Chem 23.1 2

• If a titration is to give an accurate result, we must combine the titrand and the titrant in stoichiometrically equivalent amounts (equal number of moles of titrand and titrant). We call this stoichiometric mixture the equivalence point.
• An accurate titration requires that we know the exact volume of titrant at the equivalence point, Veq. The product of the titrant’s equivalence point volume and its molarity, MT, is equal to the moles of titrant that react with the titrand.
• Endpoint is approximation of the equivalence point indicated by the change in color of an indicator.
• The difference between the end point’s volume and the equivalence point’s volume is the determinate titration error. If the end point and the equivalence point volumes coincide closely, then this error is insignificant and is safely ignored. Clearly, selecting an appropriate end point is of critical importance.
• • Complexometric titrations involve reactions between metal ions such as Mg2+, and complexing agent or ligand to form soluble complex also known as coordinate complex.
• Y^4- + Mg^2+ --> MgY^2-
• Fully deprotonated EDTA, Y^4-
• Fully protonated EDTA, H6Y^2+
• EDTA is a weak acid and is a hexadentate ligand with six binding sites, four negatively charged oxygen atoms in the carboxylate groups and two nitrogen atoms in the tertiary amino groups.
• EDTA is often depicted as H4Y (the neutral form - tetraprotic)
• Ka1= 0.0 , H6Y^2+

  Ka2= 1.5 , H5Y^+

  Ka3 = 2.0, H4Y

  Ka4= 2.68, H3Y^-

  Ka5 = 6.11, H2Y^2-

  Ka6=10.17, HY^3-

  >10.24, Y^4-
• The hardness of water refers to the measure of the amount of Calcium and Magnesium as well as other chemicals dissolved in a water sample.
• The hardness of water can be expressed in two ways: mg/L or ppm, and grams per gallon (gpg).
• Ethylenediaminetetraacetic acid is used as the titrating agent in this experiment since it effectively removes the minerals from hard water. EDTA has a greater affinity to metal ions such as Ca2+ and Mg2+ due to the presence of unshared electron pairs
• A direct analysis for Ca2+ with ethylenediaminetetraacetic acid (EDTA), which we represent here as Y4-.

  Ca2+(aq) + Y4+(aq) ⇌ CaY2-(aq)

  Unfortunately, for most samples this titration does not have a useful indicator. Instead, we react the Ca2+ with an excess of MgY2-.

  Titration of Mg2+ with EDTA:

  Mg2+(aq) + Y4-(aq) ⇌ MgY2-(aq)

  Has suitable endpoint

  Ca2+(aq) + MgY2-(aq) ⇌ CaY2-(aq) + Mg2+(aq)

  The amount of the released Mg2+ is equivalent to the amount of Ca2+ in the sample.

  The amount of EDTA used in the titration provides an indirect measure of the amount of Ca2+ in the original sample.
• Wine Red (MIn-) + Colorless (Y4-) + H+ ⇌ Colorless (MY2-) + Blue (HIn2-)