Borax, boric acid and diborane

Borax, the white powder

Borax is a white crystalline powder that people use to make laboratory glass, to clean metal before soldering and to make enamel. It occurs as the mineral tincal in dry lake beds. Its usual formula is $\text{Na}_{2}\text{B}_{4}\text{O}_{7}\cdot 10\text{H}_{2}\text{O}$. The structure of the ion is better shown by the formula $\text{Na}_{2}[\text{B}_{4}\text{O}_{5}(\text{OH})_{4}]\cdot 8\text{H}_{2}\text{O}$, which has the same atoms.

The anion $[\text{B}_{4}\text{O}_{5}(\text{OH})_{4}]^{2-}$ has four boron atoms. Two are trigonal, with three oxygen neighbours (sp$^2$), and two are tetrahedral, with four oxygen neighbours (sp$^3$). Each boron carries one $\text{-}\text{OH}$ group. Five oxygen atoms bridge the borons. The two tetrahedral borons share one of them, and each one carries the negative charge.

In water. Borax dissolves and the solution is alkaline: $[\text{B}_{4}\text{O}_{5}(\text{OH})_{4}]^{2-} + 5\text{H}_{2}\text{O} \to 2\text{H}_{3}\text{BO}_{3} + 2[\text{B}(\text{OH})_{4}]^{-}$. The solution holds a weak acid, $\text{H}_{3}\text{BO}_{3}$, and its conjugate base in equal amounts, so it is a buffer, which resists a change of pH. Its pH is close to the pKa of boric acid, 9.24. A solution of $0.01\ \text{mol/kg}$ is used as a standard of pH 9.18 at 25$^\circ$C.

With acid. Borax is a primary standard for acid solutions in titrations. This means that it is pure and stable enough to be weighed exactly: $\text{Na}_{2}\text{B}_{4}\text{O}_{7} + 2\text{HCl} + 5\text{H}_{2}\text{O} \to 2\text{NaCl} + 4\text{H}_{3}\text{BO}_{3}$. One mole of borax needs two moles of acid. Boric acid itself is made in the same way, with sulphuric acid: $\text{Na}_{2}\text{B}_{4}\text{O}_{7} + \text{H}_{2}\text{SO}_{4} + 5\text{H}_{2}\text{O} \to \text{Na}_{2}\text{SO}_{4} + 4\text{H}_{3}\text{BO}_{3}$.

On heating. Borax first loses its water and swells up into a white porous mass. At a higher temperature it melts to a clear glass of sodium metaborate and boron(III) oxide.

The glassy bead dissolves coloured metal oxides, and this is the borax bead test. A little copper(II) sulphate in the bead in an oxidising flame gives a blue bead of copper(II) metaborate, $\text{CuO} + \text{B}_{2}\text{O}_{3} \to \text{Cu}(\text{BO}_{2})_{2}$. Cobalt gives a deep blue bead of $\text{Co}(\text{BO}_{2})_{2}$.

Boric acid

Boric acid is a white solid that feels slippery. Its formula is $\text{H}_{3}\text{BO}_{3}$, or $\text{B}(\text{OH})_{3}$. Each molecule is a flat triangle with an $\text{-}\text{OH}$ at each corner. In the crystal the molecules are joined by hydrogen bonds into flat layers, which stack on top of each other. The layers slide, as in graphite, and this makes the crystals soft and flaky.

It is not a proton donor. Boric acid is a weak, monobasic Lewis acid. The boron atom, with its empty p orbital, takes a hydroxide ion from water:

Only a very small part of the molecules react. So boric acid cannot be titrated with sodium hydroxide in plain water. With a polyol (an alcohol with several $\text{-}\text{OH}$ groups) such as glycerol or mannitol it forms a stable complex, and the solution turns into a much stronger acid. Then it can be titrated with sodium hydroxide, with phenolphthalein as the indicator.

On heating. Above $370\ \text{K}$ it loses water and gives metaboric acid, $\text{H}_{3}\text{BO}_{3} \to \text{HBO}_{2} + \text{H}_{2}\text{O}$. At a red heat the product is boron(III) oxide: $2\text{HBO}_{2} \to \text{B}_{2}\text{O}_{3} + \text{H}_{2}\text{O}$.

Slide the concentration from the smallest to the largest. The pH changes by about one unit, and the part that reacts stays below 0.04 per cent.

Boric acid is used as a mild antiseptic in dilute solution, in glass and ceramics, and to keep wood from catching fire. It is poisonous if swallowed in quantity.

Diborane

The simplest boron hydride that is stable is diborane, $\text{B}_{2}\text{H}_{6}$. It is a colourless, toxic gas that takes fire in moist air.

Preparation. In the laboratory: $4\text{BF}_{3} + 3\text{LiAlH}_{4} \to 2\text{B}_{2}\text{H}_{6} + 3\text{LiF} + 3\text{AlF}_{3}$ in dry ether. In industry, at about $450\ \text{K}$: $2\text{BF}_{3} + 6\text{NaH} \to \text{B}_{2}\text{H}_{6} + 6\text{NaF}$.

