experiment 4 determination of total hardness as ppm calcium carbonate
DESCRIPTION
Chem 27.1 Experiment 4TRANSCRIPT
![Page 1: Experiment 4 Determination of Total Hardness as Ppm Calcium Carbonate](https://reader035.vdocuments.us/reader035/viewer/2022082209/56d6bdca1a28ab30168f590c/html5/thumbnails/1.jpg)
Determinat ion of Total Hardness as Parts-per-Mi l l ion Calcium Carbonate
Elysse S. SalindoKyle Lendl N. Wong
![Page 2: Experiment 4 Determination of Total Hardness as Ppm Calcium Carbonate](https://reader035.vdocuments.us/reader035/viewer/2022082209/56d6bdca1a28ab30168f590c/html5/thumbnails/2.jpg)
Object ives
Standardize EDTA Solution Determine hardness of any given sample as CaCO
3
![Page 3: Experiment 4 Determination of Total Hardness as Ppm Calcium Carbonate](https://reader035.vdocuments.us/reader035/viewer/2022082209/56d6bdca1a28ab30168f590c/html5/thumbnails/3.jpg)
Introduct ion
WATER− Is highly polar
− Called the “universal” solvent
− Can dissolve more substances than any other known liquid
− Natural water contains dissolved substances usually from mineral deposits
![Page 4: Experiment 4 Determination of Total Hardness as Ppm Calcium Carbonate](https://reader035.vdocuments.us/reader035/viewer/2022082209/56d6bdca1a28ab30168f590c/html5/thumbnails/4.jpg)
Introduct ion
TOTAL WATER HARDNESS− Defined as the concentration of of
dissolved cations (particularly Ca2+ and Mg2+) in a water sample
− Can be expressed in ppm CaCO3, grains per gallon, mmol/L, etc
− A scale is given to describe how “hard” a water sample is.
![Page 5: Experiment 4 Determination of Total Hardness as Ppm Calcium Carbonate](https://reader035.vdocuments.us/reader035/viewer/2022082209/56d6bdca1a28ab30168f590c/html5/thumbnails/5.jpg)
Introduct ion
![Page 6: Experiment 4 Determination of Total Hardness as Ppm Calcium Carbonate](https://reader035.vdocuments.us/reader035/viewer/2022082209/56d6bdca1a28ab30168f590c/html5/thumbnails/6.jpg)
Introduct ion
Two types of water hardness− Temporary hardness
Due to bicarbonate (HCO3-) present in water
Can be removed by boiling the water to expel CO
2
− Permanent hardness due to the presence of the ions Ca2+, Mg+2,
Fe3+ and SO4-
Cannot be eliminated by boiling
![Page 7: Experiment 4 Determination of Total Hardness as Ppm Calcium Carbonate](https://reader035.vdocuments.us/reader035/viewer/2022082209/56d6bdca1a28ab30168f590c/html5/thumbnails/7.jpg)
Introduct ion
HARD WATER− Is a nuisance
− Precipitation of calcium carbonate is endothermic so when hard water is heated, it forms solid CaCO
3
Water pipes, boilers, tea kettle, etc.
