Molybdenum

Water solubility

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

water solubility

Type of information:

experimental study

Adequacy of study:

weight of evidence

Study period:

2008-01-29 to 2008-05-20

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

guideline study with acceptable restrictions

Qualifier:

according to guideline

Guideline:

OECD Guideline 105 (Water Solubility)

Deviations:

yes

Remarks:

The flask shake method was used even though the water solubility of the test substance was below 0.01 g/L.

GLP compliance:

yes (incl. QA statement)

Type of method:

flask method

Water solubility:

> 6 - < 12 mg/L

Temp.:

20 °C

pH:

4

Details on results:

Preliminary test: The approximate solubility of molybdenum (metal powder) was estimated to be about 24.9 mg/L at 20 °C (see attached material).
Main tests: In a series of test, using different designs and several replicates, a trend of constantly increasing concentrations over time was observed, reaching between 6 mg/L and 12 mg/L on average after 9 or 10 days respectively and depending on the study design (e.g. amount of sample and solution volume).
Thoughout all test a slow trend of decreasing pH in the solution was observed, starting between ca. 4.3 and 5.0 at t = 120 hours (first sampling time point), and decreasing to ca. 4.0 after 240 hours.

Conclusions:

Interpretation of results: slightly soluble (0.1-100 mg/L)
A single, definitive figure (see attached material) for the water solubility of molybdenum metal powder could not be determined in this test. Please also refer to the discussion in the endpoint summary.

Reference 2

Endpoint:

water solubility

Type of information:

experimental study

Adequacy of study:

weight of evidence

Study period:

2009-12-15 to 2009-03-31

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

guideline study with acceptable restrictions

Qualifier:

according to guideline

Guideline:

OECD Guideline 105 (Water Solubility)

Version / remarks:

adopted 27 July 1995

Deviations:

yes

Remarks:

The flask shake method was used even though the water solubility of the test substance was below 0.01 g/L.

GLP compliance:

yes (incl. QA statement)

Type of method:

flask method

Water solubility:

ca. 5.5 mg/L

Temp.:

20 °C

pH:

>= 3.5 - <= 4.3

Details on results:

Calculation of the water solubility:
The water solubility of molybdenum is calculated using the molybdenum concentration at 168 and 192 hours. In experiment 2 one flask was broken, so that only two values for the molybdenum concentration at 168 and 192 hours were avaliable.
Mean molybdenum concentration (n = 6) = 5.54 +/- 2.06 mg/L
The water temperature had a standard deviation of +/- 0.5.
Observations:
During the study a dark grey precipitate of molybdenum has been observed. After filtration of the solution with a syringe filter (porostiy 0.45 µm) the solution was clear. All samples were colourless with the exception of one vessel. After 48, 72, 96, 168, and 192 hours the solution of this sample was slightly blue. The temperature of the water bath was held at 20.0 +/- 1.0 °C. No further observations have been made.

Conclusions:

Interpretation of results: slightly soluble (0.1-100 mg/L)
Under the conditions of these tests, the water solubility of molybdenum metal powder in distilled water at 20 +/- 0.5 °C after 7 - 8 days is determined as ca. 5.5 +/- 2.1 mg/L.

Description of key information

Under laboratory conditions a sample of molybdenum metal powder reacted slowly with water. It is assumed that a thin layer of molybdenum oxide on the surface of the metal powder 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 10 mg/L after a test period of ca. 1 week.

It is noted, that the tested molybdenum powder represents a worst case scenario with regards to possibly attainable concentrations of dissolved molybdenum. The tested molybdenum metal powder has a median particle size of less than 10 micrometers, representing a fine material. In contract, most molybdenum metal on the EU market is in the form of semi-finished molybdenum metal products (e.g. wires, sheets), or the form of various alloys or in ferromolybdenum. These forms can safely be assumed to be almost inert in contact with water.

In addition to the standard OECD TG 105 water solubility study, the substance (Mo metal powder) has also been tested in a bioelution study (in vitro bioaccessibility) in artificial gastric juice. This test and the results are described in the stand-alone document „MoCon read-across concept (human health)“, which is attached to the technical dossier in IUCLID section 13.

 

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 metal compounds 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). Metals themselves are generally considered “insoluble”, specifically the heavy and noble metals. Nevertheless even in the elemental state metals may react with water or a dilute aqueous electrolyte to form soluble or sparingly soluble cationic or anionic products. During this process the metal will oxidise, or transform, from the elemental state to a higher oxidation state. 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 metal powder (Bär, 2009 and Rüdel, 2009). Because of the multitude of parameters influencing the water solubility of molybdenum metal, a definitive result for water solubility could not be derived. 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 metal powder reacted slowly with water. It is assumed that a thin layer of molybdenum oxide on the surface of the metal powder 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 10 mg/L after a test period of ca. 1 week.

 

It is further noted, that the tested molybdenum powder represents a worst case scenario with regards to possibly attainable concentrations of dissolved molybdenum. The tested molybdenum metal powder has a median particle size of less than 10 micrometers, representing a fine material. In contract, most molybdenum metal on the EU market is in the form of semi-finished molybdenum metal products (e.g. wires, sheets), or the form of various alloys or in ferromolybdenum. These forms can safely be assumed to be almost inert in contact with water.

In addition to the standard OECD TG 105 water solubility study, the substance (Mo metal powder) has also been tested in a bioelution study (in vitro bioaccessibility) in artificial gastric juice. This test and the results are described in the stand-alone document „MoCon read-across concept (human health)“, which is attached to the technical dossier in IUCLID section 13.

 

 

Finally, it has been shown that molybdenum, even in the form of highly soluble salts, is of very low toxicity toward environmental organisms. Also any significant environmental exposure with molybdenum metal is quite unrealistic. Therefore, further tests regarding the dissolution of molybdenum metal (e.g. the transformation dissolution test) have not been conducted.