Methyl Orange, indicator error

With 0.1M solutions, the ideal pH range for an indicator is limited to 45-9.5. Methyl orange will exist chiefly in the alkaline form when 99.8 mL of alkali have been added, and the titration error will be 0.2 per cent, which is negligibly small for most practical purposes it is therefore advisable to add sodium hydroxide solution until the indicator is present completely in the alkaline form. The titration error is also negligibly small with phenolphthalein. 

With 0.01M solutions, the ideal pH range is still further limited to 5.5-8.5 such indicators as methyl red, bromothymol blue, or phenol red will be suitable. The titration error for methyl orange will be 1-2 per cent. 

For the primary stage (phosphoric) V) acid as a monoprotic acid), methyl orange, bromocresol green, or Congo red may be used as indicators. The secondary stage of phosphoric) V) acid is very weak (see acid Ka = 1 x 10 7 in Fig. 10.4) and the only suitable simple indicator is thymolphthalein (see Section 10.14) with phenolphthalein the error may be several per cent. A mixed indicator composed of phenolphthalein (3 parts) and 1-naphtholphthalein (1 part) is very satisfactory for the determination of the end point of phosphoric(V) acid as a diprotic acid (see Section 10.9). The experimental neutralisation curve of 50 mL of 0.1M phosphoric(V) acid with 0.1M potassium hydroxide, determined by potentiometric titration, is shown in Fig. 10.6. 

These constants determine the titration exponents pH and the best indicators for the successive hydrions. The acid can be titrated as dibasic, using methyl yellow, methyl orange or bromophenol blue, and as tetrabasic using phenolphthalein, thymolphthalein or thymol blue in the presence of a moderate excess of soluble barium salt. The values of pH in the partly neutralised acid were corrected for the salt error, and the constants Kz and jfiT4 which prevail in solutions of low concentration were thus deduced —6... 

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%. 

Incorrect results are found also in very dilute acid or basic solutions which are not buffered. When phenolphthalein is added to a very dilute alkali solution, hydroxyl ions are bound to an extent equivalent to the quantity of the red form of the indicator produced. In analogous fashion, methyl orange binds hydrogen ions when this indicator is added to dilute acid solutions. The following illustrations will show clearly that large errors can be encountered when, for example, the acid properties of phenolphthalein are neglected in very dilute alkali solutions. 

Aside from methyl orange and methyl red, thymol blue (in the acid region pH 1.3-2.8) and tropeolin 00 also have negligible salt errors. Clearly the indicators thymol blue (pH 1.3-2.8), tropeolin 00, methyl orange, and methyl red are unusually well suited for the colorimetric determination of paH because they yield reliable results, at not too high ionic strengths, which need not be corrected. 

The concentration (cqh- )tp can be read on the logCi axis at Pia (methyl orange), 2a (methyl red), Psa (phenolphthalein) and P4a (thymolphthalein). Similarly, the concentration (cH30+)tp at Pib (methyl orange), P2b (methyl red), Psb (phenolphtha-lein) and P4b (thymolphthalein) can be read at the transition points of the indicators. Table 6 gives the relevant concentrations, and the systematic errors calculated according to Eqs. (194) and (195). 

In the case of the titration of strong acids by strong bases, and vice versa, there is no significant error due to the addition of the indicator, at least when the concentration of the latter is less than 10% that of the titrand. A calculation performed on the same grounds as those that permitted us to study the ionization repression (recul d ionisation, - see Chap. 5) shows that a 10 mol/L of sodium hydroxide and 10 mol/L of methyl orange Ka = 1.6 10 ) exhibits a pH value pH of 10.00. This is exactly the same value as that exhibited by sodium hydroxide alone at the same concentration. 

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