science(chem)

Situation:
KMnO4 solution left in buret for awhile...would it be okay to go and titrate with the same solution left in the buret later? And if solution is discarded and refilled later wouldn't the solution go and react with the light thus contaminating the new solution?

And also why do you have to go and boil water for an hr to mix with to create a solution of sodium thiosulfate
(text states 10-15 min is adequate?)

KMnO4 that has been prepared under conditions in which all of the MnO2 pptd has been filtered and the KMnO4 solution is fresh, probably is ok to titrate with even though some of it has been in the buret for awhile; however, as a general rule, I would discard any KMnO4 in a buret that was not used within an hour or so of filling the buret. By discard, I don't mean to pour it back into the bottle but to throw it away. There is no rule for that, I don't suppose, but I would not use old KMnO4 just on general principles. Just before titrating a solution, I would discard the KMnO4 solution in the buret, rinse the buret several times with fresh KMnO4, then refill with fresh KMnO4. If you don't rinse and refill the buret with fresh KMnO4 until just before it is needed for a titration, there is no light contamination that can take place. As for boiling thiosulfate, one problem facing the use of thiosulfate is that bacteria react with it to produce elemental S and HSO4^-. Boiling the water used to make the solution helps kill the bacteria in the water. I don't know why 1 hr is specified; you need to ask the person who wrote those instructions.

"If you don't rinse and refill the buret with fresh KMnO4 until just before it is needed for a titration, there is no light contamination that can take place."

I don't understand know why there wouldn't be any light contamination. Is it b/c the rxn is slow or b/c I removed MnO2 was precipitated so there is a minimal reverse rxn.
And I don't think I would consider my solution to be fresh since I prepared it and standardized last week on Wed


I don't know who wrote the manual or what year that manual was written for that matter..but I guess that's my problem not yours

As for the 1hr they basically ask us to go and boil 1400ml of distilled water down to 1000ml. But basically I'm thinking of boiling the water for just 20min and letting it cool while I'm doing the analysis of iron in a razor blade lab. Would that be fine?


My opinion is that 20 minutes would be fine. Just remember you don't need 1400 mL to start since you won't be boiling much of it away. As for the KMnO4, it's because of both. The reaction is slow. You removed all the MnO2 initially (and a little of that catalyzes the reaction to make it faster). KMnO4 only a week old is not all that bad. I wouldn't hesitate to use it and use the molarity you obtained last week in the standardization if you feel those numbers were good then. Tf they were good then they probably still are ok.

Alright Thank You for the advice Dr.Bob
=)

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