1. Speciation and Equilibria of Malic Acid and Citramalic Acid with Aluminum
Marcus Harris and Yahia Z. Hamada, LeMoyne-Owen College, Memphis, TN
Chemistry
Abstract
From our ongoing efforts to study the interaction of various hydroxy carboxylates with variety of essential and non-essential metal ions (Inorg. Chem. 2003, 42, ), (Syn. and Reac. in Inorg. Metal-Org and Nano-Metal Chem , ), and (Syn. and Reac. in Inorg. Metal-Org and Nano-Metal Chem , ) we are presenting accurate potentiometric work in aqueous solutions at 25oC for the interaction of Al(III) with malic acid and Al(III) with citramalic acid. Although malic acid and citramalic acid have a slight structure difference (the former has a hydrogen on its chiral center while the latter has a methyl on its chiral center); there are great differences in their behavior in solutions independently and when reacting with the Al(III) ion. Our data presented are in good agreement with what have been reported in the literature. The proper speciation and simulation diagrams of these reaction systems will be presented and discussed.
No of runs
No of Equivalents
Remarks
Proposed complexes
Free Citramalic acid 5
1.89 ± 0.15
Citramalic acid is a di-protic acid
Citramalate2-
Al:Citramalic acid in 1:1 3
4.33 ± 0.12
4.3 H+ are released
A trimer may be formed
Al:Citramalic acid in 1:2
5.87 ± 0.15
6 H+ are released
One-to-one and the bis-complexes
Al:Citramalic acid in 1:3
8.19 ± 0.47
8 H+ are released
One-to-one and the bis
No of runs
No of Equivalents
Remarks
Proposed complexes
Free aluminum 11
3.02 ± 0.08
Al- releases 3 H+
Al(OH)3
Free Malic acid 4
2.05 ± 0.12
Malic acid is a di-protic acid
Malate2-
Al:Malic in 1:1.3 ratio 3
3.90 ± 0.09
4 H+’s are released
One-to-one complex
Al:Malic in 1:2.3 ratio
6.59 ± 0.19
6.5 H+ are released
A dimer is formed
Al:Malic in 1:3.1 ratio
8.59 ± 0.14
8.5 H+ are released
Experimental
The potentiometric titration solutions were contained in a 250 mL beaker equipped with a magnetic stirring bar. The beaker was covered with a custom made Teflon cover. In a typical titration the ligand malic or citramalic were added first then the metal ion (Al3+) solution was added followed by the addition of the appropriate amount of water to take the total volume to 50 mL. Details of solution preparation have been documented in references number one.
References
[1] Y. Z. Hamada et. al., Accurate potentiometric studies of chromium-citrate and ferric citrate complexes in aqueous solutions at physiological and alkaline pH values Syn. and Reac. in Inorg. Metal-Org and Nano-Metal Chem ,
[2] L. Alderighi, P. Gans, A. Ienco, D. Perters, A. Sabatini, and A. Vacca, Hyperquad simulation and speciation (Hyss): a utility program for the investigation of equilibria involving soluble and partially soluble species, Coord. Chem. Rev., ,
[3] A. E. Martell, R.-M. Smith, and R. J. Motekaitis, Critical Stability Constants Database, Version 6.0, NIST, Texas A & M University, College Station, TX, USA 2001
[4] Y. Z. Hamada, B. L. Carlson, and J. Dangberg, Interaction of malate and lactate with Chromium(III) and iron(III) in aqueous solutions, Synthesis and Reactivity in Inorganic Metal-Organic and Nano-Metal Chemistry ,
Acknowledgment
NSF SMARTUP Grant # HRD and Henry McBay, Grant
Conclusion
The ion exchange resin confirmed the concentration of the Al3+
The potentiometric titrations of the Al3+ malic acid showed the presence of one to one complexes and a dimer complex
The potentiometric titrations of the Al3+ citramalic acid showed the presence of one to one complexes and a trimer complex
Both malic and citramalic acids solubilized Aluminum throughout the entire pH-range