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EC number: 204-815-4
CAS number: 126-97-6
an incident report related to an accident of a truck that was
transporting thioglycolic acid, it is reported that almost 60 drums were
spread out in a deep ravine near the town of Béziers (South France) on
September 18, 2003 (Devaux, TGA truck accident, Report 143B12, 2003).
hours after the accident almost no thioglycolate was detected in samples
taken between 2 and 30 km downstream of the accident in the small river
close to the accident. Big amounts of dithiodiglycolate (up to 1000-2000
ppm; detection limit 0.4 ppm by ionic chromatography and 0.4 or 0.05 ppm
by HPLC)) were measured. For 20 days, the amount of thioglycolate and
dithiodiglycolate in 12 sampling points downstream of the accident was
followed. Thioglycolate was never detected. The concentration of
dithiodiglycolate decreased rapidly after the accident. At all sampling
points the concentration is below 2 ppm three weeks after the accident.
It must be noticed that 30 km downstream of the accident where drinking
water is used from the river, neither thioglycolate nor
dithiodiglycolate were detected during these 20 days. It should be
noticed that no thioglycolate, but only dithiodiglycolate was detected
in the soil at the place of the spillage.
experiments were conducted to understand what happened to thioglycolate
in the environment.
in water is only very slowly oxidized by air in acidic conditions, even
in the presence of traces of iron (III) or manganese (II). However,
thioglycolate in water, at a neutral pH, in the presence of 20 ppm of
iron III or 20 ppm of manganese II salt can be oxidized by air to
dithiodiglycolate within less than 3 hours. So, after the accident,
thioglycolate has been neutralized by sediments containing calcium
carbonate / magnesium salts then has been oxidized by catalytical amount
of Fe (III) or Mn (II) contained in the ground. A very strong red/purple
color that disappears after hours or days has been observed in the soil
and water at the place of spillage. It has been demonstrated that it is
caused by a thioglycolate-iron II complex at neutral pH. Once
thioglycolate is oxidized to dithiodiglycolate, this color disappears.
The extremely red violet color observed several days at the accident
site is typical for the complex of thioglycolate with Fe (II) at nearly
neutral pH. The color of this complex is so extreme, that a 1 ppm
solution has such a high extinction, that it cannot be measured
quantitatively by UV/VIS photometry with a normal 1 cm cuvette at 530
nm. The observed coloration occurs at concentrations below 1 ppm
reproducing in laboratory the contact of thioglycolate with the ground
or the sediments of the small river, it was demonstrated that
thioglycolate is quantitatively oxidized to dithiodiglycolate and that
there is no other decomposition compound after 2 days (Devaux, 2003).
amount of oxidized thioglycolic acid was determined in aerated tap water
(Sablowski and Heitsch, 2004; reliability 2). The storage condition was
an open beaker without stirring the solution. Samples were taken as fast
as the HPLC-system could carry out one analytical run; that means
approximately 35 min. The content of thioglycolate and of the oxidation
product dithiodiglycolate were quantitatively determined. The result
(See Figure 1,Oxidation of 100 ppm TGA in water, open beaker) described
in this experiment is in good accordance with the truck accident here
above cited, TGA half-live was approximately 35 min.
references provide answers related to oxidation kinetics of
pH and amount of oxidation occurring in thioglycolate solutions is
influenced by the method of preparation of the solutions (Cook and
Steel, 1958; reliability 2). For instance the effect of storage at
different temperatures upon the oxidation of thioglycolate is not as
great with unheated solutions as in heated solutions. Oxidation of
thioglycolate is increased by dilution. The dithiodiglycolate produced
on oxidation of thioglycolate itself, undergoes decomposition under
percent thioglycolic acid solutions were prepared. Half of the solution
was autoclaved at 115-116 °C for 30 minutes whilst the remainder was
sterilized by passing through a 5/3 sintered glass filters. After
sterilization, the solutions were stored in 100 mL glass-stoppered
bottles at 20 °C in the dark. That thioglycolate content was determined
by titration with potassium iodate solution in acid condition.
content (% w/v) of nominal 1% solutions sterilized by autoclaving or
filtration after storage at 20 °C was shown in the table below.
Time of storage in hours
main loss of thioglycolic acid on storage is by oxidation to
dithiodiglycolic acid. The oxidation is catalyzed by copper, manganese,
iron and cobalt but not by zinc. The rate of oxidation varies with the
pH, presence or absence of buffer and the concentration of metallic
catalyst. The disulphide formed on oxidation also acts as a catalyst in
the auto-oxidation of thioglycolic acid.
effect of storage conditions upon thioglycolic acid are well shown by
the appearance (change of color) of samples examined and their
thioglycolic acid content after storage under different conditions (Cook
and Steel, 1959; reliability 2). The older sample stored at a lower
temperature had undergone less decomposition than a fresher sample kept
at room temperature. These results are in agreement with the conclusions
reached in Cook and Steel (1958), where the oxidation of thioglycolic
acid was found to increase with dilution and temperature rise.
et al. (2003; reliability 2) has been studied self-oxidation of
thiols. It was found that these reactions in neutral and alkaline
solutions are induced by impurities of variable-valence metals. The
ability of transition metals to catalyze oxidation rate versus pH passes
through a maximum whose position on the pH scale depends on both the
nature of metal and the structure of the thiol oxidized. The reactions
of oxygen with thiol compounds in aqueous solutions are most often
described by following equations:
RSH + O2-->RSSR + H2O2
RSH + O2-->2 RSSR + H2O
reactions gave disulfides and hydrogen peroxide or water.
thiol structure can affect the kinetics both directly (thiol can be
involved in complexes limiting the reaction rate) and indirectly (by
changing the concentration of the reactive thiolate form of thiol).
Therefore, authors studied acid ionization of SH groups in solutions of
thiol compounds having different structures by spectrometry in the UV
range using absorption of the RS-chromophore in these compounds.
According to the known mechanism, one would expect that the curve of
self-oxidation rate Wo = f(pH) for thioglycolic acid will pass
through a maximum at pH 10.5. In actuality, the maximum rates are
attained at pH 6.5.
the way, the limit of quantitation for thioglycolate with UV photometer
of this complex is 0.02 ppm, this is in the level of graphite furnace
atomic absorption spectrometry (a method for real trace analysis).
search indicated that thioglycolic acid and its salts are not expected
to undergo hydrolysis in the environment due to the lack of hydrolysable
functional groups. Aqueous hydroxyl radical rate constants of 9.0 x 108,
3.6 x 109and 6.0 x 109L/mol*sec were determined
pH 1 (Buxton et al.,
1988; reliability 4; Anbar and Neta, 1967; reliability 4; Dorfman and
Adams, 1973; reliability 4), these values correspond to half-lives of
2.4 years, 220 and 130 days, respectively at an aqueous hydroxyl radical
concentration 1.0 x 10-17mol/L (Mill et al., 1980;
conclusion, the main process that leads to degradation of thioglycolic
acid in water is a fast oxidation to dithiodiglycolate as demonstrated
by literature, experiments and confirmed by the above mentioned incident
thioglycolate is quantitatively oxidized to dithiodiglycolate, other
decomposition compound after 2 days were not found. Further experiments demonstrated
that dithiodiglycolate can undergo a rapid biodegradation at a
concentration of 150 ppm, although some adaption of bacteria is
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