![]() However, VOCl3 is corrosive, toxic and difficult to prepare and handle. et al, “Study of properties of vanadium-containing silica gel”, Zh. “Synthesis and study of physicochemical properties of vanadium-containing silica-a humidity indicator”, Sb. Belotserkovskaya et al., “Indicator properties of vanadium-modified silicas and zeolites” Zh. ![]() It has been demonstrated that the vanadium compound VOCl3, when impregnated into silica gel gives a colour change from colourless to yellow to orange to red to brown as humidity increases according to the following references: If acceptable alternatives to the cobalt chloride indicator gel were not available to indicate when saturation had occurred in gas/air drying applications, for instance, this could have serious implications on the users' downstream processes, e.g. The hazard classification of cobalt chloride according to European legislation was recently amended (notification from the EEC, 15/12/98) with the consequence that the use of cobalt chloride indicator gel in industrial applications now requires much tighter control to ensure exposure limits are strictly controlled. If the cobalt chloride solution is added directly to the dried gel, the grain size is reduced. Humidified gel is silica gel that has been saturated with water from the vapour phase in order to avoid decrepitation or disintegration upon impregnation. Indicator silica gel is currently produced by impregnating humidified silica gel or a silica hydrogel with a cobalt chloride solution to produce a dry, granular end-product which contains a minimum of 0.5% cobalt chloride and which is blue in colour, changing to pink when water has been adsorbed. 2,460,065 (disclosing cobalt sulphate) and U.S. 2,460,074 (disclosing cobalt thiocyanate), U.S. 2,460,071 (disclosing cobalt chloride), U.S. Cobalt-containing gels for use as humidity indicators have been disclosed in U.S. It is estimated that approximately 4000 tonnes of cobalt chloride indicator gel are used annually on a global basis. Other drying applications include their use in transformer breathers, tank breathers, in the protection of electronics and telecommunication systems and in laboratory desiccators. ![]() to indicate moisture uptake in gas drying columns. Cobalt chloride indicator silica gels are used in a range of applications, e.g. This invention relates to silica-based indicating desiccants. ![]()
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