− Reduce effectiveness of soap
− Interacts with soap and forms an insoluble soap scum
Very difficult to clean
![Page 8: Experiment 4 Determination of Total Hardness as Ppm Calcium Carbonate](https://reader035.vdocuments.us/reader035/viewer/2022082209/56d6bdca1a28ab30168f590c/html5/thumbnails/8.jpg)
Introduct ion
![Page 9: Experiment 4 Determination of Total Hardness as Ppm Calcium Carbonate](https://reader035.vdocuments.us/reader035/viewer/2022082209/56d6bdca1a28ab30168f590c/html5/thumbnails/9.jpg)
Introduct ion
COMPLEXOMETRIC TITRATION− Reaction that involves the binding of metal
ions with a ligand/complexing agent
− Solution containing metal ion of interest (water sample) is titrated with a solution of chelating agent (EDTA)
− Endpoint is determined with an indicator (EBT) capable of forming a colored complex with the metal ion
![Page 10: Experiment 4 Determination of Total Hardness as Ppm Calcium Carbonate](https://reader035.vdocuments.us/reader035/viewer/2022082209/56d6bdca1a28ab30168f590c/html5/thumbnails/10.jpg)
Introduct ion
Ethylenediamminetetraacetic acid (EDTA)
− Hexadentate ligand − Tetrabasic or fully deprotonated
form (Y4-) can form at most 6 bonds to a single metal ion
− Forms 1:1 complex with metal ions regardless of charge
− Effectiveness (as a ligand) depends on pH level
![Page 11: Experiment 4 Determination of Total Hardness as Ppm Calcium Carbonate](https://reader035.vdocuments.us/reader035/viewer/2022082209/56d6bdca1a28ab30168f590c/html5/thumbnails/11.jpg)
Introduct ion
Eriochrome Black T Indicator (EBT)
– Can form colored complex with metal ion albeit less stable than EDTA-metal complex
– When chelated (bonded to metal ion) = wine-red in color
– When not chelated = blue in color
![Page 12: Experiment 4 Determination of Total Hardness as Ppm Calcium Carbonate](https://reader035.vdocuments.us/reader035/viewer/2022082209/56d6bdca1a28ab30168f590c/html5/thumbnails/12.jpg)
Er iochrome Black T
![Page 13: Experiment 4 Determination of Total Hardness as Ppm Calcium Carbonate](https://reader035.vdocuments.us/reader035/viewer/2022082209/56d6bdca1a28ab30168f590c/html5/thumbnails/13.jpg)
Results Standardization of EDTA
– Weight of CaCO3 = 0.1169 g
Average Molarity of EDTA = 4.816 x 10 -3
Table1. Molarity of EDTA from Standardization with CaCO3
Trial Volume of EDTA (mL) Molarity of EDTA (M)
1 25.60 4.562 x 10-3
2 23.80 4.907 x 10-3
3 23.80 4.907 x 10-3
4 23.90 4.887 x 10-3
![Page 14: Experiment 4 Determination of Total Hardness as Ppm Calcium Carbonate](https://reader035.vdocuments.us/reader035/viewer/2022082209/56d6bdca1a28ab30168f590c/html5/thumbnails/14.jpg)
Results
Sample Computation:
MEDTAV
EDTA = M
CaCO3V
CaCO3
MEDTA(25.60 mL) = ((.1169 g CaCO3)(100.09
g/mol CaCO3) / .250 L ) (25mL CaCO3)
MEDTA= 4.562 x 10
-3
![Page 15: Experiment 4 Determination of Total Hardness as Ppm Calcium Carbonate](https://reader035.vdocuments.us/reader035/viewer/2022082209/56d6bdca1a28ab30168f590c/html5/thumbnails/15.jpg)
Results
Analysis of Unknown
Table 2. Total Hardness of Unknown Water Sample by Titration with EDTA
Average Total Hardness of sample = 1258 ppm
Trial Volume of water sample (mL)
Volume of EDTA (mL)
Total hardness (ppm CaCO3)
1 10.00 26.80 1292
2 10.00 25.90 1248
3 10.00 25.60 1234
![Page 16: Experiment 4 Determination of Total Hardness as Ppm Calcium Carbonate](https://reader035.vdocuments.us/reader035/viewer/2022082209/56d6bdca1a28ab30168f590c/html5/thumbnails/16.jpg)
Results
Sample Computation:
MEDTA
VEDTA
= Munknown
Vunknown
(4.816 x 10-3 M EDTA)(.02680 L EDTA) =
Munknown
(.0100L unknown)
Munknown
= 0.0129042
Hardness of water= (100.09 g/mol CaCO3)(1000mg/1g)(.0129042 mol/L)
= 1292 mg/L = 1292 ppm
![Page 17: Experiment 4 Determination of Total Hardness as Ppm Calcium Carbonate](https://reader035.vdocuments.us/reader035/viewer/2022082209/56d6bdca1a28ab30168f590c/html5/thumbnails/17.jpg)
React ions Involved
Standardizat ion of EDTA Solut ion:
Y4- + Ca
2+ CaY→ 2-
+ 2H+
Analysis of the Unknown:
Ca2+ + HIn
2- (blue) CaIn→ -
(red) + H+
CaIn- (red) + Y4- CaY→ 2- + Hin2- (blue)
![Page 18: Experiment 4 Determination of Total Hardness as Ppm Calcium Carbonate](https://reader035.vdocuments.us/reader035/viewer/2022082209/56d6bdca1a28ab30168f590c/html5/thumbnails/18.jpg)
Discussion
EDTA
– Has many forms depending on pH conditions: H4Y, H
3Y-, H
2Y2-,
HY3- or Y4-
– Too low or too high pH can decrease the effectiveness of EDTA as a ligand
• ↓ pH,; EDTA is not fully deprotonated
• ↑ pH; hydroxides will interfere with complexation by bonding with Ca or Mg to form insoluble compounds.
