Ionisation constant

In contrast, equilibrium properties have been successfully discussed in terms of the field effect. Notable instances are those of the ionisation constants of saturated dibasic acids, - and of carboxyl groups held in... 

In applying this criterion, obs. must be compared with calc, for the same temperature. In general this entails knowledge of the temperature dependence of the relevant acidity function and of the ionisation constant. The latter factor has sometimes been allowed for (as in the calculation of calc, for the nitration of 2,4,6-trimethylpyridine in 98 % sulphuric acid at 80 °C) by using the approximate relationship, -d pKf) dT = (p, -o-9)/T. 

Quantities, Wiley-Interscience, New York, 1976 D. D. Perrin, Ionisation Constants of Inorganic Acids and Bases in Aqueous Solution, 2d ed., Pergamon Press, 1982. 

The nitro isomer is weaMy acidic the nitronic acid isomer (aci form) is much more acidic. A comparison of the ionisation constants of the two forms ki water at 25°C is given ki Table 4. 

Anhydrous oxaUc acid is very soluble in polar solvents. The ionisation constant is comparable with those of many mineral acids and is exceeded only by those of a few organic acids is approximately the same as the ionisation constant of bensoic acid (see Table 1). 

With concentrated sulfuric acid, anthraquinone forms oxonium salts, thus falling into a class of compounds known as " oxygen bases" (15). In an aqueous solution, anthraquinone has an ionisation constant equal to 7 x 10 and a of 7.4 (16). 

The equihbrium expressions for the hydrolysis reactions (eq. 1) foUow and are the ionisation constants of HOX and water, respectively. 

Acids, bases and amphoteric substances can be purified by taking advantage of their ionisation constants. Ionisation constants and pK. 

When substances ionise their neutral species produce positive and negative species. The ionisation constants are those constant values (equilibrium constants) for the equilibria between the charged species and the neutral species, or species with a larger number of charges (e.g. between mono and dications), l ese ionisation constants are given as pK values where pK = -log K and K is the dissociation constant for the equilibrium between the species [Albert and Serjeant The Determination of Ionisation Constants, A Laboratory Manual, 3rd Edition, Chapman Hall, New York, London, 1984, ISBN 0412242907]. 

E. P. Serjeant and B. Dempsey, Ionisation Constants of Organic Acids in Aqueous Solution, Pergamon Press, Oxford, 1979. 

Ionisation constants of ionisable compounds are give as pK values (published from the literature) and refer to the pKa values at room temperature ( 15 C to 25 C). The values at other temperatures are given as superscripts, e.g. pK for 25 C. Estimated values are entered as pKgjt (see Chapter 1, p. 7 for further information). 

Provides puriilcation procedures for commercially available chemicals and biochemicals includes an extremely useful compilation of ionisation constants... 

New to this edition, the ionisation constants in the form of pK have be i entered for lonisable (x>m-pounds. These are fotIow l by procedures, us l to purify the substances, in most cases to analytical purily. An index of CAS Reglkry Numbers with the reflective page numhera of the entries has been added as well, making it easy to locate any substance irrespective of which chapter it is In, and also rapidly telling the reader whether there is a purification procedure for that substance In this book. 

A large number of substances are ionisable in aqueous solutions and a knowledge of their ionisation constants, stated as pK (pKa) values, can be of importance not only in their purification but also in their reactivity. Literature values of the pK s have been inserted for ionisable substances, and where values could not be found they were estimated (pKesi)- The estimates are usually so close to the true values as not to affect the purification process or the reactivity seriously. The book will thus be a good compilation of pK values for ionisable substances. 

A. Albert and E. Serjeant, Ionisation Constants, a Laboratory Manual. Methuen, London, 1962. 

Ionisation constants for water and iweak electrolytes and variation with temperature... 

B. Ionisation constants of weak electrolytes and their temperature variation ... 

Note. All values are with reference to the molarity scale. Data for bases are expressed as acidic ionisation constants e.g. for ammonia we quote pK at =9 245 for the ammonium ion... 

