Magnesium-EDTA solutions, displacement

Mixtures of manganese, magnesium, and zinc can be similarly analysed. The first EDTA end point gives the sum of the three ions. Fluoride ion is added and the EDTA liberated from the magnesium-EDTA complex is titrated with manganese ion as detailed above. Following the second end point cyanide ion is added to displace zinc from its EDTA chelate and to form the stable cyanozincate complex [Zn(CN)4]2- the liberated EDTA (equivalent to the zinc) is titrated with standard manganese-ion solution. 

An alternative method is to add a small amount of Mg " to the EDTA solution. This immediately reacts with EDTA to form MgY with very little free Mg " in equilibrium. This, in effect, reduces the molarity of the EDTA. So the EDTA is standardized after adding the Mg " by titrating primary standard calcium carbonate (dissolved in HCl and pH adjusted). When the indicator is added to the calcium solution, it is pale red. But as soon as the titration is started, the indicator is complexed by the magnesiiim and turns wine red. At the end point, it changes to blue, as the indicator is displaced from the magnesium. No correction is required for the Mg added because it is accounted for in the standardization. This solution should not be used to titrate metals other than calcium. 

As EDTA is added it displaces the cations from the cation-indicator complex by forming more stable complexes with the cations. When all of the Ca and Mg is complexed with EDTA, (at the end point), the solution turns from wine-red to blue due to the free Eriochrome Black T indicator. In order to obtain a sharp end point, a small amount of magnesium ions must be present. This is generally not a problem with natural water samples, which tend to contain some Mg, but it is a problem when standardizing EDTA solutions with pure CaC03. A small quantity of MgCl2 is added to the standard EDTA solution to ensure the presence of Mg ions. 

If the analyte metal ion forms a stable EDTA complex rapidly, and an end point can be readily detected, a direct titration procedure may be employed. More than thirty metal ions may be so determined. Where the analyte is partially precipitated under the reaction conditions thereby leading to a slow reaction, or where a suitable indicator cannot be found, back titration procedures are used. A measured excess of EDTA is added and the unreacted EDTA titrated with a standard magnesium or calcium solution. Provided the analyte complex is stronger than the Ca-EDTA or Mg-EDTA complex a satisfactory end point may be obtained with eriochrome black T as indicator. An alternative procedure, where end points are difficult to observe, is to use a displacement reaction. In this case, a measured excess of EDTA is added as its zinc or magnesium complex. Provided the analyte complex is the stronger, the analyte will displace the zinc or magnesium. 

In displacement titrations, an unmeasured excess of a solution containing the magnesium or zinc complex of EDTA is introduced into the analyte solution. If the analyte forms a more stable complex than that of magnesium or zinc, the following displacement reaction occurs ... 

Eriochrome Black T is a typical indicator. It contains three ionizable protons, so we will represent it by Hsin. This indicator can be used for the titration of Mg + with EDTA. A small amount of indicator is added to the sample solution, and it forms a red complex with pait. pf the Mg " the color of the uncomplexed indicator is blue. As soon as all the free Mg. is titrated, the EDTA displaces the iudicator from the magnesium, causing a change in the color from red to blue ... 

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