CHAPTER 11: EDTA TITRATIONS

Transcription

1 Harris: Quantitative Chemical Analysis, Eight Edition CHAPTER 11: EDTA TITRATIONS

2 CHAPTER 11: Opener Ion Channels in Cell Membrane

3 Chapter 11.0 Ion Channels in Cell Membrane - Gramicidine A is made of 15 amino acids wound into a helix with a 0. nm- diameter channel through the middle. - The channel is lined by polar amide groups. Polar groups attract neighbering molecules by electrostatic forces. Na + and + pass through each hydrophilic pore at a rate of 10 7 ions/s. - Amide Oxygen atoms are spaced just right to replace waters of hydration from (H O) 6+. Th t id f i idi i d b l h d b d - The outside of gramicidin is covered by nonpolar hydrocarbons and are soluble inside the nonpolar cell membrane.

4 Fig EDTA (ethylenediaminetetraacetic acid) forms strong 1:1 complexes with most metal atoms. N N - EDTA : Detergents, Cleaning agents, Food additives (prevention of food oxydation) - EDTA complexes matals (Fe, Pb, Cu, Zn, etc.) so that they pass safely through wastewater treatment plants (Negative effect).

5 11-1 Metal Chelate Complexes - Chelate effect : chelating ligands (e.g., multidentate ligand) forms stronger complexes than a similar monodentate ligand.

6 Fig. 11- (a) Structure of adenosine triphosphate(atp) b) Possible structure of a metal-atp complex; M has four bonds to ATP and two bonds to H O ligands.

7 11-1 Metal Chelate Complexes -Formation constant ( f ) : the equilibrium constant for the reaction of a metal with a ligand /chelate (= stability constant) - Complexometric titration : a titration based on complex formation Example) Ligands other than NTA in Fig. 1- forms strong 1:1 complexes with most metals regardless of the charge on the metal ion.

8 Fig Structures of analytically useful chelating agents - NTA tends to form :1(ligand:metal) complexes with metal ions, whereas the others form 1:1 complexes.

9 11-. EDTA - EDTA is, by far, the most widely used chelator in analytical chemistry. - By direct titration or through an indirect sequence of reactions, virtually every element of the periodic table can be analyzed with EDTA. -Na H Y H O (the sodium salt) : A commonly used reagent

10 11-. EDTA (Acid-Base Properties) -EDTA is a hexaprotic system, designated H6Y+. The highlighted, acidic hydrogen Atoms are the ones that are lost upon metal-complex formation. - The first four p values apply to carboxyl protons, and the last two are for the ammonium protons. - The neutral acid is tetraprotic (H Y). -Na H Y H O (the sodium salt) : A commonly used reagent p 1 = 0.0 p = 1.5 p =.00 p = p 5 = 6.1 p 6 =

11 for each species : the fraction of EDTA in that forms. For example, Y- : Fraction of EDTA in the form Y - [Y [HY Y [H [H Y Y [H Y [H Y [H [Y α 5 6 Y [EDTA [Y α Y [EDTA : total concentration of all free EDTA, not complexed to metal ions Y [H [H [H [H ([H α ) H [

12 α Y [H Y [H Y [H Y [H Y [H Y [HY [Y 6 5 [Y

13 α Y [H Y [H Y [H Y [H Y [H Y [HY [Y 6 5 [Y

14 11-. EDTA Complexes n M [MY n+ + Y - MYn- f n [M [Y Formation constant ( f ) : the equilibrium constant for the reaction of a metal with a ligand /chelate (= stability constant) * f is defined for the reaction of Y - with M n+. But Y - is only one of the seven forms of EDTA. ** Whenever M n+ reacts with any forms of EDTA, the product is always MY n-!! <See Table 1-1, 1 compare the f values>

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16 11-. EDTA (Conditional Formation Constant) [Y α Y [EDTA f n [MY [MY n n [M [Y [M α Y n [EDTA Conditional formation constant t : (= effective formation constant) ' f α Y f n n [MY [M [EDTA M n+ + EDTA MY n- ' * is useful because it allows us to look at EDTA complex formation f as if the uncomplexed EDTA were all in one form ' f * is ph dependent. d [Y Y [H Y α Y [H Y [H Y [H [H Y [HY [Y 6 5

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18 Why was ionic strength indicated in Table 11-1? For example, Y- : Fraction of EDTA in the form Y - [Y [HY Y [H [H Y Y [H Y [H Y [H [Y α 5 6 Y [EDTA [Y α Y * is dependent on concentration of each EDTA species e g [Y - * Y- is dependent on concentration of each EDTA species, e.g., [Y which is dependent on [H + which in turn is dependent on ionic strength Y [H [H [H [H ([H α ) H [

