Let’s move on to the process of analysis of dissolved oxygen in water. First of all, to prepare 250 ml of 0.005 M sodium thiosulfate solution, dissolve 1.30 g of sodium thiosulfate in 250 ml of freshly distilled water (or freshly boiled and cooled deionized water).Then, add 1.5 ml of 6 M sodium hydroxide with continues stirring.