Potentiometric titration of calcium (magnesium) in dairy products
Summary
This bulletin describes a simple method for the determination of the calcium content in dairy products. Use of CuEGTA and the ion-selective copper electrode (Cu ISE) as indicator electrode allows the determination to be performed without time consuming sample preparation. If the complexing agent EDTA is used as titrant instead of EGTA, the sum of calcium and magnesium is obtained. The magnesium content can then be calculated from the difference between the results of the two titrations.
Theory
If the Cu ISE is used to indicate the complexometric titration of calcium, addition of copper ions is necessary. As the Cu2+ ions would also react with the titrant EGTA, they are already added to the sample in complexed form as CuEGTA. This complex reacts with the Ca2+ ions as follows: CuEGTA + Ca2+ CaEGTA + Cu2+
Instruments and accessories
· 702 SET/MET Titrino, 716 DMS Titrino, 736 GP Titrino, 751 GPD Titrino or 785 DMP Titrino or 726 or 796 Titroprocessor with 700 Dosino or 685 Dosimat
· 2.728.0040 Magnetic Stirrer
· 6.3014.XX3 Exchange Unit(s)
· 6.0502.140 ion-selective copper electrode (Cu ISE) with 6.2104.020 electrode cable
· 6.0726.100 double-junction Ag/AgCl reference electrode (bridge electrolyte KNO3 sat.) with 6.2106.020 electrode cable
Reagents
· Titrant: c(EGTA) = 0.1 mol/L: Dissolve 38.04 g ethylene glycol-bis-(2-aminoethyl)-tetraacetic acid in 250 mL c(NaOH) = 1 mol/L and, after cooling down, make up to 1 L with dist. water.
· Buffer solution pH = 10: Dissolve 54 g NH4Cl in approx. 400 mL dist. water, add 300 mL w(NH3) = 25% and make up to 1 L with dist. water.
· CuEGTA complex solution: Mix 100 mL c(EGTA) = 0.1 mol/L with 100 mL of a solution containing 0.2 mol/L NH4Cl and exactly 0.1 mol/L Cu(NO3)2. Titration can be used to check if this solution contains no excess Cu(II) or EGTA.
· Sulfuric acid, c(H2SO4) = 0.05 mol/L
· Sodium hydroxide solution, c(NaOH) = 0.1 mol/L
Sample preparation
Milk, milk drinks, yogurt, brine bath, etc.
Approx. 10 g sample is weighed exactly into a glass beaker and diluted with approx. 90 mL dist. water.
Cheese
Approx. 1 g finely grated cheese (calcium content approx. 1%) or an equivalent quantity of cheese with differing calcium content is weighed into a glass beaker. Add 10 mL c(H2SO4) = 0.05 mol/L and approx. 50 mL dist. water, then heat the
mixture to 40 °C and stir at this temperature for 10 min. After cooling down, neutralize to pH = 7 with c(NaOH) = 0.1 mol/L.
Analysis
Add 1 mL CuEGTA solution and 10 mL buffer solution pH = 10 to the prepared sample solution. Allow to react for 10 … 30 s under stirring, then titrate with c(EGTA) = 0.1 mol/L, using, e.g., the MET mode.
Calculation
1 mL c(EGTA) = 0.1 mol/L corresponds to 4.008 mg Ca2+
% Ca2+ = EP1 * C01 * C02 / C00
EP1 = titrant consumption in mL
C00 = sample weight in g
C01 = 4.008
C02 = 0.1 (conversion factor for %)
Answer
Ca2+ = 3.057 x 4.008 x 0.1 / 10.0091 = 0.122%
EP1 = 3.057 mL
C00 = 10.0091 g
Remarks
· If the magnesium content is also required, a second titration is carried out with c(EDTA) = 0.1 mol/L and addition of CuEDTA. This determines the sum of calcium and magnesium. The magnesium content can then be calculated from the difference between the titrant consumption of the two titrations: EP1(Mg) = EP1(Ca + Mg) – EP1(Ca) Use the same sample weight for both titrations! 1 mL c(EDTA) = 0.1 mol/L corresponds to 2.4305 mg Mg2+
· If for cheese, work is done without heating and without addition of H2SO4 (as for milk), it is not possible to determine the total calcium concentration (results found too low).
