Reactions of Cr³⁺ with aspartic acid within a wide pH range

Figures & data

Figure 1. (a) Four potentiometric titrations of free Asp in 0.1 M NaNO₃, 25°C. Sharp inflections separate the ammonium proton from the β-carboxylic acid proton. The insets are (b) correlation of the volume of added titrant in mL vs. potential in mV, and (c) is the correlation of measured pH values (pH_obs) vs. potential in mV.

Figure 2. Representative graph of potentiometric titrations of Cr³⁺: Asp in 1:2 ratio in 0.1 M NaNO₃, 25°C. The 1:1 and the 1:3 ratios show similar titration patterns.

Table 1.  The exact locations of the inflection points for Cr³⁺: Asp potentiometric titrations in different molar ratios at 25°C and 0.1 M NaNO₃.

	Minor inflection point (Eq.^a of NaOH)		Major inflection point (Eq. of NaOH)
Titration system	Average	SD	Average	SD
Cr^3+: Asp
0:1(4)^b	n/a	n/a	0.96	0.03
1:0(7)	0.90	0.02	2.97	0.07
1:1(4)	1.89	0.08	3.34	0.08
1:2(4)	2.83	0.11	4.18	0.12
1:3(4)	3.99	0.18	5.67	0.18

^aEq. is defined as the number of moles of NaOH per number of moles of the metal ion.

^bNumber of runs.

Figure 3. Distribution diagram of Cr³⁺: Asp in 1:2 ratio using program HySS³⁰.

Figure 4. UV–Vis absorption spectra of Cr³⁺: Asp in 1:2 ratio. [Cr³⁺]=1.9×10⁻⁴ M. Equilibrium time was set to 50 days.

Scheme 1. Structural formula Aspartic acid (Asp) in its Zwitter ionic form used in this study.

Scheme 2. The proposed binding mode of Asp to Cr³⁺ in aqueous solutions.