Chem 28.1 Experiment 7

1. Experiment 7

2. Objective
 Apply the concept of complexometric titration in the
determination of total hardness in drinking water

3. Simple Water and Complexes

4. Water
 Drinking
 Hard water is better for
drinking because it
contains minerals

5. Water
 Cleaning
 Soft water is better for
cleaning because it
doesn’t form scum with
soap

6. Water
 Hardness of water
 Depends on source
 Caused by Ca2+ and
Mg2+ forming
precipitates with soap

7. Water
 Hardness of water
 Most Ca2+ in water
come from CaCO3
exoskeletons of aquatic
microorganisms like
diatoms

8. Complexes
 Coordination between
metal and ligand
 An atom in the ligand
(called the donor)
donates electrons to the
metal ion, forming a
bond.
 New properties
 Complexes are usually
more stable than the
components due to the
rearrangement of the
energy levels of the d-
orbital (crystal field
theory)

9. Complexes - Ligands
 Monodentate
 Polydentate
 Chelating agents – Gr.
“chelos”, meaning teeth
 Polyanionic
 Complex-forming
 Reacts in a 1:1 ratio

10. Complexes - Usage
 Catalysts
 Polymerization, hydrogenation, hydroboration, etc.
 Medicine
 cis-diamminedichloroplatinum(II)
Cl NH3
 Chelators
 Heavy Metal Poisoning Treatment
Pt
 Cleaning agent
 Food enhancement
Cl NH3
 Anti-b acterial
 Titrants

11. Step by step discussion
*simple dilutions are not discussed

12. Solution Preparation

13. Solution Preparation
 250 mL 0.050 M std CaCO3soln
+ 20 mL dH2O, +few Cover with watch
drops 6M HCl glass (slower
1.2511 g CaCO3
(Dissolve evaporation rate to
standard
precipitate, basic keep beaker from
CO32-) getting dry)
Rinse watch glass to Evaporate to 10 mL.
beaker Cool.
Quantitatively transfer to 250 mL vol flask. Dilute to mark.

14. Solution Preparation
 pH 10 buffer
Transfer to 250 mL
142 mL conc NH3 +
Adjust soln to pH 10 vol flask. Dilute to
17.5 g NH4Cl
mark.
pOH pKb log([NH 4 ] /[ NH 3 ])
pOH log(1.8 x10 5 ) log(2.1016m ol/ 0.327m ol)
pOH 4.74 0.808
pH 8.45
Add ammonia to increase pH!

15. Solution Preparation
 pH 10 buffer
Transfer to 250 mL
142 mL conc NH3 +
Adjust soln to pH 10 vol flask. Dilute to
17.5 g NH4Cl
mark.
 Different minimum pH values for different cations
 Mn2+, Fe2+,  5-6 pH
 Fe3+, Th4+  1-1.5 pH
 Ca2+  8
 Mg2+  10

16. Solution Preparation
 pH 10 buffer
Transfer to 250 mL
142 mL conc NH3 +
Adjust soln to pH 10 vol flask. Dilute to
17.5 g NH4Cl
mark.
 As a rule of thumb,
 The higher the pH (more basic solution), the sharper
the endpoint
 The higher the formation of constant the lower the
minimum pH

17. Solution Preparation
 500 mL 0.050 M EDTA
+1.0 g
9.31 g Na2H2EDTA•2H2O +200 mL dH2O
MgCl2•6H2O
Dissolve in 500 mL vol
flask. Heat if
necessary.

18. Solution Preparation
 500 mL 0.050 M EDTA
 Why add 1.0 g MgCl2•6H2O?
 Diverse ion effect: increase solubility (easier
preparation)
 Sharper endpoint in titration
 Ensures presence of Mg2+ in sample
 Supposedly does not affect titration procedure
 What exactly happens when we add MgCl2?
 Good question.

19. Solution Preparation
 500 mL 0.050 M EDTA
 What happens when we DON’T add MgCl2?
 Endpoint not as sharp if Mg2+ is not present in solution
 Only total hardness can be computed (Ca2+ and Mg2+)
HO O
S
O
O
O
O Mg
N N
N
O
Figure 1. Structure of eriochrome black T – magnesium complex

20. Standardization
http://xkcd.com/927/

21. Standardization
10.00m L 0.0050 M wrk + 3 mL buffer Titrate to blue
std CaCO3 + 6 drps EBT endpoint
 Reactions: (Consider w/o MgCl2)
 Start:
 Ca2+ + HIn2-  CaHIn (Kf = 2.5x105)
 Before equivalence point:
 Ca2+ + H2Y2-  CaH2Y(Kf = 5.0x1010)
 At the end point:
 CaHIn + Y2-  CaH2Y + HIn2-

