Note 1: Because of the interference of K⁺ on the NH₄⁺ electrode, there is a special method for measuring samples with a K content greater than half that of the NH4

Note 2: In aqueous solution, there is an equilibrium between NH4+ and NH3 which is pH dependent.
Below about pH 6.5 there is 100% NH4+; above about pH 11.5 is 100% NH3
Thus, if TAN (Total Ammonia Nitrogen) is required then the pH of the samples must be kept below 7 (Note that pure NH4 standard solutions have a pH of about 5.5 so should always be fully ionised). However, if the level of dissolved NH3 (toxicity) is more important then the pH and temperature of each sample must also be measured and the NH4/NH3 ratio calculated.

For more information, see:
Relationship between NH4/NH3/pH/Temp. in aqueous solutions.
Ionization and Toxicity of Ammonia.
Estimation of the Allowable Upper Limit of Ammonia in Saline Waters.
Detailed Investigation of various factors to consider when making NH4 measurements.

Ion-Selective Electrode for ammonium ion (ELIT 8051 PVC membrane).

Reference electrode: double junction lithium acetate (ELIT 003n).

Dual electrode head (ELIT 201).

ELIT Computer Interface/Ion Analyser, or Ion/pH/mV meter.

Standard solution: 1000 ppm NH₄ as NH₄Cl.

Buffer solution (ISAB): 1 Molar CuSO₄.

150 ml polypropylene beakers, 100ml volumetric flask, 1, 2, 5, 10ml pipettes.

Standard Solution Preparation

For the most accurate results, Ammonium chloride must be dried for one hour at 100 °C. and cooled in a dessicator. To make 1000ppm NH4+, dissolve 2.965 g of NH4Cl in deionised water in a 1000 ml volumetric flask and dilute to the mark. According some sources, this solution is stable for six months when stored in a refrigerator but more dilute solutions for preparing the calibration curve should be freshly made each day. However, tests made at Nico2000 Ltd have demonstrated that a 10,000ppm solution, stored on the laboratory shelf at room temperature, remained stable and accurate for more than 8 years, 100ppm was still accurate after 10 days standing in a covered beaker, and 10 & 1 ppm were OK after 4 days.

Before use, the electrodes must be calibrated by measuring a series of known standard solutions, made by serial dilution of the 1000ppm standard solution. For a full calibration, prepare 100ml of solutions containing 1000, 100, 10, 1, and 0.1ppm NH4. If the approximate range of concentration of the samples is known, and this is within the linear range of the ISE, then it is only necessary to make two solutions (preferably a decade apart) which span this range.

Follow the instructions on the General Operating Instructions to measure these standard solutions and prepare a calibration graph.

NB: If the samples to be measured are expected to have a total ionic strength of greater than 0.01 Molar, then add 10 ml of buffer solution to each standard and mix thoroughly to compensate for different activity coefficients between samples and standards. BUT note that this will change the calibration slightly (ie: increase the slope but also raise the lower limit of linearity and detection limits) and lower the pH so that any NH3 will be converted to NH4.
See Typical calibration graphs (Excel file)

For low ionic strength samples, no sample preparation is necessary. Simply take approximately 50 to 100 mls of sample in a plastic beaker, or even immerse the electrodes directly in a lake or river (but take care to avoid losing the electrodes!). For high ionic strength samples, take 100mls of sample and add 10 mls of buffer solution and stir well before measurement.

Follow the instructions in the electrode operating instructions to measure a series of samples and record the results. Briefly, it is important to note that, if measuring in beakers, the electrodes must be washed and dried between each sample, to avoid cross contamination, and sufficient time must be allowed (2 or 3 minutes), before taking a reading after immersion, to permit the electrode signal to reach a stable value. For the highest precision, frequent recalibration is recommended (see operating instructions).

The results will be displayed as ppm and mol/l. If buffer solution has been added equally to standards and samples then these figures will not need adjusting because they will all be affected by the same dilution factor.

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Procedure for determining the concentration of AMMONIUM (NH₄⁺) in aqueous solutions with high potassium content.