3. Water & Environmental / Dissolved Oxygen / Electrochemistry

What type of electrode should I use to measure Dissolved Oxygen?

Quite simply, dissolved oxygen (DO) is free (non-compound) oxygen present in water. Oxygen dissolves by diffusion from the surrounding air, gained by aeration from turbid water or a waste product of photosynthesis. DO is vital to maintain aquatic life from fish to plants to bacteria and thus an important parameter to be able to measure.

Clean water

There are many factors to take into account when choosing a dissolved oxygen electrode based on your water sample, sample colour, and electrode maintenance required. There are two key types of dissolved oxygen meter sensors; electrochemical membrane covered sensors (galvanic and polarographic) and optical based sensing using luminescence.

 

Electrochemical – Galvanic Sensor

The Galvanic DO cell is very simple in its operation and basically works as a battery which generates a voltage. This voltage is directly proportional to the oxygen concentration which permeates through the membrane into the galvanic cell. The cell is constructed with a wire coil cathode wrapped around a lead anode. A salt filling solution makes a contact between both and fills the void in the electrode body which is constructed of a rigid polymer. The sensing end of the cell has an inert permeable membrane stretched over the internal half cells.

When the electrode is placed into the sample solution oxygen will diffuse through the pre-installed membrane and the thin layer of filling solution to the cathode. The oxygen is reduced at the cathode which is held at a strongly negative potential. The lead anode is oxidised (consumed) simultaneously as current flows. This yields a sensitive electrode which, when connected to a dissolved oxygen meter, can detect the smallest change measured in fractions of millivolts.

TS DO

The TRUEscience Galvanic Dissolved Oxygen electrode offers a fast response and rugged construction ensuring that calibration and measurement are both rapid and consistent. It comes complete and ready for operation; connect it up to the TRUEscience SMART Cap to transmit your measurements via Bluetooth to the TRUEscience app on an Android phone or tablet.  The intuitive app allows the user to create enriched readings such as measurement logs, set up alarms at particular DO levels and connect to the cloud.

Electrochemical – Polarographic Sensor

Polarographic sensors are very similar in structure to Galvanic sensors. Galvanic sensors contain silver and zinc which are dissimilar enough to reduce oxygen that has passed through the membrane without needing to apply voltage. Polarographic sensors, on the other hand, require a polarising potential to be applied when the DO meter if first switched on,  and the user must wait for up to 15 minutes for the sensor to stabilise.

The Myron L Ultrapen PT5 is a small and rugged DO meter which is suited for field or education applications. It has simple, basic controls with just one button at the top of the stick meter for control.

ultrapen

Electrochemistry sensors will require more maintenance by the user, with the membranes and internal solutions needing to be replaced regularly. The cleaner the water, the less you will have to replace the membranes. They also require a flow across the membrane in order to take a measurement, so they can’t measure in still samples.

Optical sensors

Optical dissolved oxygen electrodes measure DO by emitting light at a specific wavelength onto a dye in the sensing layer of the sensor which causes it to luminesce. Dissolved oxygen from the water sample diffuses into the electrode interferes with both the lifetime and intensity of luminescence of the chemical dye. If you have zero dissolved oxygen the dye would luminescence at it’s maximum, and as you increase the DO in the sample this would shorten both the luminescence lifetime and intensity. If you are going to be regularly testing water samples with high dissolved oxygen this will shorten the lifetime of your probe and you will need to replace these more often.

optical do

Optical sensors are low maintenance; there is no membrane or electrolyte that requires replacing, it’s not affected by flow and it won’t suffer from hydrogen sulphide interference. The Lovibond SD 400 dissolved Oxygen meter is an optical DO meter with advanced luminescence technology for high accuracy measurement. The disadvantages of optical sensors are that they are the most expensive, they have on average a slower response time to electrochemistry probes.

 

To summerise:

  • Both Galvanic and Optical probes require no ‘warm up’ polarization time, they are ready to get started straight away whilst polarographic will take between 5-15mins to be stable
  • Galvanic and Polarographic sensors have a faster response time
  • Optical requires almost zero flow rate whilst the electrochemistry probes require a flow across the membrane
  • Optical sensors are the most expensive option and have the highest power consumption
  • Galvanic and Polarographic will be affected by some gases including hydrogen sulfide which pass through the membrane and affect the readings
  • Optical sensors require less maintenance

For technical support on dissolved oxygen measurement please get in touch with Camlab using support@camlab.co.uk or give us a ring on 01954 233 120 and we can help on your particular application.

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