Introduction
The Ammonium Ion-Selective Electrode has a solid-state PVC polymer matrix membrane which is designed for the detection of ammonium ions (NH₄⁺ ) in aqueous solutions and is suitable for both field and laboratory applications.
The Ammonium Ion is a monovalent cation .
One mole of ( NH4+ ) has a mass of 18.038 grams; 1000 ppm is 0.055 M

Physical Specifications
Length of body excluding gold contact = 130 mm
Length of body including gold contact = 140 mm
Diameter of body = 8 mm
DC resistance at 25°C = 2.5 MOhm
Minimum feasible sample volume = 5 ml

Chemical / Operational Specifications
Preconditioning / Standard solution : Normally 1000 ppm NH4+ as NH4Cl
(But see General Operating Instructions)
Preconditioning time : at least 5 minutes
Optimum pH range : pH 0 to pH 8.5
Note: this is the range in which the electrode response to NH4+ is not affected by changes in pH.
For accurate measurements of total nitrogen, pH should be kept below 7 to ensure 100% ionisation.
Temperature range : 5° C to 50° C
Recommended ISAB : 1M CuSO4 (Add 10% v/v)
Recommended reference electrode : double junction (ELIT 003)
Reference electrode outer filling solution : 0.1M CH3COOLi.
Electrode slope at 25°C : 54 ± 5 mV/ decade
Concentration range : 0.03 to 9,000 ppm (2x10-6 to 5x10-1 Molar)
Response time : 10 seconds
(Defined as time to complete 90% of the change in potential after immersion in the new solution.)
Time for stable reading after immersion : < 1 to > 5 minutes
(Depending on concentration, use of ISAB, nature of sample and stabilisation time of liquid junction potential of reference electrode.)
Potential drift (in 1000 ppm) : < 3 mV/ day (8 hours)
(Measured at constant temperature and with ISE and Reference Electrode continually immersed.)

The following ions interfere with the ammonium measurement - Selectivity Coefficients (SC)  in brackets: Potassium (0.11), Sodium (0.0016), Magnesium (0.00018), Calcium (0.00006), Lithium (0.00003).

The SC is the approximate apparent  increase in the measured concentration caused by 1 unit of the interferent.  Thus the likely effect of any interfering ion (% increase) can be calculated as follows:  ((expected concentration) x (SC) / (expected NH4 concentration)) x 100.

On this basis, only the K interference is likely to cause a significant error.

Procedure for determining the concentration of AMMONIUM (NH₄⁺) ions in aqueous solutions.

Note 1: Because of the interference of K⁺ on the NH₄⁺ electrode, special techniques are required for measuring samples with a K content greater than half that of the NH4 - see Procedure 2

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:
Study of important factors to consider when making NH4 measurements.
Examples of measurement of rain, pond, and river water.
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.

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

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

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.

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

Standard Solution Preparation

(Taken from http://www.nilu.no/projects/CCC/manual/documents/04_4-Determination%20of%20ammonium%20in%20precipitation.htm )
For 1000ppm NH4, Ammonium chloride must be dried for one hour at 100 °C. and cooled in a desicator. Dissolve 2.965 g of the dried salt in water in a 1000 ml volumetric flask. Dilute to the mark with water. According to the author, this solution is stable for six months when stored in a refrigerator, and the more dilute solutions for preparing the calibration curve must 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 concentrations of the samples is known, and this is within the specified linear range of the ISE, then it is only necessary to make two solutions which span this range: e.g. if samples are known to lie between, say, 30 and 130ppm then you could use standards of 10 and 200ppm or even 20 and 150ppm.

Follow the instructions on the ELIT Ion Analyser software 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 computer interface software and/or 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.