Bis(tributyltin) oxide

Hydrolysis

Link to relevant study record(s)

Reference

Endpoint:

hydrolysis

Type of information:

experimental study

Adequacy of study:

key study

Study period:

Not reported

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: Study report that meets basic scientific principles, documentation is insufficient

Qualifier:

no guideline followed

Principles of method if other than guideline:

In this study the formation of tributyltin chloride from chloride containing aqueous solutions of other tributyltin compounds was investigated from the kinetic point of view to study the hydrolysis of tributyltin compounds.

Assuming that the hydrolysis of the ester-like tributyltin compounds develops similarly to the carboxylic acid ester, catalysed by protons and/or hydroxyl ions, a pH-dependent reaction rate will have to be observed. The hydrolysis of tributyltin compounds can be assessed from the formation time of the analytically readily detectable tributyltin chloride, whose amount can be determined by potentiometric titration with AgNO3.

GLP compliance:

not specified

Radiolabelling:

not specified

Analytical monitoring:

yes

Details on sampling:

- Sampling intervals for the parent/transformation products: All analyses were performed immediately (0 hours)

Buffers:

Finished buffer solutions pH 3, 5, 7 Riedel de Haen

Details on test conditions:

Hydrolysis experiments with TBTO in acidic and neutral range in the presence of chloride ions:
Aliquot portions fromt he stock solutions of TBTO were injected via a Hmailton syringe into 100 mL hydrolytic solution to make 5*10^-5 to 2.5*10^-4 m aqueous TBT x solutions (x = acid anion). As hydrolytic colution 0.1 m HCl, 0.1 NaCl and finished buffer solutions of pH values 3.5 and 7 were employed . By the addition of NaCl the chloride concentration was adjusted to 0.1 m. Due to the limited solubility of tributyltin compounds or its anionic portions, slightly cloudy solutions were frequently noted, after a brief mixing in a 100 mL separratory funnel, twice extracted with 5 mL n-hexane each and these were combined in a 20 mL volumetric flask and filled with hexane to the calibration mark. An aliquot portion of the hexane solution was shaken with 0.1 N NaOH and the aqueous phase titrated after acidification with HNO3.

Hydrolysis of TBTO in 0.1 N H2SO4:
Hydrolysis, performed using the same scheme in H2SO4 in the absence of chloride was used to investgate the role fo chloride ions on the hydrolysis. The solutions were again extracted with hexane and an aliquot portions of the hexane extract in the volumetric flasks were evaporated in a rotary evaporator. A part of the residue was titrated with 0.1 N HCl after being disolved in methanol. Another part was titrated with 0.1 N TMAH.

Stock solution of TBTO: 298.61 mg/50 mL

Number of replicates:

Not reported

Positive controls:

no

Negative controls:

no

Statistical methods:

none

Preliminary study:

In a previous investigation, the dissociation of TBT compounds in aqueous solutions was explored. Further clarification of this behavior was sought through experiments using potentiometry.

Transformation products:

not measured

Remarks on result:

not measured/tested

Details on results:

Due to their structure, organo-tin esters can neither aquire nor lose protons, so that the formation of a species capable of being titrated is only possible under the influence of a solvent. The instable character of the ester bonds in clearly seen by the slight ability to titrate the tri-n-butyl-tin compounds with their pronounced jump in potential at the equilibirum point. Under the influence of solvents, in particular from the water stemming from the titration medium, the following balance quickly arises:

