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Hydrolysis

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Reference
Endpoint:
hydrolysis
Type of information:
experimental study
Adequacy of study:
key study
Study period:
27 May - 22 June 2010
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
study well documented, meets generally accepted scientific principles, acceptable for assessment
Remarks:
Preliminary testing in a GLP study with sufficient information and relevant methodology.
Reason / purpose for cross-reference:
reference to same study
Qualifier:
no guideline followed
Principles of method if other than guideline:
A sample of 10 g/L and 100 mg/L (nominal) was prepared in glass double-distilled water and the pH was recorded over a 1 hour period. The flask neck was loosely stoppered to minimise any losses of gaseous hydrochloric acid. After one hour the 100 mg/L sample was analysed for concentration of zirconium using inductively coupled plasma - mass spectroscopy (ICP-MS). After analysis the sample was neutralised and again analysed.
GLP compliance:
yes (incl. QA statement)
Radiolabelling:
no
Analytical monitoring:
yes
Details on sampling:
Duplicate aliquots of the 100 mg/L solution after 1 hour standing and after subsequent neutralisation were added to ICP sample tubes already containing 200 µL of trace analysis grade nitric acid. Each sample was filtered through a 0.45 µm PES (polyethersulfone) filter before dilution for analysis, to remove the visible precipitate that formed during the neutralisation process.
Buffers:
not applicable
Number of replicates:
2
Positive controls:
no
Negative controls:
no
Preliminary study:
Dissolution of test item resulted in strongly acidic solution pH values. Rapid hydrolysis of test item with the formation of a stable zirconium oxychloride intermediate with the liberation of hydrochloric acid - stable equilibrium reached. On neutralization of the solution, the HCl is consumed, the equilibrium disrupted and hydrolysis is able to continue towards completion and obtention of the zirconium dioxide.
Transformation products:
yes
No.:
#1
No.:
#2
No.:
#3
Details on hydrolysis and appearance of transformation product(s):
Transformation products:
In presence of HCl, intermediate zirconium oxychloride
After neutralization:
Zirconium dioxide
Remarks on result:
other: No hydrolysis rate nor half-life value is available
Remarks:
Limited hydrolysis generates HCl and thus acidifying the sample solution and establishing a stable eliquibrium. On neutralisation of the solution, the HCl is consumed, the eliquibrium disrupted and hydrolysis is able to continue towards completion, see "Details on results"
Details on results:
Sample solutions at nominal concentration of 10 g/L and 100 mg/L resulted in stable acidic pH values of 1.83 and 3.36 respectively within one hour of preparation.

On neutralisation of the lower 100 mg/L nominal concentration sample during this test to pH 7.19 (with 0.1M sodium hydroxide), a precipitate was observed to have formed. By ICP-MS analysis of the sample solution, the total dissolved zirconium concentration was demonstrated to have reduced drastically from 27.5 mg/L (corresponding to 97.4 mg/L zirconium dichloride oxide) to 0.955 mg/L after neutralisation. It was anticipated that limited hydrolysis generates hydrochloric acid and thus acidifying the sample solution and establishing a stable eliquibrium. On neutralisation of the solution, the hydrochloric acid is consumed, the eliquibrium disrupted and hydrolysis is able to continue towards completion. The zirconium is lost from the solution through precipitation, most probably as the proposed insoluble hydrolysis product zirconium oxide.
Validity criteria fulfilled:
yes
Conclusions:
When dissolving zirconium dichloride oxide into water rapid hydrolysis to the insoluble zirconium dioxide will occur. During this process hydrochloric acid is released. However samples are stable in water due to the acidic nature of the sample solutions, in that rapid, but only limited hydrolysis occurs, resulting in acidic solution pH. Overall the hydrolysis of zirconium dichloride oxide is recognised as being incomplete in the presence of hydrochloric acid, with the formation of a stable oxychloride intermediate, not the anticipated oxide. On neutralisation hydrolysis proceeds which is confirmed by the analysis of a 100 mg/L sample one hour after preparation and after subsequent neutralisation showing that only approximately 3.5% zirconium was left in solution. Following these results it can be concluded that instantaneous hydrolysis of zirconium dichloride oxide occurs under neutral condition. This is anticipated to be representative for environmental conditions.

Description of key information

The key study from O'Connor and Woolley (2010) demonstrated that upon neutralization of the test item solution containing zirconium dichloride oxide hydrolysis is able to continue towards completion and obtention of zirconium dioxide, which is insoluble.

Key value for chemical safety assessment

Additional information

One study is available (O'Connor and Woolley, 2010), in which the hydrolysis of zirconium dichloride oxide was studied. The study was assigned a Klimisch score of 2 (reliable with restricitons) because no guideline was followed (extended water solubility study). The study demonstrated that when dissolving zirconium dichloride oxide into water rapid hydrolysis of the test item occurred with the liberation of hydrochloric acid. Dissolution of the test item resulted in strongly acidic solution pH. Overall the hydrolysis of zirconium dichloride oxide is recognised as being incomplete in the presence of hydrochloric acid.

Under natural conditions however a significant buffering capacity will exist and hence the hydrogen released will be absorbed; a decrease in pH is therefore not expected. The hydrolysis of zirconium dichloride oxide will thus continue and this immediate hydrolysis will form zirconium dioxide which has an extremely low water solubility and will precipitate out of the solution.

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