Potassium Plumbate, K₂PbO₃

Potassium Plumbate, K₂PbO₃.3H₂O or K₂Pb(OH)₆, was prepared by Fremy and Seidel, and subsequently by Bellucci and Parravano, who heated 100 grams of potassium hydroxide in a silver dish, with 20-30 grams of water, and added as much lead dioxide, made into a paste with water, as would dissolve. The filtered solution was then evaporated in vacuo over sulphuric acid, when colourless, rhombohedral crystals were obtained, which were isomorphous with potassium stannate and platinate. This salt is believed to have its water intimately combined, and not simply as water of crystallisation, both on account of its isomorphism with the similarly constituted stannate and platinate, and because the water cannot be driven off at 100° C., but is only set free when the salt is strongly heated so as to undergo the following change:

K₂Pb(OH)₆ = 2KOH + PbO + O + 2H₂O.

To sodium plumbate Hohnel attributed the formula Na₂PbO₃.6H₂O; but, according to Bellucci and Parravano, there is no sufficient reason for supposing that its constitution differs from that of the potassium salt.

By measurements of electric conductivity, and degree of hydrolysis of ethyl acetate, Parravano and Calcagni have determined the extent to which potassium plumbate is hydrolysed in aqueous solution; and they find that whilst a concentrated solution contains a small proportion of undecomposed salt, the amount of Ijais diminishes on dilution; and that in a 1/164.9 molecular solution all the plumbate has been resolved into alkali and colloidal plumbic acid. The series lead, silicon, tin, represents the order of increasing acidity of these elements.