When testing Go Direct Conductivity, it is best to measure a standard solution because it is easier to determine if the sensor is reading correctly. If your Conductivity Probe is reading differently from the standard solution, you may simply need to calibrate the sensor. See the Calibrating the Sensor section for more information. Here are some other tips to ensure best data collection practices:
- Blot the inside and outside of the electrode cell dry to avoid water droplets diluting or contaminating the sample to be tested.
- Be sure the electrode surfaces in the elongated cell are completely submerged in the liquid and that there are no bubbles around the electrode surface.
- Gently swirl the probe, or stir the solution with a stirring bar and stir plate, during data collection.
- Do not completely submerge the sensor. The handle is not waterproof.
- 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.
- When you have finished using Go Direct Conductivity, simply rinse it off with distilled water and blot it dry using a paper towel or lab wipe. The probe can then be stored dry.
- If the probe cell surface 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 works well) for another 15 minutes. Then rinse it well with distilled water and blot dry. Important: Avoid scratching the inside electrode surfaces of the elongated cell.
For additional troubleshooting and FAQs, see www.vernier.com/til/3854
Sampling in Streams and Lakes
It is best to sample away from shore and below the water surface, if possible. In free-flowing streams, there will usually be good mixing of the water, so that samples taken near the current will be quite representative of the stream as a whole. If you are sampling an impounded stream or a lake, there will be very little mixing; therefore, it is important to sample away from shore and at different depths, if possible. Do not drop the Conductivity Probe so that the entire electrode is submerged. The electrode is not constructed to withstand higher pressures, so seepage into electronic components of the electrode will result. Although it is better to take readings at the collection site, readings of total dissolved solids or conductivity should not change significantly if you collect samples and take readings at a later time. However, be sure that samples are capped to prevent evaporation.
If sample bottles are filled brim full, then a gas such as carbon dioxide, which is capable of forming ionic species in solution, is prevented from dissolving in the water sample. Since the probe has built-in temperature compensation, you can do your calibration in the lab. This means that even though you will be sampling in water that has a different temperature than your calibration temperature, the probe will take correct readings at the new sampling temperature.
Sampling in Ocean Water or Tidal Estuaries: Salinity
Salinity is the total of all non-carbonate salts dissolved in water, usually expressed in parts per thousand (1 ppt = 1000 mg/L). Unlike chloride (Cl-) concentration, you can think of salinity as a measure of the total salt concentration, comprised mostly of Na+ and Cl- ions. Even though there are smaller quantities of other ions in seawater (e.g., K+, Mg2+, or SO42-), sodium and chloride ions represent about 91 percent of all seawater ions. Salinity is an important measurement in seawater or in estuaries where freshwater from rivers and streams mixes with salty ocean water. The salinity level in seawater is fairly constant, at about 35 ppt (35,000 mg/L), while brackish estuaries may have salinity levels between 1 and 10 ppt. 5 The salinity range of the Conductivity Probe is 0 to 10 ppt. Seawater has a salinity of 35 ppt, so any seawater samples will need to be diluted before making measurements with this sensor. We recommend that you dilute seawater samples (or other samples that initially give readings above 10 ppt) to 1/4 of their original concentration, then multiply their measured salinity reading by 4 to obtain a final salinity value, in ppt. Brackish water in coastal estuaries is often in the range of 0 to 10 ppt, well within the high range of the probe.
Since there is no stored salinity calibration for a Conductivity Probe, perform a two-point calibration using 5 ppt and 10 ppt salinity standards. Make sure your sensor switch is on the high conductivity setting. You will need to prepare two standard solutions to calibrate for salinity:
- A low standard (5 ppt salinity), add 4.60 g of NaCl to enough distilled water to prepare 1 liter of solution.
- A high standard (10 ppt salinity), add 9.20 g of NaCl to enough distilled water to prepare 1 liter of solution.
Determining the Concentration: Total Dissolved Solids
Because there is a nearly linear relationship between conductivity and concentration of a specific ion or salt, Go Direct Conductivity can be used to determine the concentration of an ion. A curve can be obtained if you prepare or purchase standard solutions. Note in this figure the 2:1 ratio between conductivity in µS/cm and TDS concentration in mg/L. Even though total dissolved solids is often defined in terms of this 2:1 ratio, it should be understood that a TDS reading of 500 mg/L can have a different meaning in a sample that is mostly NaCl than in another sample that is composed primarily of hard water ions such as Ca2+ and HCO3-. The relationship between conductivity and sodium chloride concentration is approximately a 2:1 ratio and is very nearly a direct relationship.