85
The HI 7609829-10 sensor is specified for 0.02 to 200 ppm (mg/L) NH
4
+
-N
(equivalent to 0.026-260 ppm (mg/L) NH
4
+
). Based on the corresponding
molecular weights of nitrogen and ammonium, the relationship is: NH
4
+
-N
=(NH
4
+
)(14/18) = (NH
4
+
) x 0.7778. NH
4
+
-N is also called ionized ammonia.
The sensor responds in a Nernstian manner (like a pH sensor) and produces a
voltage that the meter converts to a concentration value.
APPENDIX C - ISE INFORMATION
This Appendix describes additional information about the ISE sensors used on
the HI76x9829 Probe.
HI 7609829-10: Ammonium selective electrode (ISE) is a combination liquid
membrane sensor used for the detection of free ammonium-nitrogen in fresh-
water samples. The sensor utilizes a polymeric membrane made with ammo-
nium ionophore in a PVC head and silver/silver chloride double junction gel filled
reference electrode. The outer body of the sensor is the thermoplastic PEI. This
sensor is used in place of the pH.
The measurement of ammonium-nitrogen, NH
4
+
-N is a useful tool in the
measurement of surface water contaminants such as tracing the source of
agricultural operations runoff or studying nutrient levels in natural waters.
HI 7609829-10 is an ion selective sensor that responds to the free Ammonium
ion. Ammonium ion is the ionized portion of the total ammonia concentration
and the amount present depends on pH. When the pH of the sample is below 8
pH, the primary form of ammonia is ammonium ion. See figure below.
The relationship is more complicated with increasing salinity but the two forms
together equal total ammonia.
Distribution diagram for ammonium ion/ammonia verus pH
Total = NH
4
+
+ NH
3
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
66.577.588.599.51010.511
pH
NH
4
+
9.25pH
NH
4
+
= NH
3
NH
4
+
ionized
form
NH
3
molecular
form
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