Post Lab 5 CHM3120C - Sindy SaintClair Lab Partner Pauline...

This preview shows page 1 - 4 out of 9 pages.

Sindy SaintClair Lab Partner: Pauline & Pedro TA: Siqi Sun Lab 5. EDTA Titration: Determination of Mg2+ by Direct Titration with EDTA CHM 3120C. 001
P a g e | 2 Introduction Besides acid-base and redox reactions, complexation reactions can also be applied to titration. Complexation reactions usually involve Lewis acids (electron pair acceptors), such as metal ions, and bases (electron pair donors) that also go by “ligands.” In complexation reactions, metal ions are bound to ligands to reach a certain level of stability or instability. The instability or favorability of the reaction can be found by using the Gibbs Free Energy equation (Gibbs Free Energy, 2017): ∆G = ∆H - T∆SSome metal atoms have the ability to bind with just one ligand, such as Al (OH2)6+3 , a monodentate. In this example, the water molecule, H2O, is able to bind to the metal Aluminum (Al) through just one site. However, there are special ligands called multidentate, or chelating ligands, that can bind to more than one site. They are able to produce complexes with higher stability than monodentates. The increased stability is named the “chelate effect” (The Determination of Magnesium by Direct Titration with EDTA USF Elementary Analytical Chemistry Lab Manual, 2017). Ni(H2O)62++ 6NH3Ni(NH3)62++ 6H2O Ni(H2O)62++ 2H2NCH2CH2NH2Ni[H2NCH2CH2NH2]2++ 6H2O In this experiment, the solution pH is buffered to an alkaline pH of 10, in order for all of the EDTA to be in chelating form and complete the reaction. The equivalence point of the end point of the titration are found by utilizing indicators, which are metal ion indicators. These types respond to the presence or absence of metal ions. Complexes are also formed with these metals, but not so much so with EDTA. During the process of titration, as EDTA is titrated into the
P a g e | 3 solution, it complexes with the free metal ion, then reacts with the metal ion that linked to the indicator because its Kfwith the metal is much larger than the indicators (The Determination of Magnesium by Direct Titration with EDTA USF Elementary Analytical Chemistry Lab Manual, 2017). EDTA is considered an anion that reacts with the polyvalent metal cations —having a valence of three or more—in the prepared solution. There is a 1:1 stoichiometry, which can be seen in the representation below with Magnesium: Mg2+(aq) + EDTA-4(aq) => [Mg(EDTA)]-2(aq) As EDTA was added, it reacts with the color Mg2+and then the “MgIn” complex, which is the product of Eriochrome Black T indicator and Mg2+. Because of this, the EDTA has to have a stronger bond to Mg2+than the indicator does. At one point, the solution changes from red “MgIn” to blue “In” to convey the end point of the titration—when the majority of the indicator isn’t tied to the metal ion (Complexometric determination of magnesium, 2009). For this experimentation, the percentage of MgO in unknown sample #046 will be determined using the indicators Eriochrom Black T.

  • Left Quote Icon

    Student Picture

  • Left Quote Icon

    Student Picture

  • Left Quote Icon

    Student Picture