Sodium thiosulfate, na2s2o3, is used as a fixer in photographic film developing. the amount of na2s2o3 in a solution can be determined by a titration with iodine, i2, according to the equation: 2na2s2o3(aq) + i2(aq) --> na2s4o6 +2nai(aq). calculate the concentration of the na2s2o3 solution if 23.50 ml of a 0.1710 m i2 solution react exactly with a 100.0 ml sample of the na2s2o3 solution. use 4 sig. fig. aside, the end of the titration is determined by the color. nai is a pale yellow and i2 is a deep purple. just when the purple color persists, all the iodine that can react has reacted.