1230- Wireless Dissolved CO2

Dissolved CO₂ is carbon dioxide gas that has diffused into water and exists in equilibrium with CO₂ in the air. Its concentration changes due to respiration, photosynthesis, temperature, and gas exchange, so it reflects biological and environmental activity in aquatic systems.

Measuring dissolved CO₂ provides a key indicator of water quality and ecosystem processes, helping to assess organism stress, monitor photosynthesis and respiration, and study carbon cycling.

When measured using an ion-selective electrode, dissolved CO₂ is detected as an electrical potential rather than a direct concentration. Typical measurements fall within a ±1000 mV range, with changes in dissolved CO₂ producing shifts of tens of millivolts, commonly around 50 mV per tenfold change in concentration, depending on temperature and calibration.

The following section describes how to get the best performance from your sensor, including guidance on handling, calibration, temperature control, and experimental setup to ensure accurate and reliable dissolved CO₂ measurements.

General advice

Avoid direct contact with the sensing surface. Mechanical damage such as abrasion or puncture may result in measurement inaccuracies, signal drift, or permanent sensor damage.

Gentle stirring of the sample can improve response time by reducing boundary layer effects at the sensor interface; care must be taken to avoid physical damage to the sensing element.

Temperature has a significant influence on dissolved CO₂ measurements. Sample and calibration conditions should be at the same temperature to ensure stable and repeatable results.

Some magnetic stirrers may generate sufficient heat to alter the temperature of the sample solution. If temperature change is observed, use thermal insulation beneath the beaker to minimise heat transfer.

Dissolved CO₂ sensors have a finite operational lifespan. Sensor performance and longevity depend on usage conditions, storage, and exposure to chemical environments. Ageing occurs even when the sensor is not in active use.

Typical Lifetime and Replacement Guidance (Dissolved CO₂ Electrode)

* Lifetimes are typical values for educational laboratory use and will vary depending on frequency of use, storage conditions, sample composition, and maintenance.

The electronic adaptor is not waterproof. Clean only with a damp cloth. Do not immerse in liquids or use detergents.

Do not operate or store the adaptor in environments with sustained high humidity, as this may result in malfunction or permanent damage.

If the sensor has been stored or transported at low temperatures, allow it to equilibrate to near room temperature before use.

Avoid prolonged exposure of the sensor and adaptor to direct sunlight, as this may affect measurement stability and internal temperature conditions.

Assembly

The CO₂ electrode is a gas-sensing (Severinghaus-type) electrode that measures dissolved carbon dioxide indirectly through a pH change behind a gas-permeable membrane. For accurate measurement, all carbonate and bicarbonate species in standards and samples must be converted to dissolved CO₂, which is achieved by acidifying the solution to approximately pH 4. This is normally done by adding CO₂ ionic strength adjuster, which both buffers the solution at about pH 4 and fixes the ionic strength.This is performed by drop-wise adding the ISA buffer.

1. Before use, the PTFE gas-permeable membrane should be inspected to ensure it is clean, intact, and free of wrinkles or  pinholes. 

2. The electrode needs to be soaked in pH 4 buffer for one hour. For routine daily use, conditioning for 10 to 15 minutes is usually sufficient, with the membrane fully submerged. When this is complete, rinse the electrode with distilled water and blot dry.

3. The electrode body is filled with the supplied CO₂ internal filling solution, typically a bicarbonate/chloride solution, taking care to avoid trapping air bubbles behind the membrane. Any trapped air will cause slow response or drift. After filling, the membrane cap is secured snugly without overtightening. Approximately 3.5 ml is sufficient to fill the void. 

4. Make sure that the electrode and body key ways are aligned and the the assembly does not rotate. Secure threaded top and tighten gently. 

Quick Start Guide

The CO₂ electrode must never be allowed to dry out.
Handle the electrode gently — do not touch or scratch the membrane. 

The components that you need are:

• Wireless Dissolved CO₂ sensor (adaptor + electrode)
• EasySense app (installed)
• CO₂ ISA (ionic strength adjuster) buffer
• pH 4 buffer (or acidified standards)
• Distilled or deionised water
• Calibration standards (at least two)

Step 1: Prepare the electrode

1. Check the membrane is clean, intact, and wrinkle-free.
2. Soak the electrode tip in pH 4 buffer:
• First use / long storage: 1 hour
• Routine use: 10–15 minutes
3. Rinse with distilled water and gently blot dry.

Step 2: Connect the sensor

USB

1. Connect the sensor to the computer using the USB cable.
2. Open the EasySense app.
3. Confirm the Devices icon turns green.

Bluetooth

1. Turn the sensor on (blue LED flashing).
2. Open EasySense → Devices → select your sensor.
3. Connect (green flashing LED confirms connection).

Do not pair the sensor via your device’s Bluetooth settings. 

Connect to EasySense using Devices.

Step 3: Prepare calibration solutions

1. Prepare at least two CO₂ standards.
2. Add the same volume of CO₂ ISA to each standard and to all samples.
   (typically ~1 mL ISA per 50–100 mL solution).
3. Ensure standards, samples, and electrode are at the same temperature.

