Mixtures of Acids or Bases

If two strong acids are titrated together, there will be no differentiation between them, and only one equivalence point break will occur, corresponding to the titration of both acids. The same is true for two weak acids if their Ka values are not too different. For example, a mixture of acetic acid, Ka = 1.75 X 10, and propionic acid, = 1.3 X 10, would titrate together to give a single equivalence point. 

With H2SO4, the first proton is completely dissociated and the second proton has a Ka of about 10 . Therefore, the second proton is ionized sufficiently to titrate as a strong acid, and only one equivalence point break is found. The same is true for a mixture of a strong acid and a weak acid with a Ka in the neighborhood of 10 .  

Phosphoric acid in mixture with a strong acid acts in a maimer similar to the above example. The first proton titrates with the strong acid, followed by titration of the second proton to give a second equivalence point the third proton is too weakly ionized to be titrated. 

A mixture of HCl and H3PO4 is titrated with 0.1000 M NaOH. The first end point (methyl red) occurs at 35.00 mL, and the second end point (bromthymol blue) occurs at a total of 50.00 mL (15.00 mL after the first end point). Calculate the millimoles HCl and H3PO4 present in the solution. 

The second end point corresponds to that in the titration of one proton of H3PO4 (H2P04 —HP04 ). Therefore, the millimoles H3PO4 is the same as the millimoles NaOH used in the 15.00 mL for titrating that proton  

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The approach that we have worked out for the titration of a monoprotic weak acid with a strong base can be extended to reactions involving multiprotic acids or bases and mixtures of acids or bases. As the complexity of the titration increases, however, the necessary calculations become more time-consuming. Not surprisingly, a variety of algebraic and computer spreadsheet approaches have been described to aid in constructing titration curves. 

Earlier we noted that an acid-base titration may be used to analyze a mixture of acids or bases by titrating to more than one equivalence point. The concentration of each analyte is determined by accounting for its contribution to the volume of titrant needed to reach the equivalence points. 

Weak acids and bases can be determined either alone or in the presence of strong acids and bases by titration between two pHs. It is immaterial whether they are present as free acid or base, or as a salt, or as a mixture of acid or base and salt. 

Geiser et al. [50,51] illustrated the screening of different chiral stationary phases and the separation of highly polar amine hydrochlorides using EEL methanol/C02 mixtures and the columns, Chiralpak-AD-H, Chiralpak-AS. This method is advantageous because no acid or base additive was required to achieve base line separation of the racemates and conversion to free base form for enantiomer separation was not required. Preparative-scale separations of the amine-hydrochloride were accomplished using similar mobile phase conditions [51], Furthermore, this is believed to be the first chiral separation of highly polar solutes without the addition of acid or base additive to effect the separation. 

The pH of pure (and also not so pure) water is very sensitive to small concentrations of acids and bases. One drop of concentrated sulphuric acid added to a liter of water will change the pH by 4 pH nnits (from 7 to ca. 3). Solntion pH can be stabilized by a buffer (although there may be cases where a stable pH is not desirable) addition of (not too large) quantities of acid or base to a buffered solution will not affect the pH mnch. Buffers are usually mixtures of weak acids or bases and their salts. A common example in CD is the nse of an ammoninm salt (NH4X ) to control the pH of an ammonia solntion. The equilibrium of ammonia in water is given by... 

As mentioned before, a buffer solution contains a mixture of a weak acid and its conjugate base or of a weak base and its conjugate acid. The important aspect of the buffer solution is that the pH of the solution is minimally changed when small amounts of acid or base are added or when the solution is slightly diluted. Let us now examine a buffer solution made of a weak acid HA (0.3 M), whose pKa is 4.90, and its conjugate base NaA (0.3 M). A small quantity of HC1 (0.05 M) is accidentally added to the solution. The conjugate base (A-) will react with H+ as follows ... 

If an asymmetric carbon atom has an enolizable hydrogen atom, a trace of acid or base allows that carbon to invert its configuration, with the enol serving as the intermediate. A racemic mixture (or an equilibrium mixture of diastereomers) is the result. 

