experiment 4 determination of total hardness as ppm calcium carbonate

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