What is the Ksp of Al(OH)3?

Question

Top hits on google are:

This site
Aluminum hydroxide $\ce{Al(OH)_3}$ >> $ 1.3×10^{–33}$

and

This site
Aluminum hydroxide $\ce{Al(OH)_3}$ >> $ 3×10^{–34}$

which is different by a factor of four, approximately.

What can be the reason for this variation? Experimental conditions? Where can I get accurate or reliable results/data of $K_{\mathrm{sp}}$ in general?

Answer 1

Let me add to the confusion:

• https://books.google.de/books?id=imBzjH6SHi8C&pg=PA303&lpg=PA303&dq=solubility+product+constant+aluminum+hydroxide&source=bl&ots=jXoPiVihnF&sig=eSK8saszXshTXVEKGOlThUiKVr0&hl=de&sa=X&ei=Xl5TVcepDuj_ywOXmYDQBA&ved=0CG8Q6AEwCTgK#v=onepage&q=solubility%20product%20constant%20aluminum%20hydroxide&f=false
  $2\times10^{-32}$

• http://faculty.ncc.edu/LinkClick.aspx?fileticket=smTKdcQlPY4%3D&tabid=1903
  $1.8\times10^{-33}$

• http://www.austincc.edu/dlyon/gen_chem_1_summer04/Chapter%2020%20solubiliity%20product%20f08.pdf
  $1.9\times 10^{-33}$

• http://www.csudh.edu/oliver/chemdata/data-ksp.htm
  $4.6\times10^{-33}$
• http://www.periodni.com/solubility_product_constants.html
  $3\times 10^{-34}$
• https://users.stlcc.edu/gkrishnan/ksptable.html
  $3\times 10^{-34}$

Usually I'd first have a look at the CRC Handbook of Chemistry and Physics. Unfortunately, I couldn't find the data for aluminium hydroxide.

Answer 2

I think the confusion comes from the fact $\ce{Al(OH)3}$ exists in several forms (amorphous, boehmite, bayerite, hydrargylite) each having a different solubility product!

For example, my source gives a value of $\mathrm{K_{sp}} = 10^{-32.7}$ [1].

Each modification also hydrolyses differently. Base dissociation constant for the amorphous form is $\mathrm{K_{sp}} \approx 10^{-9.5}$ while hydrargylite has $\mathrm{K_{sp}} \approx 10^{-14.6}$. This means at pH 12, the amorphous form will dissolve completely, while the hydrargylite only slightly [1].

What kind of $\ce{Al(OH)3}$ is obtained depends on the way the $\ce{Al(OH)3}$ precipitates and it is very difficult to obtain just a single form. For example, precipitating the hydroxide from acidic solution with ammonia gives mostly amorphous hydroxide, while precipitation from strongly alkaline solution gives a mixture of hydrargylite and bayerite. The precipitation can be direct, homogeneous or depending on the hydrolysis. Each method gives different results that are also influenced by foreign ions in solution.

Relatively pure boehmite can be obtained by aging the precipitate [1] and then doing the measurement.

A method for the preparations for relatively pure hydrargylite (alpha form) and bayerite (beta form) are given in Brauer[2].

Aluminium hydroxide tends to form gelatinous precipitate that easily adsorbs various ions (this property is actually utilized in waste water treatment and chromatography), hence proper washing is needed to separate the pure hydroxide from other ions such as ammonium.

Hope this helps clearing out the confusion a bit. Try to look for solubility constants of the specific modifications, then apply the correct constants for your case.

Note that although the solubility products differ by a magnitude or two, the compounds is extremely insoluble in either case and unless you do extremely precise analytical work, the difference won't affect your calculations much. It's like the difference between black and red mercury sulfides, which differ by four orders magnitude, yet the less soluble form is still practically completely precipitated.

[1]: Erdey, László. Gravimetric Analysis: International Series of Monographs on Analytical Chemistry, Vol. 7. Vol. 7. Elsevier, 2013.

[2]: Brauer G. Handbook of Preparative Inorganic Chemistry. Vol. 1. Academic Press, 1965