The Determination of Ionization Constants: A Laboratory Manual

ISBN-13: 9789401089487 / Angielski / Miękka / 2011 / 218 str.

Adrien Albert

Opis

This practical manual is devised for organic chemists and biochemists who, in the course of their researches and without previous experience, need to determine an ionization constant. We are gratified that earlier editions were much used for this purpose and that they also proved adequate for the in- service training of technicians and technical officers to provide a Department with a pK service. The features of previous editions that gave this wide appeal have been retained, but the subject matter has been revised, extended, and brought up to date. We present two new chapters, one of which describes the determination of the stability constants of the complexes which organic ligands form with metal cations. The other describes the use of more recently introduced techniques for the determination of ionization constants, such as Raman and nuclear magnetic resonance spectroscopy, thermometric titrations, and paper electro- phoresis. Chapter 1 gives enhanced help in choosing between alternative methods for determining ionization constants. The two chapters on potentiometric methods have been extensively revised in the light of newer understanding of electrode processes and of the present state of the art in instrumen tation.

Spis treści

1 Introduction.- 1.1 What is meant by ‘ionization constants’?.- 1.2 Why do we determine ionization constants?.- 1.3 Brief summary of the chemistry of ionization.- 1.4 The nature of pKa values.- 1.5 The shape of a titration curve.- 1.6 Methods commonly used for determining ionization constants.- 1.7 What degree of precision is required?.- 1.8 The effect of temperature on ionization constants.- 1.9 Molality and molarity.- 2 Determination of Ionization Constants by Potentiometrie Titration using a Glass Electrode.- 2.1 Apparatus for general use.- 2.2 Preparation of solutions.- 2.3 Choice of concentration for the titration.- 2.4 Details of the titration method.- 2.5 Derivation and choice of equations for calculating pKa.- 2.6 Some typical titrations (worked examples).- 2.7 Precision and accuracy. Checking the precision obtained.- 2.8 Common sources of error, and their elimination.- 2.9False constants.- 2.10 Partly aqueous solvents.- 3 Refinements of Potentiometrie Titration: Apparatus and Calculations.- A Apparatus.- 3.1 Semi-micro titrations.- 3.2 Micro titrations.- 3.3 The rapid-flow method.- 3.4 The hydrogen electrode.- B Calculations.- 3.5 Monofunctional acids and bases.- 3.6 Method of calculation.- 3.7 Diacidic bases, dibasic acids and ampholytes.- 3.8 Overlapping ionization processes.- 3.9 Polyelectrolytes.- 3.10 Accuracy of the potentiometric method.- 3.11 Non-aqueous solvents.- 4 Determination of Ionization Constants by Spectrophotometry.- 4.1 Introduction.- 4.2 Apparatus.- 4.3 Buffers.- 4.4 Acidity functions.- 4.5 Preparation of the stock solution of the unknown.- 4.6 The search for the spectra of two pure ionic species.- 4.7 The choice of an analytical wavelength.- 4.8 Preliminary search for an approximate value of pKa.- 4.9 Exact determination of pKa.- 4.10 Worked examples.- 4.11 Activity corrections.- 4.12 Extensions of the spectrometric method.- (a)The pKa of a very weak acid (graphical treatment).- (b)Overlapping pKa values.- (c)Computer program for overlapping values.- 4.13 Errors, precision and accuracy.- 4.14 Common sources of error.- 4.15 Spectrophotometric determination of the pKa of a substance that lacks an absorption spectrum.- 4.16 A rapid method for the approximate measurement of pKa.- 5 Relations between Ionization and Solubility. Determination of Ionization Constants by Phase Equilibria.- 5.1 Ionization constants in preparative work.- 5.2 Prediction of solubility from ionization constants.- 5.3 Determination of ionization constants from solubilities.- 5.4 Determination of ionization constants from vapour pressure, by partitioning between a pair of solvents, or by other phase equilibria.- 6 Determination of Ionization Constants by Conductimetry.- 6.1 Scope of the method.- 6.2 Apparatus.- 6.3 Procedure.- 6.4 Refinements of calculation.- 7 Some Other Methods for the Determination of Ionization Constants.- 7.1 Raman spectrometry.- 7.2 Proton nuclear magnetic resonance.- 7.3 Nuclear magnetic resonance using other atoms.- 7.4 Thermometric methods.- 8 Zwitterions (Dipolar Ions).- 8.1 Zwitterions compared to ordinary amphoteric substances.- 8.2 How to distinguish zwitterions from ordinary ampholytes.- 8.3 Zwitterionic equilibria: macroscopic and microscopic constants.- 9 The Ionization Constants of Typical Acids and Bases.- A Organic Section.- 9.1 The oxygen acids (monobasic).- (a) Aliphatic carboxylic acids.- (b) Aromatic carboxylic acids.- (c) Aliphatic hydroxylie acids.- (d) Aromatic hydroxylie acids (phenols).- (e) Other oxygen acids.- 9.2 The oxygen acids (dibasic).- 9.3 Sulphur acids, nitrogen acids and carbon acids.- (a) Mercaptans.- (b) Nitrogen acids.- (c) Carbon acids.- 9.4 The nitrogen bases (monoacidic).- (a) Aliphatic bases.- (b) Aromatic and heteroaromatic bases.- 9.5 The nitrogen bases (diacidic).- 9.6 Carbinolamine bases.- 9.7 Oxygen bases and carbon bases.- 9.8 Amphoteric substances.- B Inorganic Section.- 9.9 Inorganic acids.- 9.10 Inorganic bases 164.- C Biologically-Active Substances.- 10 Chelation and the Stability Constants of Metal Complexes.- 10.1 The nature of chelation.- 10.2 Methods of calculation.- 10.3 Choice of ionic medium and the preparation of standard solutions.- 10.4 Measurement of pH and the calculation of pCH.- 10.5 Common difficulties and how they can be overcome.- 11 Appendices.- I An outline of the Brønsted-Lowry Theory.- II Comparison of classical and thermodynamic quantities.- III Calculations of hydrogen ion activity and concentration: also of hydroxyl ion activity and concentration.- IV Some effects of temperature on ionization constants.- V How percentage ionized may be calculated, given pKa and pH.- VI An outline of the theory of pH.- References.