Add-Base Indicators

The flask contains vinegar (a dilute solution of acetic acid) and an add-base indicator (phenolphthalein) that is colorless in add solution and pink in basic solution. 

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 ... 

Here, A denotes an add of type HA-, HA or BH+, and z and q denote the charge and the radius, respectively, of species i. The influence of permittivity on p/C, depends on the charges, radii and the charge locations of the add and its conjugate base. Table 3.3 shows the pKa values of some acids and add-base indicators in water, methanol and ethanol [3], The solvent effects on pK l are smaller for BH+-type adds than for HA- or HA-type acids. For the BH+-type acids, zA=l and zB=0 in Eq. (3.16), and the influence of solvent permittivity is expeded to be small. 

Add-base indicators are either weak adds or weak bases. Because they interact with stronger acids and bases and change color in a limited pH range, they are useful as pH indicators. 

An important factor in the application of EDTA titration methods has been the development of suitable metal ion indicators, which permit visual titrations to be carried out in dilute solutions. A metal ion indicator is usually a dyestuff that forms metal ion complexes of a color different from that of the uncomplexed indicator. The complex forms over some characteristic range of values of pM, exactly as an add-base indicator forms a hydrogen ion complex over a characteristic range of pH... 

Typical add base indicators are organic dyes that change colour at or near the equivalence or end-point. They have the following characteristics ... 

USE As add-base indicator pH 4.5 red, 8.3 blue. For preparing litmus papers in microscopy to color culture media for diagnostic purposes. Has been used for coloring beverages. 

Bromcresol purple Bromocresol purple EINECS 204-087-8 NSC 374134 Phenol, 4,4 -(3H-2,1-benzoxathiol-3-ylidene)bis(2-bromo-6-methyl-, S,S-dioxide Phenol, 4,4 - 1,1-dioxido-3H-2,1-benzoxathiol-3-ylidene)bis(2-bromo-6-methyl. An add-base indicator. Crystals mp = 241,5" irritant practically insoluble in H2O, soluble in EtOH. 

Add-base indicators consist of molecules that have different molecular structures at lower pH and at higher pH. These higher-pH and lower-pH forms have different colours. 

FIGURE 16.7 pH ranges for common add-base indicators. Most indicators have a useful range of about 2 pH units. 

Add-base indicators are generally weak protolytes that change color in solution according to the pH. The acid-base equilibrium of a weak acid type of indicator (HI) in water can be represented as... 

There is no book available in the market directly on acid-base indicators even though use of acid-base indicators is widespread, growing rapidly and has exploded in the past two decades. There was a need to pubUsh a book that provided an immediate incentive for compiUng the notes to update the scientific conununity with the wealth of information on acid-base indicators. The literature in dyestuff chemistry, particularly on acid-base indicators, is largely in patents. This book provides a systematic and up-to-date library of information on 200+ add-base indicators as a reference handbook. It was compiled as a resource guide for chemists and non-chemists in industry and university. 

He employed a range of adds and he correlated the affinity (reactivity) of an acid with its catalytic power. He was therefore in a good position to appreciate Arrhenius s concept of electrolytic dissociation when the latter sent him a copy of his doctoral thesis in 1884. In 1887 Ostwald moved to Leipzig as professor of physical chemistry. For the remainder of his career he championed the ionic theory of Arrhenius against much opposition. He provided additional evidence for the theory, and he developed the theory of add-base indicators. He resigned from Leipzig in 1905, and in his retirement he worked on the theory of colours