BusSnX + HOH ↔ Bu3SnOH + HX

While the protons are taken up by the base Bu3SnOH in titration with acids, the acids HX are taken up in titration with strong bases. Therefore no jump in potential was observed in the titration of TBTO with TMAH.
The extraction behaviour of TBTCl under the conditions of the hydrolysis experiments was examined for the quatification of tri-n-butyltin chloride formatin by the hydrolysis of TBT esters in chloride-containing solutions. From the results, it is clear that both the pH values as well as the chloride ion concentration of the solution play a decisive role for the extractibility. It was harder to extract TBTCl from purely aqueous solutions because the hydrolytic balance is pushed to the side of the formation of hydrated tributyltin cations, depending upon the degree of dilution:
Bu3SnCl + nH2O ↔ Bu3(H2O)n-2OH + H3O^+ + Cl^-
TBTCl can be extracted to almost 95% from 0.1 m Cl soulutions from at least weak acidic solutions. In contrast, the chloride ion concentration must be increased by more than ten times to extract more than 90% of the TBTCl in the neutral range.
From the results of the hydrolysis experimets, it can be seen that TBTO forms tributyltin chloride in aqueous solutions in the presence of chloride ions. The formation is completed within the timeframe of processing the hydrolytic solution. Later measurements exhibited no further definitive increase in the TBTCl content. The amount of TBTCl extracted is pH dependent. Rising with an increase in the H^+ ion concentration. After the injection of the TBT compound up to 100% TBTCl can be extracted from 0.1 N HCl.

Table 1: Titration results of the potentiometric content determination of TBTO with different titration solutions.

Stock solution (mL)		Titration medium	mols found *10^6	% of theory or Sn content
TBTO M = 595.1	2.5	0.1 N HCl	47.096	95.4%
	2.5		48.476
	2.5	0.1 N HClO4	48.244	96.3
	2.5	0.1 N TMAH	No turning point fount

 

Table 2: Hydrolysis of TBT in acidic and neutral pH range in solutions with 0.1 m chloride

pH	Hydrolytic solution	Amount of the 0.1 N HCl directy titratable TBT compound [mol*10^6]	Hydrolysis time (h)	Amount of TBT Cl formed
1	0.5 ml 0.01M TBTO + 100 mL 0.1 N HCL	9.588	0	108.4%
--	0.5 ml 0.01M TBTO + 100 mL 0.1 M NaCL		0	43.2%
7	0.5 ml 0.01M TBTO + 100 mL 0.1 M NaCL in buffer pH 7		0	94.2%
7	0.5 ml 0.1M TBTO + 100 mL 0.1 M NaCL in buffer pH 7	95.877	0	36.3%
7	1 ml 0.1M TBTO + 100 mL 0.1 M NaCL in buffer pH 7	958.77	0	15.2%

Validity criteria fulfilled:

not applicable

Remarks:

not performed to standard methods

Conclusions:

In chloride containing neutral and acidic aqueous solutions of various tri-n-butyltin compounds the formation of tributyltin chloride takes place so quickly, that it cannot be measured with the experimental method chosen. The pH dependency of the amount of the tributyltin chloride and the pH independent speed of the establishment of equilibrium can be explained by solvolysis and the formation of ionized tributyltin species. The instability of the Sn-X-bond (X = anionic residue) will be emphasised by its potentiometric titration ability through strong acids and bases.

Executive summary:

In chloride containing neutral and acidic aqueous solutions of various tri-n-butyltin compounds the formation of tributyltin chloride takes place so quickly, that it cannot be measured with the experimental method chosen. The pH dependency of the amount of the tributyltin chloride and the pH independent speed of the establishment of equilibrium can be explained by solvolysis and the formation of ionized tributyltin species. The instability of the Sn-X-bond (X = anionic residue) will be emphasised by its potentiometric titration ability through strong acids and bases.

Description of key information

In chloride containing neutral and acidic aqueous solutions of tri-n-butyltin compounds, the formation of tributyltin chloride takes place so quickly, that it was not possible to measure with the method employed in the study. The pH dependency of the amount of the tributyltin chloride and the pH independent speed of the establishment of equilibrium can be explained by solvolysis and the formation of ionized tributyltin species. The instability of the Sn-X-bond (X = anionic residue) will be emphasised by its potentiometric titration ability through strong acids and bases.

Additional information

The key study Repenthin, W. (1987) was provided for information purposes, as the assessment of hydrolysis in organotins is hindered by the nature of the compounds and analytical methods for assessing organotins are limited in their accuracy. The study was well reported and followed good basic scientific principles, it was therefore assigned a reliability score of 2.