Podcast
Questions and Answers
What must be true for the M-EDTA complex during direct titration?
What must be true for the M-EDTA complex during direct titration?
In back titration, what is added after the excess of EDTA?
In back titration, what is added after the excess of EDTA?
What could be a reason for choosing back titration over direct titration?
What could be a reason for choosing back titration over direct titration?
When calculating the concentration of Ni2+ in the provided back titration example, what was the total moles of EDTA used?
When calculating the concentration of Ni2+ in the provided back titration example, what was the total moles of EDTA used?
Signup and view all the answers
What must be true during the reaction between metal and EDTA in a back titration?
What must be true during the reaction between metal and EDTA in a back titration?
Signup and view all the answers
What role does the auxiliary reagent play in titrations?
What role does the auxiliary reagent play in titrations?
Signup and view all the answers
What does the final concentration of Ni2+ in the unknown sample equal?
What does the final concentration of Ni2+ in the unknown sample equal?
Signup and view all the answers
If an analyte reacts too slowly with EDTA, what should be done?
If an analyte reacts too slowly with EDTA, what should be done?
Signup and view all the answers
What is the primary purpose of using an indicator in a titration involving EDTA?
What is the primary purpose of using an indicator in a titration involving EDTA?
Signup and view all the answers
What is the purpose of controlling the pH when using EDTA in titrations?
What is the purpose of controlling the pH when using EDTA in titrations?
Signup and view all the answers
Which of the following metal ions forms stable complexes with EDTA at a pH of 1-3?
Which of the following metal ions forms stable complexes with EDTA at a pH of 1-3?
Signup and view all the answers
Which reducing agent is mentioned as effective for removing the interference of Hg2+ in EDTA titrations?
Which reducing agent is mentioned as effective for removing the interference of Hg2+ in EDTA titrations?
Signup and view all the answers
How can masking agents aid in EDTA titrations?
How can masking agents aid in EDTA titrations?
Signup and view all the answers
At which pH range do alkaline earth metals form stable complexes with EDTA?
At which pH range do alkaline earth metals form stable complexes with EDTA?
Signup and view all the answers
What happens to Cu2+ ions when ascorbic acid is used in the medium?
What happens to Cu2+ ions when ascorbic acid is used in the medium?
Signup and view all the answers
What is a demasking agent's primary function in metal ion titrations?
What is a demasking agent's primary function in metal ion titrations?
Signup and view all the answers
Which pH is ideal for the stable complex formation of divalent metals such as Ni2+ and Copper in EDTA titrations?
Which pH is ideal for the stable complex formation of divalent metals such as Ni2+ and Copper in EDTA titrations?
Signup and view all the answers
What does the notation $Kf ’ = Kf x αY4-$ represent in relation to EDTA complex formation?
What does the notation $Kf ’ = Kf x αY4-$ represent in relation to EDTA complex formation?
Signup and view all the answers
Which ion can be demasked using formaldehyde in acetic acid?
Which ion can be demasked using formaldehyde in acetic acid?
Signup and view all the answers
Which of the following is NOT a characteristic of metal indicators used in complexometric titrations?
Which of the following is NOT a characteristic of metal indicators used in complexometric titrations?
Signup and view all the answers
Which of the following statements correctly describes the formation of a complex in complexometric titration?
Which of the following statements correctly describes the formation of a complex in complexometric titration?
Signup and view all the answers
Why are multidentate ligands more stable than unidentate ligands in complex formation?
Why are multidentate ligands more stable than unidentate ligands in complex formation?
Signup and view all the answers
What is the major advantage of using EDTA in complexometric titrations?
What is the major advantage of using EDTA in complexometric titrations?
Signup and view all the answers
Why is the conditional formation constant (Kf') used in EDTA titrations?
Why is the conditional formation constant (Kf') used in EDTA titrations?
Signup and view all the answers
Which of the following statements is TRUE regarding the use of Eriochrome Black T (EBT) as an indicator in complexometric titrations?
Which of the following statements is TRUE regarding the use of Eriochrome Black T (EBT) as an indicator in complexometric titrations?
Signup and view all the answers
What is the primary reason for using a chelating agent in complexometric titrations?
What is the primary reason for using a chelating agent in complexometric titrations?
Signup and view all the answers
Which of the following is NOT a type of ligand based on the number of lone pairs it can donate?
Which of the following is NOT a type of ligand based on the number of lone pairs it can donate?
Signup and view all the answers
Why does the equivalence point of a complexometric titration typically occur at a 1:1 ratio of metal ion to EDTA?
Why does the equivalence point of a complexometric titration typically occur at a 1:1 ratio of metal ion to EDTA?
Signup and view all the answers
What is the primary function of a metal indicator in a complexometric titration?
What is the primary function of a metal indicator in a complexometric titration?
Signup and view all the answers
Study Notes
Complexometric Titration
- Complexometry is the formation of slightly ionized complexes in solution
- A complex is formed by a reaction of a metal ion (electron acceptor) and a ligand (electron donor)
- Chelating agents form strong 1:1 complexes with metal ions
- A chelating agent is used to titrate metal ions in solution, known as complexometric titration.
- Different titration types use different electron donors and acceptors.