Step 4: Calibrate

1. Gently stir the first standard.
2. Immerse the electrode tip.
3. Wait until the reading stabilises (2–5 minutes).
4. Record the value. EasySense has a calibration facility to record the first standard. 
5. Repeat with the second standard.Record the value. EasySense has a calibration facility to record the second standard. 
6. Confirm calibration slope is approximately 55 to 60 mV per decade at 25 °C.

         If the slope is much lower, check for air bubbles or membrane condition.

Step 5: Measure samples

1. Add ISA to the sample using the same ratio as calibration.
2. Stir gently — do not bubble or stir vigorously.
3. Immerse the electrode and wait for a stable reading.
4. Record the millivolt (mV) value.
5. Determine CO₂ concentration using the calibration relationship.

After use

• Rinse the electrode with distilled water.
• Store the electrode moist, with the protective cap in place.
• Recharge the adaptor if needed.

Common quick fixes

• Unstable readings: Check for air bubbles under membrane
• Slow response: Clean or replace membrane
• Poor calibration: Ensure ISA added and temperature matched

The Calibration Relationship 

The calculation for the concentrations of CO2 are taken care of in EasySense, but an explanation as to how they are generated is now given.  

Plot the measured electrode potentials on a linear axis against the logarithm of the corresponding carbon dioxide concentrations. A straight-line response indicates correct electrode behaviour. The gradient of this line reflects the expected Nernstian dependence on ion activity at the measurement temperature.

If we assign the first (higher) concentration as C1, with a corresponding E1 (mV response), and the lower to be C2 AND E2, then the slope, Sl, of the desired line is 

                

                                          Sl = (E1 - E2) / (log10(C1) - log10(C2))        

The intercept, E0, is              E0 = E2 - Sl log10(C2)

So the electrode equation is  U = E0 + Sl log10(U) 

Where U is the unknown concentration with a response Ex.

If we measure a response from an unknown, it follows that the concentration, U, will be:

                                          log10(U) = (EX - E0) / s

                                          U = 10^((Ex - E0) / s) 

Example calculation: 

E1  = 212 mV, E2 = 152 mV, C1 = 1000 ppm, C2 = 100 ppm, Ex = 187 mV

Slope = Sl = Sl = (E1 - E2) / (log10(C1) - log10(C2))

  Sl =  (212 - 152) / (log10(C1) - log10(C2)) 

  Sl = 60 / (3- 2) = 60 mV             

The intercept, E0 = E2 - Sl log10(C2) 

   E0= 252 - 120 = 32 mV  

The unknown concentration is: 

   U = U = 10^((Ex - E0) / s) 

   U = 10^((187 - 32) / 60) = 383 ppm

nb: The above calculations work on the basis that the response is always positive. should this not be the case, then adding in a simple constant will put the responses into a regime where the above calculation can be performed. So, is the responses were all greater that -200 mV, the add in in 200 mV to all values to restore the calculation.         

Example calculation: 

E1  = -140  mV, E2 = - 200 mV, C1 = 1000 ppm, C2 = 100 ppm, Ex = -165 mV

add in (AI) =200 mV

E1  = 60  mV, 

E2 = 0 mV,

C1 = 1000 ppm, 

C2 = 100 ppm.

Ex = 35 mV

Slope = Sl = Sl = (E1  - E2) / (log10(C1) - log10(C2))

  Sl =  (-140 + 200) / (log10(C1) - log10(C2)) 

  Sl = 60 / (3- 2) = 60 mV             

The intercept, E0 = 0 - Sl log10(C2) 

  E0 = 0 - 120 = -120 mV  

The unknown concentration is: 

  U = U = 10^((Ex - E0) / s) 

  U = 10^((35 + 120) / 60) = 383 ppm

Sensor Range

This measurement range provides output in millivolts (mV) and can be used for experiments requiring direct measurement of electrode potential.

This mV range may be used with ion-selective electrodes (ISEs) probes. ISEs operate by measuring the electrical potential generated by the activity of specific ions in solution.

Ions carry either a positive or negative electrical charge. An ISE measures the electrical potential produced by the presence of these charged species. An ion-selective membrane controls the movement of ions to the electrode surface; it is this membrane that determines the electrode’s selectivity for a particular ionic species.

The relationship between electrode potential and ion activity is described by the Nernst equation:

                                                  

                                                    E = E0 + (RT/(zF)) ln⁡(C) 

where:

• E is the measured electrode potential (mV),
• E0 is the standard electrode potential,
• R is the universal gas constant,
• T is the absolute temperature (K),
• z is the charge of the ion,
• F is Faraday’s constant, and
• C is the activity of the ion in solution.

At 25 °C, this can be expressed in its simplified logarithmic form:

                                                    E = E0 + 59.16 z log10(C)

This shows that, at constant temperature, electrode potential varies linearly with the logarithm of ion activity. To convert an mV output into concentration units (such as ppm or logarithmic ion concentration), a calibration curve must be generated.

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Performance Checks

To confirm stability, periodically re-measure one of the calibration solutions. If significant deviation is observed, repeat the calibration procedure.

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Technical Notes

• Consistent ionic strength is essential for reliable electrode response.
• Temperature variation affects electrode potential and should be minimised.
• Calibration should be repeated if experimental conditions change or after prolonged use.

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