Complexometric Titration

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1 The Application of Complexometric Titration An application method of Inorganic Pharmaceutical Analysis Lecturer : Dr. Tutus Gusdinar Pharmacochemistry Research Group School of Pharmacy INSTITUT TEKNOLOGI BANDUNG

2 H 4 Y tetraprotic acid MY chelate

3 Complexometric titration curve is plotted of pm versus volume of titrant, by calculated from pm = - log [M + ] This titration curve can be used for the purpose of : 1) Determination of titration feasibility 2) Choose an appropriate metal indicator Some of technical application of the titration are : 1) Direct titration 2) Back titration 3) Substitutive titration 4) Indirect titration 5) Selective or specific titration

4 COMPLEXOMETRIC CURVE Calculate such metal ion potent as : 1) pm before the titration 2) pm before (at neighbourhood h of) the Equivalent Point 3) pm at the Equivalent Point 4) pm just after the Equivalent Point Example : ph = 12 ph = 10 ph = 8 pca Metal ionic buffer ml of EDTA 50

5 COMPLEXOMETRIC TITRATION FEASIBILITY The Effective Equilibrium Constant (K eff is as K required in the reaction) for the most feasible titration is calculated from stoichiometric reaction. Example 50 ml of M 2+ metal ion solution of 0.01 M is titrated with Na 2 EDTA solution of 0.01 M. Calculate K eff after ml of Na 2 EDTA addition where pm changed by 2 units when 2 drops (0.10 ml) of Na 2 EDTA titrant addition.

6 Before the titration, EDTA is 05mmoles 0.5 At one drop before the Equivalent Point, EDTA will be mmoles, and the excess of mmoles of M 2+ (in total volume of ml) is 5 x 10-6 MorpM= But if ΔpM is 2.0 units the pm = 7.30 or [M 2+ ]= 5 x 10-8 M, occurred after ml of titrant t addition. C Y = (0.05 x 0.01)/ = 5 x 10-6 M. [MY 2- ] = 0.5 mmoles/100 ml = 5 x 10-3 M. hence K eff = 5x10-3 /(5x10-8 )(5x10-6 ) = K = 2 x 10-3 M What is the K eff if the ΔpM is only 1.0 unit?

7 FEASIBILITY OF COMPLEXOMETRIC TITRATION If the K for a feasible titration was known, we can calculate ph minimum in which the titration is feasible. For example : Zn 2+ titrated with Na 2 EDTA, log K ef > 8.0. Read from Data Table : log K abs of ZnY 2- = log K ef = log K abs + log α 4 Then K eff at any ph can be calculated : K eff = 8.06 at ph = 4 K eff = at ph = 5 The titration will be feasible when ph = 4

8 METAL ION INDICATOR Metal ion indicator is a dyestuff forming a complexed compound with a metal ion, having different colour of free from a non complex indicator species. Metal indicators are generally acid-base indicator. Acid-base equilibrium must be calculated at any type of metal ion indicators. example : 1) Eriochrom Black T (or EBT) 2) Calmagite 3) Murexide 4) Calcon

9 pmg Complexometric curve of Calmagite In 3- pca Reddish 8 orange 8 HIn - red HIn 2- blue H - 2 In HIn 6 red blue 6 2- In3- Redish orange 4 Mg(OH) 2 White 4 MgIn - precipitate CaIn Red - red 2 2 Ca(OH) 2 White precipitate ph ph

10 Complexometric Curve of Mg-Calmagite 9 8 IV III HIn 7 H 2 In - 2- blue red 6 pka H 2 In - = 8.1 n 2- = 12,4 pka HI II In 3- Reddish orange pmg 5 Eq Pt =5,26 End Pt =5,7 4 I MgIn - red Mg(OH) 2 White precipitate ph

11 BUFFER PROPERTIES OF METAL IONS M + L ML metal ion Ligand Complex If C L = ligand analytical concentration, then K eff = (ML)/(M).C L [M] = 1/K eff. [ML]/C L log [M] = log 1/K eff + log (ML)/C L pm = log K eff - log [ML]/C L pm = log K eff + log C L /(ML) (similar to Henderson-Hasselbalch) When pm fixed to K eff and metal ion complex molar ratio to forming ligand concentration

12 COMPLEXOMETRIC TITRATION ERROR titration Error = + ΔpM atau + ΔpL (caused of ΔpM M = - ΔpL) % Titration Error = 100 x {[L] final [M] final } / C M C M = analytical concentration of metal ion total in solution ΔpM = pm final pm equivalent or [M] final = [M] equivalent. 10 -ΔpM and Due to [L] final = [L] equivalent.10 -ΔpL [M] equivalent = [L] equivalent then % Titration Error = 100 [M] equivalen. {10 ΔpM 10 -ΔpM } / C M = 100. {10 ΔpM 10 -ΔpM }/ C M.K M L if ΔpM < 0,4 (small) then % titration error = ΔpM / C M.K M L

