A Rapid Method for Determining Biocide Concentration in a Spacecraft Water Supply

Monitoring and maintaining biocide concentrations is vital for assuring safe drinking water both in ground and spacecraft applications. Currently, there are no available methods to measure biocide concentrations (i.e., silver ion or iodine) on-orbit. Sensitive, rapid, simple colorimetric methods for the determination of silver(I) and iodine are described. The apparatus consists of a 13-mm extraction disk (Empore^® membrane) impregnated with a colorimetric reagent and placed in a plastic filter holder. A Luer tip syringe containing the aqueous sample is attached to the holder and 10 mL of sample is forced through the disk in ∼30 s. Silver(I) is retained by a disk impregnated with 5-(p-dimethylaminobenzylidene)-rhodanine (DMABR), and iodine is retained as a yellow complex on a membrane impregnated with polyvinylpyrrolidone (PVP). After passage of a water sample, the colorimetric response generated by the interaction between analyte and reagent is measured by use of a hand-held, commercial reflectance spectrophotometer. This simple solid-phase extraction (SPE) method gives a high concentration factor. The sensitivity for both measurements is excellent: 0.005 mg/L for Ag(I) and 0.1 mg/L for I₂. Furthermore, the methodology minimizes sample handling and potential contamination events, produces only a small volume of waste, and requires only ∼60 s for completion. Details related to membrane impregnation, calibration, and interferences are also presented. The method was successfully tested on the ground using samples of actual Space Shuttle and International Space Station drinking water.