To eliminate sample matrix effects which render direct potentiometry inaccurate, a known addition method is adopted to analyze the Calcium content of fruit juices (fresh).


  1. Calcium Combination ion selective electrodes 3041
  2. pH/Ion Analyzer (recommended) or pH Meter with millivolt scale
  3. Accurate balance weighing to ± 0.0005g
  4. Magnetic stirrer


  1. 1000 ppm Ca2+ standard solution
  2. 0.1M KNO3 (ISAB)
  3. 0.1M NaOH

Standard Preparation:

To 36.68g of CaCI22H2O in a 1 liter volumetric flask add deionised water to the mark. This is 1000 ppm Ca2+ solution.

Reagent Preparation:

Dissolve 4g NaOH in deionised water in a 1 liter volumetric flask. Dilute to the mark. This is 0.1M NaOH.

ISAB filling solution: to 10.1g KNO3 in a 1 liter volumetric flask add deionised water to the mark. This is 0.1M KNO3.



  1. To a 250 ml beaker on a magnetic stirrer add 20 ml of fruit juice.
  2. To the sample add 20 ml 0.1M KNO3 (ISAB).
  3. Add 40 ml deionised water.
  4. Adjust the pH of the sample solution to 6.0 by the addition of 0.1M NaOH.
  5. Dilute to 100ml using deionised water by transferring the beakeri s content to a 100ml volumetric flask and diluting to the mark.
  6. Empty the contents of the flask into the 250 ml beaker and immerse the electrodes in the solution.
  7. Record the stable mV potential (E1).
  8. Add 10 ml of the 1000 ppm Ca2+ standard (ensure continuous stirring) and record the new stable mV potential (E2).


Using Ion Analyzer , this determination may be carried out directly in concentration units using the Activity (Concentration) mode.


Calculate E  by  E2  - E1 (mV).

The sample solution concentration is given by this equation.

Cu = unknown solution concentration

Cs = standard concentration

Vs = volume of standard

Vu = volume of unknown

DE = change in potential  (mV)

S = slope of electrode  (mV)

To get the actual concentration of Ca2+ in the fruit juice, multiply the calculated value by 5.

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