Talk:PH

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  • When was the scale created?
  • Explain why neutral pH rises with temperature, not just state that it happens.
  • Explain what the Pitzer model is (not done anywhere in the article).
  • What does epsilon stand for in the last equation of the Definitions section? Why are the last three equations even relevant?
  • Add TeX formatting for the Weak/Strong acid section.
  • Expand the indicators section. Titoxd(?!? - cool stuff) 23:55, 26 March 2007 (UTC)
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[edit] parts Hydrogen (pH)

I added "parts hydrogen" to clearly define that pH is a direct measurement of the concentration (parts) of hydrogen ions in a liquid (compared to distilled water). --Lperez2029 13:09, 15 December 2006 (UTC)

It isn't a direct measurment of hydrogen ion concentration. It is definitely a logarithmic scale, not linear. The only other problem being that if by distilled water you really mean pure water, then your explanation will yield a 'divide by zero' error (or efectively a pH of infinity). Pure water contains no hydrogen ions (or any ions at all as no dissociation takes place). By strict definition, pure water cannot have a pH value. It is not even posible to measure the pH of pure water, because, as I noted above, the measurement process itself introduces all of the measurable ions, completely invalidating any result.
The article already contains several theories as to what pH stands for, none of which remotely resemble 'parts hydrogen', and it wouldn't anyway as it is a logarithmic scale, so something involving 'powers' is clearly closer. I B Wright 12:46, 16 December 2006 (UTC)

pH is the symbol for the logarithm of the reciprocal of hydrogen 'ion concentration' in gram atoms (parts) per liter. For example, a pH of 5 indicates a concentration of .00001 or (10 to the power of 5) gram atoms of hydrogen atoms in one liter of solution. Therefore, pH measurement may be expressed as parts (ion potential) hydrogen, or pH (potential hydrogen), either way it is acceptable in definition terms, but "parts" hydrogen is more correct (as would the expression of ppm (parts per million). No edit wars - I'll bring some top chemists into the discussion, and we will see what is more correct for pH definition for the benefit of all, once and for all. Best regards --Lperez2029 00:39, 17 December 2006 (UTC)

I did a bit of internet research to come up with that last paragraph. Instead of getting "top chemists" into the discussion, I'd recommend you ask them for existing scientific, peer-reviewed literature that discusses the nomenclature of pH. My survey is deficient in the fact that it does not consult traditional print or academic sources, but if you can find reputable research that says otherwise, I'd have no problem including it. — Edward Z. Yang(Talk) 03:35, 17 December 2006 (UTC)

pH literally means negative the log of the hydrogen ion 'activity' in solution; it is commonly written -log[H+] meaning -log of the hydrogen ion concentration. In the common solvent water, a bare hydrogen ion is solvated by the water forming the hydronium ion [H(H2O)]+, so pH is also commonly written -log[H3O+]. For example, pH=7.0 means that the positively charged hydrogen ion concentration in solution is 1x10-7 moles H+/liter, since -log(1x10-7) = 7. Normally, pH would therefore be close in meaning to parts hydrogen (as in ppm) with the understanding that hydrogen means positively charged hydrogen ions in solution; whether the concentration is reported as pH, ppm, g H+/liter, wt%, etc., - moles H+/liter, ie concentration, is the basic chemical term.

There are some fine points here. The 'activity' or 'effective concentration' of H+ is what's measured by a pH meter electrochemically. In dilute solution, the 'activity' and actual concentration of H+ are indistinguishable; activity = concentration. As concentration increases, relative 'activity' decreases due to a chemical thermodynamics concept called chemical potential. Activity < concentration, and concentration H+ is undermeasured by the pH meter because the 'activity' is lessened due to this thermodynamic ionic shielding factor. (The actual H+ concentration could be measured by other means such as titration). So, in this context, one might say that the 'p' has to do with the potential H+ concentration (which is activity) which is less than the actual H+ concentration.

Water does inherently dissociate. Pure water, under 'standard' conditions, dissociates from H2O into hydrated H+ and OH- to the extent of 1x10-7 moles/liter, also called pH 7 or neutral pH, the pH where there is no excess of acid (H+) or base (OH-). Regardless of the early 1900's origins of the use of p in pH, consistent with the meaning of 'p' in descriptions of other chemical equilibria, such as pK, the p of pH nowadays is taken to mean -log.

