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Chapter 12: Q29P (page 285)
Give an example of the use of a masking agent
The Mg2+ in a solution of Mg2+ and Fe3+ must be titrated by EDTA. If the Fe3+ is masked with to form which does not react with EDTA.
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Sulfide ion was determined by indirect titration with EDTA. To a solution containing 25.00 mL of 0.04332 M Cu(ClO4)2 plus 15 mL of 1 M acetate buffer (pH 4.5) were added 25.00 mL of unknown sulfide solution with vigorous stirring. The CuS precipitate was filtered and washed with hot water. Ammonia was added to the filtrate (which contained excess Cu2+) until the blue color of Cu(NH3)42+ was observed. Titration of the filtrate with 0.039 27 M EDTA required 12.11 mL to reach the murexide end point. Calculate the molarity of sulfide in the unknown.
Describe what is done in a displacement titration and give an Example
According to Appendix I, Cu2+ forms two complexes with acetate:
(a) Referring to Box 6-2, find K2 for the reaction
(b) Consider 1.00 L of solution prepared by mixing 1.00 × 10-4 mol Cu(ClO4)2 and 0.100 mol CH3CO2Na. Use Equation 12-16 to find the fraction of copper in the form Cu2+
Potassium ion in a 250.0 (±0.1) mL water sample was precipitated with sodium tetraphenylborate:
The precipitate was filtered, washed, dissolved in an organic solvent, and treated with excess Hg (EDTA)2-:
The liberated EDTA was titrated with 28.73 (±0.03) mL of 0.043 7 (±0.000 1) M Zn2+. Find [K+] (and its absolute uncertainty) in the original sample.
Spreadsheet equation for formation of the complexes ML and ML2.Consider the titration of metal M (initial concentration = CM, initial volume = VM) with ligand L (concentration = CL, volume added = VL), which can form 1:1 and 2 : 1 complexes:
Let αM be the fraction of metal in the form M, αML be the fraction in the form ML, and be the fraction in the form ML2. Following the derivation in Section 12-5, you could show that these fractions are given by
The concentrations of ML and ML2 are
because is the total concentration of all metal in the solution. The mass balance for ligand is
By substituting expressions for [ML] and [ML2] into the mass balance, show that the master equation for a titration of metal by ligand is
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