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Topic: Titrating Iron tablets.  (Read 17644 times)

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Offline SlayerCake

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Titrating Iron tablets.
« on: November 10, 2009, 03:10:31 AM »
Hello, I have recently attempted to perform a series of titrations involving several different brands of iron tablet however I've hit a few snags.

I created my solutions simply by crushing one tablet of each brand into a fine powder and adding 25cm3 of 0.1mole/l-1 Sulphuric acid and heating until it dissolved. This created a series of yellow solutions which I titrated with 0.05mole/l-1 Potassium Permanganate. However instead of eventually turning a pink colour it formed a mucky brown precipitate which obscured any colour change. This may be because of all of the gunk that holds the tablets together or a problem with my indicator.

My teacher and I have an idea to solve this, involving a ridiculous amount of filtering and diluting, but I was wondering if anyone had any tips/ideas or has already done this and had similar problems but solved them?

Offline cliverlong

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Re: Titrating Iron tablets.
« Reply #1 on: November 10, 2009, 03:14:52 AM »
What reaction and products do you expect? Are you expecting to form a precipitate?

How did you dissolve the tablets?

Why do you expect a pink solution at the end?

Why do you think you need more than one filtration? (Filtration was my first thought)

What is in the tablets you are using?

Could there be something in the solution after filtraiton but before titraiton that is causing the precipitate  to form?

Can you obtain a "pure" solution of an iron salt, titrate that to see what result you should get from mixing the solutions?

Clive

Offline Borek

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Re: Titrating Iron tablets.
« Reply #2 on: November 10, 2009, 03:23:11 AM »
Too high pH, thus you got MnO2.

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

0.1M sulfuric acid is of no use here, most procedures call for addition of several mL of concentrated acid.

And you should ensure that all iron is reduced to Fe2+ before titration, otherwise your result will be some random number, not necesarilly related to the reality.
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Offline SlayerCake

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Re: Titrating Iron tablets.
« Reply #3 on: November 10, 2009, 01:51:39 PM »
Too high pH, thus you got MnO2.

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

0.1M sulfuric acid is of no use here, most procedures call for addition of several mL of concentrated acid.

And you should ensure that all iron is reduced to Fe2+ before titration, otherwise your result will be some random number, not necesarilly related to the reality.

Thanks, do you know what concentration would be best? I think the highest one I can easily get is 2M, would that do if the masses of the tablets are approximately 0.320g and 0.247g? And would it be worth leaving it overnight to dissolve fully?

Also my experiment will involve titrating iron solutions that I will make from various foods (spinach, corn flakes, etc) and the procedures I've found say I should use 0.1M Sulphuric acid to extract the Fe2+ ions but will it be better to use a higher concentration for that as well?

Again, thanks allot for the help.

Offline Borek

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Re: Titrating Iron tablets.
« Reply #4 on: November 10, 2009, 04:43:31 PM »
Thanks, do you know what concentration would be best? I think the highest one I can easily get is 2M

For titration you need pH somewhere between 0 and 1, use this information to calculate acid concentration. Just note, that acid that was used for sample dissolution/digestion is most likely already spent - at least partially - so it is probably better to add more before titration.
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Offline SlayerCake

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Re: Titrating Iron tablets.
« Reply #5 on: November 16, 2009, 02:31:56 AM »
OK so, I used an acid that was well between 1 and 0 pH also I heated and stirred both solutions constantly for about 10min to let all the iron dissolve and then I filtered both of them. However the end point was impossible to reach as after any colour change occurred if the solution was left for a length of time it would de-colourise, this occurred repeatedly, with several beakers of both solutions, and when we added excess Potassium Permanganate so as to reach an end point a brown precipitate was formed. I am *this* close to calling it quits on this project and starting fresh but if there are any more insights as to what went wrong that would be appreciated as I'm kinda bummed that I've wasted about a weeks hard work on this for no result.

Offline Borek

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Re: Titrating Iron tablets.
« Reply #6 on: November 16, 2009, 08:10:57 AM »
Most procedures I am aware of call for 'pink color stable for at least 30 sec'.

Permanganate slowly decomposes on its own - that can be the problem.

It is also possible other substances present in pills are reducing permnagante - there are digestion procedures you could use (like remove organic substances by boiling the solution with some strong oxidant, remove the oxidizing substance by drying the solution in the presence of concentrated sulfuric acid - till you have only solid iron sulfate left, then reduce iron to Fe2+ - I can be wrong about details, but that's the general idea).

I will forward the question to CHEMED-L.

This is not a titration experiment, but you may try to check this paper from Journal of Chemical Education:

Colorimetric determination of iron in vitamin supplement tablets. A general chemistry experiment
Atkins, Robert C.
August 1975 p 550
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Offline Borek

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Re: Titrating Iron tablets.
« Reply #7 on: November 16, 2009, 08:33:43 AM »
I will forward the question to CHEMED-L.

