Acid-base titrations, methyl orange

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. 

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

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. 

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. 

In the direct method, a solution of the ammonium salt is treated with a solution of a strong base (e.g. sodium hydroxide) and the mixture distilled. Ammonia is quantitatively expelled, and is absorbed in an excess of standard acid. The excess of acid is back-titrated in the presence of methyl red (or methyl orange, methyl orange-indigo carmine, bromophenol blue, or bromocresol green). Each millilitre of 1M monoprotic acid consumed in the reaction is equivalent to 0.017032 g NH3 ... 

Methyl orange changes color between pH = 3.2 and pH = 4.4 and can be used in the titration of a weak base with a strong acid (Fig. 

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. 

A sample of magnesium ribbon was burned in air. The ash was dissolved in 60.0 milliequivaients of HC1, methyl orange added, and the resulting solution back-titrated with sodium hydroxide 12.0 meq of base were required to reach the end point. An excess of base vras then added, the solution distilled, and the ammonia released was trapped in a second solution of acid (originally containing 10.0 meq of HC1) back titration of this solution required 6.0 meq of base. 

Another satisfactory process for the determination of total hardness, based on a somewhat similar principle, is due to Blacher.4 The water is first titrated with decmormal hydrochloric acid until it is neutral to methyl orange, as in the method described above for temporary hardness. After the removal of the carbon dioxide by a current of air, the methyl orange is bleached by the addition of a drop of bromine water a little phenolphthalein and a few drops of alcoholic potassium hydroxide are added, the liquid is just decolorised with dccinormal hydrochloric acid and is then titrated with an alcoholic decmormal solution of potassium palmitate until a decided red colour is produced. The quantity of the potassium palmitate solution required is proportional to the total hardness. 

Assay Based on the labeled percentage of K2C03, accurately weigh a volume of the sample solution equivalent to about 1 g of potassium carbonate, and add it to 50.0 mL of 1 A sulfuric acid. Add 2 drops of methyl orange TS, and titrate the excess acid with 1N sodium hydroxide. Each milliliter of 1 A sulfuric acid is equivalent to 69.11 mg of K2C03. 

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. 

Morphino is a strong base, the salts of which are neutral to litmus and methyl orange. The base turns red litmus blue but does not affect phenolphthalein in aqueous solution, though a rose colour is obtained when water is added to an alcoholio solution of phenolphthalein and morphine [130], Tho base can bo titrated with acids potoritiomotrioally... 

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