DETERMINATION OF CALCIUM AND MAGNESIUM IN WATER SAMPLE AND BEVERAGES BY COMPLEXOMETRIC TITRATION WITH POTENTIOMETRIC METHOD
Summary
This procedure describes a potentiometric titration method for the simultaneous complexometric determination of the calcium and magnesium ions in a sample using Na2EDTA as titran. The ion-selective calcium electrode is used as indicator electrode. To achieve a better differentiation between the Ca and Mg potential jumps an auxiliary complexing agent is added thet additionally masks Fe3+ and Al3+. Fixing the pH value at pH = 8.5 prevents interverences that might to be caused by the precipitation of CaCO3.
The method described is suitable for the automatic determination of the different water hardnesses (carbonat, calcium, magnesium, total and permanent hardness) the determination of calcium and magnesium in beverages (fruit and vegetable juices, wine) is also dealt with.
Instruments :
· Titrino or Titroprocessor
· 6.0504.100 ion-selective calcium electrode
· 6.0733.100Ag/AgCl reference electrode
· 6.0239.100 combined pH glass electrode
Reagents :
· Titran for the determination of the carbonate hardness : c(HCl) = 0.1 mol/L.
· Titran for the Ca and Mg determination : c(Na2EDTA) = 0.05 mol/L + c(KOH) = 0.1 mol/L
· Auxiliary complexing solution : c(acetylacetone) = 0.1 mol/L + c(TRIS) = 0.2 mol/L (TRIS=tris(hydroxymethyl)aminomethane)
· Hydrochloric acid w(HCl) = 20% and sodium hydroxide w(NaOH) = 20%
Analysis :
· Water sample : determination of the water hardnesses
100 mL sample is placed in the titration vessel and titrated in a first titration with c(HCl) = 0.1 mol/L (combined pH glass electrode).
Using auxiliary dosimat, 15 mL of the auxiliary complexing solution is added and, after a short waiting time, the calcium and magnesium ions are titrated in a second titration with Na2EDTA (Ca ISE). The first equivalence point corresponds to the Ca 2+ content, the difference between the second and the first equivalence point to the Mg 2+ content.
Calculations
1 st titration :
Acid neutralizing capacity = EP1*0.1*1000/C00 ;2;mmol/L
Carbonat hardness = EP1*0.1*0.5*1000/C00 ;2;mmol/L (temporary hardness)
2 nd titration :
Calcium hardness = EP1*0.05*1000/C00 ;2;mmol/L
Magnesium hardness = (EP2-EP1)*0.05*1000/C00;2;mmol/L
Total hardness = EP2*0.05*1000/C00 ;2;mmol/L
Permanent hardness = (total hardness-carbonate hardness) ;2;mmol/L
Where
1 mmol Ca 2+ = 40.08 mg Ca 2+
1 mmol Mg 2+ = 24.31 Mg 2+
.
Answer
1 st titration :
Acid neutralizing capacity = 4.482x0.1x1000/100 = 4.48 mmol/L
Carbonat hardness = 4.482x0.1x0.5x1000/100 = 2.24 mmol/L (temporary hardness)
2 nd titration :
Calcium hardness = 4.482x0.05x1000/100 =2.24 mmol/L
Magnesium hardness =(6.078-4.482)*0.05*1000/100 = 0.80 mmol/L
Total hardness = 6.078*0.05*1000/100 = 3.04 mmol/L
Permanent hardness = (3.04-2.24) = 0.80 mmol/L
Where
EP1 = 4.482 mL
EP2 = 6.078 mL
· Beverages (fruit and vegetable juices, wine) : Determination of calcium and magnesium
25 mL sample is pipetted into an evaporating dish and evaporated in a drying cabinet at 140 oC. The residue is then heated to redmess in a muffle furnace at 600 oC, until a white ash remains. After cooling down, 2 mL w(HCl) = 20% is added, the mixture heated to dissolve the ash and the resulting solution rinsed into a glass beaker with dist. Water. The pH value of this sample solution is then adjusted to 8.5 with w(NaOH) = 20%. 20 mL of the auxiliary complexing solution is added and titration is performed with c(Na2EDTA) = 0.05 using the Ca ISE.
Calculation
1 mL c(Na2EDTA) = 0.05 mol/L corresponds tomg Ca 2+ or 1.216 mg Mg 2+.
