Post on 13-Apr-2016
description
Determinat ion of Total Hardness as Parts-per-Mi l l ion Calcium Carbonate
Elysse S. SalindoKyle Lendl N. Wong
Object ives
Standardize EDTA Solution Determine hardness of any given sample as CaCO
3
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
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.
Introduct ion
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
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
Introduct ion
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
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
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
Er iochrome Black T
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
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
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
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
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)
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
Discussion
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)
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
Discussion
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)
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-
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.
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