– Every ligand and metal ion complex has an optimum pH
• Will depend on pKa of ligand and formation constant of complex
![Page 19: Experiment 4 Determination of Total Hardness as Ppm Calcium Carbonate](https://reader035.vdocuments.us/reader035/viewer/2022082209/56d6bdca1a28ab30168f590c/html5/thumbnails/19.jpg)
Discussion
![Page 20: Experiment 4 Determination of Total Hardness as Ppm Calcium Carbonate](https://reader035.vdocuments.us/reader035/viewer/2022082209/56d6bdca1a28ab30168f590c/html5/thumbnails/20.jpg)
Discussion
Titrant was prepared by combining NaOH, MgCl
2•6H
20 and EDTA.
– NaOH was added to deprotonate EDTA so it is in the form of Y4-
– Mg2+ forms a complex with EDTA (prior to titration)
CaCO3 dissolved in concentrated HCl, water and ammonia buffer then added EBT
– Ca2+ forms a complex with EBT (causing the wine-red color of solution)
![Page 21: Experiment 4 Determination of Total Hardness as Ppm Calcium Carbonate](https://reader035.vdocuments.us/reader035/viewer/2022082209/56d6bdca1a28ab30168f590c/html5/thumbnails/21.jpg)
Discussion
PRIOR to titration
– Analyte is wine-red in color due to the EBT-metal ion complex formed
DURING titration
– Analyte gradually turns purple
AFTER titration/AT end point
– Analyte is blue in color due to unchelated EBT
![Page 22: Experiment 4 Determination of Total Hardness as Ppm Calcium Carbonate](https://reader035.vdocuments.us/reader035/viewer/2022082209/56d6bdca1a28ab30168f590c/html5/thumbnails/22.jpg)
Discussion
![Page 23: Experiment 4 Determination of Total Hardness as Ppm Calcium Carbonate](https://reader035.vdocuments.us/reader035/viewer/2022082209/56d6bdca1a28ab30168f590c/html5/thumbnails/23.jpg)
Discussion
→ At pH 10, HIn2- and Mg2+ form a red complex.
Mg2+ + Hin2- (blue) MgIn- (red) + →H+
→ EDTA forms a weaker complex with Mg2+ than Ca2+. Ca2+ reacts with Y4-
first, leaving the red MgIn- solution
Ca2+ + MgIn- (red) + Y-4 CaY-2 + →MgIn- (red)
![Page 24: Experiment 4 Determination of Total Hardness as Ppm Calcium Carbonate](https://reader035.vdocuments.us/reader035/viewer/2022082209/56d6bdca1a28ab30168f590c/html5/thumbnails/24.jpg)
Discussion
→ When all the Ca2+ is titrated by Y4-, MgIn- reacts with Y4-
MgIn- (red) + Y4- MgY2- + In3- →(orane)
→ In-3 hydrolyzes
In3- (colorless) + H2O Hin2- →(blue) + OH-
![Page 25: Experiment 4 Determination of Total Hardness as Ppm Calcium Carbonate](https://reader035.vdocuments.us/reader035/viewer/2022082209/56d6bdca1a28ab30168f590c/html5/thumbnails/25.jpg)
Discussion
Prior to Titration
– Mg-EDTA complex formed in the buret
– Ca-EBT complex formed in the flask (wine-red color)
During Titration
– Formation constants: Ca-EDTA > Mg-EDTA > Mg-EBT > Ca-EBT
– Displacement Reaction: Ca-EBT Ca-EDTA and Mg-EDTA → →Mg-EBT (nag-switch sila)
After Titration
– EDTA chelates all Ca and Mg in solution thus leaving EBT unchelated so analyte turns blue in color.
![Page 26: Experiment 4 Determination of Total Hardness as Ppm Calcium Carbonate](https://reader035.vdocuments.us/reader035/viewer/2022082209/56d6bdca1a28ab30168f590c/html5/thumbnails/26.jpg)
Conclusion and Recommendation
Based on the scale for water hardness, the sample is considered to be a very hard water
It is important to get as close as possible to the optimum pH for a more successful and accurate titration
Exercise utmost care when quantitatively transferring solutions (especially the standard)
Be very alert during titration especially near the end point because even a small drop of excess can have huge effects on the result