Ca 2+ = EP1*2.004*1000/C00;2;mg/L
Mg 2+ = (EP2-EP1)*1.216*1000/C00 ;2;mg/L
EP1 = titrant consumption to reach the first EP in mL = 2.156 mL
EP2 = titrant consumption to reach the second EP in mL = 5.078 mL
C00 = 25.0 mL (sample volume in mL)
Answer
Ca 2+ = 2.156 x 2.004 x 1000/25 = 172.82 mg/L
Mg 2+ = (5.078-2.156) x 1.216 x 1000 /25 = 142.13 mg/L
Literature
Metrohm, Application Bulletin No.125/2e
Summary
This procedure describes a potentiometric titration method for the simultaneous complexometric determination of the calcium and magnesium ions in a sample using Na2EDTA as titran. The ion-selective calcium electrode is used as indicator electrode. To achieve a better differentiation between the Ca and Mg potential jumps an auxiliary complexing agent is added thet additionally masks Fe3+ and Al3+. Fixing the pH value at pH = 8.5 prevents interverences that might to be caused by the precipitation of CaCO3.
The method described is suitable for the automatic determination of the different water hardnesses (carbonat, calcium, magnesium, total and permanent hardness) the determination of calcium and magnesium in beverages (fruit and vegetable juices, wine) is also dealt with.
Instruments :
· Titrino or Titroprocessor
· 6.0504.100 ion-selective calcium electrode
· 6.0733.100Ag/AgCl reference electrode
· 6.0239.100 combined pH glass electrode
Reagents :
· Titran for the determination of the carbonate hardness : c(HCl) = 0.1 mol/L.
· Titran for the Ca and Mg determination : c(Na2EDTA) = 0.05 mol/L + c(KOH) = 0.1 mol/L
· Auxiliary complexing solution : c(acetylacetone) = 0.1 mol/L + c(TRIS) = 0.2 mol/L (TRIS=tris(hydroxymethyl)aminomethane)
· Hydrochloric acid w(HCl) = 20% and sodium hydroxide w(NaOH) = 20%
Analysis :
· Water sample : determination of the water hardnesses
100 mL sample is placed in the titration vessel and titrated in a first titration with c(HCl) = 0.1 mol/L (combined pH glass electrode).
Using auxiliary dosimat, 15 mL of the auxiliary complexing solution is added and, after a short waiting time, the calcium and magnesium ions are titrated in a second titration with Na2EDTA (Ca ISE). The first equivalence point corresponds to the Ca 2+ content, the difference between the second and the first equivalence point to the Mg 2+ content.
Calculations
1 st titration :
Acid neutralizing capacity = EP1*0.1*1000/C00 ;2;mmol/L
Carbonat hardness = EP1*0.1*0.5*1000/C00 ;2;mmol/L (temporary hardness)
2 nd titration :
Calcium hardness = EP1*0.05*1000/C00 ;2;mmol/L
Magnesium hardness = (EP2-EP1)*0.05*1000/C00;2;mmol/L
Total hardness = EP2*0.05*1000/C00 ;2;mmol/L
Permanent hardness = (total hardness-carbonate hardness) ;2;mmol/L
Where
1 mmol Ca 2+ = 40.08 mg Ca 2+
1 mmol Mg 2+ = 24.31 Mg 2+
.
Answer
1 st titration :
Acid neutralizing capacity = 4.482x0.1x1000/100 = 4.48 mmol/L
Carbonat hardness = 4.482x0.1x0.5x1000/100 = 2.24 mmol/L (temporary hardness)
2 nd titration :
Calcium hardness = 4.482x0.05x1000/100 =2.24 mmol/L
Magnesium hardness =(6.078-4.482)*0.05*1000/100 = 0.80 mmol/L
Total hardness = 6.078*0.05*1000/100 = 3.04 mmol/L
Permanent hardness = (3.04-2.24) = 0.80 mmol/L
Where
EP1 = 4.482 mL
EP2 = 6.078 mL
· Beverages (fruit and vegetable juices, wine) : Determination of calcium and magnesium
25 mL sample is pipetted into an evaporating dish and evaporated in a drying cabinet at 140 oC. The residue is then heated to redmess in a muffle furnace at 600 oC, until a white ash remains. After cooling down, 2 mL w(HCl) = 20% is added, the mixture heated to dissolve the ash and the resulting solution rinsed into a glass beaker with dist. Water. The pH value of this sample solution is then adjusted to 8.5 with w(NaOH) = 20%. 20 mL of the auxiliary complexing solution is added and titration is performed with c(Na2EDTA) = 0.05 using the Ca ISE.
Calculation
1 mL c(Na2EDTA) = 0.05 mol/L corresponds tomg Ca 2+ or 1.216 mg Mg 2+.
Ca 2+ = EP1*2.004*1000/C00;2;mg/L
Mg 2+ = (EP2-EP1)*1.216*1000/C00 ;2;mg/L
EP1 = titrant consumption to reach the first EP in mL = 2.156 mL
EP2 = titrant consumption to reach the second EP in mL = 5.078 mL
C00 = 25.0 mL (sample volume in mL)
Answer
Ca 2+ = 2.156 x 2.004 x 1000/25 = 172.82 mg/L
Mg 2+ = (5.078-2.156) x 1.216 x 1000 /25 = 142.13 mg/L
Literature
Metrohm, Application Bulletin No.125/2e
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