22. Standardization
 w/o MgCl2
Ca2+ + HIn2- 
CaHIn
Ca2+ + H2Y2-  CaH2Y
CaHIn + Y2-  CaH2Y + HIn2-

23. Standardization
10.00 mL 0.0050 M wrk + 3 mL buffer Titrate to blue
std CaCO3 + 6 drps EBT endpoint
 Reactions: (Consider w/ MgCl2)
 Start:
 Ca2+ + HIn2-  CaHIn (Kf = 2.5x105) Calcium-EBT complexation
 Before equivalence point:
 Ca2+ + H2Y2-  CaH2Y(Kf = 5.0x1010) Calcium consumption
 Ca2+ + MgH2Y  CaH2Y + Mg2+ Magnesium Displacement
 Mg2+ + CaHIn  MgHIn + Ca2+ Magnesium-EBT complexation
 Near the endpoint: Assume all Ca2+ consumed
 Mg2+ + H2Y2-  MgH2Y Released Magnesium consumption
 At the end point:
 MgHIn + H2Y2-  MgH2Y + HIn2- Magnesium Displacement

24. Standardization
 w/ MgCl2
Ca2+ + HIn2-  Mg2+ + H2Y2-  MgH2Y
CaHIn
Ca2+ + H2Y2-  CaH2Y
Ca2+ + MgH2Y  CaH2Y + Mg2+ MgHIn + H2Y2-  MgH2Y + HIn2-
Mg2+ + CaHIn  MgHIn + Ca2+

25. Standardization
 w/ MgCl2
Amount of EDTA corresponding to
Free EDTA to titrate released Mg2+
Amount of EDTA to titrate Ca2+ Amount of EDTA to
titrate Mg2+ in
indicator (negligible)

26. Standardization
 The amount of MgCl2 that was supposed to be
added was not in significant (0.0049 mol, compared
to EDTA that has 0.025 mol)
 The addition of MgCl2 should be stoichiometric (Y=X)
or negligible (Y≈0) to EDTA in order for the titration to
be unaffected

27. Sample Analysis

28. Sample Analysis
50 mL + 3 mL buffer Titrate to blue
Viva + 6 drps EBT endpoint
 Reactions: (Consider w/o MgCl2)
 Start:
 Mg2+ + HIn2-  MgHIn (Kf = 1.0x107) Magnesium-EBT complexation
 Before equivalence point:
 Ca2+ + H2Y2-  CaH2Y(Kf = 5.0x1010) Calcium consumption
 Near the endpoint: Assume all Ca2+ consumed
 Mg2++H2Y2-MgH2Y(Kf=4.9x108) Present Magnesium consumption
 At the end point:
 MgHIn + H2Y2-  MgH2Y + HIn2- Magnesium Displacement

29. Sample Analysis
 w/o MgCl2
Mg2+ + H2Y2-  MgH2Y
Mg2+ + HIn2-  MgHIn
Ca2+ + H2Y2-  CaH2Y
CaHIn + Y2-  CaH2Y + HIn2-

30. Sample Analysis
50 mL + 3 mL buffer Titrate to blue
Viva + 6 drps EBT endpoint
 Reactions: (Consider w/ MgCl2)
 Start:
 Mg2+ + HIn2-  MgHIn (Kf = 1.0x107) Magnesium-EBT complexation
 Before equivalence point:
 Ca2+ + H2Y2-  CaH2Y(Kf = 5.0x1010) Calcium consumption
Ca2+ + MgH2Y  CaH2Y + Mg2+ Magnesium release
 Near the endpoint: Assume all Ca2+ consumed
 Mg2++H2Y2-MgH2Y(Kf=4.9x108) Present Magnesium consumption
Released Magnesium consumption
 At the end point:
 MgHIn + H2Y2-  MgH2Y + HIn2- Magnesium Displacement

31. Sample Analysis
 w/ MgCl2
Mg2+ + HIn2-  MgHIn Mg2+ + H2Y2-  MgH2Y
Ca2+ + H2Y2-  CaH2Y
Ca2+ + MgH2Y  CaH2Y + Mg2+ MgHIn + H2Y2-  MgH2Y + HIn2-

32. Sample Analysis
 w/ MgCl2
Amount of EDTA corresponding to
Free EDTA to titrate released and present Mg2+
Amount of EDTA to titrate Ca2+ Amount of EDTA to
titrate Mg2+ in
indicator (negligible)

