The Drying Behavior of Cobalt Chloride

Cobalt chloride is a blue, hygroscopic substance that turns red when it absorbs moisture. The characteristic color change makes it very useful as a moisture indicator for drying agents such as silica gel. This article describes how we investigated the drying behavior of cobalt chloride hexahydrate using DSC microscopy and TGA.

Introduction

Cobalt chloride, or more precisely cobalt chloride hexahydrate (CoCl₂ • 6H₂O) is a ruby red, poisonous salt. In aqueous solution, it can be used as ink that is almost invisible on paper. When the paper is warmed, for example over a flame, the written text becomes visible and is colored blue but mysteriously disappears again shortly after.

These properties made the CoCl₂ • 6H₂O solution popular as a secret ink, especially for writing love letters. Nowadays, cobalt chloride (CoCl₂) is important technically as a moisture indicator, for example in silica gel.

Silica gel is colorless amorphous silicon dioxide and has a gel-like to solid consistency. It is strongly hygroscopic and is often used as a drying agent. For this application, the silica gel is usually impregnated with cobalt chloride, which gives it a blue color when it is dry. If the impregnated silica gel absorbs moisture, it becomes red. This is due to the fact that anhydrous cobalt chloride absorbs water and forms the ruby-red cobalt chloride hexahydrate. The silica gel can be regenerated by heating to about 160 °C to remove the water.

This allows it to be reused as a drying agent. This article describes how we investigated the drying process of CoCl₂ • 6H₂O using DSC microscopy and TGA.

Experimental Details

The DSC experiments were performed in a static air atmosphere using a DSC 1 equipped with an FRS5 sensor and the microscopy accessory [1]...

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Conclusions

DSC microscopy allows images of a sample to be simultaneously recorded by a camera while the sample is measured in the DSC 1. Visual changes can be documented and correlated with effects on the DSC curve. DSC microscopy can detect:

• color changes as a result of chemical reactions (decomposition) or polymorphism, and
• changes in the “geometry” of samples, for example as a result of melting or flowing, shrinkage or expansion, movement of the sample during glass transitions or solid-solid transitions, or the formation of cracks.

The visual information from the microscope often greatly facilitates the interpretation of physical-chemical effects observed in DSC curves. DSC microscopy is therefore a very useful accessory for studying materials by DSC.

The Drying Behavior of Cobalt Chloride | Thermal Analysis Application No. UC343 | Application published in METTLER TOLEDO Thermal Analysis UserCom 34