Omega PHE/ORE Series manual Electrode care and application, PH Electrode Calibration

2. Electrode care and application

pH/ORP electrodes are similar to batteries; they age with time and usage. The following information will help maximize electrode life.

2.1 Conditions to Avoid:

•High temperatures, strong acids or caustics will elevate electrochemical reactions and speed electrode aging.

•Coatings on the glass or junction surfaces (i.e. proteins) cause extended response time and inaccurate measurement.

•Never store the electrode tip in deionized (DI) water.

•Never expose electrode to temperatures below -12 °C (10 °F) or allow it to dehydrate. These conditions will damage the electrode.

•Never scrape or sand the glass electrode surface.

•Treat glass electrode surfaces with care. The glass is very thin and requires care to prevent accidental breakage.

2.2 Submersible Installation Tips:

•Mount electrodes in a location with ample clearance for removal for periodic cleaning and recalibration. Choose a location that keeps the electrode glass completely submerged at all times.

•Place the electrode tip in pH 4 buffer during system maintenance or storage to avoid dehydration.

•Mount the electrode near tank outlet away from reagent addition areas.

YES

YES

2.3 In-Line Installation Tips:

•pH and ORP electrodes respond best in moving fluids.

•The internal measuring electrode chamber contains a wire within a liquid and a slight amount of air. The electrode must be mounted at least 30° from horizontal to ensure proper sensing. Mounting angles less than 30° will impede performance.

30°	30°
NO	NO

NO

3. pH Electrode Calibration

All pH electrodes are designed to ensure linearity during their lifespan. The following sections define proper electrode operation.

3.1 Offset (STD)

Electrode offsets occur due to:

•Clogged reference junction

•Aged or contaminated reference solution/wire

Check offsets in a pH 7 buffer @ 25 °C. The theoretical output is 0 mV. Any deviation from 0 mV is the pH electrode offset.

pH Electrode Offset pH 7 buffer @ 25°C Theoretical: 7.0 pH (0.0 mV)

Theoretical mV Values @ 25°C

pH	mV
2	+ 295 mV
3	+ 236 mV
4	+177 mV
5	+118 mV
6	+ 59 mV
7	0 mV
8	- 59 mV
9	- 118 mV
10	- 177 mV
11	- 236 mV
12	- 295 mV

New:	pH 7	± 0.25 pH (±15 mV)
Reliable:	pH 7	± 0.85 pH (± 50 mV)

Electrode offsets greater than 0.85 pH (50 mV) indicate the electrode requires cleaning or replacement. See section 5: Maintenance and
cleaning.
	°C					pH Error
3.2 Slope (SLP)
		2	3	4	5	6	7	8	9	10	11	12
Electrode slope is the number of mV per pH unit. At 25°C the	15	0.15	0.12	0.09	0.06	0.03	0	0.03	0.06	0.09	0.12	0.15
theoretical slope is 59.16 mV per pH. Temperature has an
	25	0	0	0	0	0	0	0	0	0	0	0
appreciable affect on electrode slope. Reliable instrumentation
includes temperature compensation. The graph below illustrates	35	0.15	0.12	0.09	0.06	0.03	0	0.03	0.06	0.09	0.12	0.15
potential pH error when a temperature compensated instrument	45	0.3	0.24	0.18	0.12	0.06	0	0.06	0.12	0.18	0.24	0.3
is not used.	55	0.45	0.36	0.27	0.18	0.09	0	0.09	0.18	0.27	0.36	0.45

Recommendations:

•Calibrate temperature before calibrating the standard and slope.

•The mV offset will track across the entire pH range. The slope is usually not affected by offset changes. (i.e. pH 7= +10 mV, pH 4= +187 mV); slope = 59 mV

•Coatings on the glass may affect sensor slopes. See section 5 maintenance and cleaning.

•A constant output near 0 mV in all buffer solutions indicates a shorted electrode that must be replaced.

3.3 Response Time/Stability

Response time and stability are affected by the condition of the pH electrode's glass surface (ORP electrode - Platinum surface), reference junction, and reference solution. Restoration to acceptable levels can often be accomplished by cleaning the electrode's glass surface (ORP electrode - Platinum surface) and reference junction.

Electrode mV values should remain stable ±3 mV. Conditions that may cause fluctuations are:

1.Electrode coating

2.Ground fault: If proper operation is observed in the beaker, but is unstable in the application, a ground fault probably exists.

•Using instrumentation with isolated inputs and outputs may restore stable operation.

•Solution grounding may also restore stable operation.

page 4 OMEGA PHE/ORE Series pH/ORP Electrode Instructions