EXPERIMENT 7: QUANTITATIVE DETERMINATION OF TOTAL HARDNESS IN DRINKING WATER BY COMPLEXOMETRIC EDTA TITRATION B.F. SOSA and G.P. FORTU INSTITUTE OF BIOLOGY, COLLEGE OF SCIENCE UNIVERSITY OF THE PHILIPPINES, DILIMAN, QUEZON CITY 1101, PHILIPPINES 1
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DATE SUBMITTED: 30 MARCH 2016 DATE PERFORMED: 18 MARCH 2016
ANSWERS TO QUESTIONS
1.
What is water hardness and why is it expressed expressed as ppm CaCO3?
Water hardness is classified as the concentrations of calcium and magnesium ions and it normally goes beyond the concentrations of other meta ions. It is presently expressed as ppm of calcium carbonate because the overall concentration of all multivalent cations in the water sample is equivalent to the concentration concentra tion of calcium carbonate thus it represents the water hardness of the sample. Ppm of calcium carbonate is also equivalent to the hardness of 100ppm that contains an equivalent of 100g calcium carbonate in 1 million grams of water. It is also equivalent to 0.1g in 1L of water. 2.
How does work?
complexometric
titration
In this experiment, it is a volumetric analysis that produces an endpoint that is made visible because of the formation of a colored complex and it is called complexometric titration. It is to determine the water hardness of the Viva mineral water. Usually, it is used to know the metal ion contained in a solution . In this titration, a solution with a free metal ion is titrated using a solution of the chelating agent, titrate until all the metal ions are complexed completely, this will result to the endpoint using a ligand indicator that produces a colored complex containing the metal ion. 3.
Why was EDTA used as complexing agent/ -titrant?
Since compexometric titration is used, titrants used for metal ions like calcium ion analyte are complexing agents. This will result to the reaction of the metal cation with a Lewis Base
(electron-pair donors) in which a coordination complex is formed. This coordination compound has a complex ion, a species with a central metal atom, a transition metal enclosed to several Ligands. The complexing agents of the metal cations are the ligands acting like it. For complexation, multidentate ligands also known as chelating agents gives off more than a single electron pair and these produces multiple coordinate bonds to bond the ligand and the metal cation. The chelates then are very stable thus having large formation constant. EDTA is a important chelating agent. Almost all metal ions produces complexes with the tetrabasic form (acid form) of EDTA. It is a hexadentate ligand, It means one electron pair is given off by each acid oxygen and each amine nitrogen to the electron. Acid dissociation is needed so that EDTA’s electron pair of carboxylic groups to be available to the metal cation therefore the need of a high pH but at very high pH, most metal ions react and form complexes with hydroxide ions. This then states that there’s a pH maximum for EDTA complexation to occur. For any chelating agent and metal ion, there is a corresponding correspond ing pH for the titration that depends on pKa. For calcium ions, it has a pH above 8 in which a ammonia/ammonium buffer is used for its ph maintenance. 4.
Why were MgCl2 ! 6H2O crystals and NaOH pellets added in the precipitation of EDTA solution?
The reason behind adding MgCl2 ! 6H2O crystals and NaOH pellets is for us to determine water hardness of the sample by determining the concentration concentration of calcium carbonate easier. This is also known as displacement titration, the process of determining calcium ions via EDTA but EDTA is very slow in binding calcium ions. This is the reason behind adding another metal ion, usually
Magnesium but in the form of a salt, this will help the binding to be faster that will form metalEDTA complex. Ca-EDTA complex is more stable than the metal-EDTA formed. Then Mg ions will be replaced by the free calcium ions in the solution that will make the formation of CaEDTA complex possible. This formation will result to free magnesium ions that will be easily titrated. The original concentration of calcium ions is equal to the free magnesium ions. 5.
Why was HCl added in the preparation of CaCO3 solution?
There is the need to add HCl in the preparation of calcium carbonate because it will help dissolve calcium carbonate easier. This will be used as the primary standard that will help determine the concentration of the solution thus leading to determining the water hardness of the sample. 6.
What are the pertinent chemical equations involved during titration? Explain how the color of the solution turns to blue using the stability of complexes involved in the titration.
The equation involved in making the primary standard is "#"$% & '(") + "#"), & " $, & (, $
dissolving the calcium carbonate that will be used in titration. From the free Calcium ions this will determine the concentration of the EDTA solution. The equation of the titration will be "# ,- & ./0123 + "#./01,3
With the addition of EDTA to calcium, it will form the chelate Ca-EDTA (refer to no. 3). This solution is colorless but to be able to recognize the endpoint, an indicator is needed called eriochromschwartz-T or EBT, the equation is 45 67 & ./01
red
23
+ 45./01
,3
blue
& )7
What is the importance of maintaining the pH at 10 and choosing NH3-NH4Cl as buffer?
