Tantalum pentachloride

Water solubility

Description of key information

Tantalum pentachloride can be considered slightly soluble under conditions relevant for the exposure assessment. 

Key value for chemical safety assessment

Water solubility:

0.396 mg/L

at the temperature of:

20 °C

Additional information

As reported in the CRC Handbook of Chemistry and Physics (2011-2012), tantalum pentachloride decomposes after contact with water.

A validated method was used for quantification of dissolved tantalum as hydrolysis/transformation product of tantalum pentachloride using rhodium as internal standard. Referring to the results of pre-tests, the test item showed a rapid hydrolysis in contact with water. A strong temperature rise during the addition of water (hence it was cooled in a water bath) was observed due to the exothermic reaction. The resulting turbid solution was strongly acidic (pH-value < 1) and contained an insoluble residue.

The concentration of dissolved tantalum at the solubility limit upon mixing TaCl5 with water at nominal loading rates of 8.9 -13.5 g/L varied between 2.0 -3.5 g Ta/L (3.92 -6.91 g TaCl5/L), corresponding to an average recovery of 49.3 ± 4.6 % (relative to loading).

The recovery of dissolved tantalum in buffered solutions relative to the applied loading was determined in triplicate each. The following table represents the nominal loading rates of the test item resp. tantalum, as well as the determined concentrations of dissolved tantalum with its corresponding average recovery.

 

	pH 4 Citrate Buffer	pH 7 Phosphate Buffer	pH 9 Borate Buffer
Loading TaCl5[g/L]	6.4-12.9	9.0-10.7	6.3-9.9
Loading Ta [g/L]	3.3-6.5	4.6-5.4	3.2-5.0
Conc. of dissolved Ta [g/L]	1.5-2.1	0.5-0.8	≤ 0.001
Average recovery ± SD [%]	38.1 ± 8.1	12.8 ± 2.3	0.02 ± 0.02

 

The Ta-concentrations in water after pH-adjustment to equivalent pH-values differ considerably from those in buffer. The following table represents the nominal loading rates of the test item resp. tantalum, as well as the determined concentrations of dissolved tantalum with its corresponding average recovery.

 

	pH 4	pH 7	pH 9
Loading TaCl5[g/L]	13.5	13.5	8.9
Loading Ta [g/L]	6.8	6.8	4.5
Conc. of dissolved Ta [g/L]	< 0.1 ∙ 10^-3	< 0.1 ∙ 10^-3- 0.2 ∙ 10^-3	2.5 ∙ 10^-3- 83.0 ∙ 10^-3
Average recovery ± SD [%]	< 0.1	< 0.1	0.7 ± 1.0

 

The results considered most relevant for exposure assessment are those measured from TaCl₅ dissolved in water with subsequent pH adjustment as also potential release of the substance to the environment will take place via process waste water which is neutralized prior to entering any sewage treatment plant. The results from triplicate measurements at pH 7 range between <0.1 mg Ta/L (LOQ; < 0.198 mg TaCl₅/L) and 0.2 mg Ta/L (0.396 mg TaCl₅/L).