Introduction
The Mercury Ion-Selective Electrode has a solid-state crystal membrane. The electrode is designed for the detection of mercury ions (Hg⁺² ) in aqueous solutions and is suitable for use in both field and laboratory applications.
The Mercury Ion is a divalent cation.
One mole of ( Hg+2) has a mass of 200.590 grams; 1000ppm is 0.005M

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

Chemical / Operational Specifications
Preconditioning / Standard solution : Normally 1000ppm Hg+2 as Hg(No3)2
(But see General Operating Instructions)
Preconditioning time : 5 minutes
Optimum pH range : pH 0 to pH 2
Temperature range : 5°C to 80°C
Recommended ISAB : 0.1M HNO3 (1:1 v/v)
Recommended reference electrode : double junction (ELIT 002)
Reference electrode outer filling solution : 0.1M KNO3
Electrode slope at 25°C : 26±3 mV/decade
Concentration range : 0.2 to 201,000 ppm (1x10-6 to 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.

Interference:
NB: All poly-crystalline membranes contain Silver Sulphide and thus will not give reliable readings if Ag or S ions are present in the solution.
Note very low and narrow pH range for this electrode.

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Procedure for determining the concentration of MERCURY (Hg^++-) in WATER

Ion-Selective Electrode for mercuric ion (ELIT 8251 crystal membrane).

Reference electrode: double junction potassium nitrate (ELIT 002).

Dual electrode head (ELIT 201)

Standard solution: 1000 ppm Hg as Hg(NO₃)₂

Buffer solution (ISAB): 0.1 Molar HNO₃.

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

100 or 150 ml polypropylene beakers, 100ml volumetric flask, 1, 2, 5, 10, 25ml pipettes.

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 Hg. 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.

NB: This electrode can only be used in the pH range 0 to 2 therefore it is essential to add ISAB 50% by volume to all standards and samples before measurement.

Follow the instructions on the computer interface software to measure these standard solutions and prepare a calibration graph.

Take 25mls of sample and add 25 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 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.