Introduction
The Calcium Ion-Selective Electrode has a solid-state PVC polymer matrix membrane. The electrode is designed for the detection of calcium ions ( Ca⁺² ) in aqueous solutions and is suitable for use in both field and laboratory applications.
The Calcium Ion is a divalent cation .
One mole of ( Ca+2 ) has a mass of 40.080 grams; 1000 ppm is 0.025 M

Physical Specifications
Length of body excluding gold contact = 130 mm
Length of body including gold contact = 140 mm
Diameter of body = 8 mm
DC resistance at 25°C < 2.5 MOhm
Minimum feasible sample volume = 5 ml

Chemical / Operational Specifications
Preconditioning / Standard solution : Normally 1000 ppm Ca+2 as CaCl2
(But see General Operating Instructions)
Preconditioning time : at least 5 minutes
Optimum pH range : pH 3.5 to pH 11
Temperature range : 5°C to 50°C
Recommended ISAB : 4M KCL (Add 2% v/v)
Recommended reference electrode : Single junction (ELIT 001)
Electrode slope at 25°C : 26± 3 mV/decade
Concentration range : 0.02 to 40,000 ppm (5x10-7 to 1 Molar)
Response time : < 10 seconds
Defined as time to complete 90% of the change in potential after immersion in the new solution.
Time for stable reading after immersion : < 1 to > 5 minutes
Depending on concentration, use of ISAB, nature of sample and stabilisation time of liquid junction potential of reference electrode.
Potential drift (in 1000 ppm unstirred) < 3 mV/day (8 hours)
Measured at constant temperature and with ISE and Reference Electrode continually immersed

Interference
Only Al +3 has a strong interference on the Ca electrode, with a selectivity coefficient (SC) of 5 (ie five times more sensitive to Al +3 than to Ca +2) and can only be tolerated in very low concentrations relative to the Ca.
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: 
      ((expected concentration) x (SC) / (expected Ca concentration)) x 100.
The following ions have a small effect on the Ca electrode (SC in brackets), but these would only be significant if they were at least ten times more concentrated than the Ca ion:  Iron Fe+2 (0.02) Strontium (0.0075), Barium (0.005) Copper (0.002), Sodium, (0.0005), Magnesium (0.00055), Potassium (0.00005), Ammonium (0.000025), Lithium (0.000012)

Procedure for determining the concentration of CALCIUM (Ca⁺⁺) in WATER

Ion-Selective Electrode for calcium ion (ELIT 8041 PVC membrane)

Reference electrode: single junction silver chloride (ELIT 001)

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.

100 or 150 ml polypropylene beakers, 100ml volumetric flask, 1, 2, 5, 10ml pipettes.

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 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 0.001 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 on the computer interface software to measure these standard solutions and prepare a calibration graph.

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.

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).

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.

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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.

Ion-Selective Electrode for calcium ion (ELIT 8041 – PVC membrane)

Reference electrode: single junction silver chloride (ELIT 001)

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.

100 or 150 ml polypropylene beakers, 100ml volumetric flask, 1, 2, 5, 10ml pipettes.

For Sample Preparation:

1Molar HCl, 4 Molar KOH, conc. HNO₃

Muffle furnace, balance, platinum or silica crucibles, 250ml volumetric flasks.

pH electrode (ELIT P500)

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, then the sample weight can be chosen to ensure a solution concentration which is within the specified linear range of the ISE. In this case it is only necessary to make two solutions which span this range: e.g. if sample solutions are known to lie between, say, 30 and 130ppm then you could use standards of 10 and 200ppm.

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.

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. HNO_3. 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.

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).

The results will be displayed as ppm and mol/l in the solution. Since the 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. 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.

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Because of the complex matrix, high ionic strength, and other interfering factors in beer, the concentration of ions in the solution is best determined using the Standard Addition method rather than direct potentiometry.

Enter the slope and expected sample concentration and volume into the Standard Addition software to calculate an appropriate standard concentration and volume. Make up the calculated standard concentration by serial dilution of the 1000ppm standard.