A Potentiometric Method for the Determination of Calcium / Magnesium in Tap Water

A Potentiometric Methods for the Determination of Calcium/Magnesium in Tap Water

Equipment
Potentiometer = 751 GPD Titrino / Titroprocessor
Electrodes = 6.0504.100 Ca2+ sensitive indicator electrode; input 1
6.0733.100 Ag/AgCl reference electrode (c(KCl) = 3 mol/L)

Reagents
c(Na2EDTA) = 0.05 mol/L in c(KOH) = 0.1 mol/L; D0

Sample
100 mL tap water
15 mL c(Acetylacetone) = 0.1 mol/L in c(Trishydroxymethylaminomethane)
= 0.1 mol/L (auxiliary complexing agent, pH app. 8.5)

Remarks
1st break: Ca2+
2nd break: Mg2+

· Calculations:

Ca++=EP1*C01*C02/C00;2;mmol/l
Mg++=(EP2-EP1)*C01*C02/C00;2;mmol/l
Total = EP2*C01*C02/C00;2;mmol/l

Ca++ = calcium hardness in mmol/L
Mg++ = magnesium hardness in mmol/L
Total = total hardness in mmol/L
C01 = concentration of titrating agent (0.05 mol/L)
C02 = factor for the conversion mol Þ mmol (1000)

. Answer

Ca++=4.245x0.05x1000/100=2.12 mmol/L
Mg++=(5.535-4.245)x0.05x1000/100=0.64 mmol/L
Total = 5.535 x 0.05 x 1000 / 100 = 2.77 mmol/L

EP1=4.245 mL
EP2=5.535 mL
C00=100 mL

· Electrode preparation:
Ca electrodes should be conditioned for 10 min. in c(CaCl2) = 0.01
mol/L before use.
· The volume of auxiliary reagent can be optimised for the magnesium
content. Rule of thumb: Ratio Mg/Acetylacetone app. 0.05.

. Literature
· Metrohm Application Bulletin No. 125: Complexometric simultaneous
determination of calcium and magnesium in water samples and
beverages with the aid of an ion-selective calcium electrode

Method documentation
'pa
751 GPD Titrino 15215 751.0010
DET U... Ca-Mg
parameters
>titration parameters
meas.pt.density 1
min.incr. ...10.0 ml
dos.rate max. ...ml/min
signal drift ...20 mV/min
equilibr.time ...38 s
start V: ...OFF
pause ...0 s
dos.element:... internal D0
meas.input: ...1
temperature ...25.0 °C
>stop conditions
stop V: ...abs.
stop V ...10 ml
stop U ...OFF mV
stop EP ...9
filling rate max.... ml/min
>statistics
status:... OFF
>evaluation
EPC ...5
EP recognition:... all
fix EP1 at U... OFF mV
pK/HNP: ...OFF
>preselections
req.ident: ...OFF
req.smpl size:... OFF
activate pulse:... OFF
------------
'fm
751 GPD Titrino 15215 751.0010
DET U Ca-Mg
>calculations
Ca++=EP1*C01*C02/C00;2;mmol/l
Mg++=(EP2-EP1)*C01*C02/C00;2;mmol/l
Total=EP2*C01*C02/C00;2;mmol/l
C00= 100
C01= 0.05
C02= 1000
------------
'fr
751 GPD Titrino 15215 751.0010
U(init) 10 mV DET U Ca-Mg
smpl size 100 ml
EP1... 4.245 ml... -32 mV
EP2 ...5.535 ml ...-94 mV
Ca++ ...2.12 mmol/l
Mg++ ...0.64 mmol/l
Total ...2.77 mmol/l
stop V reached
============
'cu
751 GPD Titrino 15215 751.0010
start V 0.000 ml DET U Ca-Mg
1.0 ml/div dU=50.0 mV/div
============