ProMinent Ammonia, Operating Manual, Page: 7 / 73

AMMONIA_User 9/09
7
AMMONIA User Manual
-
Only NH
4+
ions (ammonium) are present at pH <7
-
The ratio of NH
4+
ions (ammonium) to NH
3
(ammonia) is 1:1 at pH 9.3
-
Only NH
3
(ammonia) is present at pH >11
If ammonium ions are predominant in the solution, as is the case in the acidic range, then they must be converted
into ammonia (NH
3
) by the addition of a strong alkali (e. g. NaOH, adding 10 N NaOH solution to the liquid being
measured for a pH
>
11), since the membrane is only permeable for ammonia (water in a liquid aggregate or ionic
constituents such as ammonium ions cannot pass through the membrane).
Ammonia sensor membrane caps
The advantage of the ProMinent sensor for ammonia is that prefabricated membrane caps are available, thus
avoiding the manual fitting under tension of the sensitive membrane.
Concentration range
The potentiometric measurement of ammonia shows a linear characteristic over a wide range of concentration on a
semi-logarithmic plot, as is indicated in the following conversion table for various units of concentration.
mol/l ppm as N ppm as NH
3
5 x 10
-7
7 x 10
-3
8.5 x 10
-3
10
-6
1.4 x 10
-2
1.7 x 10
-2
… … …
10
-1
1400 1700
1 14000 17000
Table 1: Conversion table for various units of concentration
At especially low levels of concentration, the response time can be considerably longer, as shown in the table above.
It may be possible to improve the response time by diluting the electrolyte by a factor of 10, using distilled water.
And, particularly at high concentration, there is a risk that readings are too low because of ammonia losses. Samples
should therefore be measured as soon as possible. Loss of ammonia can be reduced by a cover or by using the
apparatus in an enclosed vessel.
ProMinent Ammonia Analyzer measurements
The relationship between the electrode voltage and the concentration of ammonia in water is stored in the form of a
typical characteristic (Figure 5). The ProMinent analyzer is thus able to calculate the ammonia concentration from
the electrode voltage measurement and display the result directly in the concentration unit “parts per million”
(ppm) on the analyzer display.