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Test kits for water analysis
Test kits for water analysis

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6.1.1 Galvanic DO probes

Figure 8 shows the main components of a galvanic DO probe.

Figure 8  A schematic diagram of a galvanic dissolved oxygen probe.

Inside the probe, a potential difference is set up between the electrodes immersed in an electrolyte (potassium hydroxide). When the probe is brought into contact with a water sample, DO diffuses through the permeable membrane and an electrochemical reaction takes place when it comes into contact with the electrodes.

At the cathode, which is typically silver or gold, the following reaction occurs:

O2 (g) + 2H2O + 4e = 4OH(aq)
Equation label: (17)

Question 10

Is the oxygen oxidised or reduced in Equation 17?

Answer

Oxygen is reduced (it has gained electrons).

So at this electrode oxygen has been converted to hydroxide and four electrons are released per oxygen molecule.

These electrons cause a current to flow through the electrolyte and the reaction at the anode, which is typically lead, iron, cadmium or copper, is

2Pb(s) = 2Pb2+ (aq) + 4e
Equation label: (18)

So the electrons are replaced accompanied by ions formed from the anode which is gradually consumed during use.

To summarise, as a consequence of the reactions shown in Equations 17 and 18, a current flows through the electrolyte, the magnitude of which is proportional to the amount of DO.

Question 11

When using a DO probe, it is important to gently stir the water to get an accurate reading. On the basis of what you've learned about the electrochemistry involved, suggest a reason for this.

Answer

Since the probe consumes oxygen, we would expect the water near the probe to become oxygen deficient. Stirring ensures that the oxygen is replenished.