Now, let the statement speak for itself - as for adding 'parts', I'll leave that up to you gentlemen. Best regards --Lperez2029 22:17, 18 December 2006 (UTC)

You are absolutely correct, and this is already discussed in the article. We have to tread a very fine line between giving correct information and giving "usually correct" information (but more commonly used). If you feel you can clarify the article in this respect, please do so. Personally, I get the feeling that the opening paragraph is a little convoluted.
As for "parts Hydrogen", I would refrain for now. A quick Google for "Parts Hydrogen" reveals that the term is used mostly as a kind of more generalized mole. — Edward Z. Yang(Talk) 21:28, 21 December 2006 (UTC)

Thanks Edward. --Lperez2029 23:17, 21 December 2006 (UTC)

The opening paragraph is a bit convoluted (see Intro below) and needs attention, as someone already asked a very good question, "is there a separate test to measure alkalinity?" Go ahead Ed, refine the article, I'll back you up. --Lperez2029 13:32, 22 December 2006 (UTC)

[edit] Intro

"pH is a measure of the acidity of a solution in terms of activity of hydrogen (H+). "

Wait but pH also measures the alkilinity of a substance. Is there a seperate test for this? Simply south 15:34, 21 December 2006 (UTC)

pH is also the electrochemical measurement of the Alkalinity of a solution. Alkalinity is closely related to the acid neutralizing capacity (ANC) of a solution. --Lperez2029 18:00, 21 December 2006 (UTC)

To more specifically answer your question; no, there is no separate test necessary to measure alkalinity in a solution, the test may be performed utilising a pH meter or litmus paper. The introduction on the article should read more along the lines pH is a measure of the acidity or alkalinity of a solution in terms of activity of hydrogen (parts Hydrogen). For example; 4= Acidity - - 7pH=pure water - - 12=Alkalinity (or base) --Lperez2029 13:20, 22 December 2006 (UTC)

As has been noted elsewhere, such an introduction would be incorrect as pure water does not have a pH value of 7, or any pH value, as it contains no ions. I B Wright 11:32, 12 January 2007 (UTC)

Alkalinity is not the opposite of acidity. Alkalinity is a measure of the buffering capacity against the addition of H+. And yes, there is a separate test for this (titration). pH is a measure of the H+ activity. period. From this 'basicity' can be determined, if you know the pKw (ie pOH = pKw - pH)(the pKw shouldn't be assumed to be 14 - it is rare to have standard conditions), but there is no way to determine alkalinity from pH as you need to take into account a wide variety of other ions that contribute to alkalinity (ie At = OH- + HCO3- + CO3-- + B(OH)3 + ...). It is likely that there is a way to measure basicity ( ie [OH-]), though it is presumably much easier to measure acidity ([H+]). Piyrwq 16:43, 12 January 2007 (UTC)

Anybody else have a problem with this statement: "strictly speaking, there is no such thing as the H+ ion"? There certainly is such a thing as the H+ ion. It is called a proton and exists discretely in the in the state of matter known as plasma if nowhere else. If nobody objects, I would like to change this statement to read: "in aqueous solution the H+ ion does not exist independently but is actually the third hydrogen atom in H3O+, called the hydronium ion." Thoughts? Objections? Schlemazl 21:53, 13 February 2007 (UTC)

Defintitely an improvement; maybe a little absolute though, there is probably one free aqueous H+ out there somewhere. Maybe something like "in aqueous solutions H+ ions bind to water molecules forming hydronium ions, or H3O+." Piyrwq 02:41, 14 February 2007 (UTC)
Good point. I'm pretty sure there is a free H+ out there because I lost mine the other day while I was bathing. I got out of the tub, dried myself off, and dismantled the drain to see if it got caught in the P-trap. No luck. If anybody finds a free aqueous H+ out there please hold on to it until you can e-mail me a picture of it so I can try to determine wheter it is mine or not. If nobody finds it in another day or so, I'll go ahead and make the change as suggested by Piyrwq. Otherwise I think we should use my wording unless anybody else can provide evidence of a known free H+ in aqueous solution.Schlemazl 18:20, 14 February 2007 (UTC)

