Note 84 — Chapter XI · THE HALOGENS: CHLORINE, BROMINE, IODINE, AND FLUORINE
George Kamensky translation
If sodium iodate be mixed with a solution of sodium hydroxide, heated, and chlorine passed through the solution, a sparingly soluble salt separates out, which corresponds with periodic acid, and has the composition Na4I2O9,3H2O.
6NaHO + 2NaIO3 + 4Cl = 4NaCl + Na4I2O9 + 3H2O.
This compound is sparingly soluble in water, but dissolves easily in a very dilute solution of nitric acid. If silver nitrate be added to this solution a precipitate is formed which contains the corresponding compound of silver, Ag4I2O9,3H2O. If this sparingly soluble silver compound be dissolved in hot nitric acid, orange crystals of a salt having the composition AgIO4 separate on evaporation. This salt is formed from the preceding by the nitric acid taking up silver oxide—Ag4I2O_9 + 2HNO3 = 2AgNO3 + 2AgIO4 + H2O. The silver salt is decomposed by water, with the re-formation of the preceding salt, whilst iodic acid remains in solution—
4AgIO4 + H2O = Ag4I2O9 + 2HIO4.
The structure of the first of these salts, Na4I2O9,3H2O, presents itself in a simpler form if the water of crystallisation is regarded as an integral portion of the salt; the formula is then divided in two, and takes the form of IO(OH)3(ONa)2—that is, it answers to the type IOX5, or IX7, like AgIO4 which is IO3(OAg). The composition of all the salts of periodic acids are expressed by this type IX7. Kimmins (1889) refers all the salts of periodic acid to four types—the meta-salts of HIO4 (salts of Ag, Cu, Pb), the meso-salts of H3IO5 (PbH, Ag2H, CdH), the para-salts of H5IO6 (Na2H3, Na3H2), and the di-salts of H4I2O9 (K4, Ag4, Ni2). The three first are direct compounds of the type IX7, namely, IO3(OH), IO2(OH)3, and IO(OH)5, and the last are types of diperiodic salts, which correspond with the type of the meso-salts, as pyrophosphoric salts correspond with orthophosphoric salts—i.e. 2H3IO5-H2O = H4I2O9.