Determining the Hardness of an Unknown Water Sample Using Volumetric Titration

Determining the Hardness of an Unknown Water Sample Using Volumetric Titration

Introduction:

The experiment uses volumetric titration to determine the hardness of an unknown water sample which is finding the concentration of Ca²⁺ in the water. Titration is adding a solution with a known volume and concentration to another solution with an unknown concentration and known volume. Titration in this case is using a solution with a concentration that is known to find the concentration of the unknown sample. The EDTA, with the known concentration, will be added slowly to the unknown solution in a flask using a buret until there is a color change that indicates the solution has reached an endpoint and the concentration is determined. The objective of this experiment is to determine the concentration of Ca²⁺ an unknown water sample using titration.

Reaction:

H₂EDTA²⁻(aq) + Ca²⁺(aq) → CaH₂EDTA(aq)

Mathematical Equations:

Mol = M×L

g = mols×molar mass

ppm = mg of solute/ volume of water in L

Experimental Section:

This experiment used EDTA, an unknown water sample, an indicator, and a buffer to complete a successful titration. The first step was to get a 25.00mL volumetric pipet and use the pipettor to pipet a 25.00mL water sample into the 125mL Erlenmeyer flask. Then, the volume was marked in a data table. To this sample, about 5mL of pH 10 NH₃/NH₄Cl buffer was added. The last chemical that had to be added before this sample could be titrated was exactly two drops of eriochrome black T indicator. After the indicator, the solution was a deep red shade. The buret was then filled with EDTA, but before it could be filled had to be rinsed with the EDTA by adding about 5mL of the 2.02×10⁻³mol/L EDTA solution to a beaker then pouring it through a funnel into the buret to clean it. EDTA is added to the buret and the concentration of the EDTA is marked down as well as the beginning volumes for the titration process. The buret level of the liquid had to be read to 0.01mL prior to entering any titrant. The titrating began and as the EDTA was slowly added the reaction mixture was being swirled. Finally, the sample changed to a light purple or blue color