Instruction Manual
Ammonia Gas-sensing Electrode
5
Sample Requirements
Sodium hydroxide buffer must be added to standards and samples before measurement. When 10M
NaOH is added, all standards and samples should be in the range of pH 11 to 14. In this range, all
ammonium species are converted to ammonia. Adding the 10M NaOH adjusts the total level of
dissolved species below 1M. If the total level is greater than 1M, the sample should be diluted
before measurement. For further explanation, see the section entitled
Effect of Dissolved Species
.
Units of Measurement
Measurement of ammonia can be expressed in units of moles/liter, ppm as nitrogen, ppm as
ammonia, or other convenient concentration unit. Table 1 lists conversion units.
TABLE 1: Concentration Unit Conversion Factors
moles/liter ppm
N
ppm NH3
10-
2
140.0
170.0
10-
3
14.0
17.0
10-
4
1.4
1.7
MEASUREMENT PROCEDURE
Direct Measurement
Direct measurement is a simple procedure for measuring a large number of samples. A single meter
reading is all that is required for each sample. The ionic strength of samples and standards should
be made the same by adjustment with ISA for all ammonia solutions. The temperature of both
sample solutions and standard solutions should be the same.
Direct Measurement of Ammonia (using a pH/mV meter)
1.
By serial dilution, prepare three standard solutions from the 0.1M or 1,000 ppm stock
standard. The resultant concentrations should be 10-
2
M, 10-
3
M, and 10-
4
M or 1,000, 100
and 10 ppm ammonia standards. Add 1 ml of ISA per 100 ml of standard. Prepare
standards with a composition similar to the samples if the samples have an ionic strength
above 0.1M.
2.
Place the most dilute solution (10-
4
M or 10 ppm) on the magnetic stirrer and begin stirring
at a constant rate. After assuring that the meter is in the mV mode, lower the electrode tip
into the solution. After the reading has stabilized, record the mV value.
3.
Place the mid-range solution (10-
3
M or 100 ppm) on the magnetic stirrer and begin stirring
at a constant rate. After rinsing the electrode with distilled water, blot dry, and immerse the
electrode tip in the solution. When the reading has stabilized, record the mV value.
4.
Place the most concentrated solution (10-
2
M or 1,000 ppm) on the magnetic stirrer and
begin stirring at a constant rate. After rinsing the electrode with distilled water, blot dry,
and immerse the electrode tip in the solution. When the reading has stabilized, record the
mV value.
