Ammonium equilibrium with ammonia

Weak bases form hydroxide ions by reacting with water, removing H+ from water leaving OH in solution. They are weak electrolytes and only a small fraction of base molecules produce hydroxide ion at any instant. Reactions like this in which water is a reactant are called hydrolysis reactions. The most common weak bases are the nitrogen bases, neutral molecules containing a nitrogen atom that can bond to the H+ removed from water. Aqueous ammonia, a solution of ammonia gas dissolved in water, is the most common nitrogen base. A small fraction of the ammonia molecules react with water and exist in equilibrium with ammonium and hydroxide ions, NHA(aq) and OH (aq). 

As discussed in Chapter 4, ammonia in aqueous solution exists in equilibrium with ammonium hydroxide, a weak base, which is partially ionized in water. Degree of ionization is dependent on pH at physiological pH, ammonium hydroxide is 99% ionized, but at pH 9.25, is only 50% ionized. 

This is demonstrated in fig. 1, where an optrode designed to operate as an ammonium detector for drinking water is shown. A chemical reaction in which ammonia (NH3) which is in equilibrium with ammonium ion (NH4 + ) in the solution leads to a change of the colour of an indicator molecule immobilised at the end of a quartz fibre in a membrane. This colour change, which is a function of the concentration of ammonium ion present in the solution, causes enhanced absorption of the mono-chromatic light which is transported by fibre optics to the membrane and back to a detector unit. 

When organics containing reduced nitrogen are degraded, they usually produce ammonium, which is in equilibrium with ammonia. As the pK for NH3 NH4" is 9.3, the ammonium ion is the primary form present in virtually all biological treatment systems, as they operate at pH < 8.5 and usually in the pH range of 6.5-7.5. In aerobic reactions, ammonium is oxidized by nitrifying bacteria (nitrosomonas) to nitrite... 

NH4+ + S042- + Ba2+ + 20H -> BaS04 + 2NH3 + 2H20 (or NH4+ + OH ). The AP test frequently includes questions with an ammonia solution (what you re dealing with as products in this reaction). Ammonia and water are actually in equilibrium with the ammonium and hydroxide ions. 

In the first step, the ketone and ammonia arc in equilibrium with their imine, which, at pH 6, is partly protonated as an iminium ion. The iminium ion is rapidly reduced by the cyanoborohydride to give the amine. Reactions like this, using ammonia in a reductive amination, are often carried out with ammonium chloride or acetate as convenient sources of ammonia. At pH 6, ammonia will be mostly protonated anyway. 

A few substances are capable of showing both upward and downward fluxes. An example is ammonia. Ammonium in the soil, NH4 , is in equilibrium with ammonia gas, NH3(g)... 

Heating of the ammonium salts shifts the formation equilibrium to the ammonia side, the ammonium salts sublime under dissociation. Ammonium salts with oxidizing anions (N02, N03, Cr207 ) decompose under oxidation of the ammonium nitrogen to N2 or N2O. 

The amount of nitrate aerosol is controlled partly by the production of HNO3 (via Equations (11)-(13)) and partly by the availability of NH3 to form ammonium nitrate. Ammonium nitrate aerosol forms in equilibrium with gas-phase ammonia and nitric acid (Equation (14)) ... 

A number of different solid phases can precipitate from a solution of ammonia and carbon dioxide in water. As an example, the equilibrium of ammonium bicarbonate with an aqueous solution is written in the form (13) ... 

Solid ammonium chloride is in equilibrium with ammonia and hydrogen chloride gases ... 

Un-ionized ammonia is freely diffusible, whereas the ammonium ion is less so and is relatively confined to the extracellular compartment (Stabenau et al. 1958). However, ammonium ion is in d3mamic equilibrium with dissolved ammonia. Therefore, ammonium compounds that enter the circulatory system or other body fluids can thus freely penetrate tissue cells as ammonia. In h q)ophysectomized rats that were administered N-ammonium citrate orally by gavage, labeled protein was found in liver, kidney, spleen, heart, and skeletal muscle 6-72 hours after N-ammoniimi citrate administration (Vitti et al. 

In water, ammonia is in equilibrium with the ammonium ion, NH/. The ammonia-ammonium ion equilibrium is highly dependent on the pH and, to a lesser extent, the temperature of the medium. In acidic waters, the equilibrium favors the ammonium ion. 

There is evidence that ammonium nitrate in particulate matter is in equilibrium with ammonia, which would contribute to small positive errors for ammonia and small negative errors for ammonium (Doyle et al. 1979). 

So under atmospheric conditions practically all dissolved ammonia in clouds is in the form of ammonium ion. The aqueous-phase concentrations of [NH ] in equilibrium with 1 ppb of NH3(g) are shown in Figure 7.9. The partitioning of ammonia between the gas and aqueous phases inside a cloud can be calculated using (7.9) and the effective Henry s law coefficient for ammonia, // = Wnh, Ka [H+]/ATm. If the cloud pH is less than 5 practically all the available ammonia will be dissolved in cloudwater (Figure 7.10). 

Competition between sulfate and nitrate for the available ammonia results in rather complicated system behavior. Addition of ammonium sulfate to solutions containing aqueous ammonium nitrate lowers the ammonia-nitric acid vapor pressure product in equilibrium with the aerosol phase (Figure 9.21). In Figure 9.21, Y is the ionic strength fraction of ammonium sulfate and is calculated as... 

Note that the concentration product of nitric acid and ammonia in equilibrium with a mixed sulfate-nitrate solution having a value of T = 0.5 is about one-half as high as that in equilibrium with a pure ammonium nitrate solution. The temperature dependence of the partial-pressure product for the aqueous mixed-salt case is similar to that of the pure salt. 

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