Acidity of phosphoric acid

[thatricksta]

Hey everyone,
Recently we did a practical in class which required us to investiage the efffects of temperature on equalibrium. In one section of the prac we were required to heat/cool phosphoric acid with methyl violet indicator.

Nobody here had issues decreasing the acidity with an increase of temperature but we were not able to increase the acidity with a decrease in temperature.

I suspect this is because of phosphoric acids ability to ionise in water as it already begins with a pH of 1 and it is difficult to get the conjigate acids to ionise in water and the decrease in temperature is not enough, but can anyone please confirm why we can't (visiably) increase the acidity of H3PO4?

Thanks!

ps.
Did post counts and respect get reset?? o.O

[Mao]

I would think it would be more to do with the indicator. If the indicator is already saturated in one form (i.e. 99% acid form), it will be extremely difficult to tell any subsequent increase (i.e. 99.9% acid form).

[thatricksta]

I would think it would be more to do with the indicator. If the indicator is already saturated in one form (i.e. 99% acid form), it will be extremely difficult to tell any subsequent increase (i.e. 99.9% acid form).

I don't quite understand what you mean by this =/

[Graphite]

Indicators are conjugate acid base pairs and if the equilibrium is positioned greatly to the right, any colour change is had to detect. A bit like from dark blue to navy.

[thatricksta]

Im still not feeling like my question is being answered I am quite confused

The reaction is
"H3PO4 ---> H2PO4- + H+" which is exothermic and I want to know about shifting the equilibrium to the right with decreasing the temperature, so that the acidity increases and thus the indicator changes from green to yellow.

I am happy with my change from green to blue as that shows a decrease in acidity through increasing temperature and the acidity decreases, displaying a pH of "2" rather than the initial "1"

[Graphite]

Its a strong acid so yea, it does proceed almost to completion. So changes in acidity are really slight, colour changes used to detect this change in acidity is also very slight. Also the equilibria system of the indicator is dependent on the that of the phosphoric acid also.

[thatricksta]

Its a strong acid so yea, it does proceed almost to completion. So changes in acidity are really slight, colour changes used to detect this change in acidity is also very slight. Also the equilibria system of the indicator is dependent on the that of the phosphoric acid also.

H3PO4 is a weak acid, it has an acidity constant of 7.25*10^−3, which is quite low- which suggests that no, it has not completely ionised and its pH should theoretically be able to proceed below 1.

[Graphite]

Well it doesn't need a large K value to be a strong acid

[pi]

Well it doesn't need a large K value to be a strong acid

Phosphoric acid is a weak acid

[Graphite]

Serious? I never knew that I thought the 1st deprotonation was very strong.
Btw, then how do we explain the low pH?

[pi]

Serious? I never knew that I thought the 1st deprotonation was very strong.

It doesn't go to completion (in water).

Maybe the one you are thinking of is H2SO4, which has a irreversible 1st deprotonation, and hence is a strong acid.

[thatricksta]

Well it doesn't need a large K value to be a strong acid

Sorry for the slack notation, should have written "Ka" which is the acidity constant, which does mean that it has to be large to be a strong acid.


My question is still out there guys D=

[thushan]

tricksta, mao was referring to the fact that we judge the acidity of phosphoric acid BASED on the indicator's colour. so when we decrease the temperature, there is an equilibrium shift to the right, producing more H+, which would produce more of the protonated form of methyl violet (that is responsible for the colour). However, before we decrease the temperature, pretty much all the methyl violet is protonated as the pH is already quite low, much lower than the pKa of methyl violet (the pH at which methyl violet is at the midpoint of its colour change). So when we decrease the temperature, only a little more methyl violet gets protonated, which is why you don't see a measureable colour change.

think about the colour change this way: recall a standard HCl-NaOH titration using phenolphthalein. if NaOH is aliquot, and you start titration, the pH in flask steadily decreases, but the solution stays quite pink and suddenly turns clear; it doesn't gradually turn clear.

and just to clarify, strong acids generally have Ka values of >10^5 and H3PO4 is definitely a weak acid

[thushan]

Ignore above post: i just found out colour change is between pH ~ 0.15 - 3.2

I don't think temperature change was enough to decrease pH to 0.15. Phosphoric acid is somewhat weak; a 0.01 M solution would have pH = 2.25.

Probs concentration of ur solution was quite low, so pH would be much closer to 3 than it is to 0.15.

[thatricksta]

Ignore above post: i just found out colour change is between pH ~ 0.15 - 3.2

I don't think temperature change was enough to decrease pH to 0.15. Phosphoric acid is somewhat weak; a 0.01 M solution would have pH = 2.25.

Probs concentration of ur solution was quite low, so pH would be much closer to 3 than it is to 0.15.

We had a solution of 1M and its base pH was approximately 1, so that also rules out that... haha, thank you though