Thermo Fisher Scientific Ion Selective Electrodes Nitrate - Manual (Page 10)

Instruction Manual Nitrate Ion Electrode
8
Interferences
Certain anions are electrode interferences and will cause electrode malfunction, drift or measurement
errors if present in high enough levels. The level of interfering common anions that will cause a 10%
error at three levels of nitrate is given in Table 3.
TABLE 3: Concentration of Possible Interferences Causing a 10% Error at
Various Levels of Nitrate; Background Ionic Strength of 0.12M (NH
4
)
2
SO
4
.
Interferences
(moles/liter) 10
-2
M 10
-3
M 10
-4
M NO
3
-1
Cl
-1
3.0X10
-1
3.0X10
-2
3.0X10
-3
NO
2
-1
7.0X10
-3
7.0X10
-4
7.0X10
-5
Br
-1
7.0X10
-3
7.0X10
-4
7.0X10
-5
CN
-1
1.0X10
-3
1.0X10
-4
1.0X10
-5
ClO
3
-1
5.0X10
-4
5.0X10
-5
5.0X10
-6
I
-1
5.0X10
-5
5.0X10
-6
5.0X10
-7
ClO
4
-1
1.0X10
-6
1.0X10
-7
1.0X10
-8
Interferences
(ppm) 100 ppm N 10 ppm N 1 ppm N
Cl
-1
7,600 760 76.0
NO
2
-1
230 23.0 2.30
Br
-1
400 40.0 4.00
CN
-1
20.0 2.00 0.20
ClO
3
-1
30.0 3.00 0.30
I
-1
4.00 0.40 0.04
ClO
4
-1
0.07 0.007 0.0007
Interferences such as chloride, bromide, iodide and cyanide can be removed by precipitation with 0.5
grams of silver sulfate added to 100 ml of sample. Nitrite interference can be removed by adding 0.3
grams of sulfamic acid to 100 ml of sample. Carbonate and bicarbonate, which are weak interferences,
can be removed by acidifying the sample to pH 4.5 with sulfuric acid. Organic (carboxylic) anions
hinder the nitrate electrode response and can be removed by adding 10 grams of aluminum sulfate to
100 ml of sample. The above interference removal procedures require similar treatment of standards as
well as samples.
If the electrode is exposed to high levels of interfering ions which cannot be removed, the electrode
reading may drift and the response may become sluggish. Restore performance by soaking in distilled
water for 30 minutes followed by soaking in nitrate standard for 30 minutes.