Reaction mass of L-xylo-hex-2-ulosonic acid and ascorbic acid

Dissociation constant

Description of key information

KGA Greens consists primarily of KGA (2-keto-gulonic acid) and ascorbic acid.  
Following ECHA decision CCH-D-2114306527-52-01/F on 30 July 2015, Cargill performed the required tests for KGA and Mesoxalic acid substances.
The key value below shows the result we have from the main constituent: KGA.

Key value for chemical safety assessment

pKa at 20°C:

2.69

Additional information

Dissociation constants of KGA (L-xylo-hex-2-ulosonic acid) and mesoxalic acid (keto-malonic acid) were determined using the titrimetric method according the OECD 112 guideline.

Potassium hydrogen phthalate (KHP) was used as primary standard reagent for standardization of the carbon dioxide-free sodium hydroxide solution.

0.1 M of KHP solution was prepared in 100 ml HPLC grade water. KGA solution (25 ml, 0.1 M) was accurately pipetted into a 150 ml of Erlenmeyer flask with a stir bar. Mesoxalic acid monohydrate disodium salt (1.840 g) was dissolved in a 100 ml of HPLC grade water providing a final concentration of 0.1 mol/L. Twenty five mL of the above solution was accurately pipetted into a 150 ml of Erlenmeyer flask. Concentrated hydrochloric acid 37% (0.45 ml) was added to acidify the mesoxalic acid salt solution into acid form. The acidified solution was used to perform the titration experiment. The flask was placed on a thermoelectric temperature-regulating magnetic stirrer. The temperature was monitored constantly and maintained at 20 ºC. The pH probe was calibrated at two points (pH 7 and 4) using commercial potassium hydrogen phthalate buffer solution (Certipur, Merck). Titrations were performed under an atmosphere of nitrogen using Metrohm Titrono 718 Stat and phenolphthalein was used as an indicator to determine the end point. The addition of NaOH titrant was conducted at speed of dv/dt = 1-2 by automatic titration and the pH values were recorded manually. Equivalence points and pKa values are obtained from plots of titrant volume versus pH. The pKa is derived from the pH values corresponding to the half-equivalence points are obtained from the titration curves using geometric method. The half-equivalence point is where half the amount of titrant has been added.

The pKa of KGA is 2.69 ± 0.02 determined at temperature of 20 ºC. Mesoxalic acid has pKa1 value of 2.01 ± 0.05 and pKa2 value of 2.82 ± 0.05 determined at 20°C.