The typical strong acid of the water system is the hydrated proton H30+, and the role of the conjugate base is minor if it is a sufficiently weak base, e.g. Cl-, Br-, and C104. The conjugate bases have strengths that vary inversely as the strengths of the respective acids. It can easily be shown that the basic ionisation constant of the conjugate base KR canj is equal to Kw/KA conj, where Kw is the ionic product of water. 

K is the equilibrium constant at a particular temperature and is usually known as the ionisation constant or dissociation constant. If 1 mole of the electrolyte is dissolved in Vlitres of solution (V = l/c, where c is the concentration in moles per litre), and if a is the degree of ionisation at equilibrium, then the amount of un-ionised electrolyte will be (1 — a) moles, and the amount of each of the ions will be a moles. The concentration of un-ionised acetic acid will therefore be (1 — a)/ V, and the concentration of each of the ions cl/V. Substituting in the equilibrium equation, we obtain the expression ... 

Interionic effects are, however, not negligible even for weak acids and the activity coefficient product must be introduced into the expression for the ionisation constant ... 

From the point of view of quantitative analysis, sufficiently accurate values for the ionisation constants of weak monoprotic acids may be obtained by using the classical Ostwald Dilution Law expression the resulting constant is sometimes called the concentration dissociation constant . 

For very weak or slightly ionised electrolyes, the expression a2/( 1 — a) V = K reduces to a2 = KV or a = fKV, since a may be neglected in comparison with unity. Hence for any two weak acids or bases at a given dilution V (in L), we have a1 = y/K1 V and a2 = yjK2V, or ol1/ol2 = Jk1/ /K2. Expressed in words, for any two weak or slightly dissociated electrolytes at equal dilutions, the degrees of dissociation are proportional to the square roots of their ionisation constants. Some values for the dissociation constants at 25 °C for weak acids and bases are collected in Appendix 7. 

The hydrolysis constant is thus related to the ionic product of water and the ionisation constant of the acid. Since Ka varies slightly and Kw varies considerably with temperature, Kh and consequently the degree of hydrolysis will be largely influenced by changes of temperature. 

Diphenylcarbazide as adsorption indicator, 358 as colorimetric reagent, 687 Diphenylthiocarbazone see Dithizone Direct reading emission spectrometer 775 Dispensers (liquid) 84 Displacement titrations 278 borate ion with a strong acid, 278 carbonate ion with a strong acid, 278 choice of indicators for, 279, 280 Dissociation (ionisation) constant 23, 31 calculations involving, 34 D. of for a complex ion, (v) 602 for an indicator, (s) 718 of polyprotic acids, 33 values for acids and bases in water, (T) 832 true or thermodynamic, 23 Distribution coefficient 162, 195 and per cent extraction, 165 Distribution ratio 162 Dithiol 693, 695, 697 Dithizone 171, 178... 

Ionic strength adjuster buffer 565, 570 Ionisation constants of indicators, 262, (T) 265 of acids and bases, (T) 832, 833, 834 see also Dissociation constants Ionisation suppressant 793 Iron(II), D. of by cerium(IV) ion, (cm) 546 by cerium(IV) sulphate, (ti) 382 by potassium dichromate, (ti) 376 by potassium permanganate, (ti) 368 see also under Iron... 

The mechanism of decarboxylation of acids containing an amino substituent is further complicated by the possibility of protonation of the substituent and the fact that the species NH2ArCOOH is kinetically equivalent to the zwitterion NHj ArCOO. Both of these species, as well as the anion NH2 ArCOO" and even NH3 ArCOOH must be considered. Willi and Stocker644 investigated by the spectroscopic method the kinetics of the acid-catalysed decarboxylation of 4-aminosalicyclic acid in dilute hydrochloric acid, (ionic strength 0.1, addition of potassium chloride) and also in acetate buffers at 20 °C. The ionisation constants K0 = [HA][H+][H2A+] 1 (for protonation of nitrogen) and Kx = [A"][H+] [HA]-1, were determined at /i = 0.1 and 20 °C. The kinetics followed equation (262)... 

Chemioai and physical properties appearance, coiour, odour, moleouiar weight, melting point, refractive index, ionisation constants, partition coefficients, solubility under different conditions of pH or solvent... 

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