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) = 35000 / 1000 = 35 ppt
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).
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.
See general tips for using Ion Selective Electrodes at www.vernier.com/til/665