Group 2: Alkaline earth metals

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My experiment is about the quantitative determination of total hardness in drinking water by complexometric (EDTA) titration. Can you help me with the following questions: 1. The concept of Complexometric Titration and Water Hardness 2. Use of EDTA as complexing agent and titrant. 3. Use of Eriochrome Black-T as an indicator - the significance of adding MgCl.6H₂O crystals to the titrant 4.Why is the analysis of Ca and Mg done at pH 10? 5. What is the effect of using too much buffer in the analysis of Ca and Mg using EDTA titration? 6. NaOH is added to the EDTA solution if the sodium-EDTA crystals do not dissolve. How does NaOH help in dissolving EDTA? 7. What is the correlation between stability of the Mg-EDTA, the complexes Ca-EDTA, MgIn and CaIn and their corresponding K_f values?

Corrie writes ...
 
Whereas acid-base titrations involve neutralisation of an acid by a base, with the formation of water, complexometric titrations involve the combination of a metal ions with other ions or molecules (ligands) to form a new, complex ion. I think you will find answers to most of your questions, certainly 1-4 and 7, in the following web reference:
 
http://academic.pgcc.edu/psc/chm103/EDTA_Ca.pdf
 
5. Using too much NH₃/NH₄Cl buffer can result in the precipitation of some metal hydroxides.
 
6. EDTA is usually supplied as the di-sodium salt, (Na⁺)₂EDTA^2-, which is more soluble in water than the EDTA molecule. If the crystals of the di-sodium salt do not dissolve very well, then addition of further NaOH would presumably convert it to the tetra-sodium salt, which would be more soluble.
 

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updated: 17 August 2007