Lab Manual

1. Determination of hardness in sample water by Complexometric
titration using EDTA
Dr. Mausumi Adhya
HOD and Associate Professor
Supreme Knowledge Foundation, West Bengal, India

2. Theory
Hardness of water is mainly due to the presence of dissolved bicarbonate, chloride and
sulphate salts of Ca+2 and Mg+2 in water. When hard water is treated with soap, it gets
precipitated in the form of insoluble salts of Ca2+ and Mg2+. Total hardness is defined as the sum
of Ca2+ and Mg2+ concentration and is expressed in parts of CaCO3 per million parts of water i.e.
ppm.
Total hardness (Ca+2 and Mg+2) of water is estimated by titration with ethylene diamine
tetra-acetic acid (EDTA) using Eriochrome Black T (EBT) indicator. Ca+2 and Mg+2 form stable
octahedral complex with EDTA at pH 9-10 as EDTA acts as hexadentate ligand.
M+2= Bivalent metal ion
Between pH 9-10, EBT is blue in colour. Addition of metallic salts produces grape (wine)-red
colour complex.
M2++ EBT M-EBT [M= bivalent metal]
(Blue) (Grape red)
+ M+2
When the grape (wine) red complex is treated with EDTA, Ca(EDTA)2- and Mg(EDTA)2- are
formed rapidly and free EBT is generated . Hence the grape red colour is transferred to blue
colour.
M-EBT+ EDTA4- M[ EDTA]2-+ EBT
(Grape red) (Colourless) (Blue)
The above reaction shifts towards right hand side due to the stability of M(EDTA)2-complexes.
The stability of the complexes is as follows
Ca(EDTA)2-> Mg(EDTA)2-> Mg-EBT > Ca-EBT
pH 9-10
pH 9-10
pH 9-10
EDTA EDTA4-
M+2
M-EDTA complex
M

3. Titration against EDTA at pH around 12 gives the hardness due to Ca2+only. At this high pH,
Mg2+ ion is precipitated out as Mg(OH)2 and Ca2+ ion alone can be estimated by EDTA. At this
pH, murexide (ammonium salt of purpuric acid) indicator forms a pink colour with Ca2+. When
EDTA is added Ca2+ forms complex resulting in a change from pink to blue (purple) which
indicates end point of the reaction.
EDTA is commercially available as its disodium salt. The solution of disodium salt of EDTA is
secondary standard solution. Hence the solution of EDTA must be standardized by using
standard ZnCl2/ZnSO4/Zn-acetate/ MgSO4 solution.
Materials
1. Apparatus: Burette with burette stand, pipette, conical flask, beaker, volumetric flask, reagent
bottle, measuring cylinder, dropper
2. Chemicals: Disodium salt of EDTA (disodium dihydrogen ethylene diamine tetra-acetate), Zn-
acetate [Zn(CH3COO)2.2H2O], NH4Cl, NH4OH, EBT indicator, Mureoxide indicator
Procedure
1. Preparation of reagents
(I) Preparation of 250 ml 0.01 M Zn-acetate solution
Molecular weight of Zn(CH3COO)2.2H2O=219.5
To prepare 1000 ml 1 M Zn-acetate gm equivalent required=219.5 g
Hence, to prepare 250 ml 0.01 M Zn-acetate gm-equivalent required = 0.548 g
0.548 g of Zn-acetate is accurately weight out into a 250 ml volumetric flask containing 2 g of
NH4Cl dissolved in deionised water. The solid is dissolved in distilled water with continuous
stirring. The water is added upto the mark. Finally the solution is made uniform by shaking.
(II) Preparation of 1000 ml 0.01 M EDTA solution
Molecular weight of disodium salt of EDTA =372.24
To prepare1000 ml 0.01 M di-sodium salt of EDTA gm equivalent required=372.24/100=3.72 g
3.72 g of disodium salt of EDTA is taken in a 1000 ml volumetric flask and dissolved in distilled
water with continuous stirring. The water is added upto the mark. Finally the solution is made
uniform by shaking.
(III) Buffer (Ammonia-ammonium chloride salt)
About 17.5 gm NH4Cl is taken in a 250 ml measuring flask and 142 ml concentrated ammonia is
added to it. The mixture is dissolved in distilled water and the water is added upto the mark.
Finally the solution is made uniform by shaking.
(IV) Preparation of 100 ml 2(N) NaOH

