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Short-term toxicity to aquatic invertebrates

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Description of key information

The 48-hour EC50 value for D-glucono-1,5-lactone was partially read-across from that of sodium D-gluconate.  Sodium D-gluconate was reported to have a 48 hour EC50 of greater than 1000 mg/L in Daphnia magna.  The 24-hour EC50 for D-glucono-1,5-lactone was reported to be 305 mg/L.  
Daphnia magna (sodium D-gluconate): EC50 (48 hrs): > 1000 mg/L
Daphnia magna (D-glucono-1,5-lactone): EC50 (24 hrs) = 305 mg/L.

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Additional information

The short-term toxicity of D-glucono-1,5-lactone in aquatic invertebrates was partially read across from study data generated for sodium D-gluconate. Sodium D-gluconate was reported to have a 48 hour EC50 of greater than 1000 mg/L in Daphnia magna (Stiene, 2001). The study was conducted according to OECD Guideline 202 with no exceptions, as well as EU Method C.2 with no exceptions. The study also followed GLP and was rated ‘1’ for reliability under Klimisch standards.

Rübelt (1997) conducted an acute toxicity study in Daphnia magna using D-glucono-1,5-lactone. The study was not conducted according to GLP; however, it followed OECD Guideline 202 with exceptions. The 24-hour EC50 for D-glucono-1,5-lactone was reported to be 305 mg/L. Based on the deviations from OECD protocols and the lack of compliance with GLP, this study was rated ‘3’ (unreliable) according to Klimisch standards.

D-glucono-δ-lactone is a cyclic ester of gluconic acid which, in aqueous solution, forms an equilibrium mixture of the lactone and gluconic acid. Gluconic acid is a somewhat weak carboxylic acid with a dissociation constant of pKa = 3.6. The dissociation of an acid into a proton and an anion is an equilibrium, the reverse of which is the re-association of that same anion with a proton to reform the original acid. The pKa of 3.6 means that, when the ambient pH = 3.6, half the gluconic acid molecules will exist in the form of the uncharged acid, and half as the anion. At pH < 3.6, the undissociated form will predominate, and pH > 3.6 the anion will predominate. Sodium gluconate and potassium gluconate are both 1:1 salts of gluconic acid, which will each dissolve in water to generate separate sodium or potassium cations and gluconate anions. Sodium and potassium are both strong bases, and are therefore expected to remain ionized at essentially any pH, but the gluconate anions deriving from the salts will be subject to the same equilibrium as those deriving from the free acid. To be in equilibrium, both the forward and the backward reaction must possess the same pKa value, so the gluconate anion is predicted to posses the same pKa of 3.6 as the free acid. In this way, gluconic acid in aqueous solution is in equilibrium with its cyclic esters and its anion, according to the pH of the system, and in any system with sufficient buffering capacity, the effects of introducing equimolar amounts of gluconic acid, D-glucono-δ-lactone, sodium gluconate or potassium gluconate would be indistinguishable. Hence these four substances are considered to be appropriate surrogates for each other in sufficiently buffered aqueous systems, such as environmental waters, flora and fauna.