How to determine the titration endpoint when measuring dissolved oxygen
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When using the titration method to measure dissolved oxygen, starch is added and titrated until colorlessness is achieved; after a few minutes, blue color reappears. What’s going on?
After the titration was completed, I2 in the solution was reduced to I-. After being left for a while, the oxygen in the air will re-oxidize I- to I2, causing the solution to turn blue again. This post was last edited by xwtsq on 2009-2-18 22:15]
It is also possible that the iodine was absorbed by the indicator and then released slowly.
The explanation on the second floor is correct. If the starch indicator is added too early, the starch will absorb iodine, causing the blue color to not fade and making it impossible to determine the endpoint. The owner’s judgment regarding the endpoint seems to be a bit off; our operating procedure requires returning to blue within five seconds after it fades, rather than waiting a few minutes before doing so. This requirement is stipulated in the internal procedures revised by our company, but I couldn’t find its source. It is probably to prevent excessive titration. However, I hope everyone will discuss further whether this requirement is correct or not.
It indicates that your dissolved oxygen level is high. As the dissolved oxygen level increases, the titration color changes from light yellow, yellow, orange-red, red, purple, blue, to finally blue-green.
Is the color change process mentioned by the poster used to measure dissolved oxygen? The titration process for determining dissolved oxygen by titration is actually the process of using a standard sodium thiosulfate titrant to titrate iodine; even if there is supersaturation of oxygen in water, it only causes the solution to turn brown before titration (due to high levels of I2). After titrating until near the endpoint (light yellow), starch indicator is added, and then the standard sodium thiosulfate titrant is used to continue titration until the color of the solution changes from blue to colorless, at which point the endpoint is reached. This post was last edited by xwtsq on 2009-2-19 19:28]
Oh, that means we use different methods. In alkaline solutions, sodium indigo disulfonate is easily reduced by glucose to a pale yellow reduced form, which is then oxidized when it comes into contact with dissolved oxygen in water. The color of the oxidation products changes sequentially to the aforementioned colors as dissolved oxygen increases.
I’m uninformed. Are you referring to a method related to the water quality of boilers?