It is important to maintain a pH of 10 because EDTA and EBT both displays polyprotic properties that shows being unstable. It is also because only one endpoint needs to be observed. Also a pH of 10 is enough for a sharp endpoint, too much buffer that results in resistance to change in pH will produce a weaker endpoint that’s why addition of buffer must be just enough. Another reason is there is a minimum solution pH required for both metal ions of calcium and magnesium to complexate with EDTA. Calcium needs 7.3 while magnesium needs 10. Therefore the minimum solution pH is 10. This supports the stability of tha Ca-EDTA solution and any magnesium ion will not interfere the reaction. Since the pH that needs to be maintained is 10, the buffer that should be used id NH3-NH4Cl. 8.
What are the possible resources of error and their effect on the calculated parameters? Rationalize.
There are possible sources of error in this experiment, it is when there is too much buffer in the system. This will make the solution be too resistant to the change in pH thus there will be a dull change of color in the indicator or worse, it will not show any indication of change. Therefore, this will affect all the calculations needed. The water hardness that will be computed will not be accurate because the volume of the water sample will be wrong. Another is an error in solution preparation that will yield to mistakes in almost all of the calculations since solution preparation is the root of the experiment. Another is over titration that is the most common error in titration, with this error, the experiment should be repeated because it will not be fixed. REFERENCES
,3
Since calcium is not stable with the complex formation in the EBT indicator, the titration will not produce a sharp indicator in the endpoint that’s why magnesium is used in the titration because it is stable with the EBT indicator, this is known as displacement titration (refer to no. 4). "#)7 & 45,3 8 "#9 ,3 & 45)73
7.
[1] Skoog, D. (2014). Fundamentals of Analytical Chemistry. (p. 401, p.434, p.436). Belmont, CA: Brooks Cole, A Part of Cengage Learning. [2] Zumdahl, S.S., & Zumdahl S.A. (2012). Chemistry. (pp.983-985). Belmont, CA: Brooks Cole, A Part of Cengage Learning. [3] Silbeberg, M. (2010). Principles of General Chemistry. (pp.763-764, 773). New York, NY: The McGraw- Hill Companies, Inc.
APPENDIX A: SAMPLE CALCULATIONS
Molarity of primary standard 5 :;< ? @ABC;D =F) "#"$% EIII=J > > > > KF) :;< GH "#"$% EJ =) :;< E5 :;< O 4 "#"$% LC7#) KF) MFBNC75 :;<
IPQIII5 :;< IPRRR5 "#"$% E=F) "#"$% EIII=J > > > >IPIIQJ EIIPIST5 "#"$% EJ 'QI=) E5 :;< IPIQJ O EPRRT''UERT>EI3% 4 "#"
%$Concentration of the standard EDTA solution in molarity Average net volume of EDTA: 2.367mL KF) FL MFBNC75 "# 66 :;< > 4 "#"$% > EIPI=J "#"$% > EPRRT''UERT>EI3% 4
VWWXY Z[\]
V
>
O M EDTA
VWWXY ^_^`a Wb Z[\] VWWXY Z[\] V "#"$% > > O SPeUUSEEQTE>EI3% M VWWXY ^_^` ,P%cdWb Z[\] a
EDTA
Titer of the solution as mg CaCO3 per mL of standardized EDTA solution ==F) ./01 =J ./01
>
E==F) "#,E ==F) ./01
>
E==F) "#"$% =5 "#"$% >GH "#"$ O FB "#"$% ;C;fB % E==F) "#,=J ./01
IPIERRT''UERT ==F) ./01 'PUTg =J ./01 O IPSee
>
E==F) "# ,-
E ==F) ./01 =5 "#"
%$>
E==F) "#"$% E==F) "#,-
>
EIIPIST=5 "#"$% E ==F) "#"
%$=J ./01
Total hardness of water sample as ppm "#"$% Trial 1 of Sample analysis: 7f; KF) ./01 > ;C;fB O =5 "# "$% =5 "#"$% gPS =J ./01 >IPSee O TPQSU' =5 "#"$% =J ./01 Wh ^_^`a VqqqWb > = ppm iXY Xj k_lmn o_WpYm Vb cPrs%, Wh ^_^`a VqqqWb rq Wb
>
Vb
= 131.664 ppm "#"
%$Total cation content based on the concentration of Ca and Mg on the water bottle ppm Mg + ppm Ca = ppm Ca GH "# ,- & GH 45,-
O @@= "# ,GH "#"$% 5 5 eIPIgS =F) "# ,- & 'ePUIQ =F) 45 ,EUEPTTe @@= "#"$% > O SePg @@= "#,5 EIIPIST =F) "#"$% @@= "#"$% >
Relative standard deviation Trial Total Hardness S= z O
tu3u v wx y3V { v|} uv
~•/ O
y
•
z
1 131.664 ppm
= 0.9745672544
= 131.1013333 > EIII@@; O gPeU @@;
Confidence limit (95%) Confidence limit = z €
lo y
O EUIPeQ EUEPgQ
2 129.976
3 131.664
