Determination of Water Hardness with Potentiometric Method

Determination of Water Hardness with Potentiometric Method

Equipment
Potentiometer = 751 GPD Titrino / Titroprocessor
Electrodes = 6.0239.100 combined pH glass electrode; input 2
6.0504.100 Ca2+ sensitive indicator electrode; input 1
reference system used from 6.0239.100

Reagents

c(Na₂EDTA) = 0.05 mol/L in c(KOH) = 0.1 mol/L; D0
c(HCl) = 0.1 mol/L; D1
c(Acetylacetone) = 0.1 mol/L in c(Trishydroxy methylaminomethane) =
0.1 mol/L (auxiliary complexing agent, pH app. 8.5); D2

Sample
100 mL water

Remarks
· Method p+m val.
Values for the acid capacity of water.
p value = value of Phenolphthalein
m value = value of Methyl orange
The method can also be used just for p+m values, see
Application 2-3.
· Method Ca-Mg:
1st break: Ca2+
2nd break: Mg2+
The method can also be used just for Ca/Mg determinations, see
Application 1-7.

· Calculations:

p value=C71;2;mmol/l
m value=C72;2;mmol/l
Ca++=C73*C01*C02/C00;2;mmol/l
Mg++=(C74-C73)*C01*C02/C00;2;mmol/l
Total=C74*C01*C02/C00;2;mmol/l

p value = p value in mmol/L (if 0.00 mmol/L, the water has an initial pH below 8.2)
m value = m value in mmol/L (acid-binding-capability)
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 conversion for mol Þ mmol (1000)
C70 = initial pH of sample (p+m val.) = 7.75
C71 = EP1 at pH 8.2 (p+m val.) = 0.00
C72 = EP2 at pH 4.3 (p+m val.) = 5.69
C73 = EP1 from Ca2+ (Ca-Mg) = 4.471
C74 = EP2 from Mg2+ (Ca-Mg) = 6.055

. Answer

P value = 0.00 mmol/L
M value = 5.69 mmol/L
Ca++ = 4.471 x 0.05 x 1000 / 100 = 2.24 mmol/L
Mg++ = (6.055-4.471) x 0.05 x 1000 / 100 = 0.79 mmol/L
Total = 6.055 x 0.05 x 1000 / 100 = 3.03 mmol/L

· Electrode preparation:
Ca electrodes should be conditioned for 10 min. in c(CaCl2) = 0.01 mol/L before use.
· The volume of the 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
TIP Hardness
parameters
>sequence
1.stirrer:… ON
2.method: …p+m val.
3.method: …AddBuffe
4.pause …15 s
5.method: …Ca-Mg
>statistics
status: …OFF
>preselections
req.ident:… OFF
req.smpl size:… OFF
meas.mode:… OFF
temperature …25.0 °C
------------
'fm
751 GPD Titrino 15215 751.0010
TIP Hardness
>calculations
pH=C70;2;
p value=C71;2;mmol/l
m value=C72;2;mmol/l
Ca++=C73*C01*C02/C00;2;mmol/l
Mg++=(C74-C73)*C01*C02/C00;2;mmol/l
Total=C74*C01*C02/C00;2;mmol/l

C00= 100
C01= 0.05
C02= 1000
C70= 7.75
C71= 0.00
C72= 5.69
C73= 4.471
C74= 6.055
------------
'fr
751 GPD Titrino 15215 751.0010
TIP Hardness
smpl size 100 ml id#1 Metrohm
id#2 Herisau id#3 TapWater
C70 7.75
C71 0.000
C72 5.687
C73 4.471
C74 6.055
pH 7.75
p value 0.00 mmol/l
m value 5.69 mmol/l
Ca++ 2.24 mmol/l
Mg++ 0.79 mmol/l
Total 3.03 mmol/l
TIP terminated
============