Iodine Addition Using Triiodide Solutions

The STS water system is treated with iodinated water in order to prevent microbial contamination. This water is prepared by adding a concentrated solution of iodine to Ground Service Equipment (GSE) before adding the water in that unit to the spacecraft system. The solution is prepared by dissolving iodine in ethanol to make a tincture stock solution. While this procedure is rapid, the ethanol increases the carbon levels in the STS potable water and may produce unpleasant odors. The resulting high carbon levels preclude the use of total organic carbon measurements as a water quality monitoring tool. The use of triiodide solutions was studied as a substitute for using ethanol solutions. Two dissolution agents, sodium iodide and hydriodic acid, were investigated. Sodium iodide was studied at molar concentration ratios ranging from 1:1 to 2.5:1 sodium iodide to molecular iodine. Minimization of the sodium iodide is required to ensure that the iodinated water is compatible with the STS flash-evaporator system. A molar ratio of 2:1 was found to be the minimum quantity of sodium iodide required to rapidly dissolve the iodine. To eliminate the use of sodium, solutions of hydriodic acid were also investigated at molar ratios ranging from 0.6:1 to 1.2:1. The results show that a molar ratio of 1.2:1 also rapidly dissolved iodine. However, this concentrate had to be added to the water slowly to prevent re-precipitation of molecular iodine. Because of this problem, the use of sodium iodide: iodine at a 2:1 molar concentration ratio is recommended.