Pencil lead with a diameter of 0.5 mm (Tombow Co., Ltd., Japan) was implanted into a Teflon tube, so only 1 cm of its tip and end were exposed (A = 0.16 cm 2). Electrical contact was made by soldering a copper wire to the end exposed side of the PGE. Pretreatment of the PGE was done by immersing for 15 min in HNO3 6 M and then washing with water. The PGE was then introduced into the electrochemical cell containing
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