Potassium nitrate

All chemicals and reagents were of the analytical grade and double distilled deionized water was used throughout experiments.
Graphite flakes (Timrex KS 44) were purchased from Imerys Graphite & Carbon (Bodio, Switzerland). Dibutylphthalate (DBP), phosphotungstic acid hydrate (PTA), maprotiline hydrochloride, and THP were purchased from Sigma (USA). Sodium tetraphenylborate (NaTPB), zinc nitrate hexahydrate, magnesium nitrate hexahydrate, acetylsalicylic acid, glucose, galactose, and fructose were purchased from Merck (Germany). Paracetamol was kindly provided by Galenic laboratory Split—Dalmatia County Pharmcy (Croatia). Sodium acetate anhydrous was purchased from Gram-mol (Croatia); glacial acetic acid, potassium nitrate, calcium nitrate tetrahydrate, iron (III) nitrate nonahydrate, and lead(II) nitrate from Kemika (Croatia); silver nitrate and ammonium chloride from Sigma-Aldrich (Germany). Parkopan (2 mg/tablet) used in the application of the prepared electrode was obtained from the local drugstore.

All chemicals were of analytical reagent grade. All solutions were prepared with redistilled water. Acetate buffer (pH 5; 0.1 M) was prepared by mixing of previously prepared solutions (0.1 M) of potassium acetate and acetic acid (0.1 M), all purchased from Kemika (Zagreb, Croatia). Potassium dihydrogen phosphate and potassium hydrogen phosphate (all purchased from Sigma-Aldrich, St. Louis, MO, USA) were used for preparation of phosphate buffers (0.1 M; pH 7). Stock solution of the copper nitrate (1 × 10⁻³ M) was prepared by dissolution of Cu(NO₃)₂ × 5H₂O (Sigma-Aldrich, Inc.) in potassium nitrate (Kemika, Zagreb, Croatia) solution (0.1 M). The cysteine solution (Merck, Kenilworth, NJ, USA) and the food supplement solution (NAC-Twinlab^® dietary supplement, Hauppauge, NY, USA) were prepared daily by its dissolution in redistilled water previously deaerated (with N₂). Uric acid, ascorbic acid and glutathione were purchased from Sigma-Aldrich (St. Louis, MO, USA), while glucose was obtained from Kemika, Zagreb, Croatia.