Molybdenum dioxide

Water solubility

Description of key information

Under laboratory conditions, molybdenum dioxide powder reacted slowly with water. It is assumed that a thin layer of molybdenum oxide  initially reacted with the water, resulting in the release of molybdate ions (MoO4 2-) into solution. After separating the solution from the residual particles, the analysed concentrations of dissolved molybdenum were in the order of magnitude of approximately 1 -20 mg/L after a test period of ca. 1 week, depending on the loading.

Additional information

The solubility of a substance in water is generally defined as the saturation mass concentration of the substance in water at a given temperature. The solubility in water is specified in units of mass per volume of solution (ECHA Guidance on information requirements and chemical safety assessment, Chapter R.7a: Endpoint specific guidance).

 

However, as also discussed in the ECHA guidance, this concept of saturation water solubility is not directly applicable to poorly soluble, inorganic or metallic compounds.

 

Solubility of a metal oxide depends on a variety of parameters: the chemistry and composition of the compound, the particle size, the surface area in contact with the solution, the amount of substance per solution volume (“loading”) and last but not least the characteristics of the aqueous medium (environmental or physiological media). Many oxides are generally considered “insoluble”. Nevertheless oxides may react with water or a dilute aqueous electrolyte to form soluble or sparingly soluble cationic or anionic products. In this context, reaction or dissolution kinetics play a key role.

 

Two water solubility studies in accordance with OECD guideline 105 have been conducted with samples of molybdenum dioxide powder (Bär, 2009 and Rüdel, 2009). Because of the multitude of parameters influencing the water solubility of molybdenum dioxid, a definitive result for water solubility could not be derived.

In the study by Baer, 2009, molybdenum dioxide powder was suspended in water at a loading of 8 mg per 50 mL (i.e. 160mg/L). After the test period of ca. 1 week, a concentration of dissolved molybdenum of ca. 0.85 mg/L was analysed, corresponding to a "solubility" of MoO2 of ca. 1.1 mg/L.

In contrast, in the study by Ruedel, 2009, molybdenum dioxide powder was suspended in water at a loading of 1 g per 500mL (i.e. 2000 mg/L). After the test period of ca. 1 week, a concentration of dissolved molybdenum of ca. 17.5 mg/L was analysed, corresponding to a "solubility" of MoO2 of ca. 23.3 mg/L.

Clearly, the resulting concentration of dissolved molybdenum depends on the ratio of sample mass to solution volume.

Based on a weight of evidence approach and considering both studies, the following may be derived as a tentative result: Under the laboratory conditions described in the studies, molybdenum dioxide powder reacted slowly with water. It is assumed that a thin layer of molybdenum oxide initially reacted with the water, resulting in the release of molybdate ions (MoO4 2-) into solution. After separating the solution from the residual particles, the analysed concentrations of dissolved molybdenum were in the order of magnitude of approximately 1 -20 mg/L after a test period of ca. 1 week, depending on the loading.