Also, I'm being driven a little crazy by the addition of "basicity" or "alkalinity" to the definition of pH. Knowing [H3O+] does not mean you know [OH-] (basicity). If you have the literature resources at hand, measured temperature, and know the specific nature of other ions in solution you may be able to calculate basicity from the pH, but one does not define the other. It definitely does not give any insight into alkalinity. For example, it is possible to reduce pH of a solution, through the addition of CO2, while alkalinity remains constant Alkalinity#Addition_of_CO2- alkalinity is absolutely not defined by the log of hydrogen concentration, nor is basicity. These are concepts worthy of much discussion, but are not fitting for the definition of pH. Can we come to some kinda consensus, so that references to basicity and alkalinity can be permanently removed from the definition of pH? Piyrwq 02:41, 14 February 2007 (UTC)

[edit] Rewritten

Ok, I have rewritten the whole first paragraph - I hope it is an improvement, but no doubt there is still a lot to be changed. For reference, here is the old version, which I felt was over complicated and too wordy.


pH is a measure of the acidity and the basicity/alkalinity of a solution in terms of activity of hydrogen (H+). In aqueous solutions H+ ions bind to water molecules forming hydronium ions, or H3O+. For dilute solutions, however, it is convenient to substitute the activity of the hydrogen ions with the concentration, molarity (mol/L) of the hydrogen ions (however, this is not necessarily accurate at higher concentrations [1][2]).

In aqueous systems, the hydrogen ion activity is dictated by the dissociation constant of water (Kw = 1.011 × 10−14 M2 at 25 °C) and interactions with other ions in solution. Due to this dissociation constant, a neutral solution (hydrogen ion activity equals hydroxide ion activity) has a pH of approximately 7. Aqueous solutions with pH values lower than 7 are considered acidic, while pH values higher than 7 are considered basic.

The concept was introduced by S.P.L. Sørensen in 1909, and is purported to mean pondus hydrogenii in Latin.[3] However, most other sources attribute the name to the French term pouvoir hydrogène[4][5][6]. In English, pH can stand for "hydrogen power,"[4][5][6] "power of hydrogen,"[7][8] or "potential of hydrogen."[3][9][10] All of these terms are technically correct.

Conrad.Irwin 10:10, 21 February 2007 (UTC)

I like your new intro a lot, but I did make some copyedits to it. I hope you find them satisfactory. A few questions though:
  • The original introduction purported that it was impossible for water to truly have a pH of 7. I believe this assertion is correct
  • Could you source your mathematical derivation of the symbol pH? It sounds very plausible, but I'd sleep better if it was sourced.
Thanks for stepping up to the plate! — Edward Z. Yang(Talk) 21:43, 21 February 2007 (UTC)
The new introduction is good on readability, but there are some inaccuracies. First, 7 is not "defined" as neutral, and as Edward Z. Yang says, pure water doesn't have a pH of exactly 7 (maybe it does at some temperature, but not at 25 deg C). Also, although this is an omission of the entire and not just the intro, the pH concept applies as well for solvents other than water, with their own dissociation constants and different "neutral" pH values and different pH ranges. Then, the statement "Because pH is dependent activity, a property which cannot be measured easily or predicted theoretically, it is impossible to give an accurate value for the pH of a solution" is vacuous, because it depends on the degree of accuracy desired. On one extreme, it is also impossible to measure the length of a sheet of paper accurately, but on the other, it is certainly possible to measure pH with a certain degree of accuracy such as +/- 0.01 pH units (note: I'm not an expert on pH measurement so I don't know what is the maximum accuracy attainable in practice). pH is measured every day with no worries, so to imply in the introduction that it is almost impossible to measure is too deprecating IMO. Finally, saying that "the H should more correctly be [H+], standing for concentration of hydrogen ions" is a POV, and seems like a contradiction to the earlier definition that uses activity instead of concentration. --Itub 09:05, 22 February 2007 (UTC)

[edit] Contradictory statement within the article

The article states: Most substances have a pH in the range 0 to 14, although extremely acidic or extremely basic substances may have pH less than 0 or greater than 14. An example is acid mine runoff, with a pH = -3.6. Note that this does not translate to a molar concentration of 3981 M.

