[losingmymind]

The other day my AP Chemistry class was doing a redox titration of dark purple KMnO₄ (aq) and FAS, or (NH4)₂Fe(SO₄)₂ (aq) I believe, which is very slightly tinted blue-green, almost unnoticeably so, all in acidic solution.

When doing the titration, my group calculated the molarity of KMnO₄ to be very close to the actual value, using 10.00 mL of the FAS and 10.34 mL of the KMnO₄. Our second trial, however, used the same amount of FAS, but we needed over 18 mL of the KMnO₄. Another group used an entire 50 mL buret of dark purple KMnO₄, and the solution remained clear, with small bubbles appearing in the liquid. Another group had similar results. I tried another time, doing everything exactly identical to the erroneous trial, but with the last of the FAS, so only 9.10 mL, with my teacher watching over my shoulder, and I only needed 8.20 mL of the KMnO₄.

My teacher was stumped, and another former AP teacher suggested that the water my have been complexing around the Fe³⁺ ions, but he couldn't be sure.

[Hunter2]

With this analysis normally an indicator Ferroin is added to see very sharp, if all iron-II is converted to Iron -III. The pink color of the permanganate is in presence of Iron-III difficult to see. Over titrations are easy.

Other explanation: amount of FAS different in each group. Molarity of permanganate was different. Amount of sulfuric acid is different. The reactions need Oxonium ions.

[Borek]

Have all groups used exactly the same reagents in all cases?

[losingmymind]

While I cannot say for certain, I would be concerned if AP students couldn't figure out to use the only reagents available.

As for the idicator added, we were only trying to react all of the FAS in order to find the concentration of the KMnO₄ by looking to see when the dark, almost black purple stopped turning clear in the solution. It was easy to see the purple when the solutions first mixed, but they would turn clear as long as there was FAS still in the flask. The amount of FAS should have been around 10 mL for each group, the volume of sulfuric acid should have been inconsequential, according to my teacher, and some groups used the same buret with the same sample of potassium permanganate for both of their titrations, getting completely different results. I don't mean to dismiss your suggestions, but I just don't see why we would have this kind of error.

[Borek]

While I cannot say for certain, I would be concerned if AP students couldn't figure out to use the only reagents available.

I didn't mean using wrong reagents, I meant - were they all taken from exactly the same source? I don't know how the work was organized - were you all taking reagents from the same ready bottles? Or some groups from one bottle, and other groups from some other bottle, or you used all the reagent during the experiment and were given another bottle of it, or something like that?

As for the idicator added, we were only trying to react all of the FAS in order to find the concentration of the KMnO₄ by looking to see when the dark, almost black purple stopped turning clear in the solution.

No need for an indicator during permanganate titration.

the volume of sulfuric acid should have been inconsequential

Only if it was in a reasonably large excess. How much acid and of what concentration have you added?

[losingmymind]

Ah, I see. Yes, all of the FAS was in one bottle ready to be pured into a buret for measurement, all of the sulfuric acid was from a single jug, but the KMnO₄ may or may not have come from one of two different bottles. I'm not exactly certain as to which was used for which trials, or if they were even made in the same batch. Again, I'm not 100% sure, but I do think that there were two different bottles.

As for the acid, we used about 10 mL of 6 molar sulfuric acid.

[Borek]

I am running out of ideas.

It is not the first time people signal problems with permanganate titrations. At least once it could be IMHO problem with not enough sulfuric acid used, but it doesn't look to be the case here. Fe³⁺ being complexed shouldn't change the situation, as it is not getting oxidized to anything (ie it doesn't give electrons that could be consumed by the permanganate reduction).

Just in case: what were the concentrations of FAS and permanganate used? What was the exact procedure - were you diluting samples? And if so, with what? Just water? Have you all used water from the same source?

[magician4]

thinking from a completely different direction:

this here:

Another group used an entire 50 mL buret of dark purple KMnO4, and the solution remained clear, with small bubbles appearing in the liquid.

gives me the impression that there has been oxygen evolving.

This might happen with KMnO4 - solutions, as they're not completely stable over longer periods of time (i.e. several hours). Furthermore , it is known that some ions (for example Mn²⁺ , if memory serves) will act as catalysts here.
Additionally, some substances - like for example H₂O₂ - are known to react with KMnO₄ and to release oxygen along the way.

Hence, my best guess is that there was something in the water (did you use water directly from the tap?) responsible for massive additional KMnO4 consumption.

some chemical and / or biochemical oxygen demand, COD/BOD fills this profile, and your observed results then might be easily explained in terms of halflifetime of (most probably) the KMnO₄ - solution before you made use of it

regards

Ingo

[Borek]

This might happen with KMnO4 - solutions, as they're not completely stable over longer periods of time (i.e. several hours).

Several hours is not a long period of time in this context. While you are right permanganate solutions are not stable, they are stable enough to be used for titration and typical procedure calls for solution standardization just every few weeks. I don't see how decomposition could explain changing titration results twofold or more during a single lab. By a fraction of percent perhaps, but even that would be a lot.

[magician4]

the idea was that, due to named sensitivity, the solution had been prepared freshly - just not from distilled / COD-BOD free water, but from tap water instead

given that we'd had a mediumspeed*⁾ side reaction resulting thereof , this could explain why first results seemed to be next to ok. still, but later results seemingy needed that much more KMnO₄ solution

regards

Ingo


*⁾
KMnO₄ oxidations are known for their at times "strange" kinetics due to autocatalytic acceleration by, for example , Mn²⁺ : more often than not they start slow, but once they get goin', they really get goin'

[Borek]

this could explain why first results seemed to be next to ok. still, but later results seemingy needed that much more KMnO₄ solution

Final titration took more or less correct volume of permanganate, so the solution was still OK at the end of the lab period.

[losingmymind]

Molarity of K₂MnO₄ was about 0.19 according to the teacher's calculations before class.
Molarity of FAS I believe was 0.0988 M.
We added approximately 10.0 mL of the FAS to a clean flask, measuring the amount with a buret, then 10.0 mL 6 M sulfuric acid, measuring with a graduated cylinder, and about 30 mL deionized water from one of two jugs in the room, which contain nothing else, ever. Then we added the permanganate. Our school has a filter (I don't know how effective or what kind) that we put tap water through to make the DI water. I would guess that there iare still some impurities in the water, but they'd probably be miniscule.

[Borek]

Molarity of K₂MnO₄ was about 0.19 according to the teacher's calculations before class.
Molarity of FAS I believe was 0.0988 M.

If my calculations are correct that would mean 1 mL of permanganate solution per FAS sample, so one of the concentrations is most likely 10 times off:

<a href="http://www.youtube.com/v/HsL78VX8H3o" target="_blank" rel="noopener noreferrer" class="bbc_link bbc_flash_disabled new_win">http://www.youtube.com/v/HsL78VX8H3o</a>

(note: this is a working version of the upcoming EBAS release, automatic balancing in acidic/basic solutions is not supported in the current version - but upgrade will be free)

Not that it changes much - I have no idea what could be wrong.