Innovative technology at highly competitive prices.
|
|
ELIT Brand Electrochemical Sensors and Computer-based Instrumentation. Innovative technology at highly competitive prices. |
| HOME | PRODUCTS | TECHNICAL INFORMATION | SITE MAP | CONTACT US | PRICE LIST |
Method for Determining the Concentration of CALCIUM
(Ca++)
in Aqueous Solutions
Electrode Specifications
Links to publications on the WWW
Go to Ion Selective Electrodes page
|
Click on the following links for extra procedures for various substrates: Apparatus Required: Ion-Selective Electrode for calcium ion (ELIT 8041 PVC membrane) Reference electrode: single junction silver chloride (ELIT 001n) Dual electrode head (ELIT 201) Standard solution: 1000 ppm Ca as CaCl2 Buffer solution (ISAB): 4 Molar KCl. ELIT Computer Interface/Ion Analyser, or Ion/pH/mV meter. 150 ml polypropylene beakers, 100ml volumetric flask, 1, 2, 5, 10ml pipettes. Calibration: Direct Potentiometry Method 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 Ca. 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 0.001 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. Procedure for determining the concentration of CALCIUM (Ca++) in ANIMAL FEED Calcium is extracted from animal feedstuffs by ashing a known weight of sample and dissolving the ash in mixed hydrochloric and nitric acids. The pH is then adjusted to about pH7 with potassium hydroxide. Extra Apparatus Required 1Molar HCl, 4 Molar KOH, conc. HNO3 Muffle furnace, balance, platinum or silica crucibles, 250ml volumetric flasks. pH electrode (ELIT P500) Calibration: 2 ml of 4M KCl must be added to each 100ml standard and mixed thoroughly to compensate for different activity coefficients between samples and standards. Sample Preparation Weigh out an appropriate quantity of finely ground material to give a solution concentration which lies within the range of the standards chosen. If the approximate sample concentration is completely unknown then try using 1g. Transfer the sample to a suitable crucible and ignite in a muffle furnace at 320°C for 3 hours, then allow to cool. When cool, slowly add 50 ml of 1M HCl, taking care to minimize spitting, and then add 5 drops of conc. HNO3. Transfer this solution to a tall 150ml beaker or conical flask and boil gently for 30 minutes Cool this solution and adjust the pH to about 7 by adding 4M KOH and monitoring the pH change with a pH electrode (previously calibrated) connected to the computer interface. Transfer this solution to a 250ml volumetric flask and dilute to the mark with deionised water. Take 100ml of this solution in a 150ml plastic beaker and add 2 ml of buffer solution and mix well. Results Since the buffer solution has been added equally to standards and samples then the results will not need adjusting because they will all be affected by the same dilution factor. To obtain the ppm Ca in the sample (mg/Kg) the ppm in the solution must be multiplied by 250 and divided by the sample weight. For wt% Ca this figure must be divided by 1000. Procedure for determining the concentration of CALCIUM (Ca++) in BEER and FRUIT JUICE Because of the complex matrix, high ionic strength, and other interfering factors in beer and juice, the concentration of ions in the solution is best determined using the Standard Addition method rather than direct potentiometry. Calibration Before use, the electrodes must be calibrated to find an accurate value for the slope under the current operating conditions and in the range of the expected sample concentrations. If sample concentration is completely unknown then it is best to make a quick sample measurement first, using direct potentiometry with an old calibration. Then make up and measure two standards which span the expected sample range - say 1 & 10ppm or 10 & 100ppm - or more closely spaced if working below the linear range of the electrode. Standard Selection If the ELIT Ion Analyser software is being used, simple enter the slope and expected sample concentration and volume into the Standard Addition module to calculate an appropriate standard concentration and volume. Otherwise, follow the instructions on the Standard Addition page of nico2000.net (link above). Make up the calculated standard concentration by serial dilution of the 1000ppm standard. Sample Measurement 1) Pipette the pre-determined sample volume (usually 50 or 100ml) into a beaker, immerse the electrodes, swirl the solution, and record the stable voltage after a few minutes of stabilization. 