Note low pH range (3 to 7) for this electrode.
Procedure for determining the concentration of CADMIUM
(Cd++) in WATER
Apparatus Required:
Ion-Selective Electrode for cadmium ion (ELIT 8241 crystal membrane)
Reference electrode: double junction potassium nitrate (ELIT 002)
Dual electrode head (ELIT 201)
ELIT Computer Interface/Ion Analyser, or Ion/pH/mV meter.
Standard solution: 1000 ppm Cd as Cd(NO3)2
Buffer solution (ISAB): 5 Molar NaNO3.
100 or 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 Cd. 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: If the samples to be measured are expected to have a total ionic strength
of greater than 0.001 Molar, or a pH of >7 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 samples with low ionic strength and pH < 7, 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 (or
pH>7), 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.
