Determining the water hardness of unknown sample #55 was determined by complexometric titration. With the use of disodium salt EDTA as the solution to chelate the metal impurities and the Eriochrome Black T indicator as the solution used to help visualize when the impurities were completely chelated, along with a few other solutions to help the reaction. Unknown water sample #55 experimental calculations determined that the water hardness was considered in the range of “very hard” water in comparison to Tempe’s expected range for municipal water hardness.
Determining water hardness can be done in different ways. This lab will determine the hardness of water through a complexometric titration. Using sodium EDTA as the chelating agent to form complexes with the metal impurities in the solutions calcium carbonate and unknown sample. Complexes will form from sodium EDTA reacting with impurities because each element from disodium EDTA carboxylic acid groups will donate an electron pair to the metal impurities ion center. ammonia/ammonium chloride buffer used in today’s lab will reassure calcium metal ions and metal ions in unknown water sample remain in solution being determined for accurate results. The Eriochrome
Black T indicator will turn the solution being determined a pink color to ensure there is metal cations in the solution being determined. Equation 1 shows the reaction involving the indicator and metal ion.
H2In-(aq) + M2+(aq)
MIn-(aq) + 2H+(aq)
Once disodium EDTA solution is added the solution will turn form pink to blue. Equation
2 shows reaction occurring.
EDTA4-(aq) + MIn-(aq) + 2H2+(aq) --> H2In-(aq) + MEDTA2-(aq)
Blue color will determine endpoint of titration.
With this experiment the hardness of water for calcium carbonate and unknown