Determination of Water Hardness

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Introduction:

In this lab a total of six titrations are to be performed. Three of them will be done using a known Ca2+ solution, (1.000 g CaCO3 /L solution) and three of them will be done using an unknown solution obtained from the stock room. The objective of this lab is to determine the hardness of water, using the data collected from each titration performed with the unknown sample. Since the hardness of water arises from the presence of metal ions, we can use disodium salt of EDTA and the indicator Eriochrome Black T to determine the concentration of M 2+ ( Mg 2+ or Ca 2+) metal ion impurities.

Chemical principle or theory involved in this lab:

To determine waters hardness we will use a technique called a chelometric titration. “When a neutral molecule or anion (lewis base) donates electron pairs and attaches itself to a metal ion center (a Lewis acid), the resulting cluster of atoms forms a single ion called a complex. When such complexes form the electron donating groups are called ligands. When ligands with more than one binding site form complexes with metal ions, we call this process chelation, where the ligand used is called a chelating agent. ” (lab manual) In this lab the chelating agent used is Na2EDTA. Our Indictor Eriochrome Black T, is a dye, and will form a pink complex in the presence of a metal cation. As EDTA solution is added to the solution, the metal ions will complex to the EDTA solution leaving the indicator solvated. The chemical equation, with the known calcium ion solution, looks as follows: Where H2In¯ represents the solvated indicator Erichrome Black T dye.

H2In¯ + Ca 2+ ⇌ CaIn¯ + 2H
(blue) (pink)

As EDTA is added to the solution, it grabs the Ca ions away from the Erichrome Black T dye molecules to form a more stable complex. When solvated, and alone in the solution, the Erichrome Black T dye will produce a blue color; it can be represented by the equation below:

EDTA 4¯ + CaIn¯ + 2H → H2In¯ + CaEDTA 2¯
(pink) (blue)

Procedure or Method:

First prepare your disodium EDTA solution by weighing out 0.7- 0.8 grams of Na2EDTA and dissolve it in 500 mL's of deionized water. Place in a sealed container and shake vigorously. Standardize the Na2EDTA solution using calcium ion stock solution: Add 10 mL of calcium solution to a flask, and add 30 mL's of deionized water. Add 3 mL's of ammonium chloride buffer under the fume hood and stir. Add 4 drops of the Eriochrome Black T indicator dye, then titrate with the disodim EDTA solution within 15 min's. At the endpoint the color changes from pink to violet to blue within 3-5 seconds. Note the volume of Na2EDTA that was used delivered from the burret, and repeat the titration 2 more times. Obtain an unknown water stock solution from the stock room taking note of the unknown number, then titrate with the standardized disodium EDTA solution: Mix 25 mL's of the prepared water sample with 20 mL's of deionized water, then under the fume hood, add 3 mL's of ammonium chloride buffer and stir. Add 4 drops of the Eriochrome Black T indicator dye, then titrate with the disodim EDTA solution within 15 minute’s Repeat the titration 2 more times, and calculate the hardness of the prepared water sample from each of the titrations.

Observations and Calculations:

Amount of Na2EDTA solution Amount of Na2EDTA solution
required to titrate calcium Ion solution required to titrate unknown H2O sample
Trial 1 25.0 mL15.7 mL
Trail 2 23.8 mL14.9 mL
Trial 3 23.6 mL15.5 mL

Because EDTA chelates Ca 2+ ions in a one-to-one molar ratio we can calculate the moles present of Na2EDTA using the following formula(s):

Trial 1:
[Na2EDTA] = 10.00 mL CaCO3 × 1.000 g CaCO3 × 1mol CaCO3 × 1mol Na2EDTA...
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