Ligands
- Ligands are molecules that can form coordinate bonds with metal ions
-
Unidentate ligands have one lone pair
- Examples include NH3, Cl-, Br-
-
Bidentate ligands have two lone pairs
- Examples include ethylene diamine
-
Multidentate ligands have more than two lone pairs
- Examples include EDTA (ethylenediaminetetraacetic acid)
- Multidentate ligands create more stable complexes.
Multidentate Ligands
- Stability increases with the use of multidentate ligands
- Ethylenediamine forms a stable complex with cadmium with a very large formation constant
- Other multidentate ligands such as methylamine also form complexes with various metals.
Ethylenediaminetetraacetic Acid (EDTA)
- EDTA forms very stable complexes with many metal ions. This is a "cage" around the metal ions
- It's a hexadentate ligand, meaning it has six donor atoms that can coordinate with metal ions.
- The donor atoms are from two amine groups and four carboxylate groups.
- EDTA can exist in different protonated forms, and its ability to form complexes is greatly affected by the pH
EDTA (Properties of pK's)
- EDTA is a hexaprotic weak acid
- Different forms of EDTA exist in solutions
- Various protonated and deprotonated forms, i.e., Y4-, HY3-, H2Y2-, etc.
- The fraction of each form depends on the pH
- Specific pKa values for the different protonations of EDTA
Formation Constant (Kf)
- Kf is the equilibrium constant for the formation of a metal-EDTA complex.
- The higher the value of Kf, the more stable the complex.
- For a +1 cation—Ag+ + Y4- → AgY3-
- For a +2 cation—Hg2+ + Y4- → HgY2-
- For a +3 cation—Fe3+ + Y4- → FeY-
- For a +n cation—Mnn+ + Y4- → MYn-4
- Kf= [MYn-4]/[Mn+][Y4-]
Table of Formation Constants
- A table showing log Kf values of different metal ions with EDTA
- These values represent the stability of the complexes
Conditional Formation Constant
- Most EDTA is not in Y4- form below pH 10.37
- Kf treats free EDTA in one form for complex formation
- Kf' = Kf αY4- = [MYn-4]/[Mn+][EDTA]
- αY4- accounts for the different forms of EDTA at varying pH values.
Important Features of EDTA
- Formed complexes are stable and water-soluble
- Instantaneous complex formation with metals in 1:1 ratio.
- Equivalence point is easily measured.
- Complex formation or dissociation is affected by the pH of the solution.
Metal Indicators
- Organic dyes that form chelates with metal ions
- Free form of the indicator and metal-indicator complex have different colors
- The reaction between the metal ion and indicator is reversible
- Indicator complex is less stable than the metal-EDTA complex
- The color change depends on the pH
Eriochrome Black T (EBT)
- EBT is blue in free form, and forms a wine-red complex with metals at neutral pH
- Commonly used for Mg2+, Pb2+, and Zn2+
- Can't be used to determine Cu2+, Fe2+, Al3+, Co2+, or Ni2+
Direct Titration
- Similar to acid-base titrations
- M-EDTA complex must be more stable than the M-indicator complex
- Reaction must happen in a buffered medium
- Metal precipitation must be catalyzed for a rapid reaction. Auxiliary reagents (ammonia, citric or tartaric acid) are used to prevent precipitation
Back Titration
- Known excess of EDTA is added
- The excess EDTA is then titrated with a standard solution of a second metal ion.
- Three situations for back titration:
- Analyte precipitates in the absence of EDTA
- Analyte reacts too slowly with EDTA
- Analyte blocks the indicator
EDTA Titration of Mixtures
- EDTA is not selective
- EDTA selectivity can be increased by controlling the pH and the oxidation state of the metal ions, by using masking and demasking agents.
pH Effect
- Log Kf values for metal-EDTA complexes vary with the pH.
- The minimum pH for various metals to form complexes needs to be considered.
Control pH of Medium
- Trivalent / tetravalent cations (Fe3+, Th4+, Bi3+) and Hg2+ form stable complexes at low pH (1-3), while divalent metals (Co2+, Ni2+, Cu2+, Zn2+, Pb2+, Cd2+) form stable complexes at moderate pH (4-6) and alkaline earth metals (Ba2+, Ca2+, Sr2+) and Mg2+ are suitable at pH 10 using ammonia buffer.
Adjust Oxidation Number of Metal Ion
- Interference between metal ions of the same pH group is solved by adjusting the oxidation number of different metal ions. Ascorbic acid can be used as a reducing agent to reduce these metal ions.
Masking and Demasking Agents
- Masking agents are used to prevent interference by other metal ions
- They strongly form metal complexes
- Demasking agents break down complexes formed by masking agents. Cyanide complexes of Cd, Cu, Fe, Zn can be broken by formaldehyde in acetic acid
Masking Agent Table
- A table showing masking agents and the species they can mask.
Studying That Suits You
Use AI to generate personalized quizzes and flashcards to suit your learning preferences.
Related Documents
Description
This quiz explores the principles of complexometric titration, focusing on the formation of complexes between metal ions and ligands. It covers the types of ligands including unidentate, bidentate, and multidentate, as well as their roles in stabilizing metal complexes. Test your understanding of these concepts and their applications in analytical chemistry.
