tasked to review the analytical methods used to determine the purity of CASARM. Most of the methods had been written decades ago and were not traceable by current definitions. The analytical method for lewisite (L or 2-chlorovinyl-dichloroarsine) was based upon coulometric titration and was very difficult to perform in the laboratory. We determined that the method was not traceable. A new method for lewisite was required that would meet the definition of traceability; that is, the existence of an unbroken, identifiable, and demonstrable pathway.

Approach

The goal of this project was to develop and validate a traceable analytical method to determine the purity of the chemical agent lewisite. Coulometric titration was chosen as an analytical method that would meet the Army needs for traceability.

Coulometry is based on counting the number of electrons used in a chemical reaction. In a coulometric titration, a constant current is applied to a reaction cell and the potential is monitored over time. In this method, the reaction cell contains a buffered solution of potassium iodide. A measured volume of sample (analyte) that contains arsenic III is added. At the anode, iodide is oxidized to iodine. The iodine reacts with (oxidizes) the Arsenic III (the analyte) in solution to produce Arsenic V and iodide:

I₂ + As⁺³ 2I^-1 + As⁺⁵

In the titration process, when all of the Arsenic III is used up, the titration endpoint is reached. This causes a color change in the solution (due to excess iodine) and a change in the solution's potential. The change in potential is detected and is recorded on the titration curve, which plots solution potential measured by the reference electrode over time. For this study, the titration curve is generated by the computer and stored in computer memory. The time it takes to reach the endpoint (seconds) is used to determine the number of electrons used (coulombs) and the amount of Arsenic III present in the sample (moles). This value is then used to calculate the quantity (weight) of lewisite present in the original sample.

Results

The final method was prepared as a Standard Operating Procedure (SOP) for the Quality Assurance Group and included the calculated precision and accuracy data generated by the method.

The coulometric method proved to satisfy the Army needs and met the traceability requirement. The method has been transferred to CASARM for their use in quantifying lewisite purity prior to release.

Future Plans

The Environmental Chemistry Team is currently developing another traceable method for the U.S. Army. This method uses gas chromatography with atomic emission detection. The CASARM program has a range of needs for traceable standard methods for which the CERS Environmental Chemistry Team is well suited.