Protection of Ascorbic Acid from Copper(II)−Catalyzed Oxidative Degradation in the Presence of Fruit Acids: Citric, Oxalic, Tartaric, Malic, Malonic, and Fumaric Acids

Figures & data

Figure 1 Kinetic curves for copper-catalyzed autoxidation of AA, without fruit acid; Cu(II) at [(A: 0.785, B: 1.57, C: 3.14, D: 4.71, E: 6.28) × 10⁻⁷ M].

Figure 2 Kinetic curves for copper-catalyzed autoxidation of AA: (a) citric acid at 1.0 × 10⁻³ M; (b) oxalic acid at 1.0 × 10⁻³ M; (c) malonic acid at 1.0 × 10⁻³ M; Cu(II) at [(A: 0.785, B: 1.57, C: 3.14, D: 4.71, E: 6.28) × 10⁻⁷ M].

Figure 3 Kinetic curves for copper-catalyzed autoxidation of AA: (a) malic acid at 1.0 × 10⁻³ M; (b) tartaric acid at 1.0 × 10⁻³ M; (c) fumaric acid at 1.0 × 10⁻³ M; Cu(II) at [(A: 0.785, B: 1.57, C: 3.14, D: 4.71, E: 6.28) × 10⁻⁷ M].

Figure 4 k_d as function of Cu(II) concentration in 1.0 × 10⁻³ M carboxylic acid containing solutions buffered to pH 4.5 with acetate. [A: without fruit acid, B: fumaric acid, C: tartaric acid, D: malic acid, E: malonic acid, F: oxalic acid, G: citric acid]. k_d : difference between first-order rate constant in the presence and absence of Cu(II); slopes of lines yield; K: specific rate constants.

Table 1 The specific rate constants at pH 4.5 for autoxidation of ascorbic acid catalyzed (inhibited) by fruit acid-Cu(II) complexes (fruit organic acids each at 1 mM concn.)

Name of organic acid	Specific rate constant K ± sk (M^−1 min^−1)
Citric acid	(2.42 ± 0.16) × 10^4 (r ^2 = 0.9827)
Oxalic acid	(3.19 ± 0.14) × 10^4 (r ^2 = 0.9922)
Malonic acid	(3.74 ± 0.23) × 10^4 (r ^2 = 0.9848)
Malic acid	(4.32 ± 0.20) × 10^4 (r ^2 = 0.9913)
Tartaric acid	(6.56 ± 0.17) × 10^4 (r ^2 = 0.9972)
Fumaric acid	(7.99 ± 0.09) × 10^4 (r ^2 = 0.9995)
For Cu(II) alone (without fruit acid, in pH: 4.5 acetate buffer), K ± sk (M^−1 min^−1) = (11.64 ± 0.25) × 10^4 (r ^2 = 0.9982).
sk = The standard deviation of K values, r ^2 = linear regression coefficient (squared).
K = Specific rate constant independent of concentration, K = kd /[Cu(II)].

Table 2 Specific inhibition (I _s ) of copper−catalyzed autoxidation of AA at pH = 4.5 by the tested fruit acids, with the relevant acidity and stability constants

Fruit acid	pK a1	pK a2	pK a3	Log β1 ^a	Log β2 ^a	α L	Is
Oxalic acid	1.2	4.2	—	6.16 ^c (5.98)	8.50 ^c (8.15)	6.7 × 10^−1	726
Tartaric acid	3.2	4.8	—	3.2 (2.72)	5.11 (4.15)	3.3 × 10^−1	436
Citric acid	2.99 ^b	4.3 ^b	5.65 ^b	6.03 (4.64)	10.4 (7.66)	5.8 × 10^−1	792
Malic acid	3.46	5.1	—	3.6 (2.87)	—	1.88 × 10^−1	629
Malonic acid	2.8	5.7	—	5.0 (3.76)	—	5.8 × 10^−2	679
Fumaric acid	3.03	4.44	—	2.51 (2.23)	—	5.24 × 10^−1	314
^aThe values in parentheses are the corresponding conditional stability constants at pH = 4.5, i.e., Log β1′ and Log β2′, respectively, calculated with the aid of Eqs. (5) and (6).
^bTaken from Buffle et al.[50]
^cTaken from Dean.[49]

Buffle , J. , Greter , F.-L. and Haerdl , W. 1997 . Measurement of complexation properties of humic and fulvic acids in natural waters with lead and copper ion-selective electrodes . Analytical Chemistry , 49 : 216 – 222 .

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Dean , J.A. 1978 . Lange's Handbook of Chemistry , 12th , 5–54 – 5–68 . New York : McGraw-Hill Book Co .

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