Interferences - EUTECH INSTRUMENTS NITRATE EPOXY Instruction Manual

Nitrate Electrode
Electrode measurements reproducible to ±2% 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

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
Interferences
(moles/liter) 10
-1
Cl 3.0X10
-1
NO 7.0X10
2
-1
Br 7.0X10
-1
CN 1.0X10
-1
ClO 5.0X10
3
-1
I 5.0X10
-1
ClO 1.0X10
4
Interferences
(ppm) 100 ppm N
-1
Cl 7,600
-1
NO 230
2
-1
Br 400
-1
CN 20.0
-1
ClO 30.0
3
-1
I 4.00
-1
ClO 0.07
4
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.
-2 -3
M 10
-1
3.0X10
-3
7.0X10
-3
7.0X10
-3
1.0X10
-4
5.0X10
-5
5.0X10
-6
1.0X10
10 ppm N
760
23.0
40.0
2.00
3.00
0.40
0.007
-4
M 10 M NO
-2
3.0X10
-4
7.0X10
-4
7.0X10
-4
1.0X10
-5
5.0X10
-6
5.0X10
-7
1.0X10
1 ppm N
76.0
2.30
4.00
0.20
0.30
0.04
0.0007
10
Instruction Manual
) SO
4 2 4
-1
3
-3
-5
-5
-5
-6
-7
-8
.