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Method for determining the concentration of IOIDE (I-)
in Aqueous Solutions

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

Apparatus Required:

Ion-Selective Electrode for iodide ion (ELIT 8281 crystal membrane)

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

Dual electrode head (ELIT 201)

Standard solution: 1000 ppm I as KI

Buffer solution (ISAB): 5 Molar NaNO3.

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

150 ml polypropylene 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 I. 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: If the samples to be measured are expected to have a total ionic strength of greater than 0.01 Molar, then 2 ml of buffer solution should be added to each 100ml standard and mixed thoroughly to compensate for different activity coefficients between samples and standards.

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

Sample Preparation:

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 samples with high ionic strength, take 100mls of sample and add 2 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, 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.



Technical Specifications
for the Iodide Ion-Selective Electrode (ELIT 8281)

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

Introduction
The Iodide Ion-Selective Electrode has a solid-state Crystal membrane. The electrode is designed for the detection of iodide ions (I- ) in aqueous solutions and is suitable for use in both field and laboratory applications.
The Iodide Ion is a monovalent anion.
One mole of ( I-) has a mass of 126.905 grams; 1000ppm is 0.008M
Dissolve 1.308g anhydrous potassium iodide (KI) 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 = <2.5M Ohm
Minimum feasible sample volume = 5mls

Chemical / Operational Specifications
 Preconditioning / Standard solution : Normaly 1000ppm I- as KI
(But see General Operating Instructions)
Preconditioning time : 5 minutes
Optimum pH range : pH 2 to pH 12
Temperature range : 0 to 80°C
Recommended ISAB 5M NaNO3 (Add 2%v/v)
Recommended reference electrode : double junction lithium acetate(ELIT 003)
Reference electrode outer filling solution : 0.1M CH3COOLi
Electrode slope at 25°C : 54±5 mV/decade
Concentration range : 0.06 to 12,700 ppm (5x10-7 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. Cyanide ions have a selectivity coefficient (SC) of 1 (equally sensitive to CN and I) and hence will cause a significant positive error if they are present in concentrations greater than one one-hundredth of the iodide. Other minor interferences are from Br (SC~ 0.0004) and Cl (0.000001)

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 Iodide concentration)) x 100.



Return to top      Last update: CCR 14 August 2015