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Method for determining the concentration of MERCURY (Hg++)
in Aqueous Solutions

Electrode Specifications       Links to publications on the WWW     Go to Ion Selective Electrodes page


Apparatus Required:

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

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

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.

150 ml polypropylene beakers, 100ml volumetric flask, 1, 2, 5, 10, 25ml 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 Hg. If the approximate range of concentrations 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.

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 in the General Operating Instructions to measure these standard solutions and prepare a calibration graph.

Sample Preparation:

Take 25mls of sample and add 25 mls of buffer solution and stir well before measurement.

Sample 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 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.



Technical Specifications
for the Mercury Ion-Selective Electrode (ELIT 8251)

Click here to download a printer-friendly (pdf) Specification Sheet.

Introduction
The Mercury Ion-Selective Electrode has a solid-state crystal membrane. The electrode is designed for the detection of mercuric ions (Hg+2 ) in aqueous solutions and is suitable for use in both field and laboratory applications.
The Mercuric Ion is a divalent cation.
One mole of Hg+2 has a mass of 200.590 grams; 1000ppm is 0.005M
Dissolve 1.708g mercuric nitrate monohydrate (Hg(NO3)2.H2O) in 1 litre water.

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 : 0 to 80°C
Recommended ISAB : 0.1M HNO3 (1:1 v/v)
Recommended reference electrode : double junction lithium acetate(ELIT 003)
Reference electrode outer filling solution : 0.1M CH3COOLi
Electrode slope at 25°C : 26±3 mV/decade
Concentration range : 0.2 to 20,100 ppm (1x10-6 to 0.1 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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