Titrimetry (Volumetric Methods) OCN 633 Fall 2013

Transcription

1 Titrimetry (Volumetric Methods) OCN 633 Fall 2013

2 Titrimetric Methods of Analysis Some of the oldest classical wet methods High accuracy and precision Analyte reacts with solution of known composition Applicable to a wide variety of analytes Originally manual, now automated

3 Titration The controlled addition of a measured volume of reagent of exactly known concentration to a known volume of a solution of analyte whose concentration is unknown

4 Requirements Known reaction stoichiometry Rapid reaction No side reactions Large change in some solution property at equivalence point Coincidence of equivalence and end points Quantitative reaction

5 Standards in Titrimetry Primary Standards - materials that can be weighed out exactly Secondary Standards - standardized against primary standards Standard Solutions - made up approximately, then standardized

6 Requirements of Primary Standard Highest purity High molecular/formula weight Stable to drying Readily available (and cheaply) Not deliquescent (Phenomenon of a substance absorbing so much moisture from the air that it ultimately dissolves in it to form a solution) or hygroscopic

7 Types of Titrimetric Methods and oceanographic applications Acid-base alkalinity Precipitation Chloride/Chlorinity Complexometric Ca, Ca + Mg + Sr Redox Winkler/DO

8 General Helpful Hints Need to know molarity and normality -concept of equivalence -all reactions occur on an equivalent basis! Use factor label system in all equations and calculations Review an Analytical Chemistry text if necessary

9 Normality/Equivalents Concentration based on idea that all substances react on an equivalency basis Definition of an equivalent dependent on type of reaction - acid/base produces/reacts with one mole of protons - precipitation one mole of univalent ion - redox one mole of electrons - complexometric one mole of substance forming complex Normality of a solution is the # eq/l

10 Examples of Normality HNO 3 H + + NO - 3 1M = 1N H 2 SO 4 2H + + SO 2-4 1M = 2N KMnO 4 must define the reaction (i.e., how many electrons involved in the redox) MnO 4- (aq)+ 8H + + 5e - Mn 2+ (aq) + 4H 2 O 1M = 5N MnO 4- (aq) + 4H + + 3e - MnO 2 (s) + 2H 2 O 1M = 3N AgNO 3 + NaCl AgCl + NaNO 3 1M = 1N 2AgNO 3 + K 2 Cr 2 O 7 Ag 2 Cr 2 O 7 + 2KNO 3 1M = 2N

11 Equivalent Weight Mass in grams of one equivalent of a substance Equals molecular/formula weight divided by # eq/mole Equivalent weight may be less, equal to or greater (latter is rare) than molecular/formula weight

12 Volumetric Relationships For any reaction, at equivalence V 1 N 1 = V 2 N 2 This is the basis of all calculations! For titrations we adapt V std N std = V unknown N unknown

13 Plot ph versus volume of titrant (base) added Near equivalence ph changes rapidly with small additions of base Equivalence point at ph = 7 End point should be nearest as possible to equivalence point Acid-Base Titrations

14 Weak Acid-Base Titrations Plot ph versus volume of titrant (base) added Note slope of ph change in first half of titration Have situation when part of titration curve involves a buffer Equivalence point at ph > 7

15 Effect of K a on Titration Strength of acid impacts titration curve Weaker acids more difficult (buffer effect) Hard to see end point as most indicators are weak acids(bases) too

16 End Points The flag that goes up to indicate that equivalence has been reached Should coincide with the equivalence point Usually triggered by a (relatively) large change in a property of the solution - change in ph of solution - Demf or Dcurrent flow in a solution - refractive index changes Usually enhanced by use of an indicator

17 Types of Indicators Acid/base: weak acid/base organic dyes Precipitation: other ion that forms insoluble substance with titrant Complexometric: other substance that forms complex with titrant (and is colored) Redox: other substance that undergoes redox reaction with titrant (or use electrometric detection)

18 Phenolphthalein Weak organic acid (C 20 H 14 O 4 ) Used as an acid-base indicator. Colorless in acid/pinkish in base Transition occurs around ph 9 Does not dissolve very well in water, Usually prepared in alcohol solution.

19 Transitions of Phenolphthalein Acid-base transition(s) Involves cleavage of ring, dissociation of two -OH groups, then hydroxylation Activates/triggers chromophore

20 Phenolphthalein End Point Weak acid/base Transition near ph 9 End point

21 Eriochrome Black T 3-Hydroxy-4-[(1-hydroxy-2-naphthalenyl)azo]-7- nitro-1-naphthalene-sulfonic Acid, Na Complexes Mg 2+ (pink) When all free Mg 2+ has been titrated, Mg 2+ EBT complex dissociates to allow formation of stronger Mg 2+ EDTA complex Free EBT indicator is light blue

22 Reaction of Halides with Ag + AgNO 3 + NaCl AgCl + Na + + NO 3 - Basis of Mohr method for detn. of CL in seawater CL = [Cl - ] + [Br - ] + [I - ] Use standardized solution of AgNO 3 as titrant Dissolved halides form insoluble compounds with Ag + Reactions are quantitative (because of low K sp of AgCl: 1.82 X 10-10, AgBr: 5.2X 10-13, and AgI: 8.3 X )

23 Mohr Titration Visual e.p.: K 2 Cr 2 O 7 /K 2 CrO 4 indicator forms brown Ag 2 CrO 4 precipitate Accuracy and Precision ~0.5% Includes Cl - (558 mm in standard seawater) Br - (0.86 mm in standard seawater) and I - (at most 0.2 µm in standard seawater). In extreme cases, Br - and I - can rise to 2 mm each Determination of Cl - should be corrected for Br - and I - (on a practical basis the corrections will always be less than 1%, or approximately twice the level of accuracy achievable by this method).

