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Click the links below for analytical procedures for determining the concentration of
POTASSIUM (K+) in:
| WATER |
SOIL |
WINE & FRUIT JUICE | Technical Specifications Introduction Physical Specifications Chemical / Operational
Specifications Interference: 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: Thus, in order to have no significant interference effects, Rubidium concentration should be less than one hundredth of the potassium, Caesium less than one tenth of the Potassium, Ammonium less than the potassium. The remaining ions would have to be more than one hundred times the potassium in order to have a significant effect. NB: Although sodium interferes with the K measurement, sodium chloride can be used as ISAB, because the sodium will only produce an apparent potassium signal of about 0.5ppm and will not cause a significant error for most applications. If ISAB is required for low K concentrations (i.e. < 5ppm, in samples with high ionic strength) then an alternative ISAB must be used (e.g. 2% of 5M TEAC – Tetraethylammoniumchloride). Return to top Procedure for determining the concentration of POTASSIUM (K+) in WATER Apparatus Required: Ion-Selective Electrode for potassium ion (ELIT 8031 PVC membrane) Reference electrode: double junction lithium acetate (ELIT 003) Dual electrode head (ELIT 201) Standard solution: 1000 ppm K as KCl Buffer solution (ISAB): 2.5 Molar NaCl. ELIT Computer Interface/Ion Analyser, or Ion/pH/mV meter. 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 K. 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 0.01 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 on the computer interface software 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 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, 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. Return to top Procedure for determining the concentration of POTASSIUM (K+) in SOILS Water-soluble Potassium is extracted from dried soil samples by dispersion in de-ionised water and analysed by direct potentiometry. Apparatus Required Ion-Selective Electrode for potassium ion (ELIT 8031 – PVC membrane) Reference electrode: double junction lithium acetate (ELIT 003) Dual electrode head (ELIT 201) Standard solution: 1000 ppm K as KCl Buffer solution (ISAB): 2.5 Molar NaCl. ELIT Computer Interface/Ion Analyser, or Ion/pH/mV meter. 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 K. 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. 2 ml of buffer solution must be added to each 100ml standard and mixed thoroughly to compensate for different activity coefficients between samples and standards. Follow the instructions on the computer interface software to measure these standard solutions and prepare a calibration graph. Sample Preparation After removing any stones or fresh organic material (roots, twigs, leaves, worms, insects etc), and breaking up any large lumps, soil samples must be air dried by laying out in a thin layer on metal or plastic trays in a current of air at no more than 30°C until dry. Then they must be crushed in a pestle and mortar to pass through a 2mm sieve. About 200g of material should be sufficient for duplicate analysis and storage. Weigh accurately about 50g of dry soil sample and add exactly 100ml of de-ionized water and shake vigorously for 30 seconds to ensure good dispersion, then leave to stand for 15 mins. After this time, shake again for 5 seconds, and allow to stand again. Repeat this procedure three times before finally allowing to settle. When the solution is clear, take exactly 50 mls (by decanting or pipetting) and mix with 1 ml of buffer solution in a plastic beaker. Sample 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). Results The results will be displayed as ppm and mol/l in the solution. Since buffer solution has been added equally to standards and samples, these figures will not need adjusting for this addition. However, the concentration in the solution (in ppm = micrograms per ml) must be multiplied by 100 and divided by the sample weight to give the concentration in the soil (in micrograms per gram). Return to top Procedure for determining the concentration of POTASSIUM (K+) in WINE and FRUIT JUICE Wine and fruit juice samples must be diluted 1:10 with water and mixed with ISAB before analysing by direct potentiometry. Apparatus Required Ion-Selective Electrode for potassium ion (ELIT 8031 – PVC membrane) Reference electrode: double junction lithium acetate (ELIT 003) Dual electrode head (ELIT 201) Standard solution: 1000 ppm K as KCl Buffer solution (ISAB): 2.5M NaCl ELIT Computer Interface/Ion Analyser, or Ion/pH/mV meter. 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 K. 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: before calibration, 2 ml of buffer solution must be added to each 100ml standard and mixed thoroughly to compensate for any differences in ionic strength between samples and standards. Follow the instructions on the computer interface software to measure these standard solutions and prepare a calibration graph. Sample Preparation Take 10mls of sample and dilute to 100ml with deionised water, then add 2 mls of buffer solution and stir well before measurement. Sample 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, 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. Since 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. This value must be multiplied by 10 to get the concentration in the original sample. Return to top |