4. Molecular weight of NaOH=23+16+1=40
Gm equivalent weight of NaOH= g molecular weight/1=40
To prepare 100ml 2(N) NaOH gm equivalent required= [40×100×2]/1000=8 g
8 gm NaOH is dissolved in distilled water and volume is made up 100ml by distilled water.
(V) EBT indicator
0.05 gm EBT and 4.9 gm KNO3 (or 10 gm NaCl) are to be mixed by grinding with mortar.
(VI) Mureoxide Indicator
20 mg indicator and 10 gm NaCl are to be mixed by grinding with mortar.
2. Standardization of EDTA solution by standard Zn-acetate solution
10 ml Zn-acetate solution is pipetted out in a 100 ml conical flask. 2 ml buffer and a pinch of EBT
indicator are added to it. The solution is titrated with EDTA keeping the flask against white
background with constant shaking until the colour of the solution changes from grape red (wine
red) to blue. Experiment is repeated three times.
3. Determination of total hardness (Ca+2 and Mg+2) of sample Water
25 ml sample water is pipetted out into a 100 ml conical flask. 5 ml buffer and a pinch of EBT are
added to it. The mixture is titrated with EDTA running from burette by constant shaking the
conical until the colour of the solution changes from grape (wine) red to blue. Experiment is
repeated three times.
4. Determination of Ca+2 hardness of Water
25 ml sample water is taken in a conical flask. 1 ml NaOH is added to raise the pH of the water 12.
A pinch of murexide indicator is added. The solution is titrated with standard EDTA by constant
shaking the conical flask till the pink colour of the solution changes to clear blue (purple).
Experiment is repeated three times.
Results
Table 1: Standardization of EDTA solution by standard Zn-acetate solution
Number of
observation
Volume
of Zn-
acetate
(ml)
Burette reading Mean
volume of
disodium
salt of
EDTA (ml)
Strength
of Zn-
acetate
( M)
Strength of
disodium salt
of EDTA (M)
Initial Final Actual
1
10 X 0.01 S
2
3
V1S1=V2S2
Volume of EDTA × Strength of EDTA =Volume of Zn-acetate × Strength of Zn-acetate
Strength of EDTA = [10×0.01]/X =S (M)

5. Table 2: Determination of total hardness (Ca+2 and Mg+2) of sample Water
Number of
observation
Volume of
sample water
(ml)
Burette reading Mean
volume of
disodium
salt of EDTA
(ml)
Strength of
disodium
salt of
EDTA (M)
Total
hardness
of water
(ppm)
Initial Final Actual
1
25 Y S P
2
3
Table 3: Determination of Ca+2 hardness of Water
Number of
observation
Volume of
sample
water (ml)
Burette reading Mean
volume of
EDTA (ml)
Strength
of
disodium
salt of
EDTA
(M)
Ca+2 Hardness
of water (ppm)
Initial Final Actual
1
25 Z S Q
2
3
Calculation
Ca2+ + EDTA2--=M [EDTA]2-
Here 2 equivalents Ca2+ react with 2 equivalents EDTA2- i.e., 1 equivalent Ca2+ reacts with 1
equivalent EDTA2-.
Therefore, 1000 ml 1(N) disodium salt of EDTA = 1000ml 1(N) CaCO3
Or, 1000 ml 1(M) disodium salt of EDTA = 1000ml 1(M) CaCO3=100 g CaCO3
Total hardness of water
Y ml S (M) disodium salt of EDTA = [Y×S×100]/1000 g CaCO3
25 ml sample water contains [Y×S×100]/1000 g CaCO3
106 ml ------------------------------= [Y×S×100×106]/ [1000x25] g CaCO3=[Y×S×105]/25 g CaCO3
Therefore hardness of water=( Y×S×105)/25 ppm=P
Ca+2 hardness of water
Z ml S (M) disodium salt of EDTA = [Z×S×100]/1000 g CaCO3
25 ml sample water contains [Z×S×100]/1000 g CaCO3
106 ml ------------------------------= [Z×S×100×106]/ [1000x25] g CaCO3= [Z×S×105]/25 g CaCO3
Therefore Ca+2 hardness of water= Z×S×105/25 ppm=Q

