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Is $\ce{LiOH}$ a weaker base than $\ce{NaOH}$?

Note: I'm not interested in "why," but rather what the "real" $\mathrm{p}K_ \mathrm{b}$ values are.

$$ \begin{array}{lcc} \text{Data for Alkali Metal Hydroxides} \\ \hline \text{Cation} & \mathrm{p}K_\mathrm{b} & \text{Solubility} & \text{Solubility} \\ \text{} & \text{} & \pu{g/100 ml} \text{ at } \pu{20 ^\circ C} & \pu{moles/L} \text{ at } \pu{20 ^\circ C} \\ \hline \ce{Li} & -0.36 & 12.8 & 5.34\\ \ce{Na} & 0.2 & 111 & 27.75\\ \ce{K} & 0.5 & 112 & 20.0 \\ \ce{Rb} & ? & 180 & 17.6\\ \ce{Cs} & -1.76 & 200? & 13.3?\\ \hline \end{array} $$

The $\mathrm{p}K_\mathrm{b}$ data is from Wikipedia ($\ce{Li}$, $\ce{Na}$, $\ce{K}$, and $\ce{Cs}$). Wikipedia doesn't have a value for $\ce{Rb}$ and I couldn't find one.

The solubility data is from the Wikipedia Solubility Table. Wikipedia doesn't have a solubility value for $\ce{CsOH}$ at $\pu{20 ^\circ C}$ and I couldn't find one, About $\pu{200 g}$ would be a reasonable guess, which would be $\pu{13.3M}$.

One point is that the Wikipedia article for lithium hydroxide says "It ... is the weakest base among the alkali metal hydroxides."

Another point is an earlier question "Why is $\ce{LiOH}$ a weaker base than $\ce{NaOH}$". User Lighthart references a paper "Ions in water: The microscopic structure of concentrated hydroxide solutions" by Imberti et al. but the abstract at least doesn't seem to address the strength of the alkali hydroxides directly (I don't have access to the paper itself since it is behind a paywall).

Is this one of the Bertrand Chord paradox problems where it depends on how we define "stronger"?

  • With a $\mathrm{p}K_\mathrm{b}$ of -0.36, $\ce{LiOH}$ would seem to be a stronger base than either $\ce{NaOH}$ or $\ce{KOH}$.

  • Certainly more moles of $\ce{NaOH}$ dissolve in a liter of solution at $\pu{20 ^\circ C}$ than any other alkali hydroxide.

EDIT 3/22/2016 11:31 pm

Found a different table of $\mathrm{pK}_\mathrm{b}$ data for the alkali hydroxides: Source-2

EDIT 3/23/2016 12:12 am
Source-3 CRC Handbook of Chemistry and Physics, 92nd edition, "Dissociation Constants of Inorganic Acids and Bases. Only $\ce{Li}$ $(\mathrm{p}K_\mathrm{a}= 13.8)$ and $\ce{Na}$ $(\mathrm{p}K_\mathrm{a}=14.8)$ are listed as $\mathrm{p}K_\mathrm{a}$ of the cations. It is the same data as in this pdf file which is from another edition.

$$ \begin{array}{lcc} \hline \text{Cation} & \text{Wikipedia} & \text{Source-2} & \text{CRC}\\ \hline \ce{Li} & -0.36 & 0.18 & 0.2 \\ \ce{Na} & 0.2 & -0.56 & -0.8 \\ \ce{K} & 0.5 & -1.1 & \\ \ce{Rb} & ? & -1.4 & \\ \ce{Cs} & -1.76 & -1.76 & \\ \hline \end{array} $$

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    $\begingroup$ "Hence, amongst the alkali metal ions, lithium salts are the most hydrolyzable salts and the base strength increases from LiOH (pKb = -0.7) to NaOH to KOH (-1.7) G.S. Manku "Theoretical Principles of Inorganic Chemistry" - Page 260 // books.google.com/… $\endgroup$
    – MaxW
    Commented Mar 23, 2016 at 4:58
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    $\begingroup$ Just a guess- Lithium cation is smaller than the sodium cation, so the size of LiOH must be smaller than NaOH. Hence, the electrons will be pulled strongly, and it will be harder for them to leave. Hence, it would be a weaker base. Also, Lithium compounds are largely covalent, which could again be a possible reason. $\endgroup$
    – GRrocks
    Commented Mar 23, 2016 at 5:00
  • $\begingroup$ What.... is going on here? $\endgroup$
    – hBy2Py
    Commented Mar 26, 2016 at 3:09
  • $\begingroup$ @Brian - I'm trying to figure if pKb of LiOH is really less than NaOH or not. Not "why" just "if". There seem to be conflicting values. I don't know what the "real" values are. $\endgroup$
    – MaxW
    Commented Mar 26, 2016 at 3:55
  • $\begingroup$ @MaxW So it's a research project, work in progress? "Research scratchpad" is not one of the typical use-cases for Chem.SE that I've seen thus far.... More I think about it, the more comfortable I'm getting with it, but it's...different. I would say: once you're satisfied, you should probably edit the question down, remove all but one answer, and edit that answer into a short review article. Or, perhaps convert the whole thing to community wiki? $\endgroup$
    – hBy2Py
    Commented Mar 26, 2016 at 10:54

