Reference Electrode Conversions and Temperature Corrections

Reference electrode conversions

Cathodic protection measurements are often made using different reference electrodes. Because each electrode has a different standard potential relative to the Standard Hydrogen Electrode (SHE), readings must sometimes be converted from one reference scale to another.

CP 3 exam questions frequently require converting measured potentials between common electrodes such as CSE, Ag/AgCl, SCE, and zinc reference electrodes. Temperature corrections may also be required when the reference electrode is not at the standard temperature of 25°C.

Standard potentials at 25°C (vs. SHE)

Electrode	Abbreviation	Potential (V vs SHE)	Temperature Coefficient (mV/°C)
Copper / Copper Sulfate	CSE	+0.316 V	0.9
Ag / AgCl (Sea Water)	SSC (SJ)	+0.256 V	-0.33
Saturated Calomel	SCE	+0.244 V	-0.70
Ag / AgCl (Sat. KCl)	SSC (LJ)	+0.222 V	-0.70
Zinc (Soil)	ZRE	-0.800 V	N/A
Standard Hydrogen	SHE	0.000 V	0.0

Reference formulas

1) General reference conversion

\[ V_{Target} = V_{Measured} + (E_{Source} - E_{Target}) \]

\(V_{Target}\) = converted reading on the target reference electrode scale

\(V_{Measured}\) = original reading on the source reference electrode scale

\(E_{Source}\) = standard potential of the source reference (vs SHE)

\(E_{Target}\) = standard potential of the target reference (vs SHE)

2) Temperature correction (to 25°C)

\[ E_{25} = V_{Measured} + k_t (T - 25) \]

\(E_{25}\) = corrected potential at 25°C

\(V_{Measured}\) = measured potential at temperature \(T\)

\(k_t\) = temperature coefficient of the reference electrode (mV/°C)

\(T\) = electrode temperature (°C)

Worked examples

Example 1: SCE to CSE conversion

A potential is measured as -0.880 V_SCE. Convert this reading to its equivalent value on the CSE scale.

\[ \begin{array}{c} V_{CSE} = V_{SCE} + (E_{SCE} - E_{CSE}) \\[10pt] V_{CSE} = -0.880 + (0.244 - 0.316) \\[10pt] V_{CSE} = -0.880 + (-0.072) \\[10pt] V_{CSE} = -0.952\ V \end{array} \]

Answer: -0.952 V_CSE

Example 2: CSE to silver/silver chloride (seawater)

A pipeline potential is measured as -0.850 V_CSE. Convert this reading to the equivalent value measured with an Ag/AgCl seawater reference electrode.

\[ \begin{array}{c} V_{SSC} = V_{CSE} + (E_{CSE} - E_{SSC}) \\[10pt] V_{SSC} = -0.850 + (0.316 - 0.256) \\[10pt] V_{SSC} = -0.850 + 0.060 \\[10pt] V_{SSC} = -0.790\ V \end{array} \]

Answer: -0.790 V_SSC

Example 3: Zinc reference to CSE

A zinc reference electrode installed in soil measures a potential of +0.250 V relative to the pipeline. Convert this reading to the equivalent CSE value.

\[ \begin{array}{c} V_{CSE} = V_{ZRE} + (E_{ZRE} - E_{CSE}) \\[10pt] V_{CSE} = +0.250 + (-0.800 - 0.316) \\[10pt] V_{CSE} = +0.250 - 1.116 \\[10pt] V_{CSE} = -0.866\ V \end{array} \]

Answer: -0.866 V_CSE

Practice problems

Practice 1: Ag/AgCl electrode comparison

Two Ag/AgCl electrodes are used: one seawater type (+0.256 V vs SHE) and one saturated KCl type (+0.222 V vs SHE).

If the saturated KCl electrode measures -0.800 V, what would the seawater electrode read?

Reveal solution

\[ \begin{array}{c} V_{Target} = -0.800 + (0.222 - 0.256) \\[10pt] V_{Target} = -0.800 - 0.034 \\[10pt] V_{Target} = -0.834\ V \end{array} \]

Answer: -0.834 V

Practice 2: CSE to SHE conversion

Convert -0.850 V_CSE to the Standard Hydrogen Electrode (SHE) scale.

Reveal solution

\[ \begin{array}{c} V_{SHE} = -0.850 + (0.316 - 0.000) \\[10pt] V_{SHE} = -0.534\ V \end{array} \]

Answer: -0.534 V_SHE

Practice 3: Temperature correction (cold)

A potential of -0.850 V_CSE is measured at 5°C. Apply the temperature correction to determine the equivalent potential at 25°C.

Reveal solution

\[ \begin{array}{c} E_{25} = V_{Measured} + k_t (T - 25) \\[10pt] E_{25} = -0.850 + 0.0009 \times (5-25) \\[10pt] E_{25} = -0.850 - 0.018 \\[10pt] E_{25} = -0.868\ V \end{array} \]

Answer: -0.868 V_CSE

Practice 4: Temperature correction (hot)

A potential of -0.850 V_CSE is measured at 45°C. Determine the corrected potential at 25°C.

Reveal solution

\[ \begin{array}{c} Correction = 0.9 \times (45 - 25) = 18\text{ mV} = 0.018\ V \\[10pt] E_{25} = -0.850 + 0.018 \\[10pt] E_{25} = -0.832\ V \end{array} \]

Answer: -0.832 V_CSE

Practice 5: Criterion conversion (SCE)

The cathodic protection criterion is -0.850 V_CSE. What equivalent value should be achieved if using an SCE reference electrode?

Reveal solution

\[ \begin{array}{c} V_{SCE} = -0.850 + (0.316 - 0.244) \\[10pt] V_{SCE} = -0.778\ V \end{array} \]

Answer: -0.778 V_SCE

Practice 6: Zinc reference potential difference

A zinc reference electrode has a potential of -0.800 V vs SHE. A copper/copper sulfate electrode has a potential of +0.316 V vs SHE.

If the CSE is connected to the positive terminal and the zinc electrode to the negative terminal, what voltage will the voltmeter read?

Reveal solution

\[ \begin{array}{c} V = (+0.316) - (-0.800) \\[10pt] V = 1.116\ V \end{array} \]

Answer: 1.116 V

Practice 7: Multi-step electrode conversion

A potential of -0.800 V is measured using an Ag/AgCl seawater electrode at 25°C. Convert the reading to the equivalent SCE value.

Reveal solution

\[ \begin{array}{c} V_{SCE} = -0.800 + (0.256 - 0.244) \\[10pt] V_{SCE} = -0.800 + 0.012 \\[10pt] V_{SCE} = -0.788\ V \end{array} \]

Answer: -0.788 V_SCE

Reference Electrode Conversions and Temperature Corrections