Transition range acid-base indicator

In this experiment the effect of a mixed aqueous-organic solvent on the color transition range of common indicators is investigated. One goal of the experiment is to design an appropriate titrimetric method for analyzing sparingly soluble acids and bases. 

Indicator name Acid color Transition range (pH) Base color... 

M For a list of common acid/base indicators and their colors, look inside the front cover of this book. See also color plate 8 for photographs showing the colors and transition ranges of 12 common indicators. 

Transition pH range The span of acidities (frequently about 2 pH units) over which an acid/base indicator changes from its pure acid color to that of its conjugate base. 

Color plate 8 Acid-base indicators and their transition pH ranges (Section 14A-2). 

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. 

Errors (2) and (3) are negligibly small in comparison to error (1) and consequently in the selection of a suitable indicator only the magnitude of the chemical error is of great importance. Thus the appropriate acid-base indicator must have its transition pH range within the equivalence region. 

However, as has been shown by Kolthoff and Bruc-kenstein, there is no direct relationship between pH and the color of an indicator in acetic acid medium. Thus, it is not feasible to specify transition ranges of indicators in terms of pH or millivolts in this solvent. Taking into account that in acetic acid medium the titrant is usually a solution of perchloric acid in acetic acid, since it is the strongest acid in this medium, and moreover is very stable and no changes are caused in the reaction medium, the behavior of acid-base indicators is best represented by the general reaction... 

Tucker SA, Bates HC, and Acree WE (1995) Acid-base indicators — transition colors and pH ranges determined in select aqueous-organic mixed-solvents. Analyst 120 2277-2279. 

In some clock reactions, however, there is a narrow range of concentrations in which the quality of the mixing becomes critically important. Under these conditions, the time of the sharp transition from initial to final state becomes essentially unpredictable. The prototype system of this type is the chlorite-thiosulfate reaction (Nagypal and Epstein, 1986). Measurements of pH vs. time for five replicate experiments starting from the same initial concentrations are shown in Figure 15.8. For the first several minutes, all the curves are identical. The pH increases smoothly. In three of the curves, we observe a sharp drop in pH at approximately 3, 5, and 9 min in the other two, this decrease occurs at times greater than 20 min. When an acid-base indicator like phenolphthalein is added to the solution, the pH... 

The color of an acid-base indicator in its transition pH range is determined by the proportion of indicator molecules that exist in each of the two possible forms. 

Use a web search engine to find lists of acid-base indicators, (a) Select an indicator that is not mentioned in this chapter and give its color and pH at each end of its transition range, (b) Determine what is meant by the statement Vanilla extract has been described as an olfactory (acid-base) indicator. ... 

Spraying with a 0.01-1% aqueous or aqueous alcohol solution of an acid-base indicator (e.g., bromocresol green, bromothymol blue, bromophenol blue, methyl red, malachite green) can detect acid or basic compounds on the layer based on a change in color at the location of the zones and knowledge of the pH transition range of the indicator. 

A key aspect of metal oxides is that they possess multiple functional properties acid-base, electron transfer and transport, chemisorption by a and 7i-bonding of hydrocarbons, O-insertion and H-abstraction, etc. This multi-functionality allows them to catalyze complex selective multistep transformations of hydrocarbons, as well as other catalytic reactions (NO,c conversion, for example). The control of the catalyst multi-functionality requires the ability to control not only the nanostructure, e.g. the nano-scale environment around the active site, " but also the nano-architecture, e.g. the 3D spatial organization of nano-entities. The active site is not the only relevant aspect for catalysis. The local area around the active site orients or assists the coordination of the reactants, and may induce sterical constrains on the transition state, and influences short-range transport (nano-scale level). Therefore, it plays a critical role in determining the reactivity and selectivity in multiple pathways of transformation. In addition, there are indications pointing out that the dynamics of adsorbed species, e.g. their mobility during the catalytic processes which is also an important factor determining the catalytic performances in complex surface reaction, " is influenced by the nanoarchitecture. 

Indicator Transition range (pH) Acid color Base color Preparation... 

Would the indicator bromocresol green, with a transition range of pH 3.8—5.4, ever be useful in the titration of a weak acid with a strong base ... 

All indicators have a transition range. In this range, the indicator is partly in its acidic form and partly in its basic form. Thus, the indicator s color is intermediate between those of its acid and base colors. Figure 19 illustrates the transition range for two typical indicators, bromthymol blue and phenolphthalein. 

When you titrate a weak base, use an indicator with an acidic transition range. When titrating a weak acid, use an indicator with a basic transition range. 

Figure 14-7 shows hypothetical titration curves for a series of weak bases of different strengths. The curves show that indicators with acidic transition ranges must be used for weak bases. 

Several excellent primary standards are available for the standardization of bases. Most are weak organic acids that require the use of an indicator with a basic transition range. 

Figure 8.4 illustrates the colors and transition ranges of some commonly used indicators. The range may be somewhat less in some cases, depending on the colors some colors are easier to see than others. The transition is easier to see if one form of the indicator is colorless. For this reason, phenolphthalein is usually used as an indicator for strong acid-base titrations when applicable (see Figure 8.1, titration of 0.1 M HCl). In dilute solutions, however, phenolphthalein falls outside the steep portion of the titration curve (Figure 8.2), and an indicator such as bro-mothymol blue must be used. A similar situation applies to the titration of NaOH with HCl (Figure 8.3). A more complete list of indicators is given on the inside back cover.

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... 

Sulfamic acid is a strong acid and may be titrated with bases by means of indicators whose transition ranges lie within the pH range 4.5 to 9. Because of its unusual physical properties and the ease with which it may be prepared in a state of high purity, it has found application as an acidimetric standard of reference. It has also been suggested for the estimation and detection of nitrates and nitrites in the presence of each other. A comprehensive review of the physical and chemical properties of sulfamic acid and of its inorganic derivatives has recently appeared in Chemical Reviews. °... 

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