Table 1. Representative oxidation–reduction mediators used in aqueous solutions⁴⁴.

Redox half-reaction of mediator	Formal potential (V vs. NHE)	Applicable pH range	Redox half-reaction of mediator	Formal potential (V vs. NHE)	Applicable pH range
Ru(bpy)33+/2+	1.27	∼pH 2–9	Fe(CN)63−/4−	0.36	∼pH 3–10
Ru(phen)33+/2+	1.22	∼pH 4–7	Co(bpy)33+/2+	0.32	∼pH 3–9
Br2/Br−	1.09	∼pH 0–6	1,4-Benzoquinone/hydroquinone	0.28	∼pH 2–11
Fe(phen)33+/2+	1.07	∼pH 3–5	N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD)	0.27	∼pH 3–10
Fe(bpy)33+/2+	1.07	∼pH 4–7	Ru(en)33+/2+	0.18	∼pH 2–9
IrCl62−/3−	1.00	∼pH 0–5	1,2-Naphthoquinone/naphthohydroquinone	0.14	∼pH 2–10
Ru(bpy)2(NH3)23+/2+	0.88	∼pH 5–7	FeEDTA−/2-	0.12	∼pH 3–10
Ru(CN)63−/4−	0.86	∼pH 0–8	Ru(NH3)63+/2+	0.05	∼pH 2–9
Os(bpy)33+/2+	0.84	∼pH 4–7	Methylene blue	0.01	∼pH 4–10
Mo(CN)83−/4−	0.77	∼pH 4–10	Co(en)33+/2+	−0.22	∼pH 3–9
1,1-Dicarboxylic acid ferrocene	0.64	∼pH 3–10	Anthraquinone-2-sulfonate/anthrahydroquinone	−0.22	∼pH 4–9
Co(oxalate)33−/4−	0.57	∼pH 4–8	Methyl viologen2+/+	−0.45	∼pH 6–9
W(CN)83−/4−	0.49	∼pH 4–10	Co(sepulchrate)3+/2+	−0.54	∼pH 6–9
Ferrocene methanol+/0 (FcMeOH+/0)	0.44	∼pH 1–10	4,4'-Dimethyl-1,1′-trimethylene-2,2′-bipyridyl	−0.69	Not available
Co(phen)33+/2+	0.38	∼pH 3–9	Fe(TEA)23+/2+ (TEA = triethanolamine) (1M NaOH)	−0.82	∼pH 12–14
CoEDTA−/2−	0.38	∼pH 3–10	—	—	—