[Homer Jay]

I titrated an aliquot of the sample, containing Ca and Mg, with EDTA against Erio T to determine the sum. The solution turned from red to gray-green. So far okay, I put the flask aside.

But after 10 minutes the solution turned red again! Does anyone have an explanation?

[enahs]

A couple of things could have happened.

One, you did not fully mix the solution well and stopped too soon, and so as the solution settled and homogenized and "the indicator found more acid".


More likely, the reaction with carbon dioxide in the air produced carbonic acid; Just enough to turn it back red (as carbonic acid is a weak acid).

You should also boil your solution for a few minutes prior to titrating to remove the excess CO2 dissolved in the solution. And never breath on what you are titrating. I have seen people do that (as you tend to get close to it anyway) and throw off the determination by a substantial amount.

[Homer Jay]

Enahs, thank you for your answer. Maybe I really didn't stir the solution enough because I didn't use a magnetic stirrer but swirlled the solution by hand only .

I don't understand, why CO2 from the air could be a problem, because I titrated with EDTA and not a base. (complexometric titration). Of course, I also gave an ammonium-salt and ammonia ot the solution (a ready-to-use tablet, I don't know what's their name in English.)

Because of the buffer-solution the pH should be constant.

That's why I guess, that the problem was something else but I don't know what....

It looks like the EDTA complex was not stable enough and the indicator was "complexed" (coordinated?) by the metal ions again.

[enahs]

Carbonic Acid:
CO₂ + H₂O ⇌ H₂CO₃

H₂CO₃ ⇌ HCO₃⁻ + H⁺  : Pka = 3.6 @ 25 °C.

The biocarbonate ion is sometimes referred to as temporary hardness. It can be removed acidifying and boiling for a few minutes, or just boiling for ~15 minutes. It can interfere with many indicators. It can also react with the magnesium and calcium forming calcium and magnesium bicarbonates, short lived species existing only in solution. All of these factors interfere and can cause the indicator to turn back.

Note, for that to be the case, you would have to have done the titration extremely well and not have gone past the end-point to any real extent.

[Homer Jay]

Thanks again for your answer, now I understood. I don't know if I really did the titration so well. Probably I should have used a magnetic stirrer. I'm student and not in the lab now so I can't reproduce this analysis anymore but at that time I got three times this strange result.

[Borek]

Pka = 3.6 @ 25 °C

Typo, 6.3.

I don't know exact reasons, but this titration is notorious for end-point detection problems.

[Homer Jay]

I don't know exact reasons, but this titration is notorious for end-point detection problems.

That's interesting.

[enahs]

Typo, 6.3.

No. I just accidentally used the wrong one.
http://en.wikipedia.org/wiki/Carbonic_acid#Acidity_of_carbonic_acid


Ok, truth be told I copied the wrong one, with the equation to save me the trouble of typing it and inserting the double line arrow.

[Borek]

I have seen several different 'exact' values, the only thing that everybody agrees on is that 6.36 is not correct - although observed.

While 3.6 may be the real value I am reluctant to list it and use it - as it completely doesn't explain why carbonate buffer holds our blood at correct pH

[enahs]

One of the biologist came over to the chemistry department a few years back and asked if it would be possible to get a solution of carbonic acid from the chemistry department. One of the faculty members is a very funny and joking guy and seriously said something along the lines of that it is expensive and hard to make and store and the biology department would have to pay a small fee of $100. They did, he gave them a bottle of water that he breathed out in through a straw. And that my friends is the story of the time in my life that was the hardest moment to not laugh.


The professor then took everybody around that witnessed it out to lunch with that money and asked us not to tell the department chair!

[Homer Jay]

nice story