Factors Affecting Acid Strength

As we shall see later, the limitations imposed by most solvents may prevent us from being able to utilise the very strong basic characteristics of some anions. However, at this point it is more useful to consider other factors affecting the strengths of acids. 

The ionization constants of a series of hindered polynuclear aromatic acids have been determined by Newman and Boden (1961). Two factors are held to affect the acidic strength of such acids the decrease of solvation and the inhibition of resonance due to steric effects. Solvation of an ion should be an acid-strengthening factor and if steric factors decrease the amount of anion solvation, the acid should be correspondingly weaker. The resonance factor only comes into play with aromatic (or unsaturated) acids. Here resonance would involve transfer of... 

At the same time it has been shown that certain zeolite compositions do not exhibit uniform acid strength. Lunsford and coworkers extended the acidity-aluminum content correlation to the high Si/Al range of dealuminated H-Y zeolites and found that above approximately 35 Al/unit cell (Si/Al 4) the overall acidity of the zeolites decreased, presumably due to next nearest neighbor interactions, i.e., the intensive factor of acidity of the zeolite is diminished. It has also been established that certain dealumination procedures can affect the intensive factor of acidity. This was clearly demonstrated by Lago et al. on H-ZSM-S catalysts, in which dealumination by mild steaming resulted in a four-fold increase in the cracking activity for n-hexane. ... 

Many factors affect solubility and must be considered. For example, pH, ionic strength, and temperature can significantly affect solubility. For example, the aqueous solubility of a carboxylic acid can be orders of magnitude higher at a pH above the pKa than below the pKa. This is due simply to changes in the polarity of the molecule. Conversely, the solubility of weak bases such as amines is higher when the pH is below the pKa of the base. For aqueous sample preparations, addition of a water-miscible solvent such as acetonitrile or alcohol can be used to enhance solubility. For example, the solubility of acetaminophen in water is approximately 11 mg/mL, but the solubility is doubled by adding 2% ethanol (19). 

Jelly Test Conditions. Other factors affecting the Internal gel strength are the rate and duration of boiling a test jelly, stage of acid addition, rate of cooling, time of aging, type of sugar used (53, 54), and the use of buffer salts or synthetic fruit juices rather than water (43). 

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