·
Literature
Metrohm Application Bulletin No. 235 / 2e
Summary
This bulletin describes a simple method for the determination of the calcium content in dairy products. Use of CuEGTA and the ion-selective copper electrode (Cu ISE) as indicator electrode allows the determination to be performed without time consuming sample preparation. If the complexing agent EDTA is used as titrant instead of EGTA, the sum of calcium and magnesium is obtained. The magnesium content can then be calculated from the difference between the results of the two titrations.
Theory
If the Cu ISE is used to indicate the complexometric titration of calcium, addition of copper ions is necessary. As the Cu2+ ions would also react with the titrant EGTA, they are already added to the sample in complexed form as CuEGTA. This complex reacts with the Ca2+ ions as follows: CuEGTA + Ca2+ CaEGTA + Cu2+
Instruments and accessories
· 702 SET/MET Titrino, 716 DMS Titrino, 736 GP Titrino, 751 GPD Titrino or 785 DMP Titrino or 726 or 796 Titroprocessor with 700 Dosino or 685 Dosimat
· 2.728.0040 Magnetic Stirrer
· 6.3014.XX3 Exchange Unit(s)
· 6.0502.140 ion-selective copper electrode (Cu ISE) with 6.2104.020 electrode cable
· 6.0726.100 double-junction Ag/AgCl reference electrode (bridge electrolyte KNO3 sat.) with 6.2106.020 electrode cable
Reagents
· Titrant: c(EGTA) = 0.1 mol/L: Dissolve 38.04 g ethylene glycol-bis-(2-aminoethyl)-tetraacetic acid in 250 mL c(NaOH) = 1 mol/L and, after cooling down, make up to 1 L with dist. water.
· Buffer solution pH = 10: Dissolve 54 g NH4Cl in approx. 400 mL dist. water, add 300 mL w(NH3) = 25% and make up to 1 L with dist. water.
· CuEGTA complex solution: Mix 100 mL c(EGTA) = 0.1 mol/L with 100 mL of a solution containing 0.2 mol/L NH4Cl and exactly 0.1 mol/L Cu(NO3)2. Titration can be used to check if this solution contains no excess Cu(II) or EGTA.
· Sulfuric acid, c(H2SO4) = 0.05 mol/L
· Sodium hydroxide solution, c(NaOH) = 0.1 mol/L
Sample preparation
Milk, milk drinks, yogurt, brine bath, etc.
Approx. 10 g sample is weighed exactly into a glass beaker and diluted with approx. 90 mL dist. water.
Cheese
Approx. 1 g finely grated cheese (calcium content approx. 1%) or an equivalent quantity of cheese with differing calcium content is weighed into a glass beaker. Add 10 mL c(H2SO4) = 0.05 mol/L and approx. 50 mL dist. water, then heat the
mixture to 40 °C and stir at this temperature for 10 min. After cooling down, neutralize to pH = 7 with c(NaOH) = 0.1 mol/L.
Analysis
Add 1 mL CuEGTA solution and 10 mL buffer solution pH = 10 to the prepared sample solution. Allow to react for 10 … 30 s under stirring, then titrate with c(EGTA) = 0.1 mol/L, using, e.g., the MET mode.
Calculation
1 mL c(EGTA) = 0.1 mol/L corresponds to 4.008 mg Ca2+
% Ca2+ = EP1 * C01 * C02 / C00
EP1 = titrant consumption in mL
C00 = sample weight in g
C01 = 4.008
C02 = 0.1 (conversion factor for %)
Answer
Ca2+ = 3.057 x 4.008 x 0.1 / 10.0091 = 0.122%
EP1 = 3.057 mL
C00 = 10.0091 g
Remarks
· If the magnesium content is also required, a second titration is carried out with c(EDTA) = 0.1 mol/L and addition of CuEDTA. This determines the sum of calcium and magnesium. The magnesium content can then be calculated from the difference between the titrant consumption of the two titrations: EP1(Mg) = EP1(Ca + Mg) – EP1(Ca) Use the same sample weight for both titrations! 1 mL c(EDTA) = 0.1 mol/L corresponds to 2.4305 mg Mg2+
· If for cheese, work is done without heating and without addition of H2SO4 (as for milk), it is not possible to determine the total calcium concentration (results found too low).
·
Literature
Metrohm Application Bulletin No. 235 / 2e
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