Borax and Boric Acid
On this page
Direct answer
Borax, Na2B4O7·10H2O, and boric acid, H3BO3, are group 13's laboratory workhorses. Heated, borax swells as it loses water, then fuses to a transparent glassy bead that dissolves metal oxides into coloured metaborates — the borax bead test, cobalt deep blue and chromium green among the readings. Boric acid is a solid of layered sheets, planar BO3 units hydrogen-bonded into greasy layers, and despite its three hydroxyl groups a monobasic Lewis acid: it accepts OH− from water, B(OH)3 + 2H2O ⇌ [B(OH)4]− + H3O+, rather than donating any proton of its own.
What you must remember
- Borax identity: properly Na2[B4O5(OH)4]·8H2O; on heating it swells, gives anhydrous Na2B4O7, and at red heat decomposes to the glassy bead mixture 2NaBO2 + B2O3.
- Bead test palette: cobalt metaborate blue, chromium green, copper blue, iron yellow-brown — colour on glass, read after cooling from the oxidising flame.
- Borax in water: hydrolysis gives an alkaline solution, Na2B4O7 + 7H2O → 2NaOH + 4H3BO3; its stability and purity made borax a classical primary standard for acid titrations.
- Boric acid acidity: very weak, Ka about 5.8 × 10^-10 as commonly quoted; monobasic because one OH− is accepted per B(OH)3, not three protons donated.
- The polyol trick: with glycerol or mannitol, cyclic borate esters form and release protons — the effective acidity jumps enough for a sharp phenolphthalein titration with NaOH that is impossible on the bare acid.
- Thermal ladder of H3BO3: near 370 K metaboric acid HBO2, on stronger heating tetraboric acid H2B4O7, and at red heat the B2O3 glass.
- Preparations: Na2B4O7 + 2HCl + 5H2O → 4H3BO3 + 2NaCl; also vigorous hydrolysis of BCl3 to boric acid and HCl.
- Uses: boric acid as a mild antiseptic eyewash powder, borax as a soldering flux and laundry booster, borosilicate glass, and perborate bleaches.
Why one proton-equivalent, and how titration gets around it
The molecule owns three OH groups and not one ionisable proton — boron's empty p orbital does the chemistry. Water's own hydroxide completes boron's octet as [B(OH)4]−, and the proton the water loses makes the solution acidic; with Ka near 10^-10, boric acid is hundreds of times weaker than acetic acid, and a direct titration with sodium hydroxide runs into a flat, useless endpoint. Add glycerol and the picture changes: the cis-diol chelates boron into a cyclic ester, and in forming that complex releases protons stoichiometrically — the apparent acidity climbs by orders of magnitude, phenolphthalein snaps to a clean pink, and the titration works. The bead test is the same boron-accepts-oxygen story in the dry: the sodium borate glass dissolves metal oxides from the hot loop, and cobalt oxide, for instance, reports in as blue cobalt(II) metaborate. Both experiments teach one idea — boron completes its sextet by accepting, and every usefully acidic proton was borrowed from water or a diol, never its own.
Boron questions that decide ranks
JEE Main keeps to statements and structures: monobasic Lewis acid (never tribasic Bronsted), the layered hydrogen-bonded lattice with its greasy feel, the bead colours, hydrolysis giving a basic borax solution. Advanced pushes the reasoning — why glycerol precedes the titration (an NCERT-paragraph classic), why the layers slide like a solid lubricant (planar sheets over hydrogen bonds), why BCl3 hydrolyses violently while CCl4 does not (empty orbital versus saturated octet). Two slips cost marks every session: calling boric acid tribasic, and predicting a strongly acidic pH for its solution — saturated boric acid sits only near pH 5. Group 13 chemistry remains on both current syllabi in trimmed form, with borax and boric acid among the named-chemistry anchors the exam keeps returning to.
Frequently asked questions
Why is boric acid monobasic despite three OH groups?
It donates no proton; boron accepts an OH− from water to form [B(OH)4]−, releasing one equivalent of H+ per molecule — Lewis acidity, so one equivalent of base suffices.
How does the borax bead produce characteristic colours?
The fused sodium borate glass dissolves metal oxides as coloured metaborates — cobalt blue, chromium green, copper blue — read on the glass loop after cooling.
Why is glycerol added before titrating boric acid with NaOH?
The cis-diol forms a cyclic borate ester that releases protons, raising the effective acidity enough for a sharp phenolphthalein endpoint the bare acid cannot give.
What happens when boric acid is heated progressively?
It condenses through metaboric and tetraboric acids to a B2O3 glass at red heat — water leaves and B–O–B bridges build between the BO3 units.
Why is an aqueous borax solution alkaline?
Hydrolysis of the tetraborate generates sodium hydroxide alongside the very weak boric acid, leaving excess base in solution.