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Author Topic: ph effect on the solubility/Ksp of KHP soln  (Read 9772 times)

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lpstar

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ph effect on the solubility/Ksp of KHP soln
« on: February 24, 2008, 08:00:55 AM »

1. If I have a saturated KHP solution and add NaOH, what will happen to the solubility and the Ksp?
2. If I have a saturated KHP solution and add HCL, what will happen to the solubility and the Ksp?

Okay, I know that KHP is amphoteric so
1. KHP + NaOH = H2O + P2- and I am guessing equilibrium goes to the right and solubility increases. right?
2. KHP + HCL = H2P +KCL ? and I am guessing eq goes to the right and solubility increases? but I also found out that H2P is less soluble than KHP, so now I am not sure if the solubility of KHP decreases or increases.
I am thinking that since the Ksp are constants, they don't change, and what is changing is the solubility. right?
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Borek

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Re: ph effect on the solubility/Ksp of KHP soln
« Reply #1 on: February 24, 2008, 08:56:03 AM »

KHP + NaOH = H2O + P2- and I am guessing equilibrium goes to the right and solubility increases. right?

Right.

KHP + HCL = H2P +KCL ? and I am guessing eq goes to the right and solubility increases? but I also found out that H2P is less soluble than KHP, so now I am not sure if the solubility of KHP decreases or increases.

This is tricky. In the first case, dissolved substance moves to solution, so when you add NaOH more solid dissapers - it is obvious thet solubility is higher. But here - I would expect H2P to precipitate out of the solution, so the amount of solid will probably increase - but the solid will be mixture of acid and KHP. You will have more K+ in the solution, but no corresponding HP- - and I have no idea whether it means increased or decreased solubility.

Quote
I am thinking that since the Ksp are constants, they don't change, and what is changing is the solubility. right?

Right - but wrong, as in this case solubility is not a good term to describe quantitatively what is happening.
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lpstar

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Re: ph effect on the solubility/Ksp of KHP soln
« Reply #2 on: February 24, 2008, 10:01:25 AM »

Thanks for your help.  I am still a bit confused about the HCL + KHP can I say that the equilibrium will move to the left to compensate for the increase of K+? also since there is more solid would the solubility be less?  At the same time, when the acid is added since H2P is forming will the equilibrium be shifting to the right and the solubility increasing when this is happening?
« Last Edit: February 24, 2008, 10:28:27 AM by lpstar »
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Re: ph effect on the solubility/Ksp of KHP soln
« Reply #3 on: February 24, 2008, 10:58:42 AM »

I told you this one is tricky.

In the first case situation is more or less obvious - You have both P and K+ in the solution. P2- instead of HP-, but it is in the solution, so you may say KHP was dissolved.

In the second case you have K+ in the solution, but P is partially somewhere else. If you define solubility as 'amount of solid KHP that disappeared' - solubility is higher. But if you define solubility as 'amount of solid KHP that can be recovered from the solution' - solubility is lower. IMHO it is not possible to answer this questions in simple terms of higher/lower solubility.

Note: I am assuming that H2P solubility (no tricks used) is lower than that of KHP. It sounds logical and you wrote that you have checked it.
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Re: ph effect on the solubility/Ksp of KHP soln
« Reply #4 on: February 24, 2008, 11:36:21 AM »

Yes, I found that Phtalic acid has a solubility of 6.25g/L and KHP has a solubility of 80 g/L.

Thanks!
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