Cobalt sulfate heptahydrate

Nickel sulfate hexahydrate
(≥98%), cobalt sulfate heptahydrate (≥99%), manganese
sulfate monohydrate (≥99%), ammonium oxalate (≥99%),
ammonium hydroxide (28% NH₃ in H₂O, purity ≥99.99%),
light mineral oil, and lithium hydroxide (≥98%) were purchased
from Sigma-Aldrich. All chemicals were used as received without further
treatment. All the solutions were prepared with deionized water.

All the chemicals used in the present study were of analytical grade purity and were used without any further purification. Nickel sulfate hexahydrate (NiSO₄·6H₂O, ≥98% pure), cobalt sulfate heptahydrate (CoSO₄·7H₂O, ≥99% pure), manganese sulfate monohydrate (MnSO₄·H₂O, ≥99% pure), lithium sulphate monohydrate (LiSO₄·H₂O, ≥99% pure), potassium permanganate (KMnO₄, ≥99% pure), and potassium dichromate (K₂Cr₂O₇, ≥99% pure) were purchased from Sigma Aldrich Canada (Oakville, Canada). Deionized water was produced by the Milli-Q Integral water purification system of MilliporeSigma (Merck KGaA, Darmstadt, Germany). Concentrated orthophosphoric acid (H₃PO₄, 85.0 wt%) and concentrated sulfuric acid (H₂SO₄, 95.0–98.0 wt%) were supplied by VWR International LLC (Mississauga, Ontario, Canada). Quartz cuvette cells of 10 mm and 2 mm path length (200–2500 nm scan range) with a volume of 1.2 mL (with slits) and 0.7 mL, respectively, were purchased from Lianyungang Highborn Technology Co. Ltd (Lianyungang, Lianyungang Jiangsu Province, China).

Ferric nitrate
nonahydrate, (Fe(NO₃)₃·9H₂O),
ferric chloride hexahydrate (FeCl₃·6H₂O),
cobalt nitrate hexahydrate (Co(NO₃)₂·6H₂O), cobalt chloride heptahydrate (CoCl₂·6H₂O), cobalt sulfate heptahydrate (CoSO₄·7H₂O), sodium hydroxide (NaOH), hydrazine hydrate
(N₂H₄·7H₂O), sodium chloride
(NaCl), and copper sulfate pentahydrate (CuSO₄·5H₂O) were purchased from Sigma–Aldrich and used as received.
Reactive blue 5 was obtained from a local dyeing factory.