There are three common types of color-changing silica gel desiccant: blue silica gel desiccant, cobalt-free orange silica gel desiccant, and cobalt-free blue silica gel desiccant.
(1) The main cause of discoloration of the blue silica gel desiccant (blue to red) is the inclusion of cobalt dichloride.
Cobalt dichloride itself has a characteristic of changing from blue to pink due to water absorption. Toxic, should avoid contact with food and inhalation, if there is a poisoning incident, you should seek medical advice immediately.
(2), cobalt-free orange silica gel desiccant (orange to dark green) discoloration is mainly due to the presence of methyl violet.
Methyl violet changes color due to pH changes. It utilizes the principle that "methyl orange → dark green" occurs in the range of pH 0.13-0.5.
(3) The main reason for the discoloration of the cobalt-free blue silica gel desiccant (yellow to red) is the presence of Congo red, which is currently less used.
Congo red itself is used as an acid-base indicator with a color change range of 3.5 to 5.2, an alkali state of red, and an acid state of blue-violet.