Thermo Fisher Scientific Ion Selective Electrodes Calcium - Manual (Page 6)

Calcium Electrode Instruction Manual
6
Dilute concentrated samples (over 0.1M) before measurement. Recalibrate every few hours for
routine measurement.
Sample Requirements
All samples and standards must be aqueous and not contain organics which can dissolve in the
membrane or extract out the liquid ion exchanger.
The temperature of the standard solutions and of the sample solutions should be the same and below
40
o
C. About a 2% error will be introduced for a 1
o
C difference in temperature.
The pH range for the calcium ion electrode is 3-10. Neutralize samples outside this range with
NaOH or HCl to bring them in range.
Interferences should be absent. If they are present, use the procedures found in the
Interferences
section to remove them.
Units of Measurement
Calcium concentrations are measured in units of ppm as calcium, ppm as CaCO3, moles per liter, or
any other convenient concentration unit. Table 1 indicates some of the concentration units.
TABLE 1: Concentration Unit Conversion Factors
ppm Ca
+2
ppm CaCO
3
moles/liter
4.01 10.0 1.0X10
-4
10.00 24.9 2.5X10
-4
40.10 100.1 1.0X10
-3
400.80 1,000.9 1.0X10
-2
MEASUREMENT PROCEDURE
Direct Measurement
Direct measurement is a simple procedure for measuring a large number of samples. A single meter
reading is all that is required for each sample. The temperature of both sample solution and of
standard solutions should be the same.
Direct Measurement of Calcium (using a pH/mV meter)
1. By serial dilution prepare three standard solutions from the 0.1M, 1,000 ppm, or 100 ppm
standard. The resultant concentrations should be 10
-2
, 10
-3
, and 10
-4
M or 100, 10 and 1
ppm standards. Add 2 ml of ISA to each 100 ml of standard.
2.
Place the most dilute solution (1.0X10
-4
M or 1 ppm) in a 150 ml beaker on the magnetic
stirrer and begin stirring at a constant rate. After assuring that the meter is in the mV mode,
lower the electrode tips into the solution. After the reading has stabilized, record the mV
reading.