13 SELECTIVITY OF COMPLEXON Complexon selectivity can be maintained by : 1. Adjustment of ph (buffering) 2. Masking 3. Demasking 4. Separation 5. Extraction 6. Anionization 7. Kinetic Masking 8. Metal ion indicator reaction

14 1) Selectivity due to ph Mixture of Bi 3+ and Th 4+ : Bi 3+ is titrated in acid solution (ph=2) using indicator Xyline Orange + Methylthymol Blue Mixture of Bi 3+ and Pb 2+ can be titrated at ph=2 using indicator Xyline Orange, then histamine was added up to ph=5 for enabling titration of Pb 2+

15 2) Selectivity due to masking Masking does not result a physical reaction and not change the initial state By masking, some of cation mixture can not react with EDTA and indicator. The effective masking agent such as CN - forms complex compound with Cd 2+, Zn 2+, Hg 2+, Cu 2+, Co 2+, Ni 2+, Ag +, but does not react with earth alcalies (Mg 2+, Pb 2+, Ca 2+, Sr 2+ ), then metal ion mixture can be titrated with Ca 2+, Pb 2+, Mn 2+, Mg 2+

16 3) Selectivity due to demasking Demasking is a process where masked compounds are not able to change the initial state Cd 2+ and Zn 2+ which is masked with CN - can be demasked with chloralhydrate or mixture of formaldehyde + acetic acid (3:1) until complexed of [Zn(CN) 4 ] 2+ can release its Zn 2+. This technique is usually used for determining a mixture of Mg 2+, Cu 2+, Zn 2+.

17 4) Selectivity due to separation Precipitation of Ni 2+ as Ni-DMG (DMG=dimethylglyoxime) or Cu 2+ as CuCNS 5) Selectivity due to extraction Mixture of Cu 2+ and Zn 2+ or Pb 2+ and Zn 2+ with NH 4 CNS forms Zn-thiocyanate which can be extracted with isobuthylmethylketon. After dissolution of the extract in water, then Zn 2+ can be titrated with Complexon III

18 6) Selectivity due to anionization Ortho-phosphate ion can be separated by using anion exchange resin 7) Selectivity due to kinetic masking One metal ion is changed to inert, then one reaction is slow but another react fastly. Such as titration of Cr 3+ with Na 2EDTA

19 8) Selectivity due to reaction of metal ion with its indicator Metal ion indicator should 1. React to yield a sharp colour change 2. React selectivelly of specifically with metal ion 3. Yield a stable metal-indicator complex compound and result an intensive i colour due to a stability of M-Ind < M-EDTA 4. ph adjusted at the feasible e titration condition o ph= ph= H 2 In - HIn 2- In 3- red blue orange yellow ph= Have a distinct colour of free indicator from a complex of Metal Indicator.

20 ERIOCHROME BLACK T (EBT) metal ion indicator a chelate complex formed by releasing a proton from phenolic group, then metal ion forms a covalent bond with an oxygen atom and a coordinative covalent bond with a nitrogen atom EBT is a triprotic acid molecule, H 3 In, releasing H + from a sulphonic group independent d to ph of the solution, to form H 2 In - (red). This indicator forms a grave red colour stable complex (1:1) 1) with Mg 2+, Ca 2+, Zn 2+, Ni 2+ Titration of Na 2 EDTA should be performed at ph=8-10 where the EBT is in HIn 2- (blue) form.

21 CALMAGITE (a stable form of EBT) Properties and reaction are similar il to those of EBT : H 3 In H 2 In - + H + (red) H 2 In - HIn 2- + H + pka 2 = 8,1 (blue) (pka 2 EBT =6,3) 63) HIn - In 3- + H + pka 3 =12,4 (orange) (pka 3 EBT =11,6) Mg 2+ + HIn 2- MgIn - + H + (blue) (red) A sharp colour change from blue to red, titration should be at ph=8-11.

22 OH HO - O 3 S N N NO 2 Eriochrome Black T H 3 In 3

24 OH HO - O 3 S N N CH 3 Calmagite H 3 In 3

Chapter 12, & 6.4 Complexation chemistry & EDTA titrations

Chapter 12, & 6.4 Complexation chemistry & EDTA titrations First Section 6.4, an introduction to complex chemistry. The strange behavior of PbI 2 solubility: PbI 2 Pb 2+ + 2I - K sp = [Pb 2+ ][I - ] 2