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EC number: 267-956-0
CAS number: 67953-76-8
HEDP and its salts are phosphonic acid substances of very high water
solubility, and low octanol-water partition coefficient. The phosphonic
acid groups are multiply ionised at pH values relevant to biological and
environmental systems. Ionisation gives them the ability to form stable
complexes with metal ions, particularly polyvalent ones. Phosphonates
are found to adsorb strongly to inorganic matrices, and hence they
adsorb strongly to sewage sludge and to soil. They will be removed to a
high extent in biological waste water treatment by adsorption.
The toxicity of HEDP and its salts to environmental species is presented
and interpreted in terms of the concentration of active HEDP acid in the
test media. As such the results of tests conducted on HEDP and its salts
are directly comparable, because the ionisation state will depend only
on the pH of the test medium. Section 4.21 of IUCLID and Section 1 of
the CSR describe the pKa values for the ionisation of HEDP. Four pKa
values of HEDP are reported, of 1.7, 2.47, 7.28, 11.41. At
environmentally-relevant pH values HEDP will be ionised typically three
times, and will form stable complexes with metal ions.
The substances have the potential to cause effects on aquatic plants as
a consequence of nutrient limitation caused by complexation of trace
metals. As complexing agents, these substances could remobilise metals
in the environment; however, their high degree of adsorption to
sediments suggests that this is unlikely to occur. The substances are
acids and when present at high concentration they have the potential to
cause local effects on aquatic organisms as a consequence of lowered pH.
Effects on aquatic organisms arising from exposure to the acid form of
the substance are thought to result from a reduction in the pH of the
ambient environment (arising from an increase in the H+ concentration)
to a level below their tolerable range. It is not considered appropriate
or useful to derive a PNEC with studies in which pH deviations may have
been attributable to the cause of effects seen because any effects will
not be a consequence of true chemical toxicity and will be a function
of, and dependent on, the buffering capacity of the environment.
HEDP potassium salts will fully dissociate in aqueous solution and will
behave no differently to the parent HEDP acid and its potassium counter
ion at equivalent concentrations. It is not expected that the potassium
counter ion will contribute significantly to the overall toxicity of
their HEDP salts. Therefore, the impact on aquatic organisms will be
dominated by the effects of the HEDP acid and PNECs are derived in terms
of HEDP acid.
Read-across between the HEDP salts and the parent acid substance is
considered appropriate because:
The category hypothesis is that all the members are various ionised
forms of the acid CAS 2809-21-4. In in vivo toxicity studies the
local pH and ionic conditions within the stomach, gastrointestinal tract
etc. dominate the dissociation state of the phosphonate, irrespective of
the form originally dosed. At a defined pH, a salt will behave no
differently to the parent acid, at identical concentration of the
particular speciated form present, and will be fully dissociated to
yield HEDP acid and counter-ion. Hence some properties (in contact with
water or in aqueous media, i.e. ecotoxicity) for a salt can be directly
read across (with suitable mass correction, see Section 1.4 of the CSR)
to the parent acid and vice versa. In the present context the effect of
an alkaline metal counterion (sodium/potassium) will not be significant
and has been extensively discussed in the public literature. In
biological systems, polyvalent metal ions will be present, and the
phosphonate ions show very strong affinity to them. The ammonium salt
does present a particular issue since ammonia drives the toxicity of the
substance and will be assessed separately.
The behaviour of phosphonates in the environment will be profoundly
affected by their concentration in water, the concentration and
identities of metal ions, and the solids content per unit volume of
water. From surface water through to soil a wide range of these
parameters will be exhibited. Very few data concerning background
concentrations of phosphonates in the environment are published,
possibly due to the difficulties in detecting these substances at low
concentrations in environmental media. Almost all natural waters contain
more ions than the usual PEC values of the phosphonates.
In addition, in the environment the salt form is immaterial and
speciation will occur in natural media. Similarly, all environmental
related endpoints, use of buffered test media results often reflect a
salt speciation relevant for ~pH 7 only and it would be impossible to
test specific salts associated with high and low pH. Detaching or
attracting a proton does not change the chemical safety assessment of
the molecule as long as no other part of the molecular skeleton is
changed, because in studies or when there is exposure, the pH will
control the identity of the form or forms present.
Therefore. it is considered appropriate to read-across between the HEDP
category members and attribute the effects to the acid component of the
Not classified according to Regulation (EC) No 1272/2008 classifiable
based on the fact that short term toxicity data are >100 mg/L. In
addition, the NOEC for long term toxicity with invertebrates is >1 mg/L
therefore HEDP sodium is not classified for the environment.
Information on Registered Substances comes from registration dossiers which have been assigned a registration number. The assignment of a registration number does however not guarantee that the information in the dossier is correct or that the dossier is compliant with Regulation (EC) No 1907/2006 (the REACH Regulation). This information has not been reviewed or verified by the Agency or any other authority. The content is subject to change without prior notice.Reproduction or further distribution of this information may be subject to copyright protection. Use of the information without obtaining the permission from the owner(s) of the respective information might violate the rights of the owner.
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