Thermo Fisher Scientific Ion Selective Electrodes Calcium - Manual (Page 12)

Calcium Electrode Instruction Manual
12
Interferences
(ppm) 1,000 ppm CaCO
3
100 ppm CaCO
3
10 ppm CaCO
3
Mg
+2
2.43X10
5
2.43X10
4
2.43X10
3
Zn
+2
6.53X10
5
6.53X10
4
6.53X10
3
Ba
+2
9.60X10
5
9.60X10
4
9.60X10
3
K
+1
1.56X10
5
1.56X10
4
1.56X10
3
Na
+1
4.60X10
4
4.60X10
3
4.60X10
2
Ni
+2
2.94X10
4
2.94X10
3
2.94X10
2
Cu
+2
2.54X10
4
2.54X10
3
2.54X10
2
Fe
+2
1.11X10
4
1.11X10
3
1.11X10
2
Sr
+2
5.20X10
3
5.20X10
2
5.20X10
1
H
+1
1.4 pH 2.4 pH 3.4 pH
Hg
+2
8.0X10
3
8.0X10
2
8.0X10
1
Pb
+2
2.0X10
1
2.0 2.0X10
-1
Complexation
Sulfate, bicarbonate, and carbonate are the most common species that complex calcium ions. The
level of calcium ions, the level of the complexing ion, the pH of the solution, and the total ionic
strength of the solution determine the extent of the complexation. Complexation reduces the free
calcium ion concentration and, since the electrode responds only to free calcium ions, a false
reading results.
To avoid formation of CaSO
4
, the sulfate concentrations must be less than 5X10
-4
M (50 ppm). To
avoid formation of CaCO
3
or formation of the CaHCO
3
+
complex, the pH of the solution should be
less than 7, and the total carbonate/bicarbonate concentration should be less than 3X10
-3
M (280
ppm carbonate).
Temperature Influences
Samples and standards should be at the same temperature, since electrode potentials are influenced
by changes in temperature. A 1
o
C difference in temperature results in a 4% error at the 1.0X10
-3
M
level.
Provided that temperature equilibrium has occurred, the calcium electrodes can be used at
temperatures from 0
o
-40
o
C. Room temperature measurements are recommended, since
measurements at temperatures quite different from room temperature may require equilibrium times
up to one hour. Table 4 indicates the variation of theoretical slope with temperature.
TABLE 4: Temperature vs. Value for the Electrode Slope
Temp (
o
C) "S"
0 27.10
10 28.10
20 29.08
25 29.58
30 30.07
40 31.07