Add Ag/AgCl-3M reference electrode (E = 207 mV vs NHE) to the reference electrode database

[DunklesArchipel]

Request

Add an Ag/AgCl-3M entry to the reference electrode database in
unitpackage/electrochemistry/reference_electrode.py.

Currently the database contains Ag/AgCl (generic), Ag/AgCl-sat (0.197 V vs SHE) and
Ag/AgCl-1M (~0.2224 V vs SHE), but no 3 M KCl variant.

Proposed value

The Ingold "Argenthal" Ag/AgCl electrode with KCl (3 M) filling solution is reported at
E = 207 mV vs NHE (≈ 0.207 V vs SHE), which sits between the 1 M (0.2224 V) and saturated
(0.197 V) values already in the database.

"Ag/AgCl-3M": _ReferenceElectrode(
    name="Ag/AgCl-3M",
    full_name="3 M KCl silver / silver chloride electrode",
    entries=[
        _ReferenceElectrodeEntry(
            value=0.207,
            standard="ECHEMDB-2026",
            approach="experimental",
            unit="V",
            vs="SHE",
            source={"doi": "https://doi.org/10.1016/S0022-0728(80)80421-9"},
        )
    ],
),

Sources for the 207 mV value

Stated explicitly (Ingold "Argenthal", Ag/AgCl, KCl (3 M); E = 207 mV vs NHE) in:

• Droog & Schlenter, J. Electroanal. Chem. 112 (1980) 387 — DOI: 10.1016/S0022-0728(80)80421-9
• Droog, Alderliesten & Bootsma, J. Electroanal. Chem. 99 (1979) 173 (same group, full experimental description)

Context

Came up while curating droog_1980_oxygen_387 in echemdb/electrochemistry-data (PR #151),
whose reference electrode is this Ingold "Argenthal" Ag/AgCl (KCl 3 M). The entry now uses the
Ag/AgCl-3M notation, which will resolve once this database entry exists.