Sodium iodide, NaI

The iodide is prepared by neutralizing sodium hydroxide or carbonate with hydriodic acid; or by the action of iodine on sodium hydroxide, and reduction with charcoal of the iodate simultaneously formed; or from sodium hydroxide and iodine in presence of iron-filings or ferrous iodide.

The dihydrate, NaI,2H₂O, is a colourless, crystalline substance, readily soluble in water, and isomorphous with the corresponding hydrate of the chloride and with that of the bromide. The recorded values for the melting-point of the anhydrous salt vary between 603° C. (Ramsay and Eumorfopoulos) and 694.7° C. (MacCrae, who also gives 667.5° C.); Carnelley gives 630° C., Victor Meyer, Riddle, and Lamb 661.4° C., Huttner and Tammann 664° C., and Ruff and Plato 650° C. The melting-points of the halides of sodium and of the other alkali-metals fall as the atomic weight of the halogen increases. The boiling-point of sodium iodide is 1300° C., and the vapour-pressure in atmospheres corresponds with the expression

log p = - 37000/4.57T + 5.130.

For the density of the anhydrous salt Schroder gives the value 3.55, and Baxter and Brink 3.665 at 25° C.; for the dihydrate Surawicz gives 2.448. The specific heat between 26° and 50° C. is given by Schillier as 0.0881; and between 16° and 99° C. by Regnault as 0.0868. Thomsen gives the heat of formation from the elements as 69.08 Cal.; for the heat of solution of the anhydrous salt 1.2 Cal., and for the dihydrate -4.01 Cal.; for the heat of hydration of the anhydrous salt to the dihydrate he gives 5.23 Cal.

Solubility-curve of sodium Iodide.

The picture indicates the solubility of sodium iodide in water at various temperatures: The solubility-curve is in conformity with the existence of a pentahydrate, the transition-point to the dihydrate (D) being -13.5° C., that of the dihydrate to the anhydrous salt (E) being 65° C. The boiling-point of a saturated solution in contact with the solid is 141° C. At 25° C., 100 grams of ethyl alcohol dissolve 46.02 grams of sodium iodide. Sodium iodide dissolves in methyl alcohol, yielding at 15° to 16° C. needle-shaped crystals of the formula NaI,3CH₃OH.

Other properties of aqueous solutions investigated are density, refractive index, molecular elevation of the boiling-point, vapour-pressure, specific heat, and electric conductivity. References are also appended to work on the compressibility, the solubility in organic solvents and sulphurous acid, the molecular weight in liquid sulphur dioxide, the electric conductivity in acetone and dilute alcohol, the non-existence of polyiodides, isomorphism with potassium iodide, and the formation of a double salt with silver iodide.