Voltammetric Behaviour of Rhodamine B at a Screen-Printed Carbon Electrode and Its Trace Determination in Environmental Water Samples

Abstract

The voltammetric behaviour of Rhodamine B was studied at a screen-printed carbon electrode (SPCE), by cyclic and differential pulse voltammetry. Cyclic voltammograms exhibited two reduction peaks (designated R1 and R2) generated from the reduction of the parent compound through, first, one electron reduction (R1) to give a radical species, and then a further one-electron, one-proton reduction to give a neutral molecule (R2). On the reverse positive-going scan, two oxidation peaks were observed. The first, O1, resulted from the oxidation of the species generated at R2, and the second, O2, through the one-electron oxidation of the amine group. The nature of the redox reactions was further investigated by observing the effect of scan rate and pH on the voltammetric behaviour. The developed SPCE method was evaluated by carrying out Rhodamine B determinations on a spiked and unspiked environmental water sample. A mean recovery of 94.3% with an associated coefficient of variation of 2.9% was obtained. The performance characteristics indicated that reliable data may be obtained for Rhodamine B measurements in environmental water samples using this approach.

Keywords: Rhodamine B; screen-printed carbon electrode; voltammetry; water.

Scheme 1

Structure of Rhodamine B.

Figure 1

Cyclic voltammogram, obtained at a scan rate of 100 mVs⁻¹, for a 1.0 mM solution of Rhodamine B in 0.1 M phosphate at pH 8.3. (a) Starting potential 0.0 V, initial switching potential −1.5 V, second switching potential +1.5 V, final potential −0.5 V and (b) second scan of the same SPCE. Voltammetric conditions as for Figure 1a.

Figure 2

(a) Starting potential 0.0 V, initial switching potential −1.5 V, second switching potential +0.5 V, final potential −0.5 V and (b) starting potential 0.0 V, initial switching potential −0.7 V, second switching potential +1.5 V, final potential −0.5 V.

Figure 3

Typical cyclic voltammograms obtained for 1.0 mM Rhodamine B in (a) pH 1.92, (b) 3.51, (c) pH 7.17, (d) pH 8.00, (e) pH 8.78 and (f) pH 11.4.

Figure 4

Plot of Ep vs. pH for Rhodamine B. Crosses O2; triangles O1; diamonds R1 and squares R2. Voltammetric conditions as Figure 1.

Figure 5

The pH dependence for the spirolactam form of Rhodamine B.

Figure 6

Plots of peak current vs. square root of scan rate for (a) O1, (c) O2, (e) R1 and (g) R2. Plots of current function for (b) O1, (d) O2, (f) R1 and (h) R2. Other voltammetric conditions as for Figure 1a.

Figure 7

Plot of i_p vs. pH for Rhodamine B (a) oxidation processes, O1 and O2 and (b) reduction processes, R1 and R2. Crosses O2; triangles O1; diamonds R1 and squares R2. Voltammetric conditions as Figure 1. Error bars represent plus or minus a standard deviation.

Figure 8

Proposed scheme for the voltammetric behaviour of Rhodamine B at the SPCE.

Figure 9

Differential pulse voltammograms obtained with an SPCE for solid line, 2.1 µgmL⁻¹ Rhodamine B in 0.1 M phosphate buffer pH 7.1. Dashed line, buffer only. Voltammetric conditions: 0.0 V 15 s vs. Ag/AgCl. Pulse repetition time 0.2 s, step height 2.4 mV in the positive potential by applying pulse amplitude of 50 mV, pulse duration 50 ms.

Figure 10

Effect of Rhodamine B concentration on the differential pulse voltammetric current peak (i_p) for oxidation peak O2. Insert, linear section. Each point is the mean of three separate SPCEs. Error bars represent plus and minus a standard deviation.

Figure 11

Differential pulse voltammograms peak currents obtained with an SPCE for 2.1 µgmL⁻¹ Rhodamine B in 0.1 M phosphate buffer pH 7.1 with increasing concentrations of ascorbic acid. Voltammetric conditions as for Figure 7. Each point is the mean of three individual SPCEs. Error bars represent plus and minus a standard deviation.

Figure 12

Differential pulse voltammogram of Rhodamine B in an environmental water sample adjusted to be 0.1 M pH 7.1 phosphate buffer, 0.8 M ascorbic acid. Insert shows same voltammogram at larger scale.