33. Sample Analysis
 w/ MgCl2
Y
X Amount of EDTA to
titrate Mg2+ in
indicator (negligible)

34. Sample Analysis
 w/ MgCl2
Let X =
Y=
 Y
X

35. Sample Analysis
 w/ MgCl2
Let X =
Y=
 Y
X

36. Sample Analysis
 The amount of MgCl2 that was supposed to be
added was significant (0.0049 mol, compared to
EDTA that has 0.025 mol)
 The addition of MgCl2 should be negligible
(EDTA:Mg≈0) with respect to EDTA in order for the
titration to be unaffected

37. Sample Analysis
 Back Titration with EDTA is possible
 Add standardized amount of EDTA
 Back titrate with Mg2+

39. Molarity of Primary Standard
 Weight 1o std: 1.2511 g
 %Purity 1o std: 99.9%
 Final volume std: 250 mL
 Vol std sol'n: 5 mL
 Final vol working std: 50 mL

40. Molarity of Primary Standard
 The molarity of the working standard can be
computed from the given

41. Volume of titrant used in
standardization
Trial 1 Trial 2 Trial 3
Volume
working std 10 10 10
CaCO3
Final volume 14.1 28 41.8
Initial
0.4 14.1 28
volume
Net volume 13.7 13.9 13.8

42. Molarity of EDTA
Volume Net volume
Molarity
wrking std EDTA
Std A 10 mL 13.7 mL
 Molarity:

43. Titer of EDTA
nEDTA =
Molarity Titer
nCaCO3
Std A 0.003646 M 1
 Titer:

44. Total Hardness of Viva
Volume of Total
Total hardness in ppm
Water Vol EDTA Amount of
CaCO3
Sample Calcium
Sample A 50 mL 19.5 mL
 Total Amount of Calcium:
 Total hardness in ppm CaCO3:

45. Collective Data
Team Group Trial1 Trial2 Trial3 Average
1 142.58 142.58 139.02 141.40
1 2
3 140.61 139.89 140.61 140.37
Team Mean 140.88
4 148.57 149.32 147.83 148.57
2 5 151.40 140.83 140.13 144.12
6 141.29 142.02 144.92 142.74
Team Mean 145.14
7 148.38 138.63 136.54 141.19
8
3
9
10 139.15 142.66 138.45 140.09
Team Mean 140.64
Mean 142.64
Stdev 4.73
RSD 33.2

46. To Viva or not to Viva, that is the question

47. PPM Viva
 Total hardness of 192.49 ppm CaCO3

48. Conclusion
 25.8% difference between the mean and the claimed
value
 Viva’s water is softer than they claim it to be

49. References
 J. Roger Hart; J. Chem. Educ., 1984, 61 (12), p 1060.
 Blitz, Jonathan P. COMPLEXOMETRIC DETERMINATION OF Mg2+ and Ca2+. 2010. 25 January
2012 <http://www.ux1.eiu.edu/~cfjpb/teaching/quant/labs/experiment8.pdf>.
 Garrett, Simon J. CEM 333 EDTA Formation Constants. 1998. 25 January 2012
<http://www.cem.msu.edu/~cem333/EDTATable.html>.
 Jackson School of Geosciences. COORDINATION CHEMISTRY. n.d. 25 January 2012
<http://www.geo.utexas.edu/courses/376m/coord_chem.htm>.
 Jon A. McCleverty, Thomas J. Meyer. "Applications of Coordination Complexes." 2003. Platinum
Metals Review. 25 January 2012 <http://www.platinummetalsreview.com/pdf/101-104-pmr-
jul04.pdf>.
 Mccord, Dr. Stephen P. Determination of Water Hardness using Complexometric titration. 2005. 25
January 2012 <http://mccord.cm.utexas.edu/courses/spring2005/ch455/Spr05455Wk4Lab.pdf>.
 Old Dominion University. Chapter 12: EDTA Titrations. n.d. 25 January 2012
<http://www.odu.edu/sci/xu/chem321/chem321chapter12.pdf>.
 Prince George's Community College. ANALYSIS OF CALCIUM BY EDTA TITRATION TO ASSESS
WATER . n.d. 25 January 2012 <http://academic.pgcc.edu/psc/chm103/EDTA_Ca.pdf>.
 Reckhow Research Group. CHAPTER XVI VOLUMETRIC METHODS. 17 June 2011. 25 January
2012 <http://www.ecs.umass.edu/cee/reckhow/courses/572/572bk16/572BK16.html>.
 Sinex, Scott A. EDTA - A Molecule with a Complex Story. 1 August 2007. 25 January 2012
<http://www.chm.bris.ac.uk/motm/edta/edtah.htm>.
 UC Davis Department of Chemistry. EDTA TITRATIONS. 31 March 2004. 25 January 2012
<http://www-chem.ucdavis.edu/2C/Old/06EDTA.pdf >.

50. “He who asks is a fool for five minutes, but he who
does not ask remains a fool forever.”
Chinese Proverb