19 Ch. 11- EDTA (EDTA Titration Curves) Fig EDTA titration illustrated for reaction of 50.0 ml of M M n+ with M EDTA. f = 1.15 x The concentration of free M n+ decreases as the titration proceeds. M n+ + EDTA MY n- Region 1: Before the Equivalence Point - There is excess M n+ left solution after the EDTA has been consumed. - The concentration of free metal ion is equal to the concentration of excess, untreated M n+. - The dissociation of MY n- is negligible. M n+ + EDTA MY n-

20 Ch. 11- EDTA (EDTA Titration Curves) Region : At the Equivalence Point - There is exactly as much EDTA as metal in the solution. - We can treat the solution as if it had made by dissolving pure MY n-. - Some free M n+ is generated by the slight dissociation of MY n- : MY n- M n+ + EDTA -EDTA refers to the total concentration of free EDTA in all of its forms. - At the equivalence point, [M n+ = [EDTA.

21 Ch. 11- EDTA (EDTA Titration Curves) Region : After the Equivalence Point - There is excess EDTA, and virtually all the metal ion is in the form MY n-. - The concentration of free EDTA can be equated to the concentration of excess EDTA added after the equivalence point. -Free metal concentration is calculated from the known free EDTA and MY n-

22 Fig Theoretical titration curves for the reaction of 50.0 ml of M metal ion with M EDTA at ph [MY n α Y f n [M [EDTA ' f

23 Fig Titration of Ca + with EDTA as a function of ph. - As the ph is lowered, the end point becomes less distinct. Why? n [MY α Y f n [M [EDTA ' f

24 ' α f Y f [M [MY n n [EDTA

25 Ch Auxiliary Complexing Agents Auxiliary Complexing Agents may be added i) to prevent M(OH) precipitation at high ph ii) to fix ph (NH + H + NH + ; buffer) iii) to complex metal ion and keep it in solution until EDTA is introduced.

26 Ch Auxiliary Complexing Agents M + L ML β 1 [ML [M[L M + L ML β [ML [M[L M + nl ML n β n [ML n n [M[L [M : free Metal concentration [ML n : Metal-Liquid Complex concentration n : overall or cumulative formation constants n [L : concentration of free Ligand

27 Ch Auxiliary Complexing Agents α M [M C M 1 β 1 n 1 [L β [L β n[l '' f C M : total concentration of all forms of M (M, ML, MLn) M : fraction of free metal ion n- n- [MY [MY M n+ + Y - MY n- f n [M [Y α C α [EDTA : The effective formation constant at a given fixed ph (Y-)( ) and given fixed concentration of auxiliary complexing agent (M). ' α f f α Y M M M M Y n- [MY αm CM [EDTA n- [MY C [EDTA n- [MY '' f C [EDTA C + [EDTA [MYn- M M

28 Ch Auxiliary Complexing Agents Remark: EDTA is much stronger complexing agent than NH, and all of EDTA added is bound to M n+. The greater the concentration of NH, the smaller the change of pm n+ near the equivalence point. * When an auxiliary ligand is used, its amount must be kept below the level that would obliterate the end point of the titration. '' f :Theeffective effective formation constant at a given fixed ph ( Y- ) and given fixed concentration of auxiliary complexing agent ( M ).

29 Fig Titration curves for the reaction of 50.0 ml of 1.00 x 10 - M Zn + with 1.00 x 10 - EDTA at ph in the presence of two different concentrations of NH * When an auxiliary ligand is used, its amount must be kept below the level that would obliterate the end point of the titration.

30 11-6 Metal Ion Indicators How to detect the end point in EDTA titrations. 1) Metal Ion Indicator (the most common technique). ) Mercury electrode (Fig. 1-8, see the next page) ) Ion selective electrode Metal Ion indicators : compounds whose color changes when they bind to a metal ion. For example, MgIn + EDTA MgEDTA + In (red) (colorless) (colorless) (blue) * Useful indicators must bind metal less strongly than EDTA does. * Because the color of free indicator is ph dependent, most indicators can be used only in certain range ph ranges in which color changes can be seen clearly.

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32 Fig Guide to EDTA titrations of common metals. Light color: ph region in which reaction with EDTA is quantitative. (Below the ph ranges indicated f is not large enough.) Dark color: ph region in which auxiliary complexing agent is necessary to prevent hydroxide Precipitation. Underline with the indicator name: Suitable ph range for each indicator