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Orp electrode calibration, Maintenance and cleaning – GF Signet 2714-2717 pH_ORP Electrodes User Manual

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Signet 2714-2717 Twist Lock pH and ORP Electrodes

4. ORP Electrode Calibration

All ORP electrodes are designed to ensure linearity during their lifespan. The following sections defi ne proper electrode operation.

4.1 Offset (STD)
Electrode offsets occur due to:
• Clogged reference junction
• Aged or contaminated reference solution/wire

Offsets are easily checked in pH 7 buffer saturated with quinhydrone @ 25 °C; since the theoretical output is +87 mV. Any deviation
from +87 mV is the ORP electrode offset (i.e. +90 mV). Quinhydrone is the oxidizer measured by the ORP electrode and is required for
calibration. To guarantee buffer saturation, mix 1/8g quinhydrone per 50 mL of pH buffer.

ORP Electrode Offset:
Solution:

pH 7 or pH 4 buffer saturated with Quinhydrone @ 25 °C

pH 7 + Quinhydrone

pH 4 + Quinhydrone

Theoretical mV:

+ 87 mV

+ 264 mV

New:

87 ± 15 mV

+264 ± 15 mV

Reliable:

87 ± 50 mV

+264 ± 50 mV

Electrode offsets greater than ±50 mV indicate the electrode requires cleaning or replacement, see section 5.2.

4.2 Slope (SLP)
ORP slope errors are generally caused by contamination of the platinum electrode surface. Cleaning the electrode surface will usually
restore proper values, response time, and stability.

Common ORP Values

Reaction

mV

Cr

→ Cr

2+

+ 2e

-

- 913

Fe

→ Fe

2+

+ 2e

-

-

440

Cr

2+

→ Cr

3+

+ e

-

- 407

4OH

-

→ O

2

+ 2H

2

O + 4e

-

401

2I

-

→ I

2

+ 2e

-

- 400

Ti

2+

→ Ti

3+

+ e

-

- 370

Ni

→ Ni

2+

+ 2e

-

- 250

Pb

→ Pb

2+

+ 2e

-

-

126

Fe

→ Fe

3+

+ 3e

-

- 37

H

2

→ 2H

+

+ 2e

-

0

Fe

2+

→ Fe

3+

+ e

-

+ 771

Ag

→ Ag

+

+ e

-

+

799

Pb

→ Pb

4+

+ 4e

-

+ 800

3Br

-

→ Br

3

-

+ e

-

+ 1060

2Br

-

→ Br

2

+ 2e

-

+ 1066

ClO

2

-

→ ClO

2

+ e

-

+ 1160

Pt

→ Pt

2+

+ 2e

-

+

1188

Ag

→ Ag

2+

+ 2e

-

+ 1369

Many systems require both pH and ORP calibration. To conserve calibration reference solutions, use pH 7 and 4 buffers for pH
calibration fi rst. ORP calibration can be performed with the same buffers by adding quinhydrone.

5. Maintenance and Cleaning

5.1 Maintenance

Variables can affect long term pH or ORP electrode life. For this reason, a maintenance log is recommended for trend analysis. When
storing boxed sensors, lay the sensor fl at to maximize hydration of the glass surface. Keep the glass surface wet at all times. Soak the
sensor tip in pH 4.0 buffer during system maintenance intervals. In-line applications should be plumbed with a depression (trap) so liquid
is maintained around the sensor tip. If the sensor dehydrates, soak the sensor tip in pH 4 buffer for 24 to 48 hours, then visually inspect
the electrode for surface cracks, swelling, or discoloration. Severely dehydrated electrodes cannot be restored to normal operation.