Structure. A diborane molecule has 12 valence electrons (three from each boron and one from each of six hydrogen atoms). A structure with eight ordinary bonds would need 16 electrons. So diborane is electron deficient. It has two kinds of bonds:

• Four terminal B-H bonds. Each is a normal two-centre two-electron bond. Its length is $119\ \text{pm}$. They lie in one plane with the two borons, and the H-B-H angle is about $121.5^\circ$.
• Two bridge B-H-B bonds. Each is a three-centre two-electron bond: two electrons hold three atoms together. A bridge hydrogen sits above and another below the plane. The bridge B-H length is $134\ \text{pm}$, longer than the terminal one. The B-H-B angle is about $83^\circ$.

The electrons add up: $4 \times 2 + 2 \times 2 = 12$. Each boron has four bonds and is close to sp$^3$.

Reactions.

• Burning. $\text{B}_{2}\text{H}_{6} + 3\text{O}_{2} \to \text{B}_{2}\text{O}_{3} + 3\text{H}_{2}\text{O}$. The reaction gives out a great deal of energy.
• Hydrolysis. $\text{B}_{2}\text{H}_{6} + 6\text{H}_{2}\text{O} \to 2\text{B}(\text{OH})_{3} + 6\text{H}_{2}$.
• Lewis bases. With a bulky base the molecule splits evenly (symmetrical cleavage): $\text{B}_{2}\text{H}_{6} + 2\text{N}(\text{CH}_{3})_{3} \to 2\text{BH}_{3}\cdot \text{N}(\text{CH}_{3})_{3}$. With ammonia it splits unevenly (unsymmetrical cleavage): $\text{B}_{2}\text{H}_{6} + 2\text{NH}_{3} \to [\text{BH}_{2}(\text{NH}_{3})_{2}]^{+} [\text{BH}_{4}]^{-}$.
• Carbon monoxide. $\text{B}_{2}\text{H}_{6} + 2\text{CO} \to 2\text{BH}_{3}\text{CO}$.
• Metal hydrides. $\text{B}_{2}\text{H}_{6} + 2\text{LiH} \to 2\text{LiBH}_{4}$.
• Borazine. Strong heating of the ammonia product gives a ring, $3\text{B}_{2}\text{H}_{6} + 6\text{NH}_{3} \to 3[\text{BH}_{2}(\text{NH}_{3})_{2}]^{+} [\text{BH}_{4}]^{-} \to 2\text{B}_{3}\text{N}_{3}\text{H}_{6} + 12\text{H}_{2}$. Borazine, $\text{B}_{3}\text{N}_{3}\text{H}_{6}$, is a flat ring of alternating B and N atoms. It is called inorganic benzene.

Practise with new numbers

Try it in Python

The program follows 10.00 g of borax as it is heated. It uses molar masses to find what is left at each stage.

Common mistakes

• Writing boric acid as a protic acid like $\text{H}_{3}\text{PO}_{4}$. It does not give away a proton. It takes $\text{OH}^{-}$ from water.
• Using one mole of acid for each mole of borax. The equation uses two.
• Drawing diborane with one bridge or with two ordinary bonds in the bridge. The bridge bond has two electrons for three atoms.
• Counting 14 or 16 valence electrons in diborane. The right count is 12.
• Thinking that the bridge hydrogens lie in the plane of the terminal ones. They lie above and below it.

Remember these

• Borax: $\text{Na}_{2}\text{B}_{4}\text{O}_{7}\cdot 10\text{H}_{2}\text{O}$. It gives a basic solution and a buffer. It loses water and then forms a glass, $\text{NaBO}_{2}$ and $\text{B}_{2}\text{O}_{3}$. The bead test uses the glass.
• Boric acid: planar $\text{B}(\text{OH})_{3}$, joined in layers by hydrogen bonds. It is a weak Lewis acid, with pKa 9.24, and monobasic.
• Diborane: 12 valence electrons, four terminal 2c-2e bonds, two bridge 3c-2e bonds.
• Diborane burns, is hydrolysed to boric acid and hydrogen, and splits with Lewis bases.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. How many moles of hydrochloric acid react with 0.500 mol of borax?

2. Boric acid is monobasic. Which equation shows this?

3. How many electrons are in the bridge bonds of diborane, and how many atoms do they hold together?

4. What is the product when diborane is hydrolysed, and how many moles of hydrogen come from 1 mol of $\text{B}_{2}\text{H}_{6}$?

References

• OpenStax, Chemistry 2e, Chapter 18 (Representative Metals, Metalloids and Nonmetals).
• NCERT Chemistry for Class 11, the chapter The p-Block Elements (older edition), the sections on boron compounds.
• Housecroft and Sharpe, Inorganic Chemistry, the chapter on the group 13 elements (boranes).