Acid-base indicators transition interval

Table 3-3 TRANSITION INTERVALS OF SELECTED ACID-BASE INDICATORS...

Table 3 pXi values and transition intervals of some common acid-base indicators ... 

Acetate, 11, 73, 77 Acetic acid, 11, 73, 77 titration, 94 Acetylsalicylic acid, 81 Acid-base indicators, pAj values/transition intervals, 104 Acid-base titrations, 89 Acidity constant, 5 Acids, dibasic, 22, 56, 113, 127 monobasic, 19, 36, 55 pAa values, 11... 

The most common acid-base indicators are either azo dyes for example, methyl orange and methyl red nitrophenols phthaleins such as phenol-phthalein or thymolphthalein or sulfonephthaleins like bromophenol blue or bromocresol green. Acid-base indicators are available that cover visual transitions usually expressed in intervals of 2 pH units ranging from pH 0.0 to 2.0 in small increments up to pH 12.0-14.0. 

The reason for systematic titration errors is that the equivalence point is indicated too early or too late. This happens when the transition point of the indicator does not exactly match the pH of the equivalence point of the titration (systematic errors caused by wrongly calibrated pipettes or burettes will not be discussed here). The transition point of an indicator gives the experimental endpoint of the titration. Because the term endpoint can also be applied in the sense of theoretical endpoint = equivalence point we shall use here the term transition point to be clear. The same can happen in case of instrumental methods of indication when these methods do not identify the equivalence point correctly, but systematically deviate from it. Color indicators are themselves acid-base systems Hl/1 (HI + H2O 1 + HsO ), the p a value of which is usually denoted as the pA) value, and it normally falls in the range of 2-12. There are bichromic and monochromic indicators. For example, a bichromic indicator may be red as an acid and blue as a base, and a monochromic may be colorless as an acid and violet as a base. In the case of bichromic indicators, the color changes when Chi = cr, that is at the buffer point of the indicator. Of course, the color change does not abruptly occur there, but it is smeared out in an interval (the so-called transition interval of an indicator), roughly in the... 

Indicator pAT, Transition interval Color of the acid CoIot of the base... 

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