Acid blue is also known as Patent Blue V, and is a dark bluish synthetic triphenylmethane dye used as a food coloring. As a food additive, it has E number E131. It is a sodium or calcium salt of [4-(α-(4-diethylaminophenyl)-5-hydroxy- 2,4-disulfophenylmethylidene)-2,5-cyclohexadien-1-ylidene] diethylammonium hydroxide inner salt. The color of the dye is pH-dependent. In aqueous solution, its color will vary from a deep blue in alkaline or weakly acidic medium to a yellow–orange in stronger acidic conditions. It is useful as a pH indicator for the range 0.8–3.0. The structure is also redox-sensitive, changing from a reduced yellow form to an oxidized red form in solution. The reduction potential of around 0.77 volts is similar to that of other triphenylmethane dyes. It is usable as a reversible redox indicator in some analytical methods.

Providing storage is as stated on the product vial and the vial is kept tightly sealed, the product can be stored for up to 24 months.
Wherever possible, you should prepare and use solutions on the same day. However, if you need to make up stock solutions in advance, we recommend that you store the solution as aliquots in tightly sealed vials at -20°C. Generally, these will be useable for up to one month. Before use, and prior to opening the vial we recommend that you allow your product to equilibrate to room temperature for at least 1 hour.