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Author Topic: Standardization of Potassium Permanganate  (Read 10519 times)

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lunar eclipse

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Standardization of Potassium Permanganate
« on: December 07, 2009, 03:19:32 PM »

I have attempted to standardize this solution twice with no endpoint color after over 30 mL titrated.  It should consume about 6 mL.  Has anybody had trouble with this titration or know maybe what I did wrong?

-Standard potassium permanganate titrant, 0.01M (0.05N): Dissolve 1.6 g KMnO4 in 1 L distilled water. Keep in a brown glass-stoppered bottle and age for at least 1 week. Carefully decant or pipet supernate without stirring up any sediment. Standardize this solution frequently by the following procedure:
-Weigh to the nearest 0.1 mg several 100- to 200-mg samples of anhydrous Na2C2O4 into 400-mL beakers. To each beaker, in turn, add 100 mL distilled water and stir to dissolve. Add 10 mL 1 + 1 H2SO4 and heat rapidly to 90 to 95oC. Titrate rapidly with permanganate solution to be standardized, while stirring, to a slight pink end-point color that persists for at least 1 min. Do not let temperature fall below 85oC. If necessary, warm beaker contents during titration; 100 mg will consume about 6 mL solution. Run a blank on distilled water and H2SO4.   

KMnO4 was prepared months ago by someone else and should still be good.  I am not sure if I did the "1 + 1" sulfuric acid correctly.  I did a 1:1 and diluted sulfuric acid by half with DI H2O.  Our scale is in grams so I weighed the sodium oxalate between .1000 to 0.2000 grams.  I made sure the hot plate was between 90 and 95 oC by placing a thermometer in a beaker of water on the hot plate the entire time.  Any help is much appreciated. Thank you.

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Borek

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Re: Standardization of Potassium Permanganate
« Reply #1 on: December 07, 2009, 09:40:55 PM »

I assume your permanganate is strong in color as it should be? If so, the only possible problem I can think of is that you have either weighted way too much oxalate acid, or that your sulfuric acid is contaminated with some reducing agent. If the acid is too diluted and pH is too high, you may end with brown suspension of MnO2, but obviously that's not the case.

Have you tried to add excess titrant fast, to see what happens?

Was your blank titration OK?

Not that you will find anything more here, but just in case:

http://www.titrations.info/permanganate-titration
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lunar eclipse

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Re: Standardization of Potassium Permanganate
« Reply #2 on: December 08, 2009, 02:31:03 PM »

Thank you for your quick reply. 

The permanganate is a strong purple color.

I added excess titrant fast after I titrated about 15 mL and it looked like it was going to stay but then dissipated after about 5-10 seconds.

My blank titration was good.

Our scales are checked everyday for accuracy and I weighed my samples between 0.1 and 0.2 g.

That is a good website you suggested.  I will keep it in mind also for future reference.  I will take everything into consideration and back to the drawing board.  I may remake the permanganate since it was made by a previous chemist and I don't have any record it was standardized except the bottle is labeled 0.05N.  The reagent is also labeled 1.6 g KMnO4 and on that same label "actual 1.5087" or something close to that was hand-written on it.  Why it was not weighed to 1.6 g I am not sure.  I will have to figure out what was done before. Thanks Borek for all your help.  :)
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