Application Note 06

Synthesis StarterKit Production of Persulfate

Description

This Application Note describes the oxidative transfor- mation of sodium sulfate to sodium persulfate by using a BDD electrode. The setup is depicted in Figure 1. The following chemical equation is summing up all reactants and products:

2 Na2SO4 + 2 H2O → Na2S2O8 + 2 NaOH + H2

The oxidation number of the sulfate atoms is staying constant while the oxidation number of two oxygen atoms if changing from -II to –I in the peroxo group.

Experiment Figure 1: For the experiment, 1000 ml of a 1 mol/l sodium sulfate Setup to produce persulfate solution is prepared in beaker glass suitable to be used with the Synthesis StarterKit. The volume flow is set to 2 l/min. The electrosynthesis is carried out at 1.3 A which corresponds to a current density of j = 0.4 A/cm². Table 1 shows the experimental data collected during the experiment.

Table 1: Experimental data obtained during the experiment Charge Time Current Voltage Temp. V(Thio) c(Ox) [Ah/L] [min.] [A] [V] [°C] [ml] [mmol/l] 0 0 1.3 5.6 20.5 ------0.22 10 1.3 5.7 23.0 0.90 2.15 0.44 20 1.3 5.7 26.0 1.55 3.60 0.66 30 1.3 5.7 28.0 2.30 5.16 0.89 40 1.3 5.7 30.0 2.80 6.14 1.13 50 1.3 5.6 32.0 3.40 7.26 1.37 60 1.3 5.6 33.5 4.00 8.33

The concentration of oxidizer was determined by iodometry ( see appendix and App Note 07)

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Application Note 06

Synthesis StarterKit Production of Persulfate

Figure 2: Graphical representation of the evolution of persulfate concentration

In Figure 2 the graphical representation if given for the evolution of the persulfate concentration as a function of charge. The persulfate anion is kinetically hindered. Hence, most processed which are taking use of the oxidizer are carried out at elevated temperature or using catalysts.

For further information, the following literature is recommended:

Freitas Araújo et al., Journal of Electroanalytical Chemistry 2020 Serrano,et al., Electrochimica Acta. 2002

Appendix – Iodometry (see App Note 07 for more details)

The concentration of oxidizer can be determined by iodometry. In this procedure, iodide is oxidized to iodine which is forming a blue dye with starch solution. Titration with sodium thiosulfate is reducing the iodine. The end is reached when the solution turns colorless.

2− − 2− 2− − 2− S2O8 + 2 I → I2 + 2 SO4 I2 + 2 S2O3 → 2 I + S4O6

2− n(S2O8 ) 1 n(I2) 1 = 2− = n(I2) 1 n(S2O3 ) 2 1 1 n(S O2−) = n(S O2−) = c(S O2−) ⋅ V(S O2−) 2 8 2 2 3 2 2 3 2 3