Chapter 9

Distinguish between a complexing agent and a chelating agent.

Explain the principles of indicators used in chelometric titrations.

Why is a small amount of magnesium salt added to the EDTA solution used for the titration of calcium with an Eriochrome Black T indicator?

Why would it be difficult to titrate \(\mathrm{Cu}^{2+}\) with EDTA in a strongly ammoniacal medium?

Calcium ion forms a weak 1: 1 complex with nitrate ion with a formation constant of \(2.0 .\) What would be the equilibrium concentrations of \(\mathrm{Ca}^{2+}\) and \(\mathrm{Ca}\left(\mathrm{NO}_{3}\right)^{+}\) in a solution prepared by adding \(10 \mathrm{~mL}\) each of \(0.010 \mathrm{M} \mathrm{CaCl}_{2}\) and \(2.0 \mathrm{M} \mathrm{NaNO}_{3}\) ? Neglect diverse ion effects.

The formation constant of the silver-ethylenediamine complex, \(\mathrm{Ag}\left(\mathrm{NH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{NH}_{2}\right)^{+},\) is \(5.0 \times 10^{4}\). Calculate the concentration of \(\mathrm{Ag}^{+}\) in equilibrium with a \(0.10 \mathrm{M}\) solution of the complex. (Assume no higher order complexes.)

Silver ion forms stepwise complexes with thiosulfate ion, \(\mathrm{S}_{2} \mathrm{O}_{3}^{2-},\) with \(K_{f 1}=6.6 \times 10^{8}\) and \(K_{f 2}=4.4 \times 10^{4} .\) Calculate the equilibrium concentrations of all silver species for \(0.0100 M\) \(\mathrm{AgNO}_{3}\) in \(1.00 \mathrm{M} \mathrm{Na}_{2} \mathrm{~S}_{2} \mathrm{O}_{3} .\) Neglect diverse ion effects.

Calculate the weight of \(\mathrm{Na}_{2} \mathrm{H}_{2} \mathrm{Y} \cdot 2 \mathrm{H}_{2} \mathrm{O}\) required to prepare \(500.0 \mathrm{~mL}\) of \(0.05000 \mathrm{M}\) EDTA.

An EDTA solution is standardized against high-purity \(\mathrm{CaCO}_{3}\) by dissolving \(0.3982 \mathrm{~g} \mathrm{CaCO}_{3}\) in hydrochloric acid, adjusting the \(\mathrm{pH}\) to 10 with ammoniacal buffer, and titrating. If \(38.26 \mathrm{~mL}\) was required for the titration, what is the molarity of the EDTA?

Calcium in powdered milk is determined by dry ashing (see Chapter 1 ) a \(1.50-\mathrm{g}\) sample and then titrating the calcium with EDTA solution, 12.1 mL being required. The EDTA was standardized by titrating \(10.0 \mathrm{~mL}\) of a zinc solution prepared by dissolving \(0.632 \mathrm{~g}\) zinc metal in acid and diluting to \(1 \mathrm{~L}(10.8 \mathrm{~mL}\) EDTA required for titration). What is the concentration of calcium in the powdered milk in parts per million?

Calcium is determined in serum by microtitration with EDTA. A \(100-\mu \mathrm{L}\) sample is treated with two drops of \(2 M \mathrm{KOH},\) Cal-Red indicator is added, and the titration is performed with \(0.00122 M\) EDTA, using a microburet. If 0.203 mL EDTA is required for titration, what is the level of calcium in the serum in \(\mathrm{mg} / \mathrm{dL}\) and in meq/L?

In the Liebig titration of cyanide ion, a soluble complex is formed; at the equivalence point, solid silver cyanide is formed, signaling the end point: $$ \begin{aligned} 2 \mathrm{CN}^{-}+\mathrm{Ag}^{+} & \rightarrow \mathrm{Ag}(\mathrm{CN})_{2}^{-} \quad(\text { titration }) \\ \mathrm{Ag}(\mathrm{CN})_{2}^{-}+\mathrm{Ag}^{+} & \rightarrow \mathrm{Ag}\left[\mathrm{Ag}(\mathrm{CN})_{2}\right] \quad(\text { end point }) \end{aligned} $$ A \(0.4723-\mathrm{g}\) sample of \(\mathrm{KCN}\) was titrated with \(0.1025 \mathrm{M} \mathrm{AgNO}_{3},\) requiring \(34.95 \mathrm{~mL}\). What is the percent purity of the KCN?

Copper in saltwater near the discharge of a sewage treatment plant is determined by first separating and concentrating it by solvent extraction of its dithizone chelate at \(\mathrm{pH} 3\) into methylene chloride and then evaporating the solvent, ashing the chelate to destroy the organic portion, and titrating the copper with EDTA. Three \(1-\mathrm{L}\) portions of the sample are each extracted with \(25-\mathrm{mL}\) portions of methylene chloride, and the extracts are combined in a \(100-\mathrm{mL}\) volumetric flask and diluted to volume. A \(50-\mathrm{mL}\) aliquot is evaporated, ashed, and titrated. If the EDTA solution has a \(\mathrm{CaCO}_{3}\) titer of \(2.69 \mathrm{mg} / \mathrm{mL}\) and \(2.67 \mathrm{~mL}\) is required for titration of the copper, what is the concentration of copper in the seawater in parts per million?

Chloride in serum is determined by titration with \(\mathrm{Hg}\left(\mathrm{NO}_{3}\right)_{2} ; 2 \mathrm{Cl}^{-}+\mathrm{Hg}^{2+} \rightleftharpoons \mathrm{HgCl}_{2} .\) The \(\mathrm{Hg}\left(\mathrm{NO}_{3}\right)_{2}\) is standardized by titrating \(2.00 \mathrm{~mL}\) of a \(0.0108 \mathrm{M} \mathrm{NaCl}\) solution, requiring \(1.12 \mathrm{~mL}\) to reach the diphenylcarbazone end point. A \(0.500-\mathrm{mL}\) serum sample is treated with \(3.50 \mathrm{~mL}\) water, \(0.50 \mathrm{~mL} 10 \%\) sodium tungstate solution, and \(0.50 \mathrm{~mL}\) of \(0.33 \mathrm{M} \mathrm{H}_{2} \mathrm{SO}_{4}\) solution to precipitate proteins. After the proteins are precipitated, the sample is filtered through a dry filter into a dry flask. A \(2.00-\mathrm{mL}\) aliquot of the filtrate is titrated with the \(\mathrm{Hg}\left(\mathrm{NO}_{3}\right)_{2}\) solution, requiring \(1.23 \mathrm{~mL}\). Calculate the \(\mathrm{mg} / \mathrm{L}\) chloride in the serum. (Note: mercury is rarely used today due to its toxicity. The problem is illustrative.)

A \(0.1021 \mathrm{~g}\) sample containing \(\mathrm{ZnO}\) was titrated using a standard EDTA solution with Erichrome Black \(\mathrm{T}\) as indicator. It took \(25.52 \mathrm{~mL}\) of \(0.0100 \mathrm{M}\) EDTA to reach the end point. What is the percentage of \(\mathrm{ZnO}\) in the sample?