## Autoprotolysis

Emmaraf 1K
Posts: 94
Joined: Sat Oct 20, 2018 12:16 am

### Autoprotolysis

At what concentration of equilibrium hydronium ions must we consider the autoprotolysis of water as being an important factor in pH calculation. Is it 10^-6 or 10^-7?

Ethan Baurle 1A
Posts: 33
Joined: Fri Sep 28, 2018 12:19 am

### Re: Autoprotolysis

I believe it is any value higher than 1 x 10^-7 that will make a difference in the PH.

monikac4k
Posts: 56
Joined: Fri Sep 28, 2018 12:25 am

### Re: Autoprotolysis

If a weak acid deprotonates and releases less than 10^-7 M of hydronium, then you would consider autoprotolysis. This is because if you ignore autolysis and calculate pH of an acid that creates 10^-9 M hydronium, then the pH would be basic. This would not make sense since there is an acid in the solution. To fix this error, you consider the hydronium ions from autoprotolysis and you should get a more acidic pH.

Dayna Pham 1I
Posts: 98
Joined: Fri Sep 28, 2018 12:16 am
Been upvoted: 3 times

### Re: Autoprotolysis

monikac4k wrote:If a weak acid deprotonates and releases less than 10^-7 M of hydronium, then you would consider autoprotolysis. This is because if you ignore autolysis and calculate pH of an acid that creates 10^-9 M hydronium, then the pH would be basic. This would not make sense since there is an acid in the solution. To fix this error, you consider the hydronium ions from autoprotolysis and you should get a more acidic pH.

I agree with this. With [H3O+] values smaller than 10^-7 M, when you take the pH, it seems to be basic, but this is not the case. You would have to consider that the H3O+ is actually in water and surrounded by water, so you would add that small [H3O+] value, let's say 10^-10, to 10^-7 (the pH of the water around it), and determine that the solution is actually neutral.

This is because 10^-10 + 10^-7 is approximately 10^-7.