Desiccants with a high capacity have to be changed less often than desiccants with a low capacity. But it is not easy to verify if the desiccant is "wet" already. Copper sulfate changes its colour to blue (forming the pentahydrate), when absorbing water. Silicagel often contains indicators, which may be harmful or can react with the atmosphere. For example, cobalt(II) chloride or methyl violet are often used as indicators, but quite hazardous. Non-toxic indicators like ammonium iron(III) sulfate may decompose in a basic atmosphere.