Many silica gels are, from the manufacturer’s side, provided with a small amount of moisture indicators. They indicate a state of loading by means of a color change. Moisture indicators for silica gels are divided into two types:

  •  pH indicators
  • Metal salts

The pH indicators work on the principle of neutralization. In the dry state the indicator is surrounded by an acidic or alkaline environment. The environment is created by the indicator or an additionally applied acid or base. The introduction of water sets the milieu to a neutral pH value. This change in pH results in a color change of the moisture indicator. The pH value in the dry state is decisive for the required amount of introduced water to cause a color change. A well-known example is crystal violet.

 

Figure 1: Silicagel with moisture indicator crystal violet. Left without water, right with water.

In the case of moisture indicators made of metal salts, the color change occurs due to the formation of complexes. Complexes are formed as a result of the incorporation of crystal water or a reaction of the metal salts with water and chloride ions. An example of the intercalation of crystal water is the cobalt dichloride, see figure. The Blue Pink silica gel has a moisture indicator made of cobalt dichloride.

Figure 2: Silicagel with moisture indicator Cobalt dichloride. Left without water, right with water.

The metal salts form a complex with chloride ions. Depending on the water concentration, the complex is formed with water or chloride ions.6 At high humidity, the complexes are formed with water, which have a different color than the complexes with chloride ions. A frequently used metal salt is copper sulfate. It is contained in the silica gel orange-green. The moisture indicator of the Orange-Green Silica Gel probably contains a salt with chloride ions.

The concern of silica gels depends on the moisture indicators. Pure silica gel is not hazardous to health. The moisture indicators of the silica gels used by Giebel FilTec GmbH are all made of metal salts. The silica gel blue-pink, with a moisture indicator made of cobalt dichloride, is considered to be the most harmful to health. The REACH regulation classifies it as a special concern because it is carcinogenic and toxic to reproduction. Copper sulfate, which is contained in silica gel orange-green, poses a health hazard if swallowed. It causes skin irritation and eye discomfort on contact. When disposing of the product, its harmfulness to aquatic organisms must be taken into account. Silica gel orange colorless, due to the ammonium iron (III) sulfate, causes skin irritation and eye discomfort. However, the content of copper sulfate and ammonium iron (III) sulfate in the silica gels is so low that there is no danger and it does not have to be stated in the safety data sheet.