Acid-base titrations mixtures

In this experiment the overall variance for the analysis of potassium hydrogen phthalate (KHP) in a mixture of KHP and sucrose is partitioned into that due to sampling and that due to the analytical method (an acid-base titration). By having individuals analyze samples with different % w/w KHP, the relationship between sampling error and concentration of analyte can be explored. 

This approach can be used to sketch titration curves for other acid-base titrations including those involving polyprotic weak acids and bases or mixtures of weak acids and bases (Figure 9.8). Figure 9.8a, for example, shows the titration curve when titrating a diprotic weak acid, H2A, with a strong base. Since the analyte is... 

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. 

The scale of operations, accuracy, precision, sensitivity, time, and cost of methods involving redox titrations are similar to those described earlier in the chapter for acid-base and complexometric titrimetric methods. As with acid-base titrations, redox titrations can be extended to the analysis of mixtures if there is a significant difference in the ease with which the analytes can be oxidized or reduced. Figure 9.40 shows an example of the titration curve for a mixture of Fe + and Sn +, using Ce + as the titrant. The titration of a mixture of analytes whose standard-state potentials or formal potentials differ by at least 200 mV will result in a separate equivalence point for each analyte. 

Ramsing and colleagues developed an FfA method for acid-base titrations using a carrier stream mixture of 2.0 X f0 M NaOH and the acid-base indicator bromthymol blue. Standard solutions of HCl were injected, and the following values of Af were measured from the resulting fiagrams. 

The product HCl is measured by acid-base titration. An aliquot of the product mixture is analyzed for chloride ion by ion chromatography or titration with a standard solution of silver nitrate. The compound can be identified from its physical properties. 

From an acid-base titration curve, we can deduce the quantities and pK.d values of acidic and basic substances in a mixture. In medicinal chemistry, the pATa and lipophilicity of a candidate drug predict how easily it will cross cell membranes. We saw in Chapter 10 that from pKa and pH, we can compute the charge of a polyprotic acid. Usually, the more highly charged a drug, the harder it is to cross a cell membrane. In this chapter, we learn how to predict the shapes of titration curves and how to find end points with electrodes or indicators. 

The overall effect of complex formation is to remove a hydrated metal ion from the mixture of ions in solution by displacing the equilibrium in favour of the complex, cf. the similar process in the formation of water in acid-base titrations and precipitation reactions. 

Although acid-base titrations in alcohol-water mixtures have been studied extensively, we do not consider them in detail since titration curves and indicator equilibria in ethanol-water and methanol-water mixtures can be calculated in the same way as in water. Values of the autoprotolysis constants of the mixtures, are close to for mixtures containing only a moderate amount of alcohol. On the other hand, even a trace of water in ethanol causes a large increase in J SH According to Gutbezahl and Grunwald, pXsH is 14.33, 14.88, 15.91, and 19.5 for ethanol-water mixtures containing 20, 50, 80, and 100 wt % ethanol. 

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. 

Use the search engine Google to locate the Web document The Fall of the Proton Why Acids React with Bases by Stephen Lower. This document explains acid/base behavior in terms of the concept of proton free energy. How is an acid/base titration described in this view In a titration of strong acid with strong base, what is the free energy sink In a complex mixture of weak acid/base systems, such as serum, what happens to protons ... 

Finally we have seen in section 4-11 how acid-base titrations can be used in practice, even without any preliminary separations or sample clean-up, and what trade-offs are made in such analyses. This example illustrates a rather radical departure from the traditional emphasis on titrations as methods of high precision. As illustrated in Table 4.11-1, even when precise concentrations of well-defined chemical species cannot be derived from such complex mixtures, they nonetheless can be made to yield very useful quantitative information. 

In contrast, germanium oxide was found to be appreciably soluble in the molten equimolar KCl-NaCl mixture and its solutions show weak acidic properties. Its acid-base titration using NaOH as a titrant results in the appearance of a small pO drop at the equivalence point ( 1 pO unit), which corresponds to the ligand number value —0.5. This allows us to conclude that the acid-base titration runs in one step, and the final product is di-germanate ion, Gc202... 

Since there was no direct reaction of CO with the chloride melt, its concentration in the gas mixture was neglected and corrections for the partial pressure of C02 were not made. The melt was analyzed periodically for this purpose, several samples were taken off the melt, dissolved in water, and subjected to acid-base titration in aqueous solution, to determine the ratio of concentrations of Na20 to Na2C03 as described above. These studies provided the temperature dependences of the equilibrium constant of (1.2.3) in four melts of the KCl-NaCl system in the 1 temperature range 1025 -1125 K, as follows ... 

The measurement of acidity and basicity of silicon compounds follows quite well-defined phases. The first studies were conventional acid-base titrations in which pKa values were determined. The solvents varied, but were usually aqueous or water/alcohol mixtures. These studies were limited by solubility and resistance to attack by protic solvents. 

If we are using indicators in an acid-base titration, we are making the assumption that the end point of the indicator (the point at which it appears to suddenly change colour) is the same as the equivalence point. In order to look at this assumption further, we look at the way that the pH of a reaction mixture changes during an acid-base titration. 

Analysis of an Unknown Mixture by Acid-Base Titration... 

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