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Administrative data

Link to relevant study record(s)

Description of key information

No bioaccumulation potential

Key value for chemical safety assessment

Bioaccumulation potential:
no bioaccumulation potential

Additional information



The pure active substance, phosphorous acid (H3PO3), is phytotoxic, due to its acidic character. Potassium ions are used for neutralization of the acid. The registered substance is a potassium salt of phoshorous acid:



One hydrogen atom is bound to the phosphorus with a covalent bond and does not dissociate. Therefore, only two hydrogen atoms can be replaced with a cation.

The part of the molecule with the biological activity is the phosphite ion (HPO32-),


 Chemistry of phosphorous acid

The chemistry of phosphorous acid, the acid from which the phosphite ion is derived, is important for understanding the chemistry, the definition of the characteristics, as well as the testing and evaluating of the product. Phosphorous acid has two tautomers (i .e., two molecular configurations, which rapidly convert between each other):


The equilibrium favours the phosphonic acid tautomer , in which the phosphorus is in a pentavalent form [ i.e., (OH)2HPO], due to the strength of the phosphoryl group (P=O), thus under most circumstances the predominant tautomer is the phosphonic acid (RICKARD 2000).

The corresponding pKa-values for the dissociation of the hydrogen ions are:

pKa1= 1.3

pKa2 = 6.7

The low oxidation level of the phosphorus atom in the phosphorous acid, compared to phosphoric acid (H3P04), indicates a high reduction potential of phosphorous acid. However, phosphite reacts slowly with many oxidants (OHASHI 1972), and is surprisingly stable when exposed to atmospheric oxygen (ROBERTSON & BOYER 1956b). Thus, the proposed product is stable in storage and transport, and in dilutions prior to application.


Phosphorous acid forms two kinds of salts: easily water-soluble primary salts are formed with monovalent metals, such as alkali me als (potassium, sodium ), and forms soluble secondary salts with heavy metals, e.g. iron (HOLLEMAN-WIBERG 1971). A basic discussion of the chemistry of lower oxy-acids of phosphorus, including phosphorous acid, is given by WAZER (1958), COTTON & WILKINSON (1966), OHASHI (1964 & 1972).



Study with sodiumphosphite

Greger JL, Kaup S.M., Behling A.R. (1991); Calcium, Magnesium and phosphorus Utilization by rats fed sodium and potassium salts of various inorganic anions. J. Nutr. 121: 1382-1388.


Greger et al. (1991) showed that ingestion of inorganic anions increased urinary escretion of these anions regardless of whether the anions were ingested as sodium or potassium salts. The cations (sodium or potassium) had no effect on the toxicokinetics of the anions. Thus studies of the toxicokinetics of sodium phosphite may be used to make inferences to potassium phosphite.

When sodium phosphite- 32P was orally administered to male and female rats phosporous acid was mainly excreted in the urine (59-65%). A smaller amount (30-33%) was found in the faeces. Only minor amounts (1.2%) of the ad ministered radioactivity were found in the body 72 hours after cessation of dosing. The highest amount found in the spleen.

The level of radioactivity in blood reached a maximum 1 to 2.5 hours after the initial dosing. The disappearance of radioactivity from the blood seems to occur in two stages, first a rapid one (Tl/2-1-3 hours) then a much slower one. (PELFRENE 1982a).


In conclusion, phosphite is excreted via urine without undergoing metabolism . It does not cause residues of concern in tissues of exposed mammals.


Acute toxicity studies

No adverse affect, no systemic toxicity, no accumulation in organs has been reported during hystopathological examination after acute toxicity studies performed on oral and dermal exposure

Repeated dose studies

No adverse affect, no systemic toxicity, no accumulation in organs has been reported during hystopathological examination after repeated dose toxicity studies performed on oral exposure