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 thestandard addition method rather than direct potentiometry.

Apparatus Required

  • 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 contact130 mm

Length of body incl. gold contact140 mm

Diameter of body8 mm

DC resistance at 25° C< 2.5 M

Minimum feasible sample volume5 ml

Chemical Specifications

Preconditioning / Standard solutionNormally 1000 ppm Ca+2 as CaCl2

Preconditioning timeat least 5 minutes

Optimal pH rangepH 3.5 to pH 11

Temperature range5° C to 50° C

Optimal temperature25° C

Recommended ISAB4M KCL (add 2% v/v)

Recommended reference electrodeSingle junction Ag/AgCl(DNGA 181)

Electrode slope at 25° C26± 3 mV/ decade

Linear (Nernstian) measuring range0.1 to 4,000 ppm

Total measuring range0.02 to 4,000 ppm

Detection limit0.02 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 immersion2 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 FrequencyDepending 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