Investigation of the iodine–poly(vinylpyrrolidone) interaction employed in the determination of biocidal iodine by colorimetric solid-phase extraction

Colorimetric solid-phase extraction (C-SPE) has been previously explored as a means to monitor the iodine-based disinfectant used in the water systems on board the space shuttle. This same disinfectant is baselined for eventual deployment in the US water recovery system planned for node 3 of the International Space Station (ISS). With C-SPE, the I₂ concentration is determined from the diffuse reflectance spectrum (DRS) of the yellow iodine–poly(vinylpyrrolidone) (PVP) complex using the Kubelka–Munk function. However, the solution chemistry of iodine is very complex and results in a variety of inorganic species (e.g., I⁻, I₂, I₃⁻, HOI) that have very different biocidal capabilities. Thus, the nature of the interaction of iodine with PVP, and more specifically, the identity of the iodine species involved in the interaction, requires more elucidation. This paper reports the findings from a series of detailed experiments conducted to elicit a more complete understanding of the iodine–PVP system employed in C-SPE. The results indicate that I₂, one of the two dominant biocidal forms of iodine, is the species responsible for the analytical signal in our C-SPE platform. These findings lay the ground work for the planned development of a multiplexed iodine determination and speciation platform for in-flight analysis of spacecraft water samples.