Ammonia base dissociation constant

For ammonia (the most common weak base), the equilibrium reaction and base-dissociation constant are ... 

Dissociation Constants for Conjugate Acid/Base Pairs Consider the base dissociation-constant expression for ammonia and the acid dissociation-constant expression for its conjugate acid, ammonium ion ... 

Ammonia, amines, and anions of weak acids behave as weak bases in a process associated with a base-dissociation constant, K. The reaction of HA with H2O added to the reaction of A" with H2O gives the reaction for the autoionization of wata thus. Kg x Kb = Ky,. 

As with the relation between pK and K, we know that pA" , the negative logarithm of the base-dissociation constant, decreases with increasing K, (that is, increasing base strength). In aqueous solution, the two large classes of weak bases are (1) ammonia and the amines and (2) the anions of weak acids. 

Although the acid-dissociation constant for phenol (CgHjOH) is listed in Appendix D, the base-dissociation constant for the phenolate ion (CgHsO ) is not. (a) Explain why it is not necessary to list both for phenol and Ky for the phenolate ion. (b) Calculate Ky for the phenolate ion. (c) Is the phenolate ion a weaker or stronger base than ammonia ... 

Because of ammonia s log base dissociation constant (pKb) of approximately 9.00, at low pHs, ammonium is dominant and at pHs greater than 9.00, ammonia dominates. Ammonia is generally the form toxic to aquatic organisms while ammonimn is relatively non-toxic [34,51]. A number of methods exist to characterize anunonia caused toxicity in interstitial waters (see sections on pH manipulation and zeolite column chromatography). The two methods used most commonly in marine sediment TIEs are the addition of the macroalgae Ulva lactuca and the mineral zeoUte. 

However, acidity or basicity of a gas-phase adsorbate is not a sole criterion for its choice as a probe molecule. Firstly, the strength of an acidic or basic probe should be distinguished accordingly to its acid- or base-dissociation constant Ka or Kb). In addition, very important feature of probe molecule is its radius. If there is a need to locate all active sites in the structure of microporous solid material, the radius of probe molecule has to be smaller then the diameter of pore(s) opening(s). In other words, probe molecules have to be of appropriate size, so the entrance in the micropores of the solid and the access of adsorbate to each active site become possible. For example, ammonia, which is frequently used to reveal the acidic property of solids, is selected as a probe due to its basicity and due to the size of the molecule. Its molecule is smaller than the diameter of the pores in the zeolites structures, and also in many other solids. The other probe often used for investigation of solids acidity is pyridine however, the application of other chemical species is also possible. 

The data for bases are expressed as acidic dissociation constants, e.g. for ammonia, the value pKa = 9.24 is given for the ammonium ion ... 

Electrical Conductance of Aqueous Solutions of Ammonia and Metal Hydroxides. Check the electrical conductance of 1 W solutions of sodium hydroxide, potassium hydroxide, and ammonia. Record the ammeter readings. Arrange the studied alkalies in a series according to their activity. Acquaint yourself with the degree of dissociation and the dissociation constants of acids and bases (see Appendix 1, Tables 9 and 10). Why is the term apparent degree of dissociation used to characterize the dissociation of strong electrolytes ... 

So far, we ve been looking at the equilibria of weak acids, from which we developed the acid-dissociation constant, Ka. There is a similar process for weak bases. Let s use the weak base ammonia. In solution, ammonia establishes the equilibrium shown below ... 

An acid may, rather arbitrarily, be called a strong acid in glacial acetic acid if HAjp - 1- Thus perchloric acid is a strong acid, and yet the pAT for the overall dissociation constant is only 4.87 because it exists largely as ion pairs. Hydrochloric acid has an overall pAT value of 8.55 ammonia, 6.40 pyridine, 6.10 sodium acetate, 6.68 potassium chloride, 6.88 and sodium perchlorate, 5.48. Perchloric acid is the strongest acid and the one used for titration of bases that may be too weak to be titrated in water as solvent. At first it appears that an attempt to titrate a base such as pyridine with perchloric acid would fail, since both have small overall dissociation constants. Critical to the success of such titrations is the small dissociation constant of the salt formed, which results in a large favorable equilibrium constant for the reaction. 

This relationship is general for all conjugate acid/base pairs. Many compilations of equilibrium-constant data list only acid dissociation constants, since it is so easy to calculate basic dissociation constants by using Equation 9-14. For example, in Appendix 3, we find no data on the basic dissociation of ammonia (nor for any other bases). Instead, we find the acid dissociation constant for the conjugate acid, ammonium ion. That is,... 

The hydrolysis of ammonium formate will be considered next. A solution of this salt will have an acid reaction since formic acid is a stronger acid than ammonia is a base. The dissociation constants are ... 

It seems surprising at first to refer to water as a base. This is probably due to the fact that we have always considered the hydrogen ion concentration in water identical with the hydroxonium concentration, and because the dissociation constant of water as well as of other solvents is exceedingly small. Alcohol is about five hundred times weaker than water as a base. Were we to add water to an alcoholic solution of an acid, the water would act qualitatively as does ammonia when it is added to an aqueous acid solution ... 

Note that neither expression has the concentration of water in it. The concentration of pure water is 55.6 M, and in a dilute solution that might drop to 55.4 M. Thus the concentration of water might be thought to be a constant that has been incorporated into the equilibrium constant. The constant for the ionization of a weak acid is called the acid dissociation constant, and is symbolized Ka-A weak base like ammonia in water also is in equilibrium to a slight extent ... 

The conductance method is satisfactory only if the solvent can be rigorously purified. Through failure to appreciate this, the first values of pKa of picric acid in acetonitrile proved to be much too small, 5.6 and 8.9 as compared with 11.0 from electromotive force measurements on buffered solutions. D Aprano and Fuoss found that acetonitrile having a satisfactory specific conductance of about 10 cm still contained a trace of ammonia. This was converted to ammonium pic-rate when acid was added to the solvent giving a spurious contribution to the conductance of picric acid solutions. This discovery moved them to make the flat assertion that dissociation constants of weak acids cannot be determined in aprotic solvents conductimetrically . This may be an overly pessimistic view, conductance values of pKa for acids in di-methylsulphoxide and dimethylformamide agree well with those from spectrophotometric and electromotive force measurements. Approximate values of pKa and pKf can be obtained from conductometric titrations of a weak acid with a weak base. ... 

Based on a log acid dissociation constant (pKa) of 7.0 for hydrogen sulfide, as the sample pH decreases, H2S concentrations increase and toxicity increases [63,64]. Because of these imique speciation trends, the graduated pH manipulation can be used to characterize ammonia and hydrogen sulfide toxicity as well as other toxicants showing this type of behavior (e.g., some metals). 

Bases with a dissociation constant greater than 1 x 10 , such as diethyl-amine, triethylamine, piperidine, ammonia under pressure, and sodium alcoholate, have been used (33-35). Salts of Ni, Co, Fe, Ti, Zr, and Hg react in this manner. Although this reaction is limited in scope to a few elements, it does give high yields of those compounds which can be prepared. 

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