Weak bases ionization

When placed in water, some acids and bases completely ionize, or dissociate into their ions—but others do not. Acids and bases that completely ionize are called strong acids and strong bases. Strong acids are defined as acids that have a pH of 0-4. Strong bases have pH values of 10-14. On the other hand, weak acids and weak bases do not disassociate completely in water. This leads to a pH value that is closer to neutral, because some of the hydrogen ions are still attached to other atoms, decreasing the hydrogen ion concentration. 

To complete the deionization process, water from the cation unit is passed through a strong base anion exchange resin in the hydroxide form. The resin exchanges hydrogen ions for both highly ionized mineral ions and the more weakly ionized carbonic and silicic acids according to1,2 ... 

Household ammonia is the most common weak base. It ionizes as follows ... 

Strong acids and bases completely ionize/dissociate, and weak acids and bases only partially ionize/dissociate. 

Excretion of drugs will be affected by the pH of the urine. If the urine is acidic, weak bases are ionized and there will be poor re-absorption. With basic urine, weak bases are non-ionized and there is more re-absorption. The pH of the urine can be artificially changed in the range 5-8.5 oral administration of sodium bicarbonate (NaHCOs) increases pH values, whereas ammonium chloride (NH4CI) lowers them. Thus, urinary acidification will accelerate the excretion of weak bases and retard the excretion of weak acids. Making the urine alkaline will facilitate the excretion of weak acids and retard that of weak bases. 

Some recent calculations,9 based on ionization potentials derived from photoelectron spectroscopy, favour the high lone-pair to lone-pair repulsions in the molecule as being very significant in the weakness of the bond. 

Weak-base resins have primary, secondary or tertiary amine functional groups and, in neutral or alkaline solution, the amines are in the free-base (un-ionized) form and have no ion-exchange properties. In acidic solution, the amine groups are protonated and extract anions according to the following equations ... 

Weak bases are able to accept hydrogen ions from acids, but they are less determined to do so than strong bases. Weak bases do not completely ionize in water... 

To what extent the 2-(a do-poly hydroxy alkyl) benzimidazoles can be used in resolving other optically active acids has not been determined. The benzimidazoles are relatively weak bases and do not form stable salts with weakly ionized acids. Haskins and Hudson15 found that when a solution of racemic lactic acid and 2-(D-gluco-D-gulo-hepto-hexa,hydroxy-hexyl)benzimidazole was concentrated, it was the free base that precipitated rather than one of the expected salts. It seems probable, however, that the method will be found useful in other resolutions, especially of the stronger organic acids. 

Strong acids and bases ionize completely in solution, while weak acids and bases partially ionize. The partial ionization of weak acids and bases creates solutions in which an equilibrium is established. 

Such a process occurs when NH4CI reacts with KNH2 in liquid ammonia. Because liquid NH3 is a base, the ionization with even weak acids such as acetic acid goes to completion in that solvent ... 

If the acid or base is weak, ionization is incomplete and an equilibrium condition occurs where... 

Titrations in concentrated salt solutions Critchfield and Johnson found that bases with ionization constants as low as about 10 can be titrated with indicator methods in aqueous solutions containing a high concentration of a neutral salt. The acidity of the strong add titrant is increased by the high salt concentration. Thus in 7 M sodium iodide the pH shortly after the equivalence point in a titration of a weak base has an effective value of about zero instead of about 2 as in the usual titration. Since the pH before the equivalence point is essentially unchanged, the pH change in the equivalence-point region is enhanced by about 2 units. 

Base ionization constants You won t be surprised to leam that like weak acids, weak bases also form equihbrium mixtures of molecules and ions in aqueous soluhon. Therefore, the equilibrium constant provides a measure of the extent of the base s ionization. The equilibrium constant for the ionization of methylamine in water is defined by this equilibrium constant expression. 

Strong bases favor elimination rather than substitution and the 2 mechanism rather than 1, particularly if they are highly concentrated. Reaction with low concentrations of base or no base in ionizing solvents promotes 5n1 rather than 1. Strong nucleophiles or weak bases favor substitution, unless the solvent is polar aprotic, in which case an 2 reaction is preferred. 

Thousands of amines are known, and many are very important in biochemical processes. Low-molecular-weight amines are soluble weak bases. The ionization of trimethylamine, for example, forms trimethylammonium ions and OH ions. 

Changing the pK of an acidic or basic group in a molecule so that more of the compound exists in the ionized form at physiological pH lowers log D (at about pH 7) and, in general, should improve aqueous solubility. The improvement in solubility is limited, however, if the solubility of the neutral form of the compound (the inherent solubility) is very low. The situation is worsened if the starting pK is far from 7. We find this to be a particular problem with weak bases. Weakly basic pyridines, quinolines, quinazo-lines and thiazoles seem to be frequent members of combinatorial libraries. Understanding the ionization behavior of drugs and how this property relates to oral absorption is extremely complex and likely beyond the capability (and interest) of many medicinal chemists. The reader is referred to an excellent recent review in this complex area. ... 

Even weakly ionized substances, like silica and CO2, can be removed with strong-base exchangers. Sometimes these exchangers are used after a cation exchanger for complete water demineralization [4]. 

Do not confuse the terms concentrated and dilute with strong and weak they concern two different things. Concentrated and dilute refer to the concentration of an acid or base, while strong and weak refer to the ability of an acid or base to ionize in solution. There are concentrated solutions of weak acids and dilute solutions of strong bases, and so forth. 

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