Sodium hydroxide, NaOH

The pure hydroxide can be prepared in the laboratory by dissolving sodium in water and evaporating the solution, or by the electrolysis of commercial sodium hydroxide in aqueous solution with a mercury cathode, the amalgam formed being decomposed by water. A solution free from carbonate can be obtained in the laboratory by suspending metallic sodium in a layer of ether floating on the surface of water. The metal dissolves slowly in the water present in the ether, and the sodium hydroxide passes into the bottom aqueous layer. The hydroxide is manufactured by the electrolysis of a solution of sodium chloride; and, according to van Laer, an economical yield can be obtained directly by the electrolysis with a suitable diaphragm of a solution of sodium carbonate containing nitrate or sulphate of sodium to hinder the formation of sodium hydrogen carbonate. Sodium hydroxide is also produced industrially by the much older method of decomposing sodium carbonate with slaked lime, a reversible reaction:

Na₂CO₃+Ca(OH)₂ ⇔ 2NaOH+CaCO₃.

The solution of sodium hydroxide is evaporated in iron vessels, the finished product being marketed in the form of sticks or powder, or in cylindrical blocks of about 6¾ cwt. enclosed in iron drums. Slaked lime also decomposes sodium sulphate with production of sodium hydroxide, and the effect on the yield of the temperature and the dilution of the solution has been studied by Neumann and Karwat.

Sodium hydroxide is a white substance of density 2.130. It dissolves readily in both water and alcohol. It is very stable, melting at 318.4° C. and at a higher temperature volatilizing without decomposition. Admixture with other substances lowers the melting-point. The mixture containing 58.4 per cent, of potassium hydroxide melts at 167° C., that with 20.7 per cent, of a mixture of 48.5 per cent, of sodium carbonate and 51.5 per cent, of potassium carbonate at 265° C., and that with 17 per cent, of sodium carbonate at 280° C. For the latent heat of fusion per mol., Hevesy gives 1.602 Cal. Its hygroscopic character causes it to liquefy on exposure to air, but it is converted into solid carbonate by the action of atmospheric carbon dioxide. The percentage of water in fused samples varies between 0.9 and 1.2, the average being 1.1.

Freezing-points of solutions of sodium hydroxide.

Some of Pickering's data for the solubility are plotted in figure.

The curve indicates the existence of several hydrates, including two tetrahydrates, a freezing at 7.57° C., and β freezing at -1.7° C. The monohydrate melts at 64.3° C. For its density Gerlaeh gives 1.829. At 12° C. it is transformed into the dihydrate, which at a concentration of 45.5 per cent, of sodium hydroxide is in equilibrium at 5° C. with a 3.5 hydrate, 2NaOH,7H₂O, as is also the α-tetrahvdrate at the same temperature and a concentration of 32 per cent, of sodium hydroxide. At -17.7° C. the α-tetrahydrate changes to the pentahydrate, and this form at -24° C. to the heptahydrate. The saturated solution in contact with the solid boils at 314° C.

The specific heat of the anhydrous hydroxide is given by Blumcke as 0.78 between 0° and 98° C. The mean molecular refraction of the molten hydroxide between 320° and 440° C. is 5.37. The heat of formation from the elements is given by de Forcrand as 103.10 Cal., and that from the solid monoxide and liquid water as 36.50 Cal. For the former, Rengade gives 1019 Cal., and in solution 111.8 Cal.; for the heat of formation in dilute solution from sodium and water he gives 44.1 Cal. The heat of solution is given by Thomsen as 9.94 Cal., and by Berthelot as 9.8 Cal. The heat of dilution has also been studied by both these investigators. The heat of formation of the monohydrate is given by Berthelot as 3.25 Cal. The heat of neutralization of the hydroxide by mineral acids has been investigated by Richards and Rowe.

At ordinary temperatures an aqueous solution of sodium (or potassium) hydroxide dissolves sulphur, forming sulphide, polysulphides, thiosulphate, and sulphite. The reaction is very complex, but Calcagni thinks that the sulphide is probably formed first, thiosulphate next, and then polysulphides. Finally, sulphite is produced by decomposition of the thiosulphate. With concentrated solutions part of the sulphur probably dissolves without entering into combination. Ammonium hydroxide of density 0.888 behaves similarly.

When heated in copper vessels at temperatures between 350° and 600° C. in contact with air, sodium hydroxide has been observed to dissolve up to 0.73 per cent, of its weight of copper. The action on iron is less, and on nickel least of all.

Other properties of aqueous solutions have been studied, such as the density, vapour-pressure, boiling-point, molecular depression of the freezing-point, electric conductivity, electrolytic dissociation, refractivity, viscosity, diffusion, dissociation-pressure, and power of dissolving gases.