Chem 28 Lab Report

Experiment 5

COMPLEX TITRATION

Determination of the Total Hardness of Commercial Mineral Water

I. Theoretical Framework

Natural water contains dissolved ions. It dissolves ions as it flows through minerals. Calcium ion and magnesium ion are the most common of such ions in natural water.

Determination of water hardness is a useful test to measure the quality of water for household and industrial uses. Drinking hard water contributes to a small amount of calcium and magnesium toward the total human dietary needs of these ions. It also causes soap scum, clogged pipes and boilers.

The hardness of water will be determined with the use of titration. In this process, EDTA, which is a weak acid, was used as the titrant. EDTA is able to form soluble complexes with calcium and magnesium cations when it is in its ionized form. The indicator used for titration was Eriochrome Black T (EBT). Initially, the indicator will form a complex with the cations and is red in color. During titration, EDTA replaces EBT and forms more stable complexes with calcium and magnesium. When the indicator is released by the metal ions, it has a distinct blue color. Hence, the endpoint of the titration is indicated by a color change from wine red to blue

The purpose of this experiment is to determine the hardness of water by measuring the concentrations of calcium and magnesium in water samples by titration.

II. Data and Results

Standardization of EDTA Solution

Primary Standard used: CaCO3

Formula mass of 1O standard: 100.087 g/mol

% Purity of 1O standard: 99%

Trials 1 2 3

Mass of CaCO3 (g) 0.2036 0.2036 0.2036

Final Reading EDTA (ml) 35 38 40

Initial Reading EDTA (ml) 0 0 0

Net Volume of EDTA (ml) 35 38 40

Sample Analysis

Brand of mineral water used: Summit Natural Drinking Water

Ca content as CaCO3: 15 mg/L

Mg Content as CaCO3: 5 mg/L

Trials I II III

Vol. of water sample (ml) 100 100 100

Volume of EDTA (mL)

Final volume (ml) 6.2 12.2 18.6

Initial volume (ml) 0 6.2 12.2

Net Volume (ml) 6.2 6 6.4

Reported Values

Trials Molarity Titer Total Hardness

1 0.012 1.2 68.2

2 0.011 1.1 66

3 0.010 1.0 70.4

Average 0.011 ± 1x10-3 1.1± 0.1 68.2 ± 1.1489

III. Calculations

Molarity:

Trial 1

M=((203.6 mg)(0.99))/((100.087 mg/mol)(35ml))×50/250=0.012M

Trial 2

M=((203.6 mg)(0.99))/((100.087 mg/mol)(38 ml))×50/250=0.011M

Trial 3

M=((203.6 mg)(0.99))/((100.087 mg/mol)(40 ml))×50/250=0.010M

Titer:

Trial 1

((203.6 mg)(0.99))/((35 ml))×50/250=1.2 mg/ml

Trial 2

((203.6 mg)(0.99))/((38 ml))×50/250=1.1 mg/ml

Trial 3

((203.6 mg)(0.99))/((40 ml))×50/250=1.0 mg/ml

Total hardness

Trial 1

((6.2ml)(1.1mg/ml))/0.1M=68.2 mg/ml

Trial 2

((6.0ml)(1.1mg/ml))/0.1M=66 mg/ml

Trial 3

((6.4ml)(1.1mg/ml))/0.1M=70.4 mg/ml

Standard Deviation

Molarity:

√((〖(0.011-0.012)〗^2+〖(0.011-0.011)〗^2+〖(0.011-0.010)〗^2)/2)=〖1×10〗^(-3)