Ch. 13 EDTA Titrations

Transcription

1 Ch. 13 EDTA Titrations 1 Chelation in Biochemistry Chelating ligands can orm complex ions with metals through multiple ligands. This is important in many areas, especially biochemistry. 2 1

2 Metal-Chelate Complexes Metals are Lewis acids that accept electron pairs rom donating ligands that act as Lewis bases CN - is a common monodentate ligand, binding to a metal ion through one atom (C) Metals can bind to multiple ligands (usually 6) A ligand that can attach to a metal by more than one atom is multidentate or a chelating ligand Chelating agents can be used or titration o metals to orm complex ions (complexometric titration) 3 Chelating Agents in Analytical Chemistry 4 2

3 Ethylenediamenetetraacetic acid (EDTA) EDTA orms 1:1 complexes with metal ions by with 6 ligands: 4 O & 2N. EDTA is the most used chelating agent in analytical chemistry, e.g. water hardness. 5 Acid/Base Properties o EDTA EDTA is a hexaprotic system (H 6 2+ ) with 4 carboxylic acids and 2 ammoniums: pk 0.0 O O OH NH + OH O NH + O OH pk pk pk pk pk We usually express the equilibrium or the ormation o complex ion in terms o the 4- orm (all six protons dissociated). ou should not take this to mean that only the 4- orm reacts OH

4 Fraction o EDTA in 4- Form Similar to acids and bases, we can deine ractional compositions, α, deined as the raction o ree EDTA in a particular orm. Free means uncomplexed EDTA So, or 4- : H 6 H 5 H 4 H 3 H 2 H 4- EDTA 7 EDTA Complexes The equilibrium constant or a reaction o metal with EDTA is called the ormation constant, K, or the stability constant: M n4 M n M n 4 n4 M K 4 Again, K could have been deined or any orm o EDTA, it should not be understood that only the 4- reacts to orm complex ion. 8 4

5 ph Dependence o α 4-9 Formation Constants or M-EDTA Complexes 5

6 Some Metals Form 7 or 8 Coordinate Complexes The rings ormed in the M-EDTA complex can become strained. I the oxygen atoms pull back toward the nitrogen atoms, the strain is relieved. This opens up the metal to other ligands. Water molecules requently occupy these sites. 11 Conditional Formation Constant We saw rom the raction plot that most o the EDTA is not in the orm o 4- below a ph ~. We can derive a more useul equilibrium equation by rearranging the raction relationship: EDTA 4 EDTA n4 n4 M M K n 4 n M M 4- EDTA I we ix the ph o the titration with a buer, then α 4- is a constant that can be combined with K n M 4 K ' 4 4- K M n EDTA M n K ' 4K n M EDTA 12 6

7 Example Calculate the concentration o ree Ca 2+ in a solution o 0. M Ca 2- at ph and ph 6. K or Ca 2- is 4.9x (Table 13-2) 2 2 Ca EDTA Ca K ' K at ph.00, K at ph 6.00, K ' ' Conc Conc K 4 K i 4 Ca (0.36)(4.9 ( Ca 0.1x K ' 2 6 Ca 2 x Ca 2.4 EDTA x At low ph, the metal-complex is less stable )(4.9 EDTA Ca 0.1 x x 0.1- x ) ) Calcium/EDTA Titration Curve For calcium, the end point becomes hard to detect below ~ph=8. The ormation constant is too small below this point. This can be used to separate metals. At ph=4, Ca does not perorm signiicant complexaion with EDTA. However, Fe can still orm the complex, so it can be titrated without intererence rom Ca. 14 7

8 Generic Titration Curve Like a strong acid/strong base titration, there are three points on the titration curve o a metal with EDTA: beore, at, and ater the equivalence point. We ll consider a titration where we have 50.0 ml o M Ca 2+ (buered at ph=) with M EDTA. V e =25.0 ml K ' ( 0.36)(4.9 ) Beore the Equivalence Point What s pca 2+ when we have added 5.0 ml o EDTA? Ca 2 (0.040) M Fraction Remaining Initial Concentration Dilution Factor pca 2 log(0.0291)

9 At the Equivalence Point What s pca 2+ when we have added 25.0 ml o EDTA? At the equivalence point almost all the metal is in the orm Ca Ca 2- (0.040) M 75.0 Initial Concentration Dilution Factor Free Calcium is small and can be ound w/ algebra Conc Conc i 2 2 Ca EDTA Ca 2 Ca x K ' Ca EDTA x x x x 6 x 1.2 M pca 2 log(1.2 6 ) Ater the Equivalence Point What s pca 2+ when we have added 26.0 ml o EDTA? We have 1.0 ml excess EDTA EDTA (0.080) 1.05 M Initial Concentration Dilution Factor Ca (0.040) 2.63 M Initial Concentration 76.0 Dilution Factor 2 Ca Ca EDTA Ca 2 ' K (1.05 ) Ca pca M 18 9

10 Auxiliary Complexing Agents In aqueous solution, metal-hydroxide complexes or precipitates can orm, especially at alkaline ph We oten have to use an auxiliary complexing agent This is a ligand that binds strongly enough to the metal to prevent hydroxide precipitation, but weak enough to be displaced by EDTA Ammonia is a common auxiliary complex or transition metals like zinc 19 Metal Ion Indicators To detect the end point o EDTA titrations, we usually use a metal ion indicator or an ion-selective electrode Metal ion indicators change color when the metal ion is bound to EDTA: MgEbT EDTA (Red) (Clear) MgEDTA EbT (Clear) Eriochrome black T is an organic ion (Blue) The indicator must bind less strongly than EDTA 20

11 Metal Ion Indicator Compounds 21 EDTA Titration Techniques Direct titration: analyte is titrated with standard EDTA with solution buered at a ph where K is large Back titration: known excess o EDTA is added to analyte. Excess EDTA is titrated with 2nd metal ion

12 EDTA Titration Techniques (2) Displacement titration: For metals without a good indicator ion, the analyte can be treated with excess Mg(EDTA) 2-. The analyte displaces Mg, and than Mg can be titrated with standard EDTA Indirect titration: Anions can be analyzed by precipitation with excess metal ion and then titration o the metal in the dissolved precipitate with EDTA. 23 Example Titration 25.0 ml o an unknown Ni 2+ solution was treated with ml o M Na 2 EDTA. The ph o the solution was buered to 5.5 and than back-titrated with ml o M Zn 2+. What was the unknown Ni 2+ M? mol EDTA mol Zn 2 Zn 2 4- Zn (25.00 ml)( M) 1.32 mmol EDTA (17.61mL)( M) mmol Zn 2-2 Ni 2 4- Ni 2- mol Ni mmol EDTA mmol Zn mmol M Ni 2 (0.916 mmol)/(25.00 ml) M 24 12