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Electrochemical measurement technology

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The electrochemical sensor is one of the oldest technologies currently on the market, but it is the most accurate. For example, manufacturers claim an accuracy of <0.1 ppb* for dissolved oxygen in liquid phases and 0.5 ppmV* for oxygen in the gas phase.

Description of the measurement technology

Electrochemical sensor measurement is based on the method of redox reactions occurring at the interface between the measuring electrode and the target substance, oxygen, ozone, hydrogen, etc. The working principle can be divided into 4 main points:

1. Electrodes: Electrochemical oxygen sensors usually consist of two electrodes, an anode and a cathode, immersed in an electrolyte. The anode is usually made of a material that is easily oxidised and the cathode is made of a material that is easily reduced.

2- Redox reactions: Oxygen molecules in the sample enter into a redox reaction at the electrode surface. At the cathode, the oxygen molecules gain electrons and are reduced, and at the anode, the reduced oxygen is oxidised to oxygen molecules.

3. Electron flow: as oxygen is consumed and formed during these reactions, an electric current flows between the anode and cathode. The magnitude of this current is proportional to the concentration of oxygen in the sample.

4. Measurements: The magnitude of the electric current is measured and used to calculate the oxygen concentration in the sample using calibration curves or established equations.

EC Sensor with membrane.

Leland Clark developed the first bubble oxygenator for use in cardiac surgery. However, when he went to publish his results, an editor rejected his paper because it was impossible to measure the concentration of oxygen in the blood coming out of the device. This prompted Clark to develop an oxygen electrode.

The mechanism of action of Clark's cell

The electrode compartment is isolated from the measuring chamber by a thin Teflon membrane; the membrane is permeable to molecular oxygen and allows this gas to reach the cathode, where it is electrolytically reduced.

The above reaction requires a constant flow of electrons to the cathode, which depends on the rate at which oxygen can reach the electrode surface. Increasing the applied voltage (between the Pt electrode and the second Ag electrode) increases the rate of electrocatalysis. Clark attached an oxygen extraction membrane on top of the platinum electrode. This limits the rate of oxygen diffusion to the Pt electrode.

Above a certain voltage, the current plateaus and further increases in potential do not result in a higher rate of electrocatalysis of the reaction. At this stage, the reaction is diffusion limited and depends only on the membrane permeability and on the concentration of gaseous oxygen, which is the measurable value.

At the moment, Clark cell based sensors are mostly used in analytical equipment.

Calibration in air.

To perform the calibration, a serviced sensor is connected to the transmitter and the appropriate mode is started.

As described in the article above, the output of an elketochemical sensor is the current (membrane current).

Calibration of EC sensor consists in calculation of calibration coefficient, which expresses dependence of membrane current on atmospheric pressure. Thus simplified this dependence can be represented as:

Where

P_atm – is atmospheric pressure

I_mcal – Membrane current at the time of calibration

O_2AIR – generally accepted O2 concentration in the air - 20.946%

Thus, by measuring the membrane current, the sensor determines the partial pressure of the gas to be measured.

Calibration by known concentration.

This type of calibration can be used in the same way as air calibration, but has a special feature: it requires a liquid or gas with a known concentration.

This type of calibration can be used in the following cases:

- Air calibration is not possible

- A more accurate calibration in a certain range is required.

The calibration algorithm is completely identical except that the known concentration of oxygen in air is used as the concentration of oxygen entered from the instrument.

Conversion to gas units [% (ppmV)]

Further conversion to gas units (% - ppm) is based on the calibration method. Based on the above formula:

Where

C_Coef– Calibration coefficient

I_meas – Membrane current at the moment of measurement

P_Proc – Process pressure

Conversion to liquid units [ppm (ppb)]

Concentration of dissolved gas is converted according to Henry's law:

Where

k_Hen – Henry's solubility coefficient

P_par – Gas partial pressure

C_Coef –Calibration coefficient

I_meas – Membrane current at the moment of measurement

To convert the concentration of a gas from mol/litre to milligram/litre, you need to know the molecular weight (molar mass) of the gas in question. Molecular mass is measured in g/mol (grams per mole).

Use the formula to convert the concentration from mol/litre to milligram/litre:

To convert to ppm and ppb, it is assumed that mg/l ≅ ppb

This type of sensor has proven its effectiveness over many years of use and is considered a reliable and easy to use solution. It is widely used in critical applications such as:

- Determination of oxygen in cooling circuits of power plants, including nuclear power plants (NPPs).

- Measurement of oxygen in carbon dioxide (especially relevant in the food industry).

- Determination of dissolved ozone concentration in water at ozonisation plants (ozone, being a powerful oxidant, is used for disinfection of drinking water).

And many other applications where precision measurement of oxygen, ozone and hydrogen is required.