Apparatus Required:
Copper Ion-Selective Electrode (Mono (Half-cell) 1227 or 3227 Combination)
Reference electrode (For use with 1227 Half-cell only): double junction w/ Potassium nitrate in an outer chamber.
Standard solution: 1000 ppm Cu2+ as Cu(NO3)2
Buffer solution (ISAB): 3 Molar KNO3 or 5 Molar NaNO3
pH meter, preferably with a "Concentration" or "ISE" mode
100 or 150 ml glass or polypropylene beakers, 100ml volumetric flask, 1, 2, 5, 10ml pipettes.
Calibration and Sample Preparation:
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 Cu. 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.
Note: 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.
High level samples: (10-4 and up)
Since this is a linear range, two point calibration is sufficient; more points can be used if desired.
Take 100ml of sample and add 2 ml of buffer solution and stir well before measurement. Do the same for each of your standards
Low level samples: ( up to 10-5)
At least three standards are required for calibration to compensate for electrode's non-linearity.
For the specific steps follow the instructions on your Meter of how to perform calibration in Concentration or ISE mode
Sample Measurement:
If a Meter with a direct readout capability is used, just get your results in mol/l, ppm or any other linear units.
Note:It is important , to have electrodes rinsed and wiped between each sample to avoid carry over, 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 .
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:
Silver, Mercury or Sulfide ions have very high interference and can only be tolerated in a trace concentrations compared to the Cu. Since they poison a sensing element, ideally, they should be absent.
Bromide and Chloride ions both have selectivity coefficients of >1 and will cause a significant positive error it they are present in concentrations greater than one tenth of that of the Copper.