Calcium and Water Hardness

Ca 2+
ADDITIONAL INFORMATION
Test 13
Tips for Instructors
1. The range of the Calcium Ion-Selective Electrode is 0.20 to 40,000 mg/L Ca 2+ . As stated in
the introduction, most values you measure from streams and lakes will be in the range of 4 to
100 mg/LÑthe lower end of the useful range of the sensor. Calcium hardness is found in the
range of 10 to 250 mg/L as CaCO 3 .
2. Two standard solutions are included with the Calcium ISEÑa High Standard that is
1000Êmg/L, and a Low Standard that is 10 mg/L. 3 You can replace the standards using these
directions:
High Standard (1000 mg/L Ca 2+ )
a. Add 2.771 g of CaCl 2 (or 3.669 g of CaCl 2 ¥2H 2 O) to enough distilled water to prepare
oneÊliter of solution.
Low Standard (10 mg/L Ca 2+ )
b. Dilute the High Standard from 1000 mg/L to 100 mg/L by combining 100 mL of the High
Standard with 900 mL of distilled water. Mix well.
c. Combine 100 mL of the 100-mg/L solution with 900 mL of distilled water. Mix well. This
standard is now 10 mg/L Ca 2+ .
3. The Calcium ISE can be used in a wide range of pH values, pH 3 to 10. The ions that are
known to interfere with the Calcium ISE (Pb 2+ , Hg 2+ , Sr 2+ , Cu 2+ , Ni 2+ ) will not generally be
encountered in significant concentrations in freshwater samples.
4. Even better results can be obtained if you bring all samples to the same ionic strength. This is
especially important when working with very low concentrations of ions. This can be
accomplished by the addition of ionic strength adjuster, ISA. Add Calcium ISA in the ratio of
approximately 1 to 50; for example, if your water sample is 50 mL in volume, add about
1ÊmL of ISA.
A bottle of Calcium ISA solution was provided with Calcium Ion-Selective Electrodes prior
to 1999. To prepare 100 mL of Calcium ISA solution, 1.0 M KCl, add 7.46 g of solid
potassium chloride, KCl, to enough water to prepare 100 mL of solution.
5. Soaking your ISE prior to use is very important. The Advanced Preparation section
recommends approximately 30 minutes of soaking in the High Standard solution. This is
usually sufficient, but an hour-long soak is even better. If you are going out into the field,
you can Òsoak as you goÓ using the ISE Soaking Bottle. We began shipping ISE Soaking
Bottles with Vernier Ion-Selective Electrodes in January of 1999. If you purchased your ISEs
prior to this and would like to use ISE Soaking Bottles, they can be purchased from Vernier
Software (BTL-ISE, $10 per package of 5 bottles). Important: Do not let the ISE soak
longer than 24 hours. ISEs are designed to be stored dry.
6. The Calcium ISE has a PVC membrane with a limited life expectancy. It is warranted to be
free from defects for a period of twelve (12) months from the date of purchase; it is possible,
however, that you may get somewhat longer use than the warranty period. If you start to
3
Prior to 1999, the Low Standard Solution shipped with each Calcium ISE was 100 mg/L Ca 2+ . This
standard can
be diluted to 10 mg/L by carefully measuring out 10 mL of the 100 mg/L standard, and adding enough distilled
water (~90 mL) to make 100 mL of 10 mg/L standard.
13 - 6 Water Quality with Computers