The rate at which oxygen is used can be determined over a 5 day period for example. To do this one sample would
need to be collected and determined using the method above. A second sample would be taken, stoppered and
stored out of sunlight for 5 days. The remaining concentration of dissolved oxygen would then be determined by
adding the manganese sulfate, iodide and acid solutions as above at the end of the 5 day period and titrating with
sodium thiosulfate solution.