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12-Tungstoboric Acid, H9[B(W2O7)6]

12-Tungstoboric Acid, H9[B(W2O7)6].28H2O, is prepared by the addition to 1 part of normal sodium tungstate and 1.5 parts of boric acid of sufficient water to form a clear solution on boiling. After a time a mixture of sodium borate and boric acid separates. This is filtered off, the mother-liquor is treated with more boric acid, a further precipitate is rejected, and the solution treated with sulphuric acid and ether according to Drechsel. The resulting oil is decomposed by cold water, and on evaporation in a vacuum, crystals of two isomeric acids separate simultaneously; those in larger quantity being transparent octahedra of 12-tungstoboric acid, H9[B(W2O7)6].28H2O; those in smaller quantity, hexagonal bipyramidal crystals of iso-12-tungstoboric acid, H9[B(W2O7)6].22H2O. The first acid is isomorphous with the 28-hydrates of the 12-tungsto- and 12-molybdo-phosphoric and silicic acids. It melts at 45° to 51° C. The addition of nitric acid to its concentrated solution precipitates a lower hydrate containing 10H2O. The hydrogen-ion concentration in a 0.05 N solution is 3.5×10-3.

A well-defined series of crystalline salts, of composition R5H4[B(W2O7)6].xH2O (R = NH4, Li, K, Na, Ag; R2 = Ba, Ca, Cd, Cu, Mg, Ni), has been obtained by careful addition of the metal hydroxide or carbonate to 12-tungstoboric acid. These salts are isomorphous with the corresponding salts of other 12-heteropoly acids; for instance, in addition to the examples already given, the sodium salt, Na5H4[B(W2O7)6].27H2O, is isomorphous with the similarly constituted sodium metatungstate, Na6H4[H2(W2O7)6].27H2O. The equivalent conductivities of solutions of the sodium salt, containing 1/5 molecule in v litres at 25° C., have been found to be:

v =32641282565121024
Λ =183.9196.0203.2213.6225.3235.3

These values correspond with those of a neutral pentabasic salt, but the neutralisation curve shows that at least two more of the hydrogen atoms are replaceable. A guanidinium salt, (CN3H6)6H3[B(W2O7)6], has been obtained, and, according to Copaux, the addition of mercurous nitrate to a solution of the sodium salt yields a crystalline precipitate of the normal mercurous salt, Hg9[B(W2O7)6].12H2O.

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