NaOH as a Reagent: Disproportionation, Industrial Byproducts and Applications

Several nonmetals undergo disproportionation (simultaneous oxidation and reduction of the same element) when treated with hot concentrated NaOH. Boron reacts with hot NaOH to give sodium borate plus hydrogen gas (2B + 2NaOH + 2H₂O → 2NaBO₂ + 3H₂, sometimes written toward Na₃BO₃-type products depending on conditions) — boron is oxidized while some hydrogen is released. White phosphorus (P₄) disproportionates in hot NaOH solution to give sodium hypophosphite (NaH₂PO₂, phosphorus oxidized to +1) and phosphine gas (PH₃, phosphorus reduced to -3) — a classic reaction. Sulfur disproportionates in hot concentrated NaOH to give sodium thiosulfate (Na₂S₂O₃) and sodium sulfide (Na₂S) together with water — again splitting the same starting element into a more-oxidized and a more-reduced product simultaneously. All three of these are genuine, textbook-correct disproportionation reactions of NaOH with a nonmetal.

A boron trifluoride-type compound (BF₃, as the compound Z) reduced by excess lithium aluminium hydride gives diborane, B₂H₆ (compound X), releasing LiF and AlF₃ as by-products of the reduction. Diborane then hydrolyses readily in water to give boric acid, H₃BO₃ (compound Y), releasing hydrogen gas in the process (B₂H₆ + 6H₂O → 2H₃BO₃ + 6H₂). Separately, diborane burns readily in oxygen (it is highly reactive and pyrophoric-adjacent) to give boron oxide, B₂O₃, and water, releasing a large amount of energy — this combustion reactivity is part of why diborane has been explored as a high-energy fuel additive. Reading a sequence like this means tracking each compound through one transformation at a time: reduction of a fluoride gives the hydride, hydrolysis of the hydride gives the oxyacid, and combustion of the hydride gives the oxide.

Industrially, sodium hydroxide is manufactured by the chlor-alkali process: electrolysis of concentrated aqueous NaCl (brine), which produces NaOH at the cathode compartment while chlorine gas, Cl₂, is released as the co-product at the anode (hydrogen gas is also released at the cathode alongside NaOH, but among the given classic options, Cl₂ is the standard byproduct answer). Two other sodium compounds show up in specific applications worth remembering by name: sodium thiosulfate, Na₂S₂O₃ (commonly called hypo), is the standard photographic fixer, dissolving away unexposed silver halide from photographic film after development; and, in a linked industrial reaction sequence starting from Na₂CO₃ treated with water and CO₂ to give NaHCO₃, then combined with ZnCl₂ and heated, the resulting zinc compound eventually decomposes to ZnO, which — being amphoteric — dissolves in excess NaOH to form sodium zincate, Na₂ZnO₂, the final product in that sequence.