Thermo Fisher Scientific Ion Selective Electrodes Calcium - Manual (Page 11)

Instruction Manual Calcium Electrode
11
VtMt
MCa
+2
= ———
VCa
+2
where
MCa
+2
= concentration of calcium ion in the unknown (moles/liter)
Vt = volume of EDTA added at endpoint
Mt = EDTA concentration (moles/liter)
VCa
+2
= volume of unknown sample
ELECTRODE CHARACTERISTICS
Reproducibility
Electrode measurements reproducible to ±4% can be obtained if the electrode is calibrated every
hour. Factors such as temperature fluctuations, drift, and noise limit reproducibility. Reproducibility
is independent of concentration within the electrode's operating range.
Interferences
Table 3 lists some common cations that, if present in high enough levels, will cause electrode
interferences and measurement errors or electrode drift when using the calcium electrodes.
Electrode drift and slow response could indicate the presence of high interferences from the ions
listed. Soak the electrode in distilled water for five minutes, then for five minutes in calcium
standard solution to restore proper response.
TABLE 3: Concentration of Possible Interferences Causing a 10%
Error at Various Levels of Calcium.
Interferences
(moles/liter)
10
-2
M 10
-3
M 10
-4
M
Mg
+2
1.0X10
+1
1.0X10
0
1.0X10
-1
Zn
+2
1.0X10
+1
1.0X10
0
1.0X10
-1
Ba
+2
7.0X10
0
7.0X10
-1
7.0X10
-2
K
+1
4.0X10
0
4.0X10
-1
4.0X10
-2
Na
+1
2.0X10
0
2.0X10
-1
2.0X10
-2
Ni
+2
5.0X10
-1
5.0X10
-2
5.0X10
-3
Cu
+2
4.0X10
-1
4.0X10
-2
4.0X10
-3
Fe
+2
2.0X10
-2
2.0X10
-3
2.0X10
-4
Sr
+2
6.0X10
-2
6.0X10
-3
6.0X10
-4
H
+1
4.0X10
-2
4.0X10
-3
4.0X10
-4
Hg
+2
4.0X10
-2
4.0X10
-3
4.0X10
-4
Pb
+2
1.0X10
-4
1.0X10
-5
1.0X10
-6