Connect the sensor following the steps in the Getting Started section of this user manual.
Note: Properly soaking and calibrating the ISE is required before taking measurements with your ISE. For detailed calibration procedures, refer to the Calibration section of this user manual.
- Make sure the sensor is properly calibrated. If the meter has a reading of 1.0 mg/L and the sensor is not in a 1.0 mg/L solution, you need to calibrate. After calibration, rinse off the tip of the ISE and blot it dry with a paper towel.
- Insert the tip of the ISE into the aqueous sample to be tested. Important: Make sure the ISE is not resting on the bottom of the container, the white reference contacts near the tip of the electrode are immersed, and no air bubbles are trapped below the ISE. Note: Do not completely submerge the sensor. The handle is not waterproof.
- Hold the ISE still until the reading stabilizes and record the displayed reading. Note: With some aqueous samples, especially those at high concentrations, it could take several minutes for the reading of the Chloride ISE to stabilize. If you know the approximate concentrations of your samples, it is best to analyze them from lowest concentration to highest.
Chloride ions are found in freshwater samples as a result of water flowing over salt-containing minerals. These salts might include either sodium chloride (NaCl) or potassium chloride (KCl). The EPA maximum contamination level for chloride concentration in drinking water is 250 mg/L. The chloride ion concentration in seawater is approximately 19,400 mg/L—well below the upper limit of the Chloride ISE of 35,500 mg/L.
When the response of the Chloride ISE begins to slow, the membrane may need polishing. Cut a small piece (about 1 inch square) from a polishing strip. Wet the end of the electrode and the dull side of the polishing strip thoroughly with distilled water. Using only moderate pressure, polish the end of the electrode by gently rubbing it in a circular motion. This will remove the inactive layer of the membrane which impedes measurement. Rinse thoroughly with distilled water and recalibrate in the usual manner.
Sampling Freshwater Samples for Chloride Concentration
For best results, calibrate the Chloride ISE using the 10 mg/L and 1000 mg/L standards.
Measuring Chloride Concentration of Saltwater or Brackish Water
When measuring chloride concentration in seawater or brackish water, calibrate the Chloride ISE using the 1000 mg/L standard included with your Chloride ISE for one calibration point (or 1.806 parts per thousand, or ppt). For the second calibration point, prepare a standard that is 20,000 mg/L Cl– by adding 32.96 g of solid NaCl to enough distilled water to prepare 1 L of solution:
If you are calibrating in ppt, call this solution 36.13 ppt.
Determining Salinity of Saltwater or Brackish Water
Salinity is the total of all salts dissolved in water, expressed either as mg/L (equal to parts per million, ppm) or in parts per thousand (ppt). Seawater contains a fairly constant quantity of chloride ions. From your measurement of chloride ion concentration (in the previous section), salinity can be calculated using the following formula:
Salinity (mg/L or ppm) = 1.8066 × [Cl– concentration, mg/L]
Using this formula, the salinity of saltwater is calculated to be:
Salinity (mg/L or ppm) = 1.8066 × (19400 mg/L) = 35,000 mg/L
The level of salinity of seawater in parts per thousand, or ppt, would be:
Salinity (ppt) = 35,000 / 1,000 = 35 ppt
Using the Chloride ISE with Other Vernier Sensors
Some combinations of sensors interfere with each other when placed in the same solution. The degree of interference depends on many factors. For more information, see www.vernier.com/til/638
Using Ionic Strength Adjuster Solutions to Improve Accuracy
For optimal results at low concentrations of chloride ions, a standard method for taking measurements with the Chloride Ion-Selective Electrode (ISE) is to add ionic strength adjuster (ISA) solutions to each of your standard solutions and samples.
Adding an ISA ensures that the total ion activity in each solution being measured is nearly equal, regardless of the specific ion concentration. This is especially important when measuring very low concentrations of specific ions. The ISA contains no ions common to the Chloride ISE itself. Note: The additions of ISA to samples or standards described below do not need to have a high level of accuracy—combining the ISA solution and sample solution counting drops using a disposable Beral pipet works fine.
Use an ISA with the Chloride ISE by adding 5.0 M NaNO3 ISA solution (42.50 g NaNO3 / 100 mL solution) to the Cl– standard or to the solution being measured, in a ratio of 1 part of ISA (by volume) to 50 parts of the total solution (e.g., 1 mL of ISA to 50 mL of total solution, or 2 drops of ISA to 5 mL of total solution).