Vanadium pentoxide reacts with a strong alkali to form polyoxovanadates, which have a complex structure that depends on pH. If you use excess aqueous sodium hydroxide, vanadium pentoxide is converted to a colorless salt – sodium orthovanadate, Na3VO4.
If the acid is slowly added to the Na3VO4 solution, the color will gradually change from orange to red, and then brown hydrated vanadium pentoxide will begin to form. These solutions contain mainly HVO42- and V2. O74- between pH 9 and pH 13, but below pH 9, more unusual species such as V4O124- and HV10O285- (decavanadate) are more common.
