Potassium thiosulphate

Endpoint summary

Description of key information

Studies on the aquatic toxicity of potassium thiosulfate are not available, and thus, read-across to thiosulfate and sulfite/disulfite substances is applied.Upon contact with water, salts of sulfur oxyacids including potassium thiosulfate dissociate into sulfur oxyacid anions and the respective counterions. Only the properties of the thiosulfate anion are considered a relevant determinant of environmental toxicity since the potassium cations are essential and have a very low potential for acute toxicity to freshwater and marine fish. Please refer to the respective endpoint summary for potassium (K) for further details.

Thiosulfate anions are unstable under environmentally relevant conditions, will disproportionate to sulfite and will further be oxidized to sulfate or reduced to sulfide (please refer to the endpoint summary on environmental fate and behaviour).

Acute and chronic toxicity studies with thiosulfate and sulfite/disulfite substances are available for three trophic levels. In studies with controlled oxygen concentrations and pH, acute toxicity for all three trophic levels (freshwater fish and algae and freshwater and marine invertebrates) is only observed at concentrations above the respective OECD test limit of 100 mg/L.

In chronic studies with controlled oxygen concentrations and pH, effect values are unbounded (D. rerio, D. magna, P. subcapitata) or above the chronic OECD test limit of 10 mg/L (D. subspicatus, C. vulgaris, C. reinhardtii).

Effects of thiosulfate and sulfite substances on the respiration of activated sludge were not observed at the OECD test limit of 1000 mg/L, respectively.

It is thus concluded that thiosulfate and sulfite/disulfite substances, including potassium thiosulfate, are not toxic to freshwater fish, invertebrates, algae and microorganisms.

Additional information

Thiosulfate and sulfite/disulfite substances - Summary of acute toxicity data

Reliable acute toxicity data are available for several aquatic freshwater species covering three trophic levels (primary producers, primary and secondary consumers) and one saltwater species (invertebrates). The table below provides an overview of the effect values for the acute toxicity of thiosulfate and sulfite/disulfite substances applied in a weight of evidence approach.

Table: Overview of reliable acute toxicity data for thiosulfate and sulfite/disulfite substances

Species	Parameter	Endpoint	EC [mg SO3^2-/L]	Observed interference	Test substance	Reference
Oncorhynchus mykiss	mortality	96 h LC50	149.6	pH decrease*	disodium disulfite	BASF, 1981
Leuciscus idus	mortality	96 h LC50	200.5	oxygen depletion**	sodium sulfite	BASF, 1989
Leuciscus idus	mortality	96 h LC50	159.7	oxygen depletion**	potassium sulfite	BASF, 1989
Danio rerio	mortality	96 h LC50	490	oxygen depletion**	dipotassium disulfite	BASF, 1995
Lepomis macrochirus	mortality	96 h LC50	275.5	oxygen depletion**	ammonium thiosulfate	Springborn Binomics Inc, 1986
Oncorhynchus mykiss	mortality	96 h LC50	416	oxygen depletion**	ammonium thiosulfate	Springborn Binomics Inc, 1986
Daphnia magna	mobility	48 h EC50	74.9	oxygen depletion**	disodium disulfite	BASF, 1990
Daphnia magna	mobility	48 h EC50	124.2	none	ammonium thiosulfate	Springborn Binomics Inc, 1986
Mysidopsis bahia	mortality	96 h LC50	41.6	oxygen depletion**	ammonium thiosulfate	Springborn Binomics Inc, 1994
Desmodesmus subspicatus	growth rate	72 h EC50	36.8	pH decrease*	disodium disulfite	BASF, 1989
Pseudokirchneriella subcapitata	growth rate	72 h EC50	>54	none	ammonium thiosulfate	ECT, 2010

 *pH decrease: solution pH at “toxic” test concentrations is outside OECD test criteria

**oxygen depletion: oxygen concentration of medium at “toxic” test concentrations is outside OECD test criteria

 

In tests with ammonium thiosulfate and without experimental interferences (e.g., oxygen depletion in the test medium), the effect concentration was either above the OECD test limit of 100 mg/L (test with ammonium thiosulfate and D. magna; 48h EC50 =124.4 mg/L), or above the highest tested concentration of 100 mg ammonium thiosulfate/L (corresponding to 54 mg SO32-/L; test with ammonium thiosulfate and P. subcapitata; 72 h EC50).

