Equivalence point indicators

Example Equation K Species at Equivalence Point pH at Equivalence Point Indicator ... 

Define standard solution, volumetric flask, volumetric pipet, standardization, buret, stopcock, titration, titrant, substance titrated, equivalence point, indicator, end point, and 9automatic titrator. 

To select the proper indicator, the pH at which the indicator changes color should be equal to the pH of the solution being tested at its equivalence point. indicator. 

Orthophosphoric Acid may be titrated with sodium or potassium hydroxide free from carbonate. The equivalent point indicating NaH2P04 occurs at pH = ca. 4-2, which is within the transition range of methyl yellow, methyl orange and bromophenol blue. The endpoint tint should be matched against that of a comparison solution containing about the same concentration of NaH2P04. 

This equation is also a quadratic in Ch+, and so it can be solved and ch+ evaluated. The results for the neutralization of a 0.1 n solution of an acid of ka equal to 10 by a strong base are shown in Fig. 102, II the equivalence-point, indicated by an arrow, occurs at a pH of 11.0. The inflexion at the equivalence-point is seen to be small, and it is even less marked for more dilute solutions of the acid. It has been calculated that... 

In a typical titration, known volumes of a reagent of known concentration are added to a known volume of a sample of unknown concentration, and addition is continued at least until an equivalent amount of reagent is added, at which point some measurable physical or chemical property indicates that a so-called equivalence point has been reached. The unknown concentration can then be calculated. Numerous properties can be used as indicators historically, the first equivalence point indicator was the observation that bubble formation (effervescence) upon addition of potassium carbonate to vinegar would stop once the equivalence point had been reached or passed. Nowadays, the progress of most acid-base titrations is monitored either with a color indicator or, preferably, with a pH meter. 

The equivalence point in a titration is defined by the stoichiometry of the reaction. The end point is the abrupt change in a physical property (such as pH) that we measure to locate the equivalence point. Indicators and pH measurements are commonly used to find the end point in an acid-base titration. 

C>0 (5). Condensation occurs and the equivalent point indicates the formation of pyrovanadate ... 

Sketch the titration curves of the following mixtures. Indicate the initial pH and the pH corresponding to the equivalence point. Indicate the volume of titrant required to reach the equivalence point, and select a suitable indicator from Figure 17-7. 

The problem in any quantitative volumetric analysis for ions in solution is to determine accurately the equivalence point. This is often found by using an indicator, but in redox reactions it can often... 

Experimentally, the aqueous iron(II) is titrated with cerium(IV) in aqueous solution in a burette. The arrangement is shown in Figure 4.6, the platinum indicator electrode changes its potential (with reference to a calomel half-cell as standard) as the solution is titrated. Figure 4.7 shows the graph of the cell e.m.f. against added cerium(IV). At the equivalence point the amount of the added Ce (aq) is equal to the original amount of Fe (aq) hence the amounts of Ce (aq) and Fe (aq) are also equal. Under these conditions the potential of the electrode in the mixture is ( - - f)/2 this, the equivalence point, occurs at the point indicated. 

A selected list of redox indicators will be found in Table 8.26. A redox indicator should be selected so that its if" is approximately equal to the electrode potential at the equivalent point, or so that the color change will occur at an appropriate part of the titration curve. If n is the number of electrons involved in the transition from the reduced to the oxidized form of the indicator, the range in which the color change occurs is approximately given by if" 0.06/n volt (V) for a two-color indicator whose forms are equally intensely colored. Since hydrogen ions are involved in the redox equilibria of many indicators, it must be recognized that the color change interval of such an indicator will vary with pH. 

Suppose that the only available indicator changes color at a pH of 6.8. Is this end point close enough to the equivalence point that the titration error may be safely ignored To answer this question we need to know how the pH changes during the titration. 

It has been shown that for most acid-base titrations the inflection point, which corresponds to the greatest slope in the titration curve, very nearly coincides with the equivalence point. The inflection point actually precedes the equivalence point, with the error approaching 0.1% for weak acids or weak bases with dissociation constants smaller than 10 , or for very dilute solutions. Equivalence points determined in this fashion are indicated on the titration curves in figure 9.8. 

The two forms of the indicator, HIn and In, have different colors. The color of a solution containing an indicator, therefore, continuously changes as the concentration of HIn decreases and the concentration of In increases. If we assume that both HIn and In can be detected with equal ease, then the transition between the two colors reaches its midpoint when their concentrations are identical or when the pH is equal to the indicator s piQ. The equivalence point and the end point coincide, therefore, if an indicator is selected whose piQ is equal to the pH at the equivalence point, and the titration is continued until the indicator s color is exactly halfway between that for HIn and In. Unfortunately, the exact pH at the equivalence point is rarely known. In addition, detecting the point where the concentrations of HIn and In are equal maybe difficult if the change in color is subtle. 

The need for the indicator s color transition to occur in the sharply rising portion of the titration curve justifies our earlier statement that not every equivalence point has an end point. For example, trying to use a visual indicator to find the first equivalence point in the titration of succinic acid (see Figure 9.10c) is pointless since any difference between the equivalence point and the end point leads to a large titration error. 

Although not commonly used, thermometric titrations have one distinct advantage over methods based on the direct or indirect monitoring of plT. As discussed earlier, visual indicators and potentiometric titration curves are limited by the magnitude of the relevant equilibrium constants. For example, the titration of boric acid, ITaBOa, for which is 5.8 X 10 °, yields a poorly defined equivalence point (Figure 9.15a). The enthalpy of neutralization for boric acid with NaOlT, however, is only 23% less than that for a strong acid (-42.7 kj/mol... 

In a titrimetric method of analysis the volume of titrant reacting stoichiometrically with the analyte provides quantitative information about the amount of analyte in a sample. The volume of titrant required to achieve this stoichiometric reaction is called the equivalence point. Experimentally we determine the titration s end point using a visual indicator that changes color near the equivalence point. Alternatively, we can locate the end point by recording a titration curve showing the titration reaction s progress as a function of the titrant s volume. In either case, the end point must closely match the equivalence point if a titration is to be accurate. Knowing the shape of a titration... 

Draw an appropriate titration curve for aspartic acid, labeling the axes and indicating the equivalence points and the values. 

The objective of the titration is to determine the point at which reaction is complete, called the equivalence point. This is reached when the number of moles of OH- added is exactly equal to the number of moles of acetic acid, HC O originally present To determine this point, a single drop of an acid-base indicator such as phenolphthalein is used. It should change color (colorless to pink) at the equivalence point. 

As pointed out in Chapter 4, an acid-base indicator is useful in determining the equivalence point of an acid-base titration. This is the point at which reaction is complete equivalent quantities of acid and base have reacted. If the indicator is chosen properly, the point at which it changes color (its end point) coincides with the equivalence point To understand how and why an indicator changes color, we need to understand the equilibrium principle involved. 

A weak acid-strong base titration. The curve represents the titration of 50.00 mL of 1.000 M acetic acid, HC2H3O2. with 1.000 /W NaOH. The solution at the equivalence point is basic (pH = 9.22). Phenolphthalein is a suitable indicator. Methyl red would change color much too early, when only about 33 mL of NaOH had been added. Bromthymol blue would change color slightly too quickly. 

From Figure 14.7, it should be clear that the only suitable indicator listed is methyl red. The other two indicators would change color too early, before the equivalence point. In general, for the titration of a weak base with a strong acid, the indicator should change color on the acid side of pH 7. 

---