Phenolphthalein would not be an appropriate indicator to use to determine the Ka1 of phosphoric acid Why not? what indicator would be appropriate?
I Think its 8-11 as it is a small range but i'don't know if its right.
The pH range for phenolphthalein is 8-10 so it would not be appropriate for H3PO4 since the equivalence point is much more acid than that. If you know the pH range for K1 for H3PO4, here is a link from which you can pick the right indicator to use.
http://www.jiskha.com/display.cgi?id=1329337234
Thanx!!
Well, using phenolphthalein to determine the Ka1 of phosphoric acid would be like trying to use a balloon to measure the temperature. It simply wouldn't work! Phenolphthalein is a pH indicator that changes color within a range of pH 8.2 to 10.0. However, the Ka1 of phosphoric acid is much lower, around 7.2. So, phenolphthalein wouldn't be able to detect the subtle pH change that occurs in this range.
To determine the Ka1 of phosphoric acid, a more appropriate indicator would be bromothymol blue, which changes color within the pH range of 6.0 to 7.6. This indicator would be more sensitive to the pH change that occurs during the ionization of phosphoric acid.
So, phenolphthalein may be great for your Valentine's Day card, but not for determining the Ka1 of phosphoric acid!
You are correct, phenolphthalein would not be an appropriate indicator to use to determine the Ka1 of phosphoric acid. Phenolphthalein is a pH indicator that undergoes a color change at a pH range of about 8-11. However, phosphoric acid is a polyprotic acid, meaning it can donate multiple protons.
The Ka1 of phosphoric acid is associated with the first proton donation, which occurs at a lower pH range (around 2-3). Therefore, phenolphthalein would not exhibit a color change at the pH range relevant to the determination of Ka1 for phosphoric acid.
Instead, a more suitable indicator for determining the Ka1 of phosphoric acid would be one that changes color in the lower pH range, such as methyl orange or bromothymol blue. These indicators change color around pH 3-4, which is more appropriate for the determination of the first dissociation constant Ka1 of phosphoric acid.
Phenolphthalein is not an appropriate indicator to use to determine the Ka1 of phosphoric acid because phenolphthalein has a pH range of approximately 8.2 to 10. Secondly, phosphoric acid (H3PO4) is a triprotic acid, meaning it can lose three protons successively. The Ka1 value refers to the first ionization constant, where phosphoric acid loses only one proton. Phenolphthalein's pH range does not align well with the pH at which the first proton is dissociated from phosphoric acid.
To determine the Ka1 of phosphoric acid, you would need an indicator that changes color in the pH range close to the expected dissociation of the first proton. A more suitable indicator for this purpose could be bromothymol blue, which has a pH range of around 6.0 to 7.6. This range covers the expected pH of phosphoric acid during the first ionization.
Regarding your comment about the pH range being 8-11, that is not accurate for the first ionization of phosphoric acid. The pH range you mentioned is more applicable for phenolphthalein, not phosphoric acid.