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Method for determining the concentration of CHLORIDE
(Cl-)
in Aqueous Solutions
Electrode Specifications
Links to publications on the WWW
Go to Ion Selective Electrodes page
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Click on the links below for extra details for various substrates: Apparatus Required: Ion-Selective Electrode for chloride ion (ELIT 8261 crystal membrane) Reference electrode: double junction lithium acetate (ELIT 003n) Dual electrode head (ELIT 201) Standard solution: 1000 ppm Cl as KCl Buffer solution (ISAB): 5 Molar NaNO3. 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, 1ppm Cl. 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 0.01 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. Procedure for determining Cl- in BUTTER Chloride is extracted from butter by heating with dilute nitric acid and measured by direct potentiometry. Calibration Before measuring, each standard must be mixed 1:1 with dilute HNO3 ISAB (60ml conc. HNO3 in 1 litre. Sample Preparation 1) Weigh accurately about 10g of butter and place in a beaker with 100ml dil HNO3. 2) Heat until butter melts (do not boil) and stir well to ensure complete extraction of the chloride, but avoid emulsification. 3) Allow to cool so that the butter fat solidifies above the HNO3 solution. 4) Puncture the fat layer and pipette out 25mls of the solution. Dilute this 1:1 with deionised water. Procedure for determining Cl- in FRUIT JUICE Because of the relatively high Cl content, complex matrix, high ionic strength, and other interfering factors in fruit juice, the concentration of ions in the solution is best determined using the Sample Addition method rather than direct potentiometry - but note that this method will only work in the linear range of the electrode. 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 with a ten-fold difference in concentration which span the expected sample range and calibrate with these. Alternatively, a more accurate slope can be found by making up a standard in the middle of the range for the samples and analysing it using the Standard Addition software - then use the manual input to adjust the slope until the ppm is correct. Standard Selection If measurements are to be made using the ELIT Ion Analyser, simply enter the calibration slope, the standard concentration and volume and the expected sample concentration into the Sample Addition software to calculate an appropriate volume for the sample. Otherwise, follow the instructions on the Standard Addition page of nico2000.net (link above) to determine the best standard concentration and sample volume. Make up the calculated standard concentration by serial dilution of the 1000ppm standard. Sample Measurement 1) Pipette the pre-determined standard volume (25, 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 sample, swirl or stir 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 or volume of sample. Results 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. The ppm concentration in the solution will have to be multiplied by 100 and divided by the sample weight to get the concentration in the butter in micrograms per gram (= ppm = mg/Kg) Procedure for determining Cl- in MAYONNAISE Chloride is extracted from mayonnaise by dissolving in nitric acid and measured by direct potentiometry. Calibration Mix the standards with 1Molar nitric acid ISAB in the proportion 1:1 to make them compatible with the samples. Sample Preparation Weigh accurately about 1g of mayonnaise and disperse in 500ml of 1M nitric acid in a 1000ml volumetric flask. Shake thoroughly, dilute to 1 litre with deionised water and mix again. Filter about 50 to 100mls of solution into a sample beaker. Results Since buffer solution has been added equally to standards and samples, these figures will not need adjusting because they will all be affected by the same dilution factor. The measured values must be multiplied by 1000 and divided by the sample weight to get the concentration in the original sample. Procedure for determining Cl- in MILK The casein in milk tends to clog the ion selective membrane and thus reduce its sensitivity. This problem can be overcome by mixing one volume of milk sample with two volumes of dilute nitric acid (60 ml concentrated HNO3 diluted to 1 litre with de-ionised water) and shaking for one minute. The nitric acid is used instead of NaNO3 ISAB. After mixing with the nitric acid, samples are measured by Direct Potentiometry in the conventional manner. Calibration: HNO3 ISAB must be added to all standards in the proportion 2:1 to make them the same Ionic Strength and the same dilution factor as the samples. Procedure for determining Cl- in MEAT To measure chloride in meat samples it is necessary to use 0.5M nitric acid as ISAB (made by diluting 31.25ml conc. HNO3 to 1 litre) and add this in the proportion 1:1 to all samples and standards. Chloride is extracted from meat products by liquidizing about 1g of meat with 100 ml of water. After filtration, 25 ml of the sample solution is mixed with 25 ml of ISAB and analysed by Direct Potentiometry as normal. The measured sample concentration must be multiplied by 100 and divided by the sample weight to obtain the concentration in micrograms per gram of meat. Procedure for determining Cl- in SOIL Chloride is extracted from soil by shaking with deionised water and measured by direct potentiometry on the filtered solution. Calibration NB: before measuring, 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. Sample Preparation Weigh accurately about 4g of ground, air-dried soil and transfer to a 100ml screw cap shaking bottle. Add 50ml of deionised water and mix in a mechanical shaker for about 1 hour. Filter off the residue and wash thoroughly, retaining all washings. Dilute to 100ml in a volumetric flask. Transfer to a 150 ml beaker, add 2ml of buffer solution and stir well before measurement. Results Since buffer solution has been added equally to standards and samples, these figures will not need adjusting because they will all be affected by the same dilution factor. The measured ppm value must be multiplied by 100 and divided by the sample weight to obtain the concentration in the soil in mg/Kg.
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