I love people on CHEMED-L, I got an answer in less than 15 minutes :)

Quote
Hi Marcin:
Yes, and it works very well.  The one issue that always comes up is how to reduce the Fe+3 to Fe+2.
It is best to use a little bit of Mg powder amalgamated with mercury(II) nitrate.. after amalgamating the magnesium metal, make sure you wash it with copious amounts of distilled water and keep the metal wet.
 
Dissolve the Fe tablet in water and then add some sulfuric acid to help digest the starch (you might have to heat it on a hot plate or over a bunsen burner)
 
After the table has dissolved (more than likely you will have some suspended solids) filter or centrifuge and add the decantate to the amalgamated magnesium.
Wash the remaining solid with Distilled water transferring the washes to the amalgam flask.
 
Do the same thing regarding the amalgam mixture.... decant the liquid off, wash it twice, and then immediately titrate the solution with standardized permanganate.
 
My students get within 1% of the value on the bottle every time they do the experiment.
 
You can also do a similar experiment using dichromate (Cr2O7-2) except that you have to use some diphenylaminosulfonic acid indicator (the dichromate isn't intense enough to use as a self-indicator)
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Offline Borek

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Re: Titrating Iron tablets.
« Reply #8 on: November 16, 2009, 11:29:56 AM »
More comments from CHEMED-L:

Quote
Since I retired, my school stuff is packed away, but I do think I have seen this procedure in a lab manual. I went to the site, and I think the second person's comments (from Borek) are correct. One would also have to try and find a white pill for best results. I dont think any of the inert ingredients in an iron tablet should cause problems.

Quote
The direct titration with KMnO4  has some some shortcomings. In short this method is probably not applicable to a tablet until and unless sample clean up procedures are done. The easiests I can think of is dry ashing to remove all organics (because there are plenty of other things which will reduce permanganate, especially organic acids which lead to brown MnO2). Then after digestion of the ash, any Fe3+ must be reduced to Fe2+ say by a small amount of zinc. You probably have to take 3-4 tablets to get a decent burette reading.
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Offline Borek

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Re: Titrating Iron tablets.
« Reply #9 on: November 16, 2009, 12:08:43 PM »
Quote
Here is the ingredient list from my iron tablet bottle: calcium carbonate, Cellulose, Ferrous Sulfate (134mg), Corn Starch, croscarmellose Sodium, Dextrin, Maltodextrin, acacia, silicon dioxide, hypromellose, red 40 lake, talc, polyethylene glycol, dextrose,  magnesium stearate, sodium carboxymethylcellulose, sodium lauryl sulfate, soy lectithin, titanium dioxide, blue 2 lake, sodium citrate.
Not being an organic chemist, I have no idea what these things do to permanganate; but arent most of the ingredients filler, binders, smoothing agents and agents to make the pills feel good and not taste awful, and are reasonably inert? The two dyes are the problem for an endpoint.
It also seems to me that one could buy a supplement with a hell of a lot fewer ingredients (mine has the smallest amount of iron, 27mg, that one can get...there are others with aver 100mg equivalent iron, and maybe fewer ingredients as well).
I just think the idea of titrating a pill holds more fascination for students that a made up unknown. I know when I had students titrate antacid tablets the lab provoked lots of interest (and we had to use white pills there as well).

Quote
It seems to work very well for my students if they filter before reducing the Fe+3 to Fe+2 and titrating.
They normally get results within a few mg of the listed value on the bottle.
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Offline Borek

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Re: Titrating Iron tablets.
« Reply #10 on: November 16, 2009, 02:44:44 PM »
Quote
Unfortunately KMnO4 oxidizes non-discriminately although the kinetcis may be slow for certain things. It even reacts with elemental carbon in acidic solution. Sugars and starch are among the first things that are going to react with it. My whole concern is that the results will always be higher because you are not oxidizing Fe2+ only.

We used to do a similar analysis with a real tablet but using a spectrophotometric method, which is pretty standard available everywhere on the web.
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Offline Arctic-Nation

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Re: Titrating Iron tablets.
« Reply #11 on: November 16, 2009, 05:23:02 PM »
I remember a time when I was lab supervisor for first year medicine students, and the determination of iron in iron tablets always gave problems. Procedure was something like this: put tablet in erlenmeyer flask, add excess amount of a mixture of sulfuric and phosphoric acid, boil till tablet is dissolved, cool, dilute, and titrate with permanganate. Pretty simple, but a few people always managed to somehow burn their solution (giving something brownish black instead of reddish). The real catch, however, was that the amount of iron calculated through this titration was always larger than the mass of the tablets themselves. Every time I had to make sure nobody weighed their tablets to avoid annoying questions afterwards, and every time a few smartasses did exactly that.

Since then I hate this problem. ;)

Offline soy matcha latte

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Re: Titrating Iron tablets.
« Reply #12 on: March 27, 2019, 05:21:39 AM »
**Ignore me, I am impatient**

So do you guys know why the calculated iron content was so high compared to the stated amount on the supplement packaging?

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