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Introduction
Introduction
Complexometric titrations are particularly useful for determination of a mixture of different metal ions in solution. One of the most widely used ligands—ethylenediaminetetraacetic acid (EDTA)—forms strong 1:1 complexes with many metal ions. The resulting metal–ligand complex, in which EDTA forms a cage-like structure around the metal ion, is very stable. The availability of a ligand that gives a single, easily identified end point made complexation titrimetry a practical analytical method for quantification of metal ions in solutions.
EDTA is a chelating agent that binds to metals through 4 carboxylic acids. Its formation constant for complexation is different for each metal, and because theligands are acids, the formation constants are also strongly dependent on pH. Metals always bind more strongly as pH increases because acidic hydrogens are removed from the EDTA.
EDTA is colourless whether or not it is bound to a metal ion. Endpoints for complexation titrations can be observed using metal ion indicators. The indicator itself is a complexing agent whereby the colour change depends on whether the indicator are in free form or bound ligands in a complex. As the colour of the metal ion indicators are pH dependent, most indicators can be used only in certain pH ranges. When enough EDTA has been added to complex all metal ions in solution, the EDTA begins to remove the remaining metal ions that are bound to the indicator, thus causing the indicator to undergo a colour change just at the equivalence point. It is important that the indicator must bind the metal ions less strongly than the EDTA.
Because of its wide applicability, EDTA lacks selectivity. 2 ways to increase the selectivity of EDTA is by controlling the pH of the medium and by utilising masking agents. Control of pH by buffer solutions may sometimes be used to enable two or more metal ions in a mixture to be titrated individually and successfully in the same solution. Metals with higher formation constants can be titrated at a lower pH. Masking agents protect some components of the analyte from reacting with EDTA. These reagents form complexes with interfering ions which are more stable than complexes formed with indicator or EDTA. For example, Al3+ in a mixture of Mg2+ and Al3+ can be measured by first masking the Al3+ with F−, thereby leaving only the Mg2+ to react with EDTA.
EDTA Techniques
EDTA Techniques
1- Direct titration: You simply add an indicator to a metal ion solution and titrate with EDTA, similar to a normal titration. Under the experimental conditions, the EDTA combines quantitatively with the metal cation to form the complex ion. Analyte is buffered to a pH at which the conditional formation constant for the metal-EDTA complex is large and the colour of the free indicator is distinctly different from that of the metal-indicator complex. The end point occurs when all the metal cation is bound by EDTA.
2- Back titration: An excess of EDTA is added to the metal ion solution and the mixture is titrated with a known concentration of a second metal ion. The second metal ion will form complex with the excess EDTA molecules until the endpoint, where all excess EDTA has been complexed. Addition of more second metal ion will then start to complex with the indicator, immediately changing the indicator's colour. The second ion has to form a weaker complex with EDTA than the analyte ion so that the second metal ion doesn't displace the analyte ion from its complex with EDTA. Back titration are used when metal-indicator complex is too strong, metal-EDTA complex forms too slowly or when the metal precipitates in the absence of EDTA.
Methodology
Methodology
Standardization of EDTA
Weigh out accurately 0.1500 g of granulated zinc metal and dissolve in a few drops of 1:1 nitric acid.
Rinse the watch glass and the resulting solution quantitatively into a 250 mL volumetric flask, make up to the mark with distilled water and mix well.
Measure out 25 mL of zinc solution into a 250 cm3 conical flask, add 2 drops of xylenol orange indicator solution and dilute to about 100 mL with distilled water.
Add hexamine solution (10% w/v) until the solution becomes red‐purple.
Titrate the solution with EDTA. At the end point, colour changes to a yellow‐orange.
Carry out the standardization in triplicate. Calculate the molarity of the EDTA solution.
Procedure for Determination of Bismuth and Lead (pH adjustment)
Pipette 10 mL of Unknown A solution into a 250 mL conical flask.
Determination of bismuth: Add 2 drops xylenol orange indicator solution and dilute to about 100 mL with distilled water. Titrate with standard EDTA solution until the colour changes from red‐purple to clear orange‐yellow.
Determination of lead: Add hexamine solution (10% w/v) slowly until the colour becomes red-purple. Continue the titration with standard EDTA solution until a clear orange yellow colour is obtained again.
Carry out the determination in triplicate and calculate the concentrations of the metal ions in g dm-3.
Procedure for Determination of Cadmium and Mercury (back titration)
Pipette 10 mL of Unknown B solution into 250 mL conical flask. Add 35 mL accurately measured excess of standard EDTA solution. After about 5 minutes, add some solochrome black T/KCI indicator mixture. Then add 10 mL ammonia‐ammonia chloride buffer (pH 10). Solution will turn dark blue.
Back titrate the excess EDTA solution with standard zinc solution (from the EDTA standardization) until the colour changes via blue to purple.
Determination of mercury: Add about 1 g of potassium iodide to the titrated sample to convert the mercury chelate to potassium mercuric iodide and thus liberating EDTA. Titrate the liberated EDTA with the standard zinc solution until a purple colour solution is obtained again. Carry out the determination in triplicate and calculate the concentrations of the metal ions in g dm-3.
Results
Results
Task
Task
Based on the results given, explain all the reactions involved using equations and calculate the concentration of all the metal ions in the mixtures in g dm-3.
Question:
Question:
What is the function of the ammonia buffer for the back titration?
What is the difference between direct titration and back titration?
What is the purpose of EDTA standardization?
What is the role of an indicator in titration?
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