6. Conclusion
The total hardness of water sample is……………………..ppm.
Hardness of sample water due to presence of Ca+2………….. ppm.
Hardness of sample water due to presence of Mg+2 = (total hardness- Ca+2 hardness) ppm
Precautions
1. As the buffer solution contains ammonia, after use buffer should be kept in stoppered bottle.
2. Ammonia should be used cautiously.
Viva Voce
1. What is the cause of hardness of water?
2. How many types of hardness of water?
Answer. There are two types of hardness (a) temporary hardness and (b) permanent hardness.
Temporary hardness is due to the presence of bicarbonates salts of Ca+2 and Mg+2. Whereas,
permanent hardness is due to the presence of chloride and sulphate salts of Ca+2 and Mg+2.
3. How can you remove temporary hardness?
Answer. Temporary hardness is removed by boiling.
4.How can you remove permanent hardness?
Answer. Lime soda process or ion exchange resin is used to remove permanent hardness of water.
Lime soda process: To remove chloride and sulphate salt of Ca+2 and Mg+2, soda ash
(Na2CO3)quick lime (CaO) or hydrated lime [Ca(OH)2] are added.
MgSO4+ Ca(OH)2+ Na2CO3=Mg(OH)2+CaCO3+Na2SO4
CaCl2 + Ca(OH)2+ Na2CO3 = CaCO3 +NaCl
Mg(OH)2 and CaCO3 are precipitated out from the solution.
5. What is the unit of hardness of water?
6. Which reagent is used to estimate hardness of water?
7. Why EDTA is a secondary standard solution?
Answer. Disodium salt of EDTA contains two molecules water. It should not be used as
primary standard because during weighing number of water crystallization changes and
again purification of this salt is difficult.
8. Why the titration is called complexometric titration?
Answer. In the reaction complex is formed.
M+2 + (EDTA)4-= M(EDTA)2-
9. What is the structure of complex which is formed in the titration?
Answer. Octahedral
Hardness of water= [Volume of EDTA required for sample water × strength of EDTA×105/ volume of sample water taken] ppm

7. 10. What is the co-ordination number of EDTA?
Answer. Six
11. Why the pH of the analyte is 9-10?
Answer. In the reaction octahedral complex is formed i.e co-ornation number of EDAT is six. It is
only possible when the pH of the solution is more than 7.
12. Which indicator is used in this titration?
13. Write the reactions which occur in this titration.
14. How is the hardness of water expressed?
15. Why is EBT/KNO3 mixture used as indicator in EDTA titrations?
KNO3prevents the possible variation in sharpness of the end point and also prevents the
variation in stability of the indicator solution. KCl or K2SO4 or corresponding Na- salts have the
same function.
16. For preparation of standard Zn-acetate solution, ammonium chloride is required. Explain.
Answer. To prevent hydrolysis, Zn-acetate is dissolved in water containing ammonium chloride.
17. Classify the drinking water according to degree of hardness.
The degree of hardness of drinking water has been classified in terms of the equivalent CaCO3
concentration as follows:
Soft 0-60 mg/L
Medium 60-120mg/L
Hard 120-180mg/L
Very hard >180mg/L