[mithrilhack]

Does ethanol have a pH? Wikipedia states it has a pKa of 15.9...what does that mean?

[Mitch]

Well, pH can change by either adding more H+ in solution or taking some out.

[DrCMS]

Ethanol does not have a pH.  

As pH by its very definition is the log of the hydrogen ion concentration in aqueous solution.  If you don't have water you dont have a pH.  You can stick a pH probe in pure ethanol and get a value on the meter but that's not its pH.

[xiankai]

checking the website where u derived the pK_a from, a notation comes with it to "(H+ from OH group)"

it seems that ethanol has H+... in a wierd way?

[mithrilhack]

Well actually I want to know if ethanol mixed with water would change the pH of the water.

[lemonoman]

Not sure if this will help or not but...

When ANYTHING is put into water, there will be an 'equilibrium'  involved.  Water is called the 'universal solvent' for this reason (even though some things seem NOT to dissolve, they do, to a very small degree)

Anyways, when ethanol (C₂H₅OH) is put into water, a few different equilibrium could occur:

C₂H₅OH + H₂O <==> C₂H₅OH₂⁺ + OH^-

which 'produces' a hydroxide (the easiest way to visualize basicity)

Or, possibly:

C₂H₅OH + H₂O <==> C₂H₅O^- + H₃O⁺

where H₃O⁺ is H₂O with the classic H⁺, that you mentioned earlier, attached to it.

Technically speaking, both of these equilibria occur to some degree.  One will occur more than the other - and so one of OH^- or H⁺ will be more plentiful in the solution (as opposed to a perfectly neutral solution, where the exact same amount of each is present).  If there is more OH^-, then the pH will be basic (and the pH value will tell you exactly how much OH^- there is) - on the other hand, if the pH is acidic then there is more H⁺ and you can figure the math of that out too, if you like.

[Mitch]

C₂H₅OH + H₂O <==> C₂H₅O^- + H₃O⁺

That's where knowing the pKa comes in. Ka is equal to the above quoted equilibrium. So you tell us the extent of disocciation of ethanol in aqueous solution. From there we'll move on determining the pH with different mol amounts of ethanol.

[AWK]

pH of diluted ethanol solutions in water is practically the same as that of water. It can be calculated from two equilibriums - Kw (ion product for water) and Ka.

[Borek]

http://www.chembuddy.com/?left=pH-calculation&right=pH-weak-acid-base

[TakeItEasy]

Ethanol does not have a pH.  

As pH by its very definition is the log of the hydrogen ion concentration in aqueous solution.  If you don't have water you dont have a pH.  You can stick a pH probe in pure ethanol and get a value on the meter but that's not its pH.

:oif that's not the pH of ethanol, then what is it?

[DrCMS]

Thats the point the meter will give a reading but by definition in a non-aqueous system the number can NOT be pH.  To be a true pH reading you must have water.

[Dude]

Come to think of it, I don't know the answer to Quark's question.  pH is operationally defined as a cell potential generated by a certain amount of H+ in an aqueous buffer solution upon a glass electrode.  If one places the electrodes in ethanol or hexane, a value comes out.  One can logically argue based upon activity that the value isn't referable to the aqueous buffer solutions that were used to calibrate the instrument.  However, what creates the potential to cause a value to register?  Now I'm confused as to why I really can't divide by 0 other than 20 years of educators telling me that it is undefined.

[Borek]

why I really can't divide by 0 other than 20 years of educators telling me that it is undefined.

For division:

if x/y=a then ay=x

but it is not satisfied for y=0.

From what I know pH scale can be generalized for use in other solvents than water, thus it seems for me ethanol has a pH - and the neutral one of 9.45, as it autodissociates just like water does (but the ethanol ionic product is - at 10^-18.9 - substantially smaller then Kw).

[X]

If anyone still cares, when measuring pH of ethanol, pHe is measured. It is an apparent pH with special equipment and circumstances, since to have a pH, an aqueous solution is necessary.

[Medic851]

Of course ethanol has a pH

Everything organic has a pH and you could probably argue that inorganic compounds have a pH depending on the definition of an acid (bronsted lowry, lewis etc..)

If Wikipedia is correct in stating that the pKa is 15.9 than that value is related to pH by the negative logarithm of pKa.

So.... pH = (-log)pKa---------- if you use the inverse log function on your calculator which is 10^x power times the pKa 15.9 you should derive a number around 7.9 which represents the pH. So Ethanol is slightly basic which is consistent with the idea that that OH (hydroxide ion) the characteristic feature of alcohols is present in solution.