Kinetics and Mechanism of Oxidation of Glutamic Acid with Bi(V) Phosphato Complex

Aims and Background

Our aim to undertake study of the reaction of Glutamic acid with Bi(V) was basically to determining the pattern of reactivity of Glutamic acid towards Bi(V) in Phosphoric acid medium.

Material and Methods

Bi(V) solution :  There is only one known method¹ that utilizes a mixture of HClO₄ and HF for digesting sodium bismuthate to obtain Bi(V) in the solution. Such solution in HClO₄ – HF mixture are not stable at ambient temperature. Morever, the speciation of Fluorobismuth(V) species in such solution is yet to be resolved. Further the method of standardization of this solution requires critical conditions. However, there problem are not encontered for Bi(V) solution in phosphoric acid medium. Recently, kinetics of oxidation of various compounds by Bi(V) in HClO₄ – HF mixtures have been reported², but the problem of species of Bi(V) species still remain unsolved. We attempt to get Bi(V) as solution by using aqueous phosphoric acid. The requisite quantity of sodium bismuthate was digested in aqueous H₃PO₄ (3.0 mol dm^–3). Solution for 20 minutes after filtration a pink coloured solution were obtained. The solution was standardized iodometrically^3–6.

All the other materials used were of AnalR grade doubly distilled water was used through out the investigation

Kinetic Procedure

The requisite quantities of various components of the reaction mixture, except Glutamic acid, were taken in glass – stoppered Erlenmeyer flasks, which were then suspended in a water bath thermostated at the desired temperature ± 0.1⁰C unless stated otherwise. The reaction was initiated by adding there requisite solution of the temperature pre-equilibrated Glutamic acid into the reaction mixture, and the time of initiation was recorded when half of the contents from the pipette were released. Aliquot samples (5 to 10 cm^–3) were withdrawn at different intervals of time and then quenched in an ice-cold KI (10%) solution; the liberated iodine was titrated against thiosulphate solution using starch as an indicator. Measurements in triplicate without any interference from the ingredients of the reaction mixture were in agreement to within ± 5%. Initial rates were computed employing plane mirror method⁷.

Results

Stoichometry

Several reaction mixture with an excess of Bi(V) over Glutamic acid (GA) at fixed concentration of Phosphoric acid (3.0 mol dm^–3) were allowed to react in a water bath thermostated at 35±0.1⁰C. They were kept of 8 hrs. and excess of Bi(V) was estimated iodometrically after ensuring the completion of the reaction. The stoichometry of the reaction was formed as (Table-1) as represented by the equation (1).

COOH          NH₃⁺

|                    |

CH₂ – CH₂– CH – COOH + Bi(V) + H₂0

COOH

|

→Bi(III) + CH₂ –CH₂ – CHO + NH₃ + CO₂ + 3 H⁺          …(1)

The slightly higher ratio is due to itself decomposition of Bi(V). The oxidation product a aldehyde was detected by spot test⁸. The oxidation product of amino acid has been identified as an aldehyde by other reactions.

Bi(V) And Glutamic Acid [Ga] Dependence

The concentration of Bi(V) an Glutamic acid were varied in the range of (1.5 – 9.8) x 10³ mol dm^–3 and (1.0–20.0) x 10³ mol dm^–3 respectively at fixed concentration of Phosphoric acid (3.0 mol dm^–3). The initial rate (ir) were computed by plane mirror method⁷. The plot of initial rate (ir) against the  concentration of Bi(V) yielded a straight line while in the case of the substrate it is a curve. Thus the reaction order with respect to oxidant is one and the order with respect to Glutamic acid seemed to be complex one (Table 2, 3, Fig.-1,2).

The concentration of Glutamic acid was varied from (2–10) 10^-2 mol dm^–3 at five different concentration of Bi(V) and also at fixed concentration of Phosphoric acid 3.0 mol dm^–3.

The initial rates were calculated by plane mirror method. The reaction rate initially increases and finally ends towards a limiting value.

---

This work is licensed under a Creative Commons Attribution 4.0 International License.