Hach-lange HQD - Gel filled ORP_Redox electrode MTC Manuel d'utilisateur télécharger pdf (Page 7)

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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

= E + E

ref

where:

= oxidation reduction potential of the sample relative to the SHE

E = potential developed by the ORP/Redox electrode

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

5220.9

0224.2

1 2 3 4 5 6 7 8 9 10 11 12 ... 15 16

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Hach-lange HQD - Gel filled ORP_Redox electrode MTC Manuel d'utilisateur Page 7

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