I found the explanations of what the pKa is somewhat confusing. I would like to suggest the definition as --

At the pH of a solution in which the concentrations of the acid and base are equal, the pKa=0.

I don't know if this has been written somewhere that I haven't seen, but what I saw said that the pKa is the pH of a solution in which the acid and base concentrations are equal.

The pKa is the acid dissociation constant and it is defined by a formula. The explanation that you propose may apply to some, but perhaps not all instances of pKa.

If you still want to change the definition, you'd best contact the iupac and the iso directly, they're probably not actively reading this forum.

Here's what I tell my OChem students:

Instead of getting bogged down in the formulas and math (which can be intimidating as Hell), try and get a sense of what the 'story' is. That will at least give you a sense of what you're looking at, and when you have that conceptual understanding, the technical stuff can come later. If you mentally define pKa as some obnoxious rational function full of [A]s, logarithms, and [H+]s, you don't really know what you're looking at.

Pka is how strongly a molecule (or functional group) holds onto a proton. Some functional groups (like carboxylic acid) are pretty existentially okay with having the protonated oxygen, and so it takes a comparatively stronger (or "needier" ) base for them to give it up. Other acids (like sulfuric acid) is much happier in it's conjugate base form, and so it's more likely to donate it's proton.

Molecules are like communists - they give what they have to others that need it more.

If you have a molecule A that has a proton, but is also fairly okay without it, and you introduce it to another molecule that REALLY REALLY wants a proton (molecule B), molecule A will donate it's proton to molecule B.

Pka is the mathematical formulation of this sort of 'molecular happiness'.

Blessings
~ND

It is all about equilibrium. pKa tells you where the equilibrium sits, far to the left or far to the right.

pKa is just logarithmic notation of Ka. p essentially just stands for -log(something) Concentration varies on such a large scale it is useful to use logarithm

So you could do it for anything, like temperature, if you wanted. 5000*C is the same as -3.70p*C
-log(5000)=-3.7

logs don't change too much when changing the order of magnitude, so its a good way to scale large numbers and very very small numbers so a human can make sense of them. (-log(5)=0.69)


So what an acid does in water, is donate a proton by dissociation. HA gives up a proton to give H+

Ka is just the ratio of initial acid (HA) to proton and conjugate base (H+ and A-

So if 30% of the acid gives away a proton, you have a ratio of 30%/70% (product over reactant)

HA --> H+ and A-

this is a 1 to 1 to 1 ratio (ie, 1 unit of HA makes 1 unit of H+ and 1 unit of A-)

Ka is ratio of dissociation products (H+ and A-) over the acid molecule

Ka = [H+][A-]/[HA]

since you make 2 products from 1 reactant, that has to be factored into the equilibrium.
lets say we have a concentration of 1mol/L of acid.

Ka= [0.3][0.3]/[0.7] = 0.1286

pKa = -log(0.1286) = 0.89

this is actually pretty strongly acidic.

for reference, acetic acid (vinegar) has a pKa of 4.7.

a 1M solution of acetic acid, only 0.4% will dissociate to form protons, so 99.6% remains unchanged

This is from the DMT-Nexus Wiki called "PKa":

"The pKa is the pH at which 50% of the alkaloid is in salt form and 50% is freebased."
https://wiki.dmt-nexus.me/PKa

Thanks for your replies. I already had an understanding of what the pKa is however.

Your issue might have been more clear if you had pointed out that it was the definition on the nexus wiki you want to change.

However, stating that the pKa would equal 0 in some specific condition simply makes no sense, whatever the condition.

I read the nexus wiki as I was finding out what "pKa" meant and was confused for awhile until I researched other sites. I thought that experienced nexus users would know I was talking about changing the nexus wiki (and not that I was suggesting changing the accepted scientific definition of pKa). I realized that wasn't the case when I saw the replies.

