Zinc bis[O-(6-methylheptyl)] bis[O-(sec-butyl)] bis(dithiophosphate)

Hydrolysis

Link to relevant study record(s)

Reference

Endpoint:

hydrolysis

Type of information:

experimental study

Adequacy of study:

key study

Study period:

2010

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

other: The study was performed according to guideline with analytical monitoring by a substance specific method.

Qualifier:

according to guideline

Guideline:

OECD Guideline 111 (Hydrolysis as a Function of pH)

Deviations:

no

GLP compliance:

yes

Remarks:

This study was not performed by a GLP certified laboratory. Hydrolysis was considered to be a phsyico-chemical endpoint, therefore, not conducting the study to GLP was considered acceptable.

Radiolabelling:

no

Analytical monitoring:

yes

Details on sampling:

- Sampling intervals for the parent/transformation products: Samples were evaluated for depletion of the parent compound (dialkyldithiophosphate) by 31P-NMR (phosphorus-Nuclear Magnetic Resonance) spectroscopy once at the end of the 5 day Tier 1 preliminary test

-Sampling methods for the volatile compounds, if any: no volatile components

- Sampling intervals/times for pH measurements: pH was measured at the start and at the end of the 5 day test period with a Beckmann Model 360 meter calibrated prior to use.

- Sample storage conditions before analysis: Samples were freshly prepared for the 5 day Tier 1 study. Samples were immediately analyzed for hydrolysis products after the 5 day incubation period at 50 degrees C.

- Other observation, if any (e.g. precipitation, color change etc.): At pH 4 and 7, mixtures of the test material in buffer were cloudy. A sediment of white solids was observed at pH 9. A slight oil slick was observed at all pHs. These observations were expected as the test substance is a mixture of oil and Zn metal complexes with organic ligands of variable water solubility. At the end of the 5 day test period, mixtures were centrifuged at 2100 rpm for 30 min. resulting in a clear aqueous phase on which NMR testing was performed.

Buffers:

- pH 4
- Type and final molarity of buffer: 0.1M potassium biphthalate + 0.1N NaOH
- Composition of buffer: 0.4 mL of 0.1NaOH + 50 mL biphthalate/100 mL distilled, deionized water (as described in OECD 111)

pH 7
- Type and final molarity of buffer: 0.2M Tris-maleate buffer
- Composition of buffer: Stock solution: 24.2 g Tris(hydroxymethyl)aminomethane + 23.2 g maleic acid / 1000 mL. 50 mL of stock was adjusted to pH 7 with 0.1M NaOH and diluted to 100 mL with distilled, deionized water.


pH: 9
- Type: Sodium borate buffer
-Composition: (Purchased) Supplier: EMD chemicals Catalog Number EM-BX1658-1
-Description: pH value 9.0 colorless, solution accurate to +/- 0.01 pH units at 25 deg. C. Standardized against solutions whose certification is NIST traceable.

Details on test conditions:

TEST MEDIUM
-0.5 g of test material were weighed directly into three separate test flasks
-500 mL of one of the following buffers was added (one to each flask): pH 4 phthalate, pH 7 tris/maleate, pH 9 borate buffer
-pH 4 and pH7 buffers were freshly prepared in distilled, deionized water; pH 9 buffer was purchased

TEST SYSTEM
- Type, material and volume of test flasks, other equipment used: test flasks consisted of acid washed and dried 1000 mL glass flasks fitted with a gas inlet adaptor, thermowell, reflux condenser and a mechanical stirrer.

- Measures taken to avoid photolytic effects: test flasks were wrapped with aluminum foil to eliminate light

- Measures to exclude oxygen: the mixtures were heated to 50 deg. C under a stream of nitrogen at 0.1 cfh to minimize oxidation. The nitrogen sweep was discontinued ater the mixtures reached 50 deg. C to prevent evaporation.

-Temperature: Mixtures were maintained at 50 deg. C for the 5 day test period and monitored regularly by thermometer.

Duration:

5 d

pH:

4

Temp.:

50 °C

Initial conc. measured:

1 000 mg/L

Duration:

5 d

pH:

7

Temp.:

50 °C

Initial conc. measured:

1 000 mg/L

Duration:

5 d

pH:

9

Temp.:

50 °C

Initial conc. measured:

1 000 mg/L

Number of replicates:

no replicates

Positive controls:

no

Negative controls:

no

Statistical methods:

n/a

Preliminary study:

In the 5-day Tier 1 preliminary study, 93.4% of the parent compound was recovered at pH 7. At pH 4 and 9, the recovery was 100%.