24 Mohr Titration Mohr titration with AgNO 3 using the is the preferred analytical method for CL, unless equivalent but slightly more precise Mohr titration with electrometric e.p. is used. Electrometric e.p. uses Ag + ISE E.p. detected on basis of Ag half reaction: Ag + + e - = Ag o Electronic e.p. ~0.1% accuracy-precision

25 Silver ion electrode Ag + + 1e - Ag o E = E o (0.059/n)log{ [products] / [reactants] } E = E o log 1/[Ag + ] E = E o log [Ag + ] Upon addition of excess AgNO 3, E rises sharply

26 Complexometric Titrations Derived from inorganic chemistry Basis is sharing of electron pair between metal and ligand complex formation Ligand(s) is(are) electron rich species Complex formation is a multi-step process Ni(H 2 O) Cl - <===> NiCl H 2 O

27 Step-wise Complexation Ni 2+ + Cl - NiCl + NiCl + + Cl - NiCl 2 NiCl 2 + Cl - NiCl 3 - NiCl Cl - NiCl 4-2 Each step is described by a K f Overall K is product of individual K s ß 1 = K 1 ß 2 = K 1 K 2 ß 3 = K 1 K 2 K 3 ß 4 =K 1 K 2 K 3 K 4

28 Complexometric Titrations Based on complexation of analyte with large multidentate molecule rather than step wise complexation with individual ligands Common functional groups that serve as individual ligands are e - rich Caboxylic acids, amines, imides, thio, sulfate

29 Stability of Chelates Complexes of multidentate ligands are more stable due to the chelate effect. Rings with five- or six-fold complexation are best. Entropy effects enhance stability M(H 2 O) L ML 6 + 6H 2 O 7 7 M(H 2 O) 6 + L ML + 6H 2 O 2 7 Greater entropy change for the latter reaction.

30 Electron Pair Sharing Carbamic or Aminoformic acid, NH 2 COOH

31 Amino Polycarboxylic Acids EDTA - Ethylenediamine tetraacetic acid Has 6 Lewis base sites (4 oxygen and 2 amine electron pairs) It is often designated as H 4 Y and its anion as Y 4- All six complexation sites are able to bind to a single metal ion for many metals.

32 Hexadentate complex Two amine groups Four acetate groups Very strong K s with most metal ions (esp. high + valency ions) Binding constants are f(ph) EDTA

33 Changes in EDTA Speciation as f(ph)

34 Formation Constants with EDTA Cation K MY Cation K MY K Ce x10 15 Na Al x10 16 Li x10 2 Co x10 16 Tl + 3.5x10 6 Cd x10 16 Ra x10 7 Zn x10 16 Ag + 2.1x10 7 Gd x10 17 Ba x10 7 Pb x10 18 Sr x10 8 Y x10 18 Mg x10 8 Sn x10 18 Be x10 9 Pd x10 18 Ca x10 10 Ni x10 18 V x10 12 Cu x10 18 Cr x10 13 Hg x10 21 Mn x10 13 Th x10 23 Fe x10 14 Fe x10 25 La+ 3.2x10 15 V x10 25 VO x10 15 Co x10 41

35 Ca & Mg Titration with EDTA

36 Cd Titration with EDTA Plot p(m) vs volume of EDTA (titrant) Use colorimetric e.p. Use Cd 2+ ISE E.p. is where pcd changes sharply

37 Other Amino Polycarboxylic Acids NTA - Nitrilotriacetic acid. Tetradentate (best suited for small metals) DETPA - Diethylenetriamine pentacetic acid. (works best for 8 coordinate metals such as the 3rd transition series and lanthanides) TTHA - Triethylenetetramine hexacetic acid. (for 10 coordinate metals such as the actinides)

38 Complexometric Titration of Ca (old color end point method) Ca 2+ is determined using a modification of the method by Tsunogai, Nishimura, and Nakaya (1968), Talanta, 15, p Uses ethylene bis(oxyethylene-nitrilo)-tetra acetic acid (EGTA) as titrant Uses 2,2'ethane-diylidine-dinitrilo-diphenol (GHA) as indicator The Ca(GHA) complex is quantitatively extracted into a layer of n-c 4 H 9 OH (pink color).

39 New Ca titration with EGTA Same as old method wrt basis of method End point measured with Ca ISE Large change in pca leads to large E change Precision of method vastly improved

40 Autotitrators you will use