I don't know who wrote that wiki, and I'm not good enough with chemistry to critique either of the definitions, but I just want to say that the wiki is open for any nexus member to edit. So feel free to edit anything you think can be better, and let others know so that it can be properly reviewed

Thanks. I'm not a member of the wiki group and when I tried to create an account it gave an error that it was unable to load the CAPTCHA (verifying you're not a machine) page and to contact an administrator. When I went to the link for "administrators" it said there was no such page.

weird, can you try again to register? maybe try a different browser just in case.. It should be working, otherwise we will talk to the traveler

It's great if you want to work on the wiki, but please don't change the pKa page to read "At the pH of a solution in which the concentrations of the acid and base are equal, the pKa=0", as you wrote earlier, because that is not a correct statement. IMHO it would not make the page more clear and instructive either, even if it were correct.

The page could be improved by adding some text about the pKa being an equilibrium constant, and explaining a bit more about equilibria in general, though I am not sure where it would all lead to - it is supposed to be a simple page about the pKa, not a general chemistry course.

It would also be helpful to make the discussion of salt solubility more clearly related to the equilibrium mechanism. In the same context, the Xlogp partition coefficent could also be mentioned, as it is yet an other factor contributing to the equilibrium in biphasic extractions.

Tried to create the account using internet explorer and the same thing happened. (I was originally trying using firefox).

The pKa tells you that a solution of a substance at certain values of pH, has x% of the substance in the acid (salt) form and y% in the basic (free) form. At pKa=0, there is a 50% to 50% ratio of acid to base forms.

For example, at a pH of 8.68, a solution of DMT has a pKa of 0 and 50% is in a salt form (acid) and 50% is in the free base form.

DMT does NOT have a pKa of 0 under any circumstances. Please review the chemistry theory underlying the pKa constant before editing the wiki to make it "more understandable".

This is from the nexus wiki:

So you're saying this is incorrect?

the keyword (or symbol) here is the '+' or '-' sign

If you notice in that graph the x-axis is actually pH - Pka, not pKa

the pka of DMT is 8.68, so at a pH of 8.68, it is in the center of the equilibrium, 50% protonated 50% deprotonated

at +1 pka, so pH 9.68, then 90% is deprotonated and 10$ is protonated, etc..


Here is the equation used the determine these ratios
pH = pKa + log(A-/HA)

I see mindlusion already posted an explanation. The pKa is a constant, but as the pH varies, so does the difference of the pKa and pH accordingly.

I think that some of the problem is that we're talking about the pKa of a buffered solution - So the conjugate base (A-) is DMT, and the acid is DMTHCl. Therefore the pKa is
log[[DMT-][HCl+]/DMTHCl].

If I understand correctly, when 50% (moles) is DMT and 50% (moles) is HCl (neutralizied by a base) the Ka [DMT][HCl]/DMTHCl = 1 and logKa = 0 and the pH is 8.68.

No, not exactly.

its 50% DMT to 50% DMT-H+, not HCl, when the pH has been adjusted to 8.68. (Assuming it will all be in solution, which it actually wont be.)

The Henderson-Hasselbalch equation [pH=pKa +log(A-/H)] is modified version of the the Ka equilibrium constant equation. But the doesn't make sense for a few reasons.

First, DMT never exists in a state with a negative charge, and neither does HCl ever have a positive charge. DMT is a base. The Ka constant equation is for DMT is correctly written as:

Ka= [DMT][H3O+]/[DMTH+]

Understand that pH is inherent into this equation THE->> [H3O+] and from this alone, if 50% of DMTH+ dissociates, it does not equal 1.

In the henderson-hasselbalch, the the log(H+) is taken out and becomes pH on the left hand side. It is used to quickly estimate the ratio of A- to HA for small tweaking of pH. As in when creating a buffer solution.

So log([A-]/[HA} does NOT equal Ka. Ka is equal log([H+]) + log([A-]/[HA]) , and log[H+] happens to be pH.

and as pitubo mentioned, DMT is a base, it would make more sense intuitively to calculate the pKb.
and it should be Kb[b]= [DMT-H+][OH-]/[DMT] , but in reality it is no more useful to the chemist than the pKa, so for the purposes of simplicity and standardization, the pKa is used regardless unless explicitly necessary.

Now I'm wondering what the Ka of a substance is at 50% dissociation. If Ka=[A-][H+]/AH then e.g. if you have 6 moles of acetic acid and it's 50% dissociated, then you would have 3 moles of H+ and 3 moles of CH3CO2. Wouldn't that mean 3x3/6?