Acidic dissociation constant of ammonium ion at 0 to 50 C, and the base strength of ammonia

Abstract

The acidic dissociation of many acids has been studied in detail, but weak bases have genera lly been neglected because of t he experimental difficulties encountered in the investigation of some of their buffer solutions by electromotiye-force methods. These difficulties may arise from volatility of the free base or the existence of extraneous electrode reactions such as the formation of ammine complexes at the s ilver-silver-chloride electrode. In th is study of ammonia, special saturators were used to prevent, insofar as possible, t he removal of ammonia from the solutions by the hydrogen gas. A co rrection was applied for the amou nt of diammine silver complex formed at the sil ver-silver-ch loride electrode, a nd t his silver ion was prevented by mechanical means from reaching the platinum electrodes.With these precautions, the acidic dissociation constant of ammonium ion at 0° to 50° C was determined f rom electromotive-force measurements on 19 b uffer solutions containing equal molalities of ammonia and ammonium chloride. The changes of free e nergy, heat content, entropy, and heat capacity that accompany the dissociation processes in the standard state were derived from the temperatu re coefficients of the acidic dissociation constant of ammonium ion and of the basic dissociation constant of ammonia. The activity coefficient o[ ammonium chloride in equ imolal buffer solutions at 25° C was calcu lated.

Dissociation constants of weak bases from electromotive-force measurements of solutions of partially hydrolyzed salts

“…The values of E O and 2.3026 RT/F are summarized in another publication [4]. Thermodynamic dissociation constants are derived from exp erimental values of pwH by expression of mrr 01' frrmrr in terms of the constants that fix their values in the cell solutions, and extrapolation of a suitable function of the dissociation constants to zero concentration where the activity coefficients become unity.…”

“…The hydrogensilver chloride cell, so useful in precise determinations of the dissociation constants of weak acids by the electromotive-force method [1,2,3]1 ean be successfully applied to a m easurement of the constant of ammonia only under special conditions [4]. Thus, if equimolal mixtures of ammonia and ammonium chloride are to be investigated by this method, one must not only determine and apply corrections for the volatility of ammonia and for the solubility of silver chloride but must retard the diffusion of silver to the hydrogen electrode as well.…”

Effects of uncertainties in the thermodynamic properties of aerosol components in an air quality model – Part 1: Treatment of inorganic electrolytes and organic compounds in the condensed phase

“…The stoichiometric dissociation constant on a free H + ion basis (the first term in parentheses in Eq. 4) is equivalent to K o a,m ×γ NH + 4 /(γ H + γ NH 3 ) where K o a,m is the thermodynamic value of the dissociation constant which is 5.6885×10 −10 mol kg −1 at 25 • C (Bates and Pinching, 1949). Substituting into Eq.…”

Acidic dissociation constant and related thermodynamic quantities for diethanolammonium ion in water from 0 to 50 °C

“…(B ) m .the solu.tI?n differ somewhat from the stOIchIOmetrIC molahtl~s ml and m2 . This hydrolysis reaction is written as follows: (2) It is evident from this equation that the molality of BH+ will be equal to (m ) +mOH) whereas that of B will be (m2-mOH)' Accordingly the ionic strength (1) will be given by (3) It has been shown earlier [19] that the concentration of hydroxyl ion can be computed very co~veni.ently and with sufficient accuracy by the approxImatIOn (4) where K w is the ion product constant for water [20].…”

“…The results are summarized in table 3. The thermodynamic quantities for the dissociation of monoethanolammonium ion [12], diethanolammonium ion, and trieth anolamrnonium ion [13] at 25°C ar e compared in table 4 …”