Why is a borax solution basic?

Question

When searching the wikipedia link for borax, it tells you that dissolving borax in water gives a basic solution.

On further inspection, one site lists the reaction equation (https://www.vedantu.com/question-answer/aqueous-solution-of-borax-is-a-neutral-b-class-11-chemistry-cbse-5faa1a6e09145d2af952ad22), where upon dissolving, 4 equivalents of boronic acid are produced, and two equivalents of sodium hydroxide. The site claims that due to the weakness of the boronic acid, the solution remains basic due to sodium hydroxide.

However, that does not seem to make sense. For example, when titrating weak acids with a strong base, if they are present in equimolar amounts, all of the weak acid still gets deprotonated. In this case, there should be no sodium hydroxide left, and two equivalents of corresponding base and two equivalents of boronic acid - then we would have a buffer at the pKa value (ph=9.25)?

Answer 1

Borax (Ref.1 with crystal structure) is a complicated molecule (also see here). I'd like to redirect you to my answer here, which confirms Mithoron's comment about the correct anion of borax, $\ce{[B4O5(OH)4]^2-}$ (in addition to Wikipedia article about borax). According to my article aforementioned above, when dissolved in water, borax is subject to completely dissociate, and then, hydrolyzes to orthoboric acid, $\ce{B(OH)3}$ according to the following equation: $$\ce{[B4O5(OH)4]^2- (aq) + 5H2O (l) <=> 4B(OH)3 (aq) + 2OH- (aq)}$$

This would yield a solution of $\mathrm{pH}$ of about 9.24 (based on concentration) according to Environmental Health Criteria 204: Boron, which says:

The chemical properties of sodium metaborate differ from those of the other sodium borates, in that the metaborate has a much higher solubility and alkalinity in aqueous solution. Thus, the solubility in water at $\pu{20 ^\circ C}$ is $\pu{41.9 parts}$ sodium metaborate octahydrate (compared with $4.7$ for borax) per hundred parts saturated solution by weight. The $\mathrm{pH}$ of an aqueous solution of the metaborate at $\pu{20 ^\circ C}$ ranges from 10.5 at 0.1% w/w to 12.0 at 18% w/w (compared with$\mathrm{pH}$ 9.24 for borax over a wide range of concentrations).

Metaboratiate anion mentioned here is $\ce{B(OH)4-}$. Also, above except explains the complexity of borates. The $\mathrm{pH}$ value given is in good agreement with boric acid, which is a very weak acid with a $\mathrm{p}K_\mathrm{a}$ of 9.15.

References:

1. S. Maričić, V. Pravdić, and Z. Veksli, "Proton conductivity in borax, $\ce{Na2[B4O5(OH)4].8H2O}$," Journal of Physics and Chemistry of Solids 1962, 23(11) 1651-1659 (https://doi.org/10.1016/0022-3697(62)90248-2).

Answer 2

A $1$ molar borax solution contains $2$ moles sodium cations $\ce{Na+}$ and $1$ mole tetraborate anion $\ce{B4O7^{2-}}$. The cation $\ce{Na+}$ does not react with water. On the contrary, the borate anion is a relatively weak base, reacting partially with water according to : $$\ce{B4O7^{2-} + H2O -> HB4O7^{-} + OH-}$$ The reaction may further react from the ion $\ce{HB4O7^{-}}$ producing new species. This does not change the conclusion : sodium borate solutions contain more $\ce{OH-}$ ions than $\ce{H+}$. As a consequence, these solutions are basic.