Chemical Forums
Chemistry Forums for Students => High School Chemistry Forum => Topic started by: Vishal on January 12, 2015, 08:34:30 AM
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Is it possible to have pH of acid to be negative and base to have pH more than 14? If yes, please provide an example for both and explain how.
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You have to show your attempts at solving the question to receive help. This is a forum policy.
Just apply what you know about pH. What is the concentration of H+ at pH 1? pH 0? pH -0.1? Does it look reasonable?
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Respected Sir
The question is arose due to curosity, not as a homework. The fact mentioned in our textbook was
pH of saturated solution of NaOH is ≈15.
pH of concentrated HCl is ≈-1.
No reference books accepted the facts. So the mention facts are correct or not?
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Have you tried what I suggested?
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Unfortunately I feel that the answer to this question is actually a bit more complex than just just asking about concentrations and how they relate to pH.
Yes, you can sit down with a pencil and paper and show if the concentrations of the solutions required to produce these pH values are reasonable. But at the end of the day, if you stick a pH meter in 2M sodium hydroxide, it's going to spit out a value in the 13 range. Then you're sitting there with your pencil and paper thinking about how you calculated something above 14.
This is mostly due to something called activity interference, which I'm sure you don't learn about in your high school chemistry class. It's something you learn in analytical chemistry.
Basically what this all means is that once you start reaching concentrations above a certain point you begin to see non-ideal behavior.
Granted, I've never sat there with a pH meter and tried to push the pH of a solution past 14, so I don't actually know the answer to your question.
If you can, I suggest not sitting down with a pen and paper, but actually trying to make solutions that will go out of the range of a pH meter. Ask your teacher! You never know what they might let you do.
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Basically what this all means is that once you start reaching concentrations above a certain point you begin to see non-ideal behavior.
I didn't want to mention it before we establish the basic things. Yes, pH is not a negative log of the concentration, but of the activity - and yes, activity equals concentration only in diluted solutions. It doesn't change the fact that pH scale is not artificially bound.
And for high ionic strengths activity coefficients can get higher than 1, making things even more interesting.
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I didn't want to mention it before we establish the basic things. Yes, pH is not a negative log of the concentration, but of the activity - and yes, activity equals concentration only in diluted solutions. It doesn't change the fact that pH scale is not artificially bound.
So the way typical, handheld pH meters work are they actually measuring a negative log of the concentration or of the activity?
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No idea if a simple one will be able to work in a concentrated solution, I doubt it. But it is not a matter of the pH meter itself (although cheap ones are probably optimized for a narrow range), more like a problem with selecting the correct electrode.
Somehow I think for correct results it will be better to use a hydrogen cell (that is, saturating the solution with a gaseous hydrogen and using platinum electrode covered with black platinum) than to use pH electrode.
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@Borek. Very interesting. Thanks.
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I didn't want to mention it before we establish the basic things. Yes, pH is not a negative log of the concentration, but of the activity - and yes, activity equals concentration only in diluted solutions. It doesn't change the fact that pH scale is not artificially bound.
So the way typical, handheld pH meters work are they actually measuring a negative log of the concentration or of the activity?
AFAIK they measure difference between potentials of two electrodes where potential of one depends on activity of H+ in solution. The change in activity of H+ changes the potential difference: the change of 1 unit of pH equals if Im not mistaken 60mV potential change. For acurate measurments its good to calibrate the elctrodes by buffer solutions of known pH
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Oops, somehow I misread the original curiouscat question and answered the one I thought he asked :( But yes, electrode reacts to activity, not to the concentration, and the electrode response is simply the one defined by the Nernst equation (so just RT/F). There is always a second, reference electrode - in most cases these days just put in the same housing, see images here: http://www.ph-meter.info/pH-electrode-construction
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Oops, somehow I misread the original curiouscat question and answered the one I thought he asked :( But yes, electrode reacts to activity, not to the concentration, and the electrode response is simply the one defined by the Nernst equation (so just RT/F). There is always a second, reference electrode - in most cases these days just put in the same housing, see images here: http://www.ph-meter.info/pH-electrode-construction
Ok, thanks again.
Then, out of idle curiosity, has anyone of you managed to get a reliable, calibrated pH electrode to actually read beyond the 0-14 range?
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Not me. But others did: http://pubs.acs.org/doi/abs/10.1021/es990646v