2) Add the calculated volume of standard, swirl or stirr well then wait for a new stable voltage when the solutions have completely mixed and the electrodes have re-equilibrated. The software will then calculate the sample concentration and give an estimate of the quality of the determination - if the sample concentration is very different from the expected value then you will be prompted to make a second measurement using a more appropriate volume or concentration of standard. Procedure for determining the concentration of CALCIUM (Ca++) in Meat Products Calcium is extracted from meat directly into distilled water and measured by direct potentiometry. Calibration Each standard must be mixed thoroughly with an equal volume of buffer solution in order to compensate for different activity coefficients between samples and standards. Sample Preparation Weigh out accurately about 1g of meat sample and liquidise with 100ml of distilled water. Centrifuge the mixture and take 25 mls of the supernatant and mix with 25 mls of ISAB. Results Since the buffer solution has been added equally to standards and samples then the results will not need adjusting because they will all be affected by the same dilution factor. To obtain the ppm Ca in the sample (mg/Kg), the ppm in the solution must be multiplied by 100 and divided by the sample weight. Procedure for determining the concentration of CALCIUM (Ca++) in SKIMMED MILK Because of the high Ca content, complex matrix, high ionic strength, and other interfering factors in milk, the concentration of ions in the solution is best determined using the Sample Addition method rather than Standard Addition or direct potentiometry. The procedure is the same as for Beer and Juice (above) except that a small volume of sample is added to a large volume of standard and the Sample Addition calculations are used. Procedure for determining the concentration of CALCIUM (Ca++) in SOIL Calcium is extracted from soil by shaking with sodium acetate solution and measured by direct potentiometry. Calibration In order to maintain a similar matrix to the sample solutions, each standard must be mixed in the ratio 1:1 with 0.1M sodium acetate (dilute 200mls of the extraction solution to 1 litre with deionised water), then add 2 ml of KCl buffer solution to 100ml of this mixture, and mix thoroughly before measurement. Sample Preparation Weigh accurately about 4g of ground, air-dried soil into a screw capped centrifuge bottle and add 50ml of 0.5M sodium acetate extraction solution. Shake mechanically for about 2 hours and then centrifuge to remove all soil particles from the solution. Pipette 10mls of the supernatant into a 100ml flask and dilute to the mark with deionised water. Transfer to a 150ml beaker and add 2ml of KCl buffer and mix well. Results Since buffer solution has been added equally to standards and samples the results will not need adjusting for this dilution factor, but the ppm figure must be multiplied by 250 to compensate for the dilution of the sample and the standard, and divided by the sample weight in order to get the concentration in the soil in mg/Kg. RETURN TO TOP Procedure for determining the concentration of CALCIUM (Ca++) in SEA WATER Because of the high Ionic Strength of sea water it is difficult to measure directly by normal Direct Potentiometry. This is only possible if the calibration standards are made up in artificial sea water which does not contain Ca -and even then it is likely that the analytical errors will be relatively high. For more accurate results, it is better to take a measured-volume sub sample and analyse it in a beaker using the Sample Addition method. The procedure is the same as for Beer and Juice (above) except that a small volume of sample is added to a large volume of standard and the Sample Addition calculations are used. This should overcome the Ionic Strength effect and can give precision levels down to about ± 2%. The relatively high level of Ca should mean that there will be no significant interference from other ions. If higher precision is required, a second alternative, which requires rather more chemical expertise and manipulation but is reported to yield results within ±0.1%, is to use the Ca ISE as an end-point indicator in a titration with EGTA as published by XIE et.al. CHIN. J. OCEANOL. LIMNOL. 1997. Note that the Calomel RE is no longer necessary and can be replaced with the more recently developed gel-filled Single Junction AgCl reference (ELIT 001) Technical Specifications Introduction Physical Specifications Chemical / Operational
Specifications Interference |