Hach-Lange HQD - Gel filled ORP_Redox electrode MTC - Manual (Page 7)

DOC272.53.80033 7
Measurement - conversion to SHE reference
For some applications it is customary to report redox potential readings relative to the
standard hydrogen electrode (SHE), also called normal hydrogen electrode (NHE). To do
this, select the value in
Table 1 Standard potential of reference electrode that
corresponds to the temperature of the solution measured. Substitute that value E
ref
into
the equation and solve for E
h
:
E
h
= E + E
ref
where:
E
h
= oxidation reduction potential of the sample relative to the SHE
E = potential developed by the ORP/Redox electrode
E
ref
= potential developed by the reference electrode portion relative to the SHE (refer to
Table 1 Standard potential of reference electrode ).
Table 1 shows the potentials, E
ref
, developed by the reference electrode portion relative to
the SHE at various temperatures.
Measurement - Oxidation Titrations
Oxidation-reduction, or redox titrations, provide a simple, reliable method for determining
many substances in solution. A redox titration consists of adding to an unknown sample
small increments of a titrant that converts the unknown to a different oxidation state. After
each addition of titrant, the ORP/Redox electrode develops a potential proportional to the
logarithm of the ratio of the activities of the two oxidation states.
At the inflection, or end point, the titrant has completely oxidized or reduced the unknown,
causing a sharp change in the logarithm of the ratio of the activities of the two oxidation
states. A corresponding sharp change in the potential is developed by the platinum
electrode. Often several oxidizing or reducing species can be precisely determined in the
same solution by a single titration with several inflection points.
The following directions represent a general procedure for performing a redox titration
once the sample is prepared for measurement.
Table 1 Standard potential of reference electrode
Temperature (ºC) Electrode Potential in mV (E
ref
)
80 163.1
75 167.7
70 172.1
65 176.4
60 180.3
55 184.4
50 188.4
45 192.3
40 196.1
35 199.8
30 203.4
25 207.0
20 210.5
15 214.0
10 217.4
5 220.9
0 224.2