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Hey! Free space! Guess I'll use this as my ELN, then.

Potassium bromate

Goal: Investigation of different conditions for the electrolysis of potassium bromide to potassium bromate.


Anodic oxidation: 2 Br- → Br2 + 2 e-
Cathodic reduction: 2 H2O + 2 e- → H2 + 2 OH-

Br2 + OH- → BrO- + Br- + H+
3 BrO- → BrO3- + 2 Br-

3 H2O + Br- → BrO3- + 3 H2

Run #1:

Name Amount [mmol] Mass [g] Concentration [mol/L]
Potassium bromide 400.0 47.60 2.000
Potassium dichromate 0.85 0.25

The reagents were dissolved under stirring in deionized water ad 220 mL in a 250 mL beaker. Platinized titanium (0.25 µm) served as the anode, stainless steel (V2A) as the cathode - both in the form of 8 mm rods submersed about 4.5 cm in the solution (11.56 cm2) 40 mm apart. A constant current of 2.89 A (250 mA/cm2) at initially 5.8 V was run through the solution, while the electrolyte was stirred at 250 RPM. The temperature increased to around 53 °C during this time, while the voltage dropped to 4.9 V. Vigourous bubbling was observed at the cathode, and slight bubbling at the anode.

After 3.5 h corrosion had appeared above the solution on the cathode and had contaminated the liquid. A titanium cathode was swapped in place, the solution filtered while warm and the electrolysis resumed. After 6 h a marked increase of oxygen formation at the anode had been noted and the cell left to run over night. In the morning crystals had appeared in the beaker which approx. doubled in volume after cooling to room temperature. Electrolysis was interrupted after 12 h and resumed in the afternoon at 8.3 V with the intention of shutting down the cell after 6 faradays had passed (22 h 15 min). It was accidentally run for 1 h longer, and when it was stopped the liquid had taken on a brighter shade of yellow. During the cleanup it was noticed that the POM-C lid had been attacked by the solution and had even cracked in two places. There were also two small brown spots on the anode that were assumed to be damage incured when the crust was removed from the upper part of the rod.

The resulting suspension was cooled in the refrigerator and the precipitate filtered off by gravity filtration, washed with 100 mL of cold water in several portions and dried in the dessicator. Yield of crude product: 48.35 g (72 % of theoretical 66.8 g). The wash water was combined with the electrolyte for future runs.

Run #2: Some parameters were amended as follows for the second experiment: The titanium cathode was used from the beginning, and the KBr was dissolved in 200 mL of the mixture of the previously used electrolyte and washings. The suspended KBr had to be heated to ~35 °C on the hotplate to force dissolution. First crystals were observed after approx. 3:50 h and the electrolysis stopped after 6 F had passed. Workup completed the same as before. More discolored spots were observed on the anode that didn't seem to affect the reaction. Yield of crude product: 57,85 g (86.6 % of theoretical 66.8 g).

Recrystalisation #1: 105.90 g crude product was dissolved in 236 mL boiling water, filtered hot and slowly cooled down for recrystallisation. The crystalized mass from the filter was redissolved in 30 mL hot water, filtered, and the filtrates combined. Yield after cooling in the refrigerator, vacuum filtration and drying in a desiccator: g ( %). The filtrate was concentrated by boiling until a fair amount of crystals had deposited. The resulting slurry was then combined with the electrolyte from the previous runs.

Run #3: Same parameters as Run #2: 47.60 g KBr, 200 mL of the electrolyte from previous runs (now saturated with KBrO3), 2.89 A constant current (250 mA/cm2). The electrolyte now had to be heated to 50 °C to insure dissolution of all the KBr.