The isolation and full characterization of a number of large hydrido clusters have in many cases been hampered by their extremely high acidity. The clusters [Ni3gPt8(CO)48H,6- )]" ( = 3-6), for instance, exist as an equilibrium mixture of anions a (n = 3), b (n = 4), c ( = 5), and d (n = 6) in acetonitrile solution. This mixture has been found to be easily converted into one of its components by controlled addition of acid or base [Eq. (26)] (373). 

Sometimes when extractions are performed to remove undesired byproducts, the concentration of the desired product in the organic and aqueous layers are also determined. The concentration in the organic layers are deemed the most important (contains the desired product), and the concentrations in the aqueous layers are determined later to ensure mass balance and overall yield of the reaction. The aqueous layers are usually enriched with the undesired by-products and are good samples to use during the development of the HPLC method in the early stages of the synthetic development. The pH of these layers is usually checked as well to ensure that the proper amount of acid or base has been added to the reaction mixture either to quench the reaction or to drive the desired product into the organic layer. 

Potentiometric acid-base titrations are particularly useful for the analysis of mixtures of acids or poly-protic acids (or bases) because often, discrimination between the endpoints can be made. An approximate numerical value for the dissociation constant of a weak acid or base can be estimated from potentiometric titration curves. In theory, this quantity can be obtained from any point along the curve, but it is most easily derived from the pH at the point of halfneutralization. 

For an acid titrated halfway to its equivalent point, pH = pKa. For mixtures of acids and bases, and hence for carbons having functional groups of different acid or basic strength, this holds true as well. For weak acid and base groups, the effect of water dissociation is significant around pH = 7. Therefore, a simple potentiometric titration can give information about the dissociation constants and neutralization equivalence of the carbon. In several cases these indications can be sufficient to determine the nature of the functional groups and provide a comprehensive description of the behavior of carbon in terms of acidity and basicity. A differential plot of the titration curve can be considered in the same way as a conventional absorption spectrum of the sample. Acidity or basicity constants are then calculated at half-titration, as pH = pKw — pKb for a base and pH = pKa for an acid. 

Potentiometry. Potentiometric methods rely on the logarithmic relationship between measured potential and analyte concentration. The most common involves an instrument called a pH-Stat , in which a glass (pH) electrode follows reactions that either consume or produce protons. Since pH changes cause changes in enzyme activity, the pH is maintained at a constant value by the addition of acid or base. The rate of titrant addition is then proportional to the rate of the enzymatic reaction. Precise measurements using the pH-Stat require low buffer concentrations in the enzymatic assay mixture. 

Experimental neutralization curves closely approximate the theoretical curves described in Chapters 14 and 15. Usually, the experimental curves are somewhat displaced from the theoretical curves along the pH axis because concentrations rather than activities are used in their derivation. This displacement has little effect on determining end points, and so potentiometric neutralization titrations are quite useful for analyzing mixtures of acids or polyprotic acids. The same is true of bases. 

Buffer solutions are liquids of such composition as to resist appreciable changes in hydrogen ion concentration. Addition of traces of acids or bases leaves their pH practically unaltered. For this reason they are indispensable as comparison media in colorimetric pH determinations. Buffer mixtures can be retained unchanged for two months if stored in closed flasks made of good glass and containing a disinfectant (a minute th3unol crystal). It is preferable, however, to prepare fresh solutions each month. 

Fig. 22. A binary mixture of 2/3DN30C10 and DAP(Bn)22+ behaves in a manner analogous to an XNOR gate. In the absence of acid or base, a strong complex with pseudorotaxane geometry is formed, a Upon addition of trifluoroacetic acid (A), by virtue of the formation of a strong complex with the crown ether, protons displace the DAP(Bn)22+ from the cavity of 2/3DN30C10. b Neutralization of the solution with n-butylamine (B) results in the reformation of the pseudorotaxane. c If the initial mixture is treated with base, a strong interaction between n-butylamine and DAP(Bn)22+ leads to the destruction of the complex, d Neutralization of the solution with TFA leads to a simple acid-base reaction, and pseudorotaxane formation. If the two inputs are acid and base (A and B), with a 1 signaling presence and a 0 absence, this system behaves as an XNOR gate, if pseudorotaxane formation is represented by an output of 1...

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