Talk:PH

From Academic Kids

We have a fine article on PH here-- but it doesn't say what the P and the H stand for!

AFAIK there is also pOH, in a sense the opposite of pH, which measures the concentration of OH- ions

Contents

1 pH 2 Title 3 Range 4 I hope it will finish discussion about possibility/impossibility 5 CO2 5.1 answer to last comment 6 'Neutral' substance 7 Cancer patients

pH

What is the pH of 4% Hydrochloric acid?

I doubt Wikipedia is the right place to ask that question. You might try in the newsgroup sci.chem... -- Schnee 17:48, 21 Sep 2003 (UTC)

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I modified the pH calculation from Ka so that it doesn't assume that the concentration of the original acid stays constant. I don't know which method is more widely used, but we should prefer a more accurate one. --Yath 09:28, 3 May 2004 (UTC)

Title

Is it possible to name the article 'pH' as opposed to 'PH'?

No, it's a limitation of MediaWiki as it's being used here. It's probably the most annoying of the ones on Wikipedia:List_of_pages_whose_correct_title_is_not_allowed_by_MediaWiki. -- User:Docu

Range

"In layman's terms , the "pH" value is an approximate number between 0 and 14..."

I'm pretty sure that the pH can fall outside the range 0 to 14. Maybe they meant that it usually falls in this range. I've worked with 18 M sulfuric acid, and I'm pretty sure that stuff had a pH < 0. Maybe someone could correct this.

I'm quite sure that pH cannot exceed this range. 0 means it has no hydrogen ion (H⁺) concentration, and only hydroxide ion (OH^-) concentration, which cannot be possible. That's as it says in a college biology textbook. See http://www.google.com/search?hl=en&lr=&client=firefox-a&rls=org.mozilla:en-US:official&oi=defmore&q=define:ph for more websites on this topic. --Bsdlogical 17:54, 11 Dec 2004 (UTC)

The pH scale is a logarithmical scale, so sub-zero values are neither impossible nor strange, just unusual. —Caesar 23:35, 2004 Dec 15 (UTC)

Nonsense. Expression -log(c) is negative just for c>1. And c is the H₃O⁺ concentration which evidently cannot exceed 100%, therefore pH cannot be negative. And pH>14 is nonsense for the same reason, since the product of H₃O⁺ and OH^- concentrations in aqueous solution is constant — 10^-14. So pH>14 would indicate the concentration of OH^- greater than 100%. The pH>14 can be achieved only in other than aqueous solutions, where the constant is different from 10^-14. Negative pH definitely doesn't exist.

-- Egg 22:42, 2004 Dec 16 (UTC)

Oh, I'm sorry. My basic presumption that c cannot be >1 is wrong, so the pH eventualy could be out of the 0-14 range. But in fact the definition of pH uses ion activity instead of concentration and that is k times concentration, where k<1. And the ions become inactive in high concentrations, therefore pH really doesn't exceed nor 0 nor 14.

-- Egg 00:47, 2004 Dec 17 (UTC)

But don't forget, that concentration is not percentual, but molar. The "concentration of water in water" therefore is not 1, but approx. 56 mol.dm^-3. If we imagine hypothetic situation, that we have the 1:1 mixture of H₃O⁺ cations and X^- anions of the same molar weight, we get pH = -log 28 = -1.4 . I'm not taking into account activity, I just wanted to show that the limitation is not related to K_w. —Mykhal 01:39, 20 Dec 2004 (UTC)

I hope it will finish discussion about possibility/impossibility

pH - is just a function of activity of hydrogen ions, pH= -1 means that you have activity of hydrogen ions equal 10 mol per liter. It is definitely POSSIBLE if you work with high concentrations of acid solutions like H2SO4. pH=15 means that you have activity of OH-ions equal 10 mol/l (or if you want to recalculate into hydrogen ions activities for aqueous (!) solution = 10^-15 mol/l). It is also definitly POSSIBLE when you work with high contration alkali solutions like NaOH or KOH. DO NOT use percent concentration, ONLY molarity. Please, stop this funny descussions about possibility - impossibility. (Kaverin)

CO₂

I've read, here and there, mainly on equipment for pH balancing, stuff that suggests that there's a relationship between pH and CO₂.

Anyone got a minute to explain why higher CO₂ should lower pH? Should an over-planted aquarium see a really high pH?

answer to last comment

look at hydrolisys CO₂ + H20 -> H⁺ + HCO₃^- (Kaverin)

Degrading plants will generate CO₂ and H⁺, this will serve to lower the pH. Thriving plants(ie photosynthesizing) will consume CO₂ and H⁺, thus raising your pH. Net results depends how well your plants are doing I'd think. (Piyrwq)

'Neutral' substance

Is the statement that "the only neutral substance is distilled water" correct? If neutrality is simply pH=7, then it's trivial to create a "neutral" solution that isn't distilled water. For example, add baking soda to water to raise the pH. Then, add just enough vinegar to make the pH=7. The water is neutral (pH=7), but isn't distilled, either. Perhaps we are confusing "neutral" with "pure?"

I think this is important to clarify, especially since the article calls this out as being "important to note."

...Incidentally, pure distilled water, when exposed to the atmosphere, has a pH of 5.7, due to carbon dioxide ingassing.

Agree on both points. Note removed. Vsmith 00:47, 27 Feb 2005 (UTC)

Cancer patients

The table claims that cancer patients have a significantly lowered saliva pH. I'm sure this is only true for a few types of cancers, if at all. Do we have a reference? AxelBoldt 20:53, 26 May 2005 (UTC)