At pH 9, the zinc metal of the parent complex was exchanged for the sodium of the buffer forming a sodium dialkyldithiophosphate product and an insoluble sediment of zinc oxide/hydroxide. The 31P-NMR spectrum showed a single peak at 108 ppm indicating the alkyldithiophosphate ligands remained intact after the metal exchange. The sodium dialkyldithiophosphate product was confirmed by ICP analysis which showed 0% zinc and the presence of sodium in the test mixture after the 5-day study which was not present in the starting mixture.

A < 10% change in the concentration of the dialkyldithiophosphate ligands of the parent compound at pH 4, 7 and 9 meets the OECD 111 criterion for a hydrolytically stable material. A full hydrolysis study is not required.

Test performance:

The pH and temperature were maintained throughout the 5-day study. There were no unexpected occurrences that would have affected the test results. Test mixtures were cloudy and an slight oil slick was observed at all pHs. At pH 9, the zinc metal of the parent complex precipitated as zinc oxide. These observations were expected as the test substance contains a mixture of oil and zinc metal complexes with organic ligands of different water solubilities. All analyses were performed on the clear supernants

Transformation products:

no

% Recovery:

100

pH:

4

Temp.:

50 °C

Duration:

5 d

% Recovery:

93.4

pH:

7

Temp.:

50 °C

Duration:

5 d

% Recovery:

100

pH:

9

Temp.:

50 °C

Duration:

5 d

Remarks on result:

hydrolytically stable based on preliminary test

Table 1. Results of 5 -day Preliminary Hydrolysis Test (at 50 ⁰C)

pH	4	7	9
Buffer System	phthalate	tris/maleate	borate
Approx. saturation concentration (ppm)	2700	2700	2700
Starting Concentation (ppm)	1000	1000	1000
% recovery after 5 days	100	93.4	100 ^(1)

⁽¹⁾ 100% recovery of dialkyldithiophosphate with an exchange of zinc in the parent compound for the sodium of the pH 9 buffer occurred.

Validity criteria fulfilled:

not specified

Conclusions:

Under the conditions used in this study, hydrolytic degradation of the dialkyldithiophosphate (DDP) anion never exceeded 10%, even with fully saturated solutions. However, the zinc complex of the DDP anion was found to be labile in aqueous solution and it was not possible to definitively determine the cation associated with the DDP anion in solution when multiple cations were present.

Executive summary:

A Tier 1 preliminary hydrolysis study was performed according the OECD 111 guideline, Annex 1, tiered hydrolysis scheme.  Mixtures of the test substance were prepared in buffers at pH 4, 7 and 9 and maintained at 50 degrees C for 5 days. Following the 5 day incubation, mixtures were evaluated by 31P-NMR (phosphorus-Nuclear Magnetic Resonance) spectroscopy for depletion of the alkyldithiophosphate signals between 112 - 108 ppm and the appearance of other peaks which could be identified as hydrolysis products.

 

At pH 4, 7 and 9, the 31P-NMR spectra showed a change of < 10% in the concentration of the alkyldithiophosphate ligands bound to the zinc metal indicating the substance meets the OECD 111 criterion for a hydrolytically stable material.

 

At pH 9, the zinc metal was exchanged for the sodium of the buffer forming a sodium salt with the alkyldithiophosphate ligands remaining intact. In the exchange, the zinc metal was precipitated as insoluble zinc oxide/hydroxide. No other peaks besides the alkyldithiophosphates (between 112 – 108 ppm) were detected in the NMR spectrum.

 

Based on the results of the 5-day Tier 1 preliminary test, the substance was determined to be hydrolytically stable and a full study is not required.

 

Under the conditions used in this study, hydrolytic degradation of the dialkyldithiophosphate (DDP) anion never exceeded 10%, even with fully saturated solutions. However, the zinc complex of the DDP anion was found to be labile in aqueous solution and it was not possible to definitively determine the cation associated with the DDP anion in solution when multiple cations were present.

Description of key information

Under the conditions used in this study, hydrolytic degradation of the dialkyldithiophosphate (DDP) anion never exceeded 10%, even with fully saturated solutions. However, the zinc complex of the DDP anion was found to be labile in aqueous solution and it was not possible to definitively determine the cation associated with the DDP anion in solution when multiple cations were present.

Additional information

Read-across from other, structurally closely related ZDDPs is justified because of structural similarity and analogy of properties and effects. see section 13 for justification information