If an operator runs out of standard sodium thiosulfate while conducting a Winkler test, which reagent can be used instead?

In the context of the Winkler test, which is used to determine dissolved oxygen levels in water, the correct substitute for sodium thiosulfate when it runs out is phenylarsenine oxide. This is due to the chemical properties of phenylarsenine oxide, which serves as a suitable reducing agent in the titration process of the Winkler method.

While sodium thiosulfate is primarily used to titrate the iodine generated in the reaction, phenylarsenine oxide can perform a similar function, allowing the operator to continue the test without significant disruption. This capability is crucial in ensuring continuous measurement of dissolved oxygen, which is vital for monitoring aquatic ecosystems.

Other options, while possibly relevant in different reactions or uses, do not serve the same purpose in this specific test. Sodium chloride and magnesium sulfate are not reducing agents and do not have the chemical reactivity necessary for the intended titration in the Winkler test. Potassium iodide is involved in generating iodine when the Winkler reaction occurs, but does not act as a replacement for sodium thiosulfate in the titration step. Thus, phenylarsenine oxide stands out as the appropriate alternative in this scenario.