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Specialty Chemistry Forums => Chemical Education and Careers => Topic started by: ac70 on June 05, 2015, 03:57:41 PM

Title: Concentration from PH
Post by: ac70 on June 05, 2015, 03:57:41 PM

Hello,
is it possible to determine the acid's concentration from its PH?

Let's say for example the acid is 0.1 PH what is its concentration?

thanks

Title: Re: Concentration from PH
Post by: sjb on June 05, 2015, 04:36:50 PM

Quote from: ac70 on June 05, 2015, 03:57:41 PM

Hello,
is it possible to determine the acid's concentration from its pH?

Let's say for example the acid is 0.1 pH what is its concentration?

thanks

What is the definition of pH?

Title: Re: Concentration from PH
Post by: ac70 on June 05, 2015, 04:55:58 PM

In chemistry, pH is the negative log of the activity of the hydrogen ion in an aqueous solution...

I am not a chemist .. so what I would like to know is: how can I express a 0.1 PH in percentage ?
Maybe the question does not make sense but an answer instead of a question would be appreciated.

thank you

Title: Re: Concentration from PH
Post by: Borek on June 05, 2015, 05:40:18 PM

Percentage of what in what?

To some extent it is possible to calculate concentration of an acid knowing its pH, but as most acids are not 100% dissociated it is not always trivial, and - what is more important - highly inaccurate.

Title: Re: Concentration from PH
Post by: Babcock_Hall on June 05, 2015, 06:05:07 PM

@OP, A forum rule is that you must show an attempt before we can help you.  IMO the question about a definition was asked to prompt you in that direction.

Title: Re: Concentration from PH
Post by: ac70 on June 05, 2015, 06:14:23 PM

I have a 0.1 PH of HCl and I want to dilute it so that it become a 7% solution of HCl.

The problem is that I know the PH of the acid because I can measure it with the PH meter but I do not know how much water I have to add.

Let's say for example that the concentration is 36% (the maximum) and I want to obtain 10 ml of solution of HCl acid at 7%.
I could calculate:

7% / 36% = 0.19
0.19  * 10 = 1.9 ml
10 ml(of total solution) - 1.9 ml (of HCl) = 8.1 (of water)

that would work.
The problem is that I am not sure my hcl acid is 36%.

Hope the question is clear

Title: Re: Concentration from PH
Post by: Borek on June 06, 2015, 03:55:10 AM

As I wrote earlier - using measured pH (especially that low) to determine acid concentration is so inaccurate your final result can be easily off by a factor of 2 if not higher. The best approach is to measure the concentration of the HCl solution by titration (and not directly, diluting it first).

Title: Re: Concentration from PH
Post by: ac70 on June 06, 2015, 04:48:30 AM

thanks I think I got it now.

Do you think I can use Na2Co3 to do the titration?
(I do not have many chemicals available)

Can you please indicate a link that explain step by step how to do it?

thanks

Title: Re: Concentration from PH
Post by: Borek on June 06, 2015, 06:20:04 AM

While in principle using Na₂CO₃ is possible, that wouldn't be my substance of choice.

Plenty of information here: http://www.titrations.info

Title: Re: Concentration from PH
Post by: ac70 on June 07, 2015, 05:42:13 PM

Found this if somebody need it :  https://www.youtube.com/watch?v=7pvTTRrLzXg  (https://www.youtube.com/watch?v=7pvTTRrLzXg)