4 Answers 4

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In Proton Affinities of the Alkali Hydroxides J. Am. Chem. Soc., 1969, 91, pp 2810–2811, it is shown that gas phase affinities in $\pu{kcal/mol}$ are:

  • $\ce{LiOH}$: 240.7
  • $\ce{NaOH}$: 247.6
  • $\ce{KOH}$: 262.6
  • $\ce{CsOH}$: 269.2

So, yes, $\ce{LiOH}$ is weaker, at least in the gas phase.

In aqueous solution, ACIDITY FUNCTIONS FOR STRONGLY BASIC SOLUTIONS Chemical Reviews volume 66, pages 119-131:

the order of basicity for the same molarity of aqueous solution, $\ce{LiOH < NaOH < KOH}$.

For example, for a $\pu{1M}$ solution, the acidity function (H) is:

  • $\ce{LiOH}$: 13.48
  • $\ce{NaOH}$: 14.01, 14.16 (two different measurement techniques)
  • $\ce{KOH}$: 14.17

So in both the gas and aqueous phases, $\ce{LiOH}$ is a weaker base than $\ce{NaOH}$.

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  • $\begingroup$ Can there be something that is stronger/weak acid/base as a gas, than as a liquid? Hmm.. $\endgroup$
    – Neal Conroy
    Commented Jul 29, 2023 at 14:09
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The reference 

  Acids and Bases (Essential Chemistry) 
  Kristi Lew
  Publisher: Chelsea House Publications; Library Binding edition (December 1, 2008)
  Language: English
  ISBN-10: 0791097838
  ISBN-13: 978-0791097830

from page 42 enter image description here

I clipped the image from Goggle books. So far as I know the pKb values for LiOH, NaOH and KOH do not have a primary source listed.

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Wikipedia references

LiOH, pKb -0.36

The reference listed is Kristi Lew, Acids and Bases (Essential Chemistry)

Note: I changed Wikipedia to value of -0.36 from what had been -0.63 which seems to be a transpositional error.

NaOH, pKb 0.2

Reference is website Polyprotic Acids & Bases. UC Davis ChemWiki

Website does not have source of values listed. Of the alkali metal hydroxides only NaOH and KOH are listed. NaOH and KOH values agree with Kristi Lew, Acids and Bases (Essential Chemistry)

KOH

Wikipedia lists a pKa value of 13.5 for which pKb = 14.0 - pKa = 0.5

There is no source listed.

RbOH

No pKb value given.

CsOH, pKb -1.76

Reference given is http://www.periodensystem-online.de/index.php?sel=wert&prop=pKb-Werte&show=list&el=4&id=acid which is Source 2.

No idea where website got its data.

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$\begingroup$ 
  Quantitative Chemical Analysis, Eighth Edition Edition
  Daniel C. Harris
  Publisher: W. H. Freeman; Eighth Edition edition (April 30, 2010)
  Language: English
  ISBN-10: 1429218150
  ISBN-13: 978-1429218153

enter image description here

To my knowledge no reference is given for the complex constants. I clipped the problem from Google books.

Given the complex constants cited, for 1.00 molar solutions:

       [M+] = [OH-]   [MOH]    
   Li      0.65        0.35       
   Na      0.85        0.14

So it would seem that indeed that NaOH is a stronger base than LiOH based on this data.

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  • $\begingroup$ "The Aqueous Chemistry of the Elements" by George K. Schweitzer, Lester L. Pesterfield, Oxford University Press; 1 edition (January 14, 2010), ISBN-13: 978-0195393354 // page 100 "LiOH*H2O and LiOH are very soluble, completely dissociated in HOH, and act as a strong base." // page 104 "NaOH is very soluble, completely dissociated in HOH, and acts as a strong base." $\endgroup$
    – MaxW
    Commented Mar 26, 2016 at 18:47
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    $\begingroup$ Why did you post three answers? $\endgroup$
    – Mithoron
    Commented Apr 21, 2016 at 16:55
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    $\begingroup$ I'm curious as to what the "real" value is since various sources have different answers. I was trying to document some of the various sources. $\endgroup$
    – MaxW
    Commented Apr 21, 2016 at 17:29

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