In acid-base titrations

In acid-base titrations the end point is generally detected by a pH-sensitive indicator. In the EDTA titration a metal ion-sensitive indicator (abbreviated, to metal indicator or metal-ion indicator) is often employed to detect changes of pM. Such indicators (which contain types of chelate groupings and generally possess resonance systems typical of dyestuffs) form complexes with specific metal ions, which differ in colour from the free indicator and produce a sudden colour change at the equivalence point. The end point of the titration can also be evaluated by other methods including potentiometric, amperometric, and spectrophotometric techniques. 

The intensity and colour of the fluorescence of many substances depend upon the pH of the solution indeed, some substances are so sensitive to pH that they can be used as pH indicators. These are termed fluorescent or luminescent indicators. Those substances which fluoresce in ultraviolet light and change in colour or have their fluorescence quenched with change in pH can be used as fluorescent indicators in acid-base titrations. The merit of such indicators is that they can be employed in the titration of coloured (and sometimes of intensely coloured) solutions in which the colour changes of the usual indicators would... 

For end-point detection, any method usual in acid-base titration can be used with electrometric indication the precautions for protection against the... 

Observations that the presence of protein affects the colour change of some indicators used in acid-base titrations led to the development of methods for the quantitation of proteins based on these altered absorption characteristics of such dyes. As the presence of protein alters the colour produced by these indicators when measuring pH, so in the quantitation of proteins using dye-binding methods the control of pH is vital. 

Indicators are used in acid-base titrations as they change colour at i reaction. Indicators are usually weak acids in which the colour of the acid is different from that of its conjugate base. If we represent the Indicator as a weak acid of formula HIn, and say it has a red colour, and its conjugate base as In", and say it has a blue colour, its dissociation in water can be represented as Hln(aq) + HjO(l) H30" (aq) + In"(aq)... 

Historically, pH sensitive dyes have been extensively used as indicators in acid-base titrations and in simple spot test papers, even leading to a common phrase in our everyday language, when people or topics are described as having passed the litmus test . The use of complexometric titrations for metal ions was a later but widely... 

In acid-base titrations, pD is pH and pK is pK because [D] = [H+]. This is the reason for the name of the curve, well known from analytical chemistry. The drawback of this plot is that many points are needed in the vicinity of the inflection point. 

For the titration of a strong base with a weak acid, the equivalence point is reached when the pH is greater than 7. The half equivalence point is when half of the total amount of base needed to neutralize the acid has been added. It is at this point that the pH = pK of the weak acid. In acid-base titrations, a suitable acid-base indicator is used to detect the endpoint from the change of colour of the indicator used. An acid-base indicator is a weak acid or a weak base. The following table contains the names and the pH range of some commonly used acid-base indicators. 

For a titration with EDTA, you can follow the derivation through and find that the formation constant, Kf, should be replaced in Equation 12-11 by the conditional formation constant, K, which applies at the fixed pH of the titration. Figure 12-12 shows a spreadsheet in which Equation 12-11 is used to calculate the Ca2+ titration curve in Figure 12-11. As in acid-base titrations, your input in column B is pM and the output in column E is volume of titrant. To find the initial point, vary pM until V, is close to 0. 

In acid-base titrations, a difference plot, or Bjerrum plot, is a graph of the mean fraction of protons bound to an acid versus pH. The mean fraction is nH calculated with Equation 13-59. For complex formation, the difference plot gives the mean number of ligands bound to a metal versus pL(= —log[ligand]). 

As in acid-base titrations, indicators and electrodes are commonly used to find the end point of a redox titration. 

Sulfamic acid is a stable, non-hygroscopic, monoprotic acid which is very convenient to use as a primary standard in acid-base titrations. The acid hydrolyzes fairly rapidly in boiling water to give ammonium bisulfate. It may be recrystallized from water at 70°C with rapid cooling to avoid decomposition. Sulfamic acid does not dissolve appreciably in oxygenated organic solvents, e.g., alcohols, ethers. 

This exercise will develop your understanding of some of the practical skills involved in acid-base titrations and the processing and evaluation of experimental results. 