Other acute effect concentrations (EC/LC50s) are i) unbounded, ii) above the test limit of 100 mg/L as defined in the respective OECD guideline (OECD 201, 202, 203), and/or iii) presumably caused by to oxygen depletion and/or a pH decrease and do not represent the intrinsic toxicity of sulfite/disulfite and thiosulfate substances. Thus, it may be assumed that the “substance-specific” EC/LC50 of thiosulfate substances for freshwater fish and algae and freshwater and marine invertebrates is higher than the EC50/LC50s derived in studies applying a test system without proper buffering capacity for oxygen concentrations and pH.

It is thus concluded that thiosulfate and sulfite/disulfite substances, including potassium thiosulfate, are not acutely toxic to freshwater fish and algae and freshwater and marine invertebrates. Additional studies on the acute effects of disodium disulfite toDaphnia magna and Raphidocelis subcapitata according to OECD 202 and OECD 201, respectively, with appropriate aeration and buffering of thetest system, are currently ongoing.

Thiosulfate and sulfite/disulfite substances -Summary of chronic toxicity data

Chronic toxicity data are available for several aquatic freshwater species covering three trophic levels (primary producers, primary and secondary consumers). The table below provides an overview of the effect values for chronic toxicity of sulfite/disulfite and thiosulfate substances.

Table: Overview of reliable chronic toxicity data for thiosulfate and sulfite/disulfite substances

Species	Parameter	Endpoint	EC [mg SO3^2-/L]	Observed interference	Test substance	Reference
Danio rerio	all (ELS)	34 d NOEC	≥ 200.5	none	sodium sulfite	ECT, 2010
Daphnia magna	reproduction, mortality	21 d NOEC	≥ 8.41	none	disodium disulfite	BASF, 1993
Desmodesmus subspicatus	growth rate	72 h EC10	28	pH decrease*	disodium disulfite	BASF, 1989
Pseudokirchneriella subcapitata	growth rate	72 h NOEC	≥ 54	none	ammonium thiosulfate	ECT, 2010

 *pH decrease: solution pH decreases at higher test concentrations

**oxygen depletion: oxygen concentration of medium at “toxic” test concentrations is outside OECD test criteria

 

Chronic effect values of ammonium thiosulfate are unbounded, without effects at the highest test concentration (P. subcapitata). Other chronic effect values are i) unbounded, without effects at the highest test concentration (D. rerio, D. magna) or ii) above the chronic OECD test limit of 10 mg/L (D. subspicatus).

It is thus concluded that thiosulfate or sulfite/disulfite substances, including potassium thiosulfate, are not chronically toxic to freshwater fish, invertebrates and algae.

 

Toxicity to STP microorganisms

Regarding the toxicity to microorganisms, reliable studies of the respiration inhibition of activated sludge and supporting studies of the growth inhibition of Pseudomonas putida are available (see Table below).

Table: Overview of reliable toxicity data to microorganisms for thiosulfate and sulfite/disulfite substances

Species	Parameter	Endpoint	EC [mg SO3^2-/L]	Test substance	Reference
Activated sludge	respiration inhibition	3 h EC50 3 h NOEC	> 634.4 ≥ 634.4	sodium sulfite	ECT, 2010
Activated sludge	respiration inhibition	3 h EC50 3 h NOEC	> 540.2 ≥ 540.2	ammonium thiosulfate	ECT, 2010
Pseudomonas putida	growth inhibition	17 h EC50 17 h EC10	47.2 25.9	disodium disulfite	BASF, 1988a
Pseudomonas putida	growth inhibition	17 h EC50 17 h EC10	207.2 77.3	potassium sulfite	BASF, 1988b
Pseudomonas putida	growth inhibition	17 h EC50 17 h EC10	65.8 23	potassium disulfite	BASF, 1988c

 

Effects on the respiration of activated sludge were not observed at the OECD test limit of 1000 mg/L in studies with ammonium thiosulfate or sodium sulfite, respectively.

The hazard assessment is based on effect concentrations for the respiration inhibition of activated sludge in accordance with ECHA guidance on IR & CSA, Chapter 7b (Version 4.0, 2017).

It is thus concluded that thiosulfate and sulfite/disulfite substances, including potassium thiosulfate, are not toxic to aquatic microorganisms.