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 Calcium ISE to stabilize. If you know the approximate concentrations of your samples, it is best to analyze them from lowest concentration to highest.
Using the Calcium 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 the Calcium ISE to Measure Water Hardness as Ca2+
Your Calcium Ion-Selective Electrode (ISE) can be used to determine the concentration of aqueous Ca2+ ions, in the range of 1.0 to 40,000 mg/L. It can be especially useful in determining “hardness of water.” Calcium ions are often found in freshwater samples as a result of water flowing over soil and mineral deposits containing limestone, chalk, magnetite, or dolomite. In one common reaction, limestone is dissolved according to the reaction
CaCO3(s) + H+(aq) ↔ Ca2+(aq) + HCO3–(aq)
This reaction and others similar to it produce water with a relatively high concentration of Ca2+ ions, and lesser concentrations of Mg2+ and Fe3+ ions—known as hard water.
Many methods of determining water hardness use “total hardness,” or the sum of hardness due to Mg2+ and Ca2+. Since the Ca2+ concentration of freshwater usually exceeds that of Mg2+, determining the Ca2+ concentration alone is a good indicator of water hardness—we will refer to this measurement as “calcium hardness.” For best results, calibrate the Calcium ISE using the 10 mg/L and 1000 mg/L standards.
Using the standard solutions described here, your results will be in units of mg/L of Ca2+. Units of calcium hardness are usually expressed as “calcium hardness as CaCO3”. To convert from units of mg/L of Ca2+ (150 mg/L is used in this example) to units of calcium hardness as CaCO3, in mg/L, you would use this expression:
It is important to remember that total hardness, taking into account both the Ca and Mg levels, will be about 1.5 times higher than your calcium hardness value. Water hardness varies considerably in different parts of the United States, from levels of less than 60 mg/L (total hardness as CaCO3) in Washington, Oregon, Louisiana, Mississippi, Tennessee, and New England, to levels exceeding 250 mg/L in Midwestern states (Ohio, Indiana, Illinois, Iowa, Nebraska, South Dakota, and Oklahoma). Water with a hardness as CaCO3 level greater than 120 mg/L is considered to be “hard,” while levels exceeding 180 mg/L are referred to as “very hard.” Total water hardness, the sum of calcium and magnesium hardness in mg/L CaCO3, can be determined by titration with EDTA. A protocol can be found in our lab book Water Quality with Vernier. A plot of ln [Ca2+] (natural log of calcium ion concentration) vs. volume is used to determine the equivalence point. The second derivative can be used to calculate the point of maximum inflection at the equivalence point of the titration.
Using Ionic Strength Adjuster Solutions to Improve Accuracy
For optimal results at low concentrations of calcium ions, a standard method for taking measurements with the Calcium Ion-Selective Electrode (ISE) is to add ionic strength adjuster (ISA) solution 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 calcium ions. The ISA contains no ions common to the Calcium 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.
Add the 1.0 M KCl ISA solution (7.46 g KCl / 100 mL solution) to the Ca2+ standard or to the solution being measured, in a ratio of 1 part of ISA (by volume) to 50 parts of 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).