Titrations in Nonaqueous Solvents

Thus far we have assumed that the acid and base are in an aqueous solution. Indeed, water is the most common solvent in acid-base titrimetry. When considering the utility of a titration, however, the solvent s influence cannot be ignored. 

The dissociation, or autoprotolysis constant for a solvent, SH, relates the concentration of the protonated solvent, SH2, to that of the deprotonated solvent, S . For amphoteric solvents, which can act as both proton donors and proton acceptors, the autoprotolysis reaction is 

You should recognize that is just the specific form of Ks for water. The plT of a solution is now seen to be a general statement about the relative abundance of protonated solvent 

Perhaps the most obvious limitation imposed by Ks is the change in pH during a titration. To see why this is so, let s consider the titration of a 50 mb solution of 10 M strong acid with equimolar strong base. Before the equivalence point, the pH is determined by the untitrated strong acid, whereas after the equivalence point the concentration of excess strong base determines the pH. In an aqueous solution the concentration of H3O+ when the titration is 90% complete is 

The change in pH when the titration passes from 90% to 110% completion is 

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Fritz, J. S. Acid-Base Titrations in Nonaqueous Solvents. Allyn and Bacon Boston, 1973. 

Fritz, J.S., Acid-Base Titration in Nonaqueous Solvents Allyn and Bacon Boston, 1973 Chapter 2. 

Popov, A. I., Caruso, H. Acid-base equilibria and titrations in nonaqueous solvents. B. Amphiprotic solvents, in Ref. 1, pp. 303-348. 

J. S. Fritz, Acid-Base Titrations in Nonaqueous Solvents (Boston Allyn Bacon, 1973) J. Kucharsky and L. Safarik, Titrations in Non-Aqueous Solvents (New York Elsevier, 1963) W. Huber, Titrations in Nonaqueous Solvents (New York Academic Press, 1967) I. Gyenes, Titration in Non-Aqueous Media (Princeton, NJ Van Nostrand, 1967). 

Some of the reasons for considering coulometric titrations in nonaqueous solvents are that many organic compounds are not soluble in water, metals can exist in oxidation states that are not found in water, and advantage can be taken of the acidity or basicity of the solvent to improve the basic or acidic strength of a base or acid, respectively. 

Table 5.21 Commonly used pH indicators for titrations in nonaqueous solvents...

J. s. fritz Acid-Base Titrations in Nonaqueous Solvents, G. F. Smith Chemical, Columbus, Ohio, 1952 Acid-Base Titrations in Nonaqueous Solvents, Allyn and Bacon, Boston, 1973. 

Huber, W. Titrations in Nonaqueous Solvents, Academic Press, Inc., NY, 1967. 

Many amines that ai e too weak to be titrated as bases in water are readily titrated in nonaqueous solvents, such as anhydrous acetic acid, which enhance their basicity. 

Acid-Base Titrations in Nonaqueous Solvents. It is a fact that the apparent acidity or basicity of a compound is strongly dependent on the acid-base properties of the solvent. For example, very strong acids such as HCl and HNO3 cannot be individually titrated in water because water is sufficiently basic that these acids appear to be totally ionized. Very weak bases, such as amines, cannot be successfully titrated with strong acid in water. Many acids or bases that are too weak for titration in an aqueous medium, however, become amenable to titration in appropriate nonaqueous solvents. As a consequence, there are now many neutralization methods that call for solvents other than water [23-25]. 

J. Fritz, "Acid-Base Titrations in Nonaqueous Solvents", The G. Frederick Smith Chemical Comnany,... 

Use of potentiometry for pH titration allows analyses to be carried out in colored or turbid solutions. Also, it solves the problem of selecting the correct indicator for a particular acid-base titration. The endpoint can be determined more accurately by using a first or second differential curve as described earlier. It also permits pH titrations in nonaqueous solvents for the determination of organic acids and bases as described subsequently. In addition, it can be readily automated for unattended operation. 

The experimental setup for titrations in nonaqueous solvents differs from those in aqueous solution and specialized texts (e.g., Huber) should be consulted. 

Kucharsky, J. Safarik, L. Titrations in Nonaqueous Solvents, Elsevier Amsterdam, 1965. 

Acid-base indicators for titrations in nonaqueous solvents are normally weak protolytes. For dissociation model I as given in Table 1, the acid-base equilibrium of a weak acid type of indicator in the pure solvent can be represented in water by the following reaction ... 

Reactions at the silicon nitride - solution interface - The use of potentiometric titrations in nonaqueous solvents indicates that there are possibly several different kinds of acid and base sites on the silicon nitride depending on the prior environmental history of the powd. The results are generally consistent with potentiometric titrations us d to evaluate the silicon dioxide surface. Future work will focus on further potentiometric as well as conductometric titrations on silicon nitride as well as model systems such as silicon dioxide. 

Aqueous titrations of amines are amply discussed elsewhere d. Nonaqueous titrations. Three main purposes may be served by carrying out titrations in nonaqueous solvents increased solubility, change of the pH scale, and resolution of mixtures. The prediction of a potentiometric titration curve in an arbitrary solvent is a difficult task, in which many factors intervene, such as dielectric constant, definition of acid and base in relation to the solvent, electrodes, actual structure of conjugate acids and bases, etc. Acetic acid, sulphuric acid, acetonitrile, and alcohol-water mixtures have been extensively studied and were reviewed elsewhere Some solvents will be treated here briefly ... 

A. H. Beckett and E. H. Tinley, Titration in Nonaqueous Solvents, 3rd cd.. The British Drug Houses, Poole, 1960. 

Some other chemical and physical methods, such as thermal analysis and nuclear magnetic resonance analysis, have been occasionally used for the analysis of pantothenates in pharmaceutical preparations. Pure compounds can also be determined chelatomettically or using alkalimetric titration in nonaqueous solvents or can be analyzed after decomposition to ammonia (5,48,49). 

The scope of acidic titrations in nonaqueous solvents is well illustrated by the behavior of succinic acid (Fig. 7). 

Titrations in Nonaqueous Solvents, W. Huber, Academic Press, (1967). Analytical Chemistry, Analytical Reviews (see e.g., April 1970). 

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