Timing of adding the starch indicator
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When titrating the copper solution obtained from copper concentrate using sodium thiosulfate, after adding potassium iodide, sodium thiosulfate solution is used for titration; according to the standard procedure, starch is added after the solution reaches a light yellow color. Since it’s difficult to determine exactly what that light yellow color corresponds to, what are the effects of adding starch earlier or later at this stage? Also, near the end of the process, we add a potassium thiocyanate solution – why isn’t this included in the national standards?
Light yellow looks great; just make it often, and the key is to control the acidity well. Potassium thiocyanate must be added; otherwise, the results will be low.
“Titrate until a light yellow color is reached and then add starch. Since it’s difficult to determine the exact light yellow color, what is the difference between adding starch early or late at this stage? There is no difference between adding starch early or late; what’s important is that starch be added at the same time to both the blank sample and the parallel samples.
Add the starch indicator once it turns light yellow; the color change is very noticeable with a clear contrast.
It should be easy to observe when it reaches a light yellow color; this isn’t like a finish-line determination, where you need to add the indicator exactly at the moment the light yellow color is reached – it’s fine if the color is roughly that shade. Potassium thiocyanate might play a role in chelating metal ions
Titrate until a light yellow color is reached; add starch to turn it dark green, then continue titrating
Adding starch too early will coat the iodine, resulting in inaccurate titration
Titrate until a light yellow color is reached, then add starch; this makes the color easier to observe