Acid-base titrations, methyl orange indicator

Indicators that change color at pH lower than 7 are useful in determining the equivalence point of strong-acid/weak-base titrations. Methyl orange is an example of this type. The equivalence point of a strong-acid/ weak-base titration is acidic because the salt formed is itself a weak acid. Thus the salt solution has a pH lower than 7 at the equivalence point. 

The utility of acid-base titrimetry improved when NaOH was first introduced as a strong base titrant in 1846. In addition, progress in synthesizing organic dyes led to the development of many new indicators. Phenolphthalein was first synthesized by Bayer in 1871 and used as a visual indicator for acid-base titrations in 1877. Other indicators, such as methyl orange, soon followed. Despite the increasing availability of indicators, the absence of a theory of acid-base reactivity made selecting a proper indicator difficult. 

Given three add-base indicators—methyl orange (end point at pH 4), bromthymol blue (end point at pH 7), and phendphthalein (end point at pH 9)—which would you select for the following acid-base titrations ... 

When silicotungstic acid is titrated with a base using methyl orange or chlorophenol red as the indicator, an end point is obtained when four equivalents have been added. The end point with chlorophenol is sharper because this indicator does not form a precipitate with the acid.4 When the acid is titrated using phenolphthalein as the indicator, the end point is not permanent. However, when the titration is carried out in a hot solution, an end point is reached when twenty-four equivalents of base have been added. The end products are sodium tungstate and silicic acid. 

Ethylenediamine, (I I,NCH,), is a strong organic base miscible with water and alcohol. It is a colourless and viscous liquid with a density of 0.898 and a melting point of 8 °C. The pH of a 25% aqueous solution is 11.5. Like triethanolamine, it is an aliphatic amine soluble in water and, therefore, can be determined by the acid-base titration with methyl orange as an indicator. 

PROBLEM 11.22 Methyl orange is an azo dye used as an indicator in acid-base titrations. (It is yellow-orange above pH 4.5 and red below pH 3.) Show how it can be synthesized from p-aminobenzenesulfonic acid (sulfanilic acid) and N,N-dimethylaniline. 

Table 18.6 summarizes the use of phenolphthalein and methyl orange as acid-base indicators. The principles described here can he used to select other suitable indicators for acid-base titrations. 

Neutralization indicators, or acid-base indicators or pH indicators, are auxiliary reagents added to the titrand solution in order to detect the equivalence point in acid-base titrations. They can also be used for an accurate quantitative measure of the pH. Tournesol, a natural pigment extracted from some blue-green lichens, was the first pH indicator to be used (1850). Phenolphthalein and methyl orange were introduced somewhat later (1877 and 1878, respectively). Undeniably, the chief interests in the use of acid-base indicators are their low cost and ease of handling. However, they give rise to less precise and less accurate endpoints than some instrumental methods. 

In general, indicators have two forms, which possess different colors. Indicators for acid-base titrations are themselves weak acids or bases where the two forms differ in color as shown in Figure 3 of Topic C4 for methyl orange. 

It is clear that neither thymolphthalein nor phenolphthalein can be employed in the titration of 0.1 M aqueous ammonia. The equivalence point is at pH 5.3, and it is necessary to use an indicator with a pH range on the slightly acid side (3-6.5), such as methyl orange, methyl red, bromophenol blue, or bromocresol green. The last-named indicators may be utilised for the titration of all weak bases (Kb> 5 x 10-6) with strong acids. 

FIGURE 11.13 Phenolphthalein can be used to detect the stoichiometric point of a weak acid-strong base titration, but methyl orange would give a very inaccurate indication of the stoichiometric point. The pH curves are superimposed on approximations to the color of the indicators in the neighborhoods of their end points. 

We see that to obtain the most accurate results in titrating a strong acid and a strong base an indicator with indicator constant about 10 pK = 7) should be chosen, such as litmus or bromthymol blue. The titration curve calculated above, and given in Figure 20-3, shows however that the choice of an indicator is in this case not crucial any indicator with pK between 4 (methyl orange) and 10 (thymolphthalein) could be used with error less than 0.2%. 

The basic constant of ammonium hydroxide has abotit the same value as the acid constant of acetic acid. Hence to titrate a weak base such as ammonium hydroxide with a strong acid methyl orange (pK 3.8) may be used as the indicator. 

It is possible by suitable selection of indicators to titrate separately a strong acid and a weak acid or a strong base and a weak base in a mixture of the two. Let us consider, for example, a solution of sodium hydroxide and ammonium hydroxide. If strong acid is added until the pH is 11.1, which is that of 0.1 N ammonium hydroxide solution, the strong base will be within 1% of neutralization (Fig. 20-3). Hence by using alizarine yellow [pK 11) as indicator the concentration of strong base can be determined, and then by a second titration with methyl orange the concentration of ammonium hydroxide can be found. 

In titrations of a strong acid by a strong base, the equivalence point occurs at pH 7, and bromthymol blue would be an appropriate indicator, as Table 6 confirms. However, when a weak acid is titrated by a strong base, the equivalence point is at a pH greater than 7 and thymol blue or phenolphthalein would be a better choice. On the other hand, methyl orange could be the best choice if your titration uses a weak base and a strong acid, because the equivalence point might be at pH 4. 

II) Many free bases may be directly titrated with hydrochloric acid, methyl orange being used as an indicator. 

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