Determination of Water Hardness
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:
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:
References: Lab Manual City of Gilbert, “http://www.3mwater.com/medi/documents/WaterReport_GilbertAZ.pdf” FairFax Water, “http://www.fcwa.org/water/hardness.htm”