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13 Dec 2019

EDTA (ethylenediaminetetraacetic acid) is the most widely used metal chelator in analytical chemistry, which forms strong 1:1 complexes with most metal ions. Water hardness can be determined by titrating Ca2+, Mg2+ and other metal ions with EDTA. During the titration, ammonium-ammonium chloride buffer is often used. Which of the following descriptions about the function of ammonium-ammonium chloride is/are correct?

-To react with EDTA

-To be used as a metal ion indicator

-To minimize the affect of other metal ions

-To maintain the pH of the solution around 10

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Jamar Ferry
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17 Dec 2019

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Related questions

Problem S2
Water hardness is related to the amount of calcium ions in thewater. This is not a health issue, but free calcium can bind tosoap molecules to leave insoluble deposits on hair, skin, or fibersin clothing.

Calcium concentrations can be determined inexpensively by usinga metal chelator EDTA, which binds stoichiometrically toCa2+ with a formation constant of Kf(Ca2+) = 5 x 1010M-1. The highformation constant insures that Ca2+ titrations withEDTA give a very sharp endpoint. However, the Ca-EDTA complex iscolorless. In order to detect the Ca2+ titrationendpoint, a second metal indicator HIn is used. The indicator doesnot bind calcium, but it binds Mg2+ to give a red complex (λmax =615nm). In the absence of free Mg2+ ions, the Indicatoris sky-blue (λmax = 420nm). EDTA binds Mg2+, but theformation constant is much lower than for calcium at Kf(Mg2+) = 4.9 x 108M-1, so thatEDTA will only bind Mg2+ when there is no free calciumin solution.

The calcium titration is carried out, by adding a 0.5mM MgIn tothe initial water sample. The resulting sample solution containingcalcium and MgIn is then titrated with a standard solution of EDTA(known concentration). As long as there is free calcium in thesolution, EDTA will bind Ca2+. Once the calcium iscompletely titrated, then EDTA will bind Mg2+.

QUESTION

A small volume of MgIn is added to the 1L water sample. Thefollowing volumes of a 0.1M EDTA stock solution were then added tothe solution, and the corresponding absorbance at 615nm wasmeasured. Determine the calcium concentration in the initial 1L ofsample water. State any assumptions you make.

ml of 0.1M Absorbance
EDTA added at690nm mL EDTA OD690 mLEDTA OD690

0.0 0.710 1.0 0.710 2.0 0.710

0.2 0.710 1.2 0.710 2.2 0.660

0.4 0.710 1.4 0.710 2.4 0.518

0.6 0.710 1.6 0.710 2.8 0.376

0.8 0.710 1.8 0.710 3.0 0.092

Please help with question 20 and 30. Explanations and details would be greatly appreciated!! Thank you

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