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Method for determining Potassium
(K+) Click the links for extra details for particular samples:
SOIL
WINE & FRUIT JUICE Apparatus Required: Ion-Selective Electrode for potassium ion (ELIT 8031 PVC membrane) Reference electrode: double junction lithium acetate (ELIT 003n) 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. 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 linear range of the ISE, then it is only necessary to make two solutions (preferably one 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. Return to top Extra Details for determining POTASSIUM (K+) in SOILS Water-soluble Potassium is extracted from dried soil samples by dispersion in de-ionised water and analysed by direct potentiometry. 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. 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 Extra Details for determining POTASSIUM (K+) in WINE and FRUIT JUICE Most wine and fruit juice samples contain relatively high concentrations of K and have high ionic strength so it is necessary to dilute with water to give a K content between about 5 and 100ppm and mix with ISAB before analysing by direct potentiometry. Alternatively, samples can be measured directly, without dilution, using the Standard Addition or Sample Addition method depending on the actual K content. 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 Last Update: CCR 19 Feb 2015 |