6. Calcium and magnesium ions are the primary contributors to \"hardness\" of wa
ID: 716330 • Letter: 6
Question
6. Calcium and magnesium ions are the primary contributors to "hardness" of water and they are important components of limestone. Mia decides that she will quantify the calcium and magnesium content of tap water samples from her home and from UST using a 0.10 M EDTA (ethylenediaminetetracetic acid) solution she has prepared from the following reagent. E9884 SIGMA-ALDRICH Ethylenediaminetetraacetic aci ACS reagent, 99.4-100.6%, powder MSOS SIMILAR PRODUCTS CAS Number 60-004 LUnar Formula (HO CCHalh NCH CH,NICH-CO2 MclecuiarWelight 282.24 Belistain Registry Number 1716295 EC Number 200-4494 MDL number MFCD00003541 PubChem Substance 10 2489470 Ciga 39030907 Properties Related Categories Acids & Bases, Bases, Chemical Synthesis, Essensial Chemicals, Inorganic Bases, More.. grade ACS reagent (primary standard 99.95-100.05% dry basis S0.005% silica S0.01% insolubles S1.0% loss on heating 851 'cpit.) assay mp Show More (16) Based on the product information for the EDTA, would she be able to accurately determine the calcium and magnesium content with her solution? If not, what change or addition should she make to her protocol?Explanation / Answer
EDTA known as ethylenediaminetetraacetic acid is hexadentate ligand which encapsulates various metal ions at different pH range. The chelate formed during the encapsulation of metal ion is stable in particular pH range. For the estimation of hardness of water due to Mg2+ and Ca2+ ions, an alkaline pH is required.
When Ca and Mg reacts with EDTA they release H+ ions into the solution which decrease the pH of the medium and in turn make the complex labile, to get rid of such problem, an ammonium buffer is added to the solution.
Ca2+ + EDTA --------> CaEDTA2- + 4H+
Mg2+ + EDTA ---------> MgEDTA2- + 4H+
Hence, Mia has to add a buffer solution. During titration she has to add an indicator which will help her to determine the end point of the titration by providing a colour change during the end point. Thus, she is supposed to add Eriochrome Black T indicator as indicator otherwise it would not be possible for her to detect the exact end point.
Next issue is the solubility of EDTA in water, many a time it is difficult to get high concentration of EDTA solution, hence its disodium salt is used and pH is made alkaline to make it more soluble, a pH approximately equal to 8 is generally required.
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