Procedure for determining the level of calcium (Ca2+) in beer
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.
Apparatus Required
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Ion selective electrode for calcium ion
Reference electrode: single junction silver chloride
Electrode connection to meter
Standard solution: 1000 ppm Ca as CaCl2
4 M KCl ionic strength adjustment buffer (ISAB)
100 or 150 ml polypropylene beakers, 100ml volumetric flask, 1, 2, 5, 10ml pipettes.
Calibration
Before use, the electrodes must be calibrated to find an accurate value for the slope under the current
operating conditions. This is done by measuring 10ppm and 100ppm standards, made by serial dilution of
the 1000ppm standard solution, and calculating the difference between the two measured voltages. Normally
the slope is expected to be between about 20 and 30 mV per decade of concentration.
Standard Selection
For the standard addition method to work best it is necessary to have some idea of the likely sample
concentration first. If this is completely unknown then it is best to make a quick sample measurement directly
after calibration using direct potentiometry as described in the interface operating instructions. Make up the
calculated standard concentration by serial dilution of the 1000ppm standard, using 2 mL of ISAB/100 mL
solution.
Sample Measurement
1. Pipette the pre-determined sample volume (usually 50 or 100ml) into a beaker, add 2 mL of ISAB
per 100 mL of solution, immerse the electrodes, swirl the solution, and record the stable voltage after
a few minutes of stabilization.
2. Add the calculated volume of standard, with stirring, and wait for a new stable voltage when the
solutions have completely mixed and the electrodes have re-equilibrated.
Results
The results will be displayed as mg/L. If the estimated sample concentration was far from the measured
value and thus the selection of standard volume and concentration were not optimum, then the software will
produce a comment to warn that a second attempt may provide a more reliable result.
Interference
Only Al 3+ has a strong interference on the Ca electrode, with a selectivity coefficient of 5 (ie five times more
sensitive to Al3+ than to Ca2+) and can only be tolerated in very low concentrations relative to calcium.
Technical Specifications for the Calcium ISE DNGA 25
Introduction
The calcium ion-selective electrode has a solid-state PVC polymer matrix membrane. The electrode is
designed for the detection of calcium ions (Ca2+) in aqueous solutions and is suitable for use in both field and
laboratory applications.
Physical Specifications
Length of body excl gold contact
130 mm
Length of body incl. gold contact
140 mm
Diameter of body
8 mm
DC resistance at 25° C
< 2.5 M
Minimum feasible sample volume
5 ml
Chemical Specifications
Preconditioning / Standard solution
Normally 1000 ppm Ca+2 as CaCl2
Preconditioning time
at least 5 minutes
Optimal pH range
pH 3.5 to pH 11
Temperature range
5° C to 50° C
Optimal temperature
25° C
Recommended ISAB
4M KCL (add 2% v/v)
Recommended reference electrode
Single junction Ag/AgCl (DNGA 181)
Electrode slope at 25° C
26± 3 mV/ decade
Linear (Nernstian) measuring range
0.1 to 4,000 ppm
Total measuring range
0.02 to 4,000 ppm
Detection limit
0.02 ppm
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
2 to 5 minutes (partly dependent on reference)
NB: the Ca electrode is sensitive to movement so it is better not to stir the solution.
Potential drift
(in 1000 ppm unstirred) < 3 mV/ day (8 hours)
(when measured at constant temperature and with ISE continually immersed but reference electrode
removed between readings so as to avoid drift in it's liquid junction potential)
Recalibration Frequency
Depending on precision requirements
Interference
Only Al3+ has a strong interference on the Ca electrode, with a selectivity coefficient of 5 (ie five times more
sensitive to Al3+ than to Ca2+) and can only be tolerated in very low concentrations relative to the Ca. Other
ions have a small effect (selectivity coefficients shown in brackets), but these would only be significant if they
were at least ten times more concentrated than the Ca ion:
strontium (0.0075), barium (0.005) copper (0.002), sodium, (0.0005), magnesium (0.00055),
potassium (0.00005), ammonium (0.000025), lithium (0.000012)
Typical Levels of Minerals in Beer
Ion
Typical levels (mg/L)
Aluminium
0.01
Iron
0.05
Copper
0.03
Calcium
20-70
Sodium
20-50
Chloride
100-200
Potassium
1-10
Magnesium
1-10