In Acid-Base Titration and the Global Carbon Cycle, the scenario and the application laboratory were developed in conjunction with a field ecologist who simply stated, We can have the students set up in the laboratory what we have done in the field. They just have to set up an apparatus that traps the leaves and collects the C02. This experi-... 

Because of the striking color change between the bases (1) and monocations (4), it has been suggested that they could be useful indicators in acid-base titrations. A number of 2,4-disubstituted derivatives have been suggested for use over the pH range 5-9 (80MI2), and certain 3-aryl-... 

An important question is whether we can use any indicator electrode. A redox electrode, i.e. inert in the range of potential where measurements are being done, is a possibility, especially for redox titrations. In other cases, the use of electrodes selective to the ion being titrated is better, such as pH electrodes in acid-base titrations. The method of analysis of the data obtained is, naturally, the same in all cases and independent of electrode material. 

In acid-base titrations of weak acids or bases the expressions are also more complicated. Nevertheless, the introduction of concentration dependence as a function of the composition of the medium in the Nernst equations always leads to the correct result. 

Salt formation generally favors the quinoid form and these salt solns are usually brightly Qolored. Presence of strong acids favors the colorless phenol form. Because of these color changes, nitrophenols have been used as indicators in acid-base titrations... 

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. 

Figure 22.7 Examples of indicators used in acid-base titrations thymol blue and phenolphthalein...

More than brief discussion of the numerous ways in which end points can be taken other than by visual methods is beyond our scope. For example, end-point techniques may involve photometry, potentiometry, amperometry, conductometry, and thermal methods. In principle, many physical properties can be used to follow the course of a titration in acid-base titrations, use of the pH meter is common. In terms of speed and cost, visual indicators are usually preferred to instrumental methods when they give adequate precision and accuracy for the purposes at hand. Selected instrumental methods may be used when a suitable indicator is not available, when higher accuracy under unfavorable equilibrium conditions is required, or for the routine analysis of large numbers of samples. 

Under the auspices of the International Union of Pure and Applied Chemistry, an Analytical Methods Committee carried out a critical examination of some primary standards suggested for use in acid-base titrations, with a view to making recommendations concerning practical applications. Some of the findings of this committee were as follows (1) Benzoic acid as a primary standard is not recommended because... 

The proton conditions of equations 48 and 49 correspond to the two equivalence points in acid-base titration systems. The half-titration point is usually (not always) given by pH = pAT. Thus the qualitative shape of the titration curve can be sketched readily along these three points (Figure 3,3a). 

It is important to know the dissociation constant of an indicator in order to use it properly in acid-base titrations. Spectrophotometry can be used to measure the concentration of these intensely colored species in acidic versus basic solutions, and from these data the equilibrium between the acidic and basic forms can be calculated. In one such study on the indicator wj-nitrophenol, a 6.36 X 10 M solution was examined by spectrophotometry at 390 nm and 25°C in the following experiments. In highly acidic solution, where essentially all the indicator was in the form HIn, the absorbance was 0.142. In highly basic solution, where essentially all of the indicator was in the form In , the absorbance was 0.943. In a further series of experiments, the pH was adjusted using a buffer solution of ionic strength I, and absorbance was measured at each pH value. The following results were obtained ... 

M The standard reagents used in acid/base titrations are always strong acids or strong bases, most commonly HCl, HCIO4, H2SO4, NaOH, and KOH. Weak acids and bases are never used as standard reagents because they react incompletely with analytes. 

We find two types of titration errors in acid/base titrations. The first is a determinate error that occurs when the pH at which the indicator changes color differs from the pH at the equivalence point. This type of error can usually be minimized by choosing the indicator carefully or by making a blank correction. 

Although indicators are still widely used in acid/base titrations, the glass pH electrode and pH meter allow the direct measurement of pH as a function of titrant volume. The glass pH electrode is discussed in detail in Chapter 21. The titration curve for the titration of 50.00 mL of 0.1000 M weak acid = 1.0 X 10 ) with 0.1000 M NaOH is shown in Figure 14F-4a. The end point can be located in several ways from the pH versus volume data. 

Use the mole method and molarity in acid-base titration reactions... 

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