[hamil]

When one discussed pH, the pH is normally on the range of 0 to 14. It seems that pHdiscussions are about weak acids. If you have a strong acid of say 6 M HCl, the pH should be -.77 is this correct?  Is there a reason why pH is considered to be in the range of 0 to 14 and not-2 to 16 for example?

[stewie griffin]

I'm not an expert in the area of pH... maybe some analytical chemistry folks can help out in this discussion. But I do know that there's nothing wrong with having a pH outside of the 0-14 range. For some reason in general chemistry pH is taught as if ONLY 0-14 exists, but I've seen several cases where the pH is in fact negative (I believe it's in the case of superacids that I saw this... something like SbF₅ mixed with HF).
I haven't seen a pH above 14 though....don't know why not.

[stewie griffin]

Here's something similar that I found on the internet.
http://www.newton.dep.anl.gov/askasci/chem99/chem99230.htm

[BetaAmyloid]

pH is based on a scale of 1 mol dm^-3 solutions (solvents specifically in water) between the strong acid hydrochloric acid and the strong base sodium hydroxide. The pH scale will never change, since it is solely based on solvents in water between HCl and NaOH. pH could be considered as a basis for measuring acidities, or as a control to a group of experiments.

Anything beyond pH of 0-14, with the substance being dissolved in water, is considered either a superacid or superbase. Since sulfuric acid (also hydrochloric acid) is considered pH=0, the basis of a superacid is sulfuric acid (also hydrochloric acid); since sodium hydroxide is considered pH=14, the basis of a superbase is sodium hydroxide.

Examples of a superacid are trifluoromethanesulfonic acid (CF₃SO₃H) or fluorosulfonic acid (FSO₃H), while examples of a superbase are organolithium or lithium nitride (Li₃N).

[Borek]

pH is based on a scale of 1 mol dm^-3 solutions (solvents specifically in water) between the strong acid hydrochloric acid and the strong base sodium hydroxide. The pH scale will never change, since it is solely based on solvents in water between HCl and NaOH. pH could be considered as a basis for measuring acidities, or as a control to a group of experiments.

No.

Anything beyond pH of 0-14, with the substance being dissolved in water, is considered either a superacid or superbase. Since sulfuric acid (also hydrochloric acid) is considered pH=0, the basis of a superacid is sulfuric acid (also hydrochloric acid); since sodium hydroxide is considered pH=14, the basis of a superbase is sodium hydroxide.

No.

[BetaAmyloid]

Here Borek, lets use reading comprehension.

A definition of pH from wikipedia:

Pure water has a pH around 7; the exact values depends on the temperature. When an acid is dissolved in water the pH will be less than 7 and when a base, or alkali is dissolved in water the pH will be greater than 7. A solution of a strong acid, such as hydrochloric acid, at concentration 1 mol dm−3 has a pH of 0. A solution of a strong alkali, such as sodium hydroxide, at concentration 1 mol dm−3 has a pH of 14. Thus, measured pH values will mostly lie in the range 0 to 14.

Therefore, pH is based on a scale of 1 mol dm_-3 with water. It is rated between hydrochloric acid to sodium hydroxide, or 0-14 on the normal pH scale.

A definition of superbase from wikipedia:

In chemistry, a superbase is an extremely strong base. Although there is no official definition for the designation of a superbase, sodium hydroxide is considered a benchmark, much like sulfuric acid is used to define superacids.

If sodium hydroxide is considered a benchmark, and sodium hydroxide has a pH=14, then anything beyond pH=14 is a superbase.

A definition of superacid from wikipedia:

A superacid is an acid with an acidity greater than that of 100% pure sulfuric acid...

If sulfuric acid is considered a benchmark, and sulfuric acid has a pH=0, then anything beyond pH=0 is a superacid.

If I'm not right, tell me why I'm wrong. Until then, UTFSE.

[Borek]

Therefore, pH is based on a scale of 1 mol dm_-3 with water.

Sorry, I am unable to understand what you have just wrote.

It is rated between hydrochloric acid to sodium hydroxide, or 0-14 on the normal pH scale.

No. These are just given as examples, they are not part of the scale. Besides, pH of 1 M HCl is not 0 and pH of 1 M NaOH is not 14. Measured values are close, but different by tenths of pH unit, because of the high ionic strength of the solution.

A definition of superbase from wikipedia:

In chemistry, a superbase is an extremely strong base. Although there is no official definition for the designation of a superbase, sodium hydroxide is considered a benchmark, much like sulfuric acid is used to define superacids.

If sodium hydroxide is considered a benchmark, and sodium hydroxide has a pH=14, then anything beyond pH=14 is a superbase.

Not necesarilly. Diluted solution of superbase can have pH 8. Could be your initial remark, the one that did not make sense to me, meant that you think that pH of 1 mol/dm³ solutions is in any way special, and that it can be used to check if the substance can be classified as superbase or superacid. That's not true either, as superacids and superbases don't even have to be soluble in water.

[BetaAmyloid]

1. You know I meant 1 mol dm^-3.

2. And haha? ha? Different by tenths? They are close enough, this is what they base it on via wikipedia.

3. Duh..I never said anything about dilutions. I said that a superacid is a pH under 0 and a superbase is a pH over 14, obviously if they were diluted their pH would change. Nor did I say that 1 mol dm^-3 was special, I stated that a base for the pH scale is rated at 1 mol dm^-3.

4. I said compared to this pH scale it would have to be soluble in water, obviously a different solute would classify a substance into a different pH scale with different ratings, etc.

[SigmaBond]

@Borek, he didn't mean that, he was correct.

@Cancer Curer, you are correct...+1.

 

[Borek]

1. You know I meant 1 mol dm^-3.

I know you meant 1 mol/dm³, but it still doesn't make sense. Concentration of the substance is not part of the pH definition. Concentration of H⁺ (more precisely activity of H⁺) is.

I said that a superacid is a pH under 0 and a superbase is a pH over 14

pH is not something that can be atrtributed to the substance. It is something that can be attributed to the solution of the substance. You can't state "acid is a pH" you may say "acid solution of concentration x has a pH y". To compare strength of teh acids you can use their dissociation constants.

pH of the solution of the acid of known concentration can't be part of testing of the acid strength - pH of all acids strong enough will be identical. To check which one is stronger you have to use other solvents, like glacial acetic acid.