But it also states that pH is, strictly speaking, a function of the activity of hydrogen ions in solution, not their concentration. And that is acknowledged by references. Now, a careful look at Wikipedia's article on Activity, or preferably to a Physical Chemistry book, will lead you to see that Activity ranges from ZERO to ONE, and is never higher than one, because pure substances have an activity of 1 by definition (it was meant to be analogued to molar fraction, and molar fraction can never be greater than 1 either).

Now pick a calculator and verify the value of -log(1), the result is 0, and that is the lowest pH a substance can ever have. Strictly speaking, pH can never be lower than zero, except when calculated in a less rigurous way, using molarity or molality: but that is incorrect, and should be pointed out. pH lower than zero may be obtained by other ways, maybe (pH-meter?), but it should be clearly specified how was that value obtained, what does it mean (because if it has no meaning, is serves no purpose and shouldn't be used), and emphasis should be put on that strictly speaking, the pH of a solution can never be lower than 0 (because activity ranges from 0 to 1, and therefore -log(act) ranges from 0 to positive infinity). I'll wait for answers regarding this before modifying the article myself. 158.170.51.28 14:50, 27 December 2006 (UTC)

activity expressed in mole fraction ranges from 0 to 1. Activity can be expressed using molarity (as with pH) or molality and can exceed 1. Chemist207.8.130.130 18:27, 28 December 2006 (UTC)
You are confusing the activity coefficient γ, for which 0<γ<1 and α=γ*C where α is the activity and C the concentration in mole/L. You can have α>1 and thus pH<0. Still, I don't understand why pH= -3.6 is not translated to concentration (even effective concentration). How is it measured? and if it doesn't represent the concentration then what does it represent?
Also you can't have a concentration of H+ in an aquatic solution higher than that of water (55.5 M) so you can't even reach pH= -2.0, let alone pH= -3.6. What's the nature of the acid mine runoff? Anyway I don't know of any solution which can reach a density so high that would allow C>100 mole/L. As it is, this example is confusing instead of enlightning.--84.254.22.153 17:42, 14 January 2007 (UTC)
I have looked into the matter and found out that γ can be >1. In fact these values are so high exactly because γ is extremely high and not concentration. I am editing the page accordingly including the reference. Since I am new at this maybe someone can format my edit so that it looks better/it is linked to other parts of wikipedia.--84.254.20.189 17:39, 27 February 2007 (UTC)

[edit] Puissance d'Hydrogène

Was added by User:69.140.68.72 and removed by me. A Google search brings up 20 hits. The top four English resources (disregarding Wikipedia's status as first):

— Edward Z. Yang(Talk) 15:03, 19 February 2007 (UTC)

[edit] Images

This article needs a nice image to plaster on the top of the document. One can't really photograph pH, though, so we've got to pick something fairly representative. Candidates:

  • Image of the pH scale
  • Image of a pH indicator

Possible candidates? (add ones you find to the gallery below) — Edward Z. Yang(Talk) 21:53, 21 February 2007 (UTC)



[edit] This page has been vandelised somewhat

Not experienced with this, but i cant seem to delete the vanelism :( —The preceding unsigned comment was added by 129.234.4.76 (talk) 14:50, 24 February 2007 (UTC).

[edit] Semiprotection

I propose we semi-permanently semi-protect this article. The amount of vandalism it receives is absolutely ridiculous. — Edward Z. Yang(Talk) 22:36, 1 March 2007 (UTC)

Semiprotected. Please yell at me if you disagree. — Edward Z. Yang(Talk) 17:54, 16 March 2007 (UTC)

[edit] biological values?

I think it would be interesting to have some biological Ph values for comparison. For example the stomach. Also according to de.wikipedia wine has Ph 4, there is a hole there in the english table. --172.174.140.43 07:47, 4 March 2007 (UTC)

[edit] pH-imbalance

I actually wanted to look at the page "pH-imbalance", meaning in the body, but I just got redirected to pH.

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