CONDUCTIVITY SENSOR
The Conductivity sensor can be used to measure the conductivity in a solution or the total ion concentration in aqueous samples being investigated in the field or in the laboratory. It can be connected to any Vernier interface (ULI, Serial Box Interface, MPLI, or Voltage Input Unit), as well as to the Texas Instruments Calculator Based Laboratory (CBL) System. Conductivity is one of the easiest environmental tests on aquatic samples. Although it does not say which specific ions are present, it quickly determines the total concentration of ions in a sample. Can be used to perform a wide variety of tests or plan experiments to determine changes in ion levels or total salinity
It allows students to qualitatively see the difference between the ionic and molecular nature of a substance in an aqueous solution. This may include differences in the strength of weak acids and bases, or the number of ions that an ionic substance can dissociate into a formula unit. Use the sensor to confirm the relationship between conductivity and ion concentration in an aqueous solution. The unknown concentrations of samples can then be determined.
- Measure changes in conductivity resulting from photosynthesis in aquatic plants, with the resulting decrease in carbon dioxide bicarbonate ion concentration.
- Use this sensor for an accurate measurement of Total Dissolved Solids (TDS) in a stream or lake.
- Monitor the rate of reaction in a chemical reaction in which dissolved ions and the conductivity of a solution vary with time due to an ionic species being consumed or produced.
- Perform a conductivity titration to determine when the stoichiometric amounts of two substances have combined.
- Use the conductivity sensor to determine the rate at which an ionic species diffuses through a membrane such as dialysis tubing.
- Monitor changes in conductivity or total dissolved solids in an aquarium containing aquatic plants and animals. These changes could be due to photosynthesis or respiration.
How does the conductivity sensor work?
The Vernier Conductivity sensor measures the ability of a solution to conduct an electrical current between two electrodes. In the solution, the current flows by the transport of the ion. Therefore, a large concentration of ions in the solution will lead to higher conductivity values.
The conductivity sensor is actually measuring conductance, defined as the reciprocal of resistance. When resistance is measured in ohms, conductance is measured using the unit S, Siemens (formally known as mho). Since the Siemens is a very large unit, aqueous samples are commonly measured in microsiemens, or µS.
Although the conductivity sensor is measuring conductance, it is often interesting to find conductivity in a solution. The conductivity, C, is found by using the following formula:
C = G*kc
where G is the conductance and kc is the cell constant. The cell constant is determined, for one sensor, using the following formula:
kc = d/A
where d is the distance between the two electrodes, and A is the surface area of the electrode.
A potential difference is applied to the two electrodes of the conductivity sensor. The resulting current is proportional to the conductivity of the solution. This current is converted to a voltage that will be read by a Vernier interface or the CBL system.
Alternating current is provided to prevent complete migration of the ion to the two electrodes. With each cycle of alternating current, the polarity of the electrodes is reversed, which in turn reverses the direction of ion flow. This is a very important feature of the conductivity sensor that prevents most electrolysis and polarization from occurring at the electrodes.
Thus, the solutions being measured for conductivity do not become dirty.
One of the most common conductivity sensor applications is finding the concentration of total dissolved solids, or TDS, in a water sample. This can be achieved by generating a relationship between conductivity and ionic concentration in a solution, as shown here. The relationship persists until very large ionic concentrations are reached.
The Vernier conductivity sensor has three sensitivity range settings:
- 0 to 200 µS (0 to 100 mg/L TDS)
- 0 to 2000 µS (0 to 1000 mg/L TDS)
- 0 to 20,000 µS (0 to 10,000 mg/L TDS)
Preparing the conductivity sensor for use.
Conductivity or concentration can be measured using the Vernier conductivity sensor in a few minutes:
- To ensure that the electrode surface is free of debris, wet the sensor tip with distilled water for about 10 minutes. Clean the surface of the electrode (inside the hole near the end of the sensor).
- Connect the conductivity sensor to a port (channel) of the Vernier interface, or TI CBL system. Now you are ready to perform or upload a calibration for the sensor.
Calibration.
The conductivity sensor can be easily calibrated to two known levels using any of the Vernier data acquisition programs. Calibration units can be µS, mg/L as TDS, mg/L as NaCL, or any other unit of your choice.
- Select the range of the sensor conductivity: low = 0 to 200 mS, medium = 0 to 2000 mS, and high = 0 to 20,000 mS.:
- Zero Point Calibration: Only perform this calibration with the sensor tip out of any liquid or solution. A very small voltage reading will be displayed on the computer or CBL screen. Call this value 0 mS or 0 mg/L.
- Standard Solution Calibration Point: Put the conductivity sensor in a standard solution (solution of known concentration), for example the sodium chloride standard provided with the sensor. Make sure that the hole in the electrode surface is immersed in the solution. Wait for the displayed voltage to stabilize. Enter the value of the standard solution.
This calibration method is quite easy, we recommend that you perform a calibration whenever you use the sensor. Alternatively, you can save a calibration that is performed with a conductivity range setting (range setting or standard value in the calibration name), and reload it at a later date.
Taking Measurements using the Conductivity sensor
When the conductivity sensor has been calibrated, it is ready to take readings:
- Rinse the cap of the conductivity sensor with distilled water.
- Insert the sensor cap into the sample to be tested. Important: make sure that the electrode surface in the cell is completely submerged in the liquid.
- While gently shaking the sensor, wait for the reading on the computer, CBL, or calculator display to stabilize. This takes no more than 5 to 10 seconds.
- Rinse the sensor tip with distilled water before taking other measurements.
- If you are taking readings at temperatures below 15°C or above 30°C, allow more time for the temperature compensation to adjust and provide a stable conductivity reading.
Maintenance and Storage of the Conductivity sensor.
- When you are done using the conductivity sensor, just rinse it with distilled water and dry using a paper towel or lab dryer. The sensor can then be stored.
- If the surface of the sensor cell is contaminated, soak it in water with a mild detergent for 15 minutes. Then soak it in a dilute acid solution (0.1 M hydrochloric acid or 0.5 M acetic acid work well) for another 15 minutes. Then rinse it well with distilled water.
Specs.
Conductivity sensor range:
Low Range: 0 to 200 µS/cm (0 to 100 mg/L TDS or
0 to 1.63x10-3 mol/L as NaCl)
Medium Range: 0 to 2000 µS/cm (0 to 1000 mg/L TDS or
0 to 0.017 mol/L as NaCl)
High Range: 0 to 20,000 µS/cm (0 to 10,000 mg/L TDS or
0 to 0.19 mol/L as NaCl)
Resolution (with 12 bit interface).
- Low Range: 0.082 ?S/cm, 0.041 mg/L TDS
- Medium Range: 0.82 ?S/cm, 0.41 mg/L TDS
- High Range: 8.2 ?S/cm, 4.1 mg/L TDS
Accuracy: ?1% of full scale readings for each range.
Response Time: 98% of full scale of readings in 5 seconds, 100% of full scale in 15 seconds.
Compensation temperature: automatic from 5 to 35 °C
Temperature range (can be placed in): 0 to 80°C
Cell constant: 1.0 cm -1
Description: cap type, epoxy body, parallel carbon (graphite) electrodes.
Dimensions: 12mm OD and 150mm length.
