[1] The dark red hygroscopic substance is a strong acid, with a pKa of 0.32,[2] as such it exists almost entirely as the pertechnetate ion in aqueous solution.

The red color in solution is thought to be due to the formation of the polyoxometallate Tc20O4−68,[3] while fresh HTcO4 is white.

[4] Use of strong enough acid solution, for example, concentrated sulfuric acid, can generate the protonated form, which then exists as the octahedral TcO3(OH)(H2O)2 dihydrate complex.

[5] In aqueous solutions the pertechnic acid is complitely dissociated [6] and therefore it is considered as very strong acid its deviation from ideal activity being due to ion pair and triple ion formation with counter-cation of hydronium.

[7] Density and activity values of pertechnetic acid aqueous solutions at T = 298.15 K are tabulated in.