1112 - Wireless Conductivity Pack

Some typical conductivity ranges of hydrous solutions are:

• The Conductivity electrode needs to be kept clean and free of deposits and other types of build-up. This epoxy cell type of electrode has ‘easy to clean’ graphite plates which are corrosion safe and less easily contaminated. 
• If used in solutions with a high ion concentration, it is possible for the graphite plates to become contaminated. Soak the electrode cell portion in water with a mild detergent for 15 minutes. Then soak in a dilute acid solution such as 0.1 mol dm-3 hydrochloric acid or 0.5 mol dm-3 acetic acid for another 15 minutes. Rinse well with distilled water.
• The most common reason for inaccurate measurements is cross contamination of samples. Take care not to transfer droplets of one sample to another. Wash the electrode with distilled water between test solutions and shake vigorously to remove droplets. Ideally air-dry and then wash the cell in a sample of the solution to be measured. 
• Be sure that samples are capped to prevent evaporation. It is best to fill sample bottles to the brim to prevent a gas such as carbon dioxide dissolving in the water sample. 
• Do not use in a situation that could result in damage to the graphite plates in the cell chamber. Do not attempt to blot or wipe the inside of the cell.
• The automatic temperature compensation for this electrode operates over the range 10ºC to 35ºC, but it can be placed in solutions within a temperature range of 0 to 80ºC.
• There may be a decrease in accuracy when used in solutions that have a conductance of above 20 mS (20,000 mS). Measurements above 50 mS will be approximate. If samples need to be diluted, use fresh deionised water (to reduce inaccuracy caused by variation in the conductivity of the deionised water).
• The Conductivity electrode not only measures conductivity between the graphite plates but also, to a lesser extent, in a field to the side of the electrode. In a narrow vessel, the walls may interfere with this field. If the electrode is held too close to the top of the liquid level or other objects (e.g. the bottom of a beaker) an incorrect reading may result.
• Interference may occur between electrochemical sensors (pH, Oxygen, and Conductivity) if they are placed in the same solution at the same time and connected to the software by a USB lead. This is because these sensors make an electrical connection to the solution; therefore an electrical path exists between the sensors through the solution. Maximise the distance between the sensors to minimise the effect; the distance required will depend on the conductivity of the solution.

If the Sensors are being used in a solution that has a fairly high conductance e.g. seawater, test in separate containers of the same solution.

Total dissolved solids (TDS)

Solids are found in mobile water (streams, rivers) and are present as suspended and dissolved.

Suspended solids are measured using a Turbidity sensor.

Dissolved solids which relate back to suspended solids can be measured by the changes in the conductivity they create. For a solid to dissolve in water it needs to be ionic; the ions in the water change the conductivity; more ions, more conductivity.

The agreed standards for conductivity and TDS use a simple conversion factor

• Total solids (TDS) can be estimated in ppm (part per million) i.e. mg/L by multiplying the reading in microSiemens by 0.67, i.e. 200 mS x 0.67 = 134 ppm.

The calculate function can be applied to produce the conversion. Use a calculate function that multiplies the channel by the constant (ax for example, where a = 0.67)

Take care to get the conductivity channel to the correct scale factor – micro Siemens.

Cleaning, storage, and maintenance of the electrode

 Depending on the sample application, the electrode may require cleaning periodically to ensure accurate measurements. 

• Water-soluble contamination can be removed by soaking in distilled water.
• Petroleum-based contamination can be removed by soaking in warm water and a mild detergent for 15 minutes.
• Ethanol may be used to clean the electrode, limit the wash time to a maximum of 5 minutes.
• Lime or hydroxide coating can be removed by soaking in a dilute acid solution such as 0.1 mol dm-3 hydrochloric acid or 0.5 mol dm-3 acetic acid for 15 minutes.
• Place the electrode in Deionised water and “measure the conductivity” this can force contamination out of the electrodes but may require many changes of water.

After cleaning, rinse well with distilled water, shake vigorously and leave to air-dry. 

Store the electrode dry.