Reef Chemistry Question of the Day #39 pH of Hydroxides

[Randy Holmes-Farley]

Reef Chemistry Question of the Day #39


Which of the following has the lowest pH when mixed into pure fresh water at equal weight concentrations?

A. NaOH
B. Ca(OH)₂
C. Mg(OH)₂
D. B(OH)₃

Bonus question: why does it have a lower pH than the others?

Good luck!






























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[cope413]

Trick question again? Seems too easy. D is boric acid. Gotta be the lowest pH of the group. I'm sure it has something to do with boron, but no idea why. I just remember boron being kind of an odd element.

 

[DFW]

D because it has the extra hydrogen atom?

 

[cagdason]

Mg(OH)2 will be most acidic as it does not dissolve much in water the solubility is 0.00064gr/100ml however B(OH)3 and the others are relatively strong bases where the pKas are 9.24, 12.4, 13.3; NaOH has pKa of 13; Ca(OH)2 pKa is 12.4

 

[Heatherrom]

D. B(OH)3 Boric acid.
Mg(OH)2 is mostly insoluble in water, and if anything will give off hydroxide ions making the solution more alkaline. NaOH is a strong base that will completely dissociate in water giving off one mol of hydroxide for every mol of NaOH dissolved, until the solution reaches its saturation point, seriously increasing pH. CaOH is only slightly soluble, but being a strong base it will dissociate into its ions ca+2 and 2 OH-, increasing pH. B(OH)3 or written as H3BO3, is boric acid. Though a weak acid, it will still dissociate and give off 2H+ ions,for every mol dissolved, lowering pH!

 

[Randy Holmes-Farley]

Anyone else what to give this a shot?

 

[cagdason]

Boric acid is also not that soluble in water and with the given pKas I don't know how acidic it is although the name is boricacid. As far as I know the reason it is acidic is because it absorbes OH- instead of giving H+ to water but the buffering range is around pH=9 so I would guess it won't cause a pH decreased when mixed with water. So I am confused because of the name it sounds like Boric Acid will be more acidic but as Mg(OH)2 will not disolve it will remain what ever pH the water was assuming it is pure RO around 7 it is still lower then the buffering pH of Boric acid.

 

[Randy Holmes-Farley]

And the answer is...D. B(OH)₃

B(OH)₃ is boric acid, which I didn't want to identify by name.

I thought some of you might think it a stronger base since it has three OH groups.

But in reality, as some folks noted, it doesn't actually release any of these OH groups, but picks up a fourth one, lowering the pH of fresh water and forming borate (a significant buffer in seawater):

B(OH)₃ + OH- --> B(OH)₄^-

Taking up OH^- is identical to releasing H⁺, so the pH drops:

B(OH)₃ + H₂O → B(OH)₄^- + H⁺ (pKa ~8.55)

FWIW, mixing boric acid into fresh water gives a pH around 5 (depending on the amount added).

All of the others raise the pH of fresh water by releasing some amount of hydroxide. Which of these raises the pH the most will depend on how much is added (even when adding the same amount of each).

Folks are correct that magnesium hydroxide is poorly soluble, but if you are below the solubility limit, it will actually raise the pH the most of these if you add equal weight to water, because it has the most hydroxide ions per unit of weight due to the very low weight of magnesium ion.

But, if you are above the solubility limit of magnesium hydroxide (1 gram dissolves in about 80 liters of water, giving a pH of about 10), then calcium hydroxide gives the biggest effect up to its solubility limit of about 1.5 gram per liter (giving a pH of about 12.5).

Above 1.5 g/L, sodium hydroxide will give the biggest pH boost of all the chemicals in the list.

Isn't chemistry wonderfully intricate!

Happy Reefing!