Chemical Forums

Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: rolnor on July 02, 2019, 08:49:03 AM

Title: PKa of hydrazine
Post by: rolnor on July 02, 2019, 08:49:03 AM

Does anyone of you guys/gals have the second pKa for hydrazine? I can not find it.
FOUND IT! pKa1 is 8,0 and pKa2 is -1,0

Title: Re: PKa of hydrazine
Post by: AWK on July 02, 2019, 11:10:21 AM

These are correct values

N2H4, pkb1 = 6.07 and Pkb2 = 15.05

Title: Re: PKa of hydrazine
Post by: rolnor on July 02, 2019, 12:40:55 PM

Why are my valeus incorrect? They are from German wikipedia, ref. 2 and 3.
https://de.m.wikipedia.org/wiki/Hydrazin

The pKa2 is from Handbook. pKa1 is from another source from 2003.

Title: Re: PKa of hydrazine
Post by: AWK on July 02, 2019, 01:02:28 PM

I did not say your values are incorrect. I show you only more precise pKb values for comparison.

Title: Re: PKa of hydrazine
Post by: wildfyr on July 03, 2019, 08:55:45 AM

One is pKa other is pKb

Title: Re: PKa of hydrazine
Post by: rolnor on July 03, 2019, 09:14:07 AM

I dont follow, why are we diskussing pKb?

Title: Re: PKa of hydrazine
Post by: Babcock_Hall on July 03, 2019, 12:03:19 PM

The pK_b values can be converted into pK_a values.  I found 8.07 using Williams' pK_a table.  Hinman, R.L. J. Org. Chem. 1958, 23, 1587.

Title: Re: PKa of hydrazine
Post by: rolnor on July 03, 2019, 02:00:57 PM

Is pKa 14-pKb? Its a bit surprising that the values are different, pKa is not difficult to meassure?
Here is one more reference giving pKa1 as 8.10
https://pubs.acs.org/doi/abs/10.1021/ja01577a030
Its important to get it right even if my correlation is very strong regardless if its 8.0 or 8.10 or 8.07

Title: Re: PKa of hydrazine
Post by: pgk on July 03, 2019, 02:30:20 PM

Kw = KaKb  →  Kb = Kw/Ka  →  logKb = log(Kw/Ka)  →  logKb = logKw - logKa  →  -logKb = -logKw + logKa  →  pKb = 14 - pKa  →  pKa = 14 - pKb

Title: Re: PKa of hydrazine
Post by: pgk on July 03, 2019, 03:14:49 PM

Do not worry about slight variations of pKa and pKb values that figure in the literature because this is not so important.
Besides, measurement of these values for strong acids and bases, is not convenient because the Henderson-Hasselbach equation is not very accurate in this case.
In case of multibasic acids and polyamines, the Henderson-Hasselbach is not applicable at all and thus, pKa and pKb are indirectly measured by other methods, e.g. monoesterification, selective monoketimination, selective monoamidation or calculative methods, e.g. Hammet equation.