How to determine ammonia
Ammonia Photometric Method (0-2 ppm)
Nitrogen exists in many forms in aqueous systems. These are nitrate, nitrite, ammonia and organic nitrogen. Organic nitrogen sometimes referred to analytically as albuminoid or fixed nitrogen, occurs naturally in surface water from the decomposition of organic matter. Ammonia results from the breakdown of these nitrogenous organic compounds by microbiological organisms or boiler conditions.
Ammonia nitrogen is of importance in connection with the determination of the specific conductance of steam condensate. Ammonia possesses a high specific condensate and where this gas is present in steam condensate, conductance measurements must be corrected for the conductance due to ammonia. Otherwise erroneously high solids content of the steam would be indicated. Where effective degassing equipment is used to remove ammonia and carbon dioxide prior to the measurement of steam purity, ammonia may be reduced to such a low value in the test samples as to require no correction for its conductance effect.
Ammonia may also be significant because of its high corrosivity to copper and copper bearing alloys which it solubilizes by forming the copper ammonium ion. Ammonia may be removed by the use of break point chlorination or cation exchange by hydrogen zeolite.
Measurement of organic nitrogen can also be used in tracing process chemical leaks. In these instances, organic nitrogen is converted to ammonia by kjeldahl digestion. The ammonia thus generated is detected by Nesslerization. This digestion procedure should be performed in a qualified, fully equipped laboratory. Samples may be preserved by the addition of strong acid to yield a pH of less than 2 in the sample.
Theory of Test
In a basic medium, ammonia reacts with Nessler Reagent, forming a colored complex that varies from yellow to deep amber. The intensity of the color is proportional to the ammonia concentration. The ammonia content is expressed as nitrogen in parts per million.
Apparatus Required
Filter photometer complete with associated laboratory glassware.
Chemicals Required
Ammonium Standard
Nessler Reagent
Procedure for Test
Carefully measure 50 ml of a freshly obtained sample into the Erlenmeyer flask. Add 1 ml of Nessler reagent with mixing, and allow the color to develop for 10 minutes. Measure the absorbance at 425-nm using a 10-mm cell. Because the Nessler reagent has a pale color, the photometer should be zeroed with a reagent blank. Prepare a calibration curve from 0 to 2.0-ppm NH3 as N using approximate dilutions of the ammonium standard. These standards should be treated in the same manner as indicated above for the water samples. Plot the resulting absorbance values versus the ammonia concentration on linear graph paper.
Calculation of Results
The ammonia content as N in parts per million is obtained by reference to the ammonia calibration curve.
Limitations of Test
The direct Nesslerization of the ammonium ion is applicable to waters of low mineral content such as condensed steam and condensate samples. Results are affected by calcium, magnesium, iron,
turbidity and sulfides. Special treatment can be used for the removal of these interfering substances.Calcium and magnesium interferences may be eliminated by complexation with EDTA. Alternatively, the sample may be distilled and collected in dilute acid to remove the ammonia from the interfering matrix.
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