Equipment and Chemicals:
Ammonium Ion selective electrode, 3051 Combination (recommended) or 1051 Half Cell
Reference electrode (not needed for 3051): double junction with NaCl or MgSO4.
Standard solution: 1000 ppm NH4 as NH4Cl .
Buffer solution (ISAB): 1 Molar NaCl or MgSO4.
100 or 150 ml beakers, 100ml volumetric flask, 1, 2, 5, 10ml pipettes.
Calibration:
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.
If the samples to be measured are expected to have a total ionic strength of greater than 0.01 M, then add 10 ml of buffer solution to each standard and mix thoroughly to compensate for different activity coefficients between samples and standards.
Sample Preparation:
For low ionic strength samples, no sample preparation is necessary. Simply take approximately 50 to 100 ml 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 100ml of sample and add 10 ml of buffer solution and stir well before measurement.
Sample 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).
Results:
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.
Interference:
The following ions cause interference to the ammonium measurement (selectivity coefficients in brackets), but only the K interference is likely to cause a significant error.
Potassium (0.11), Sodium (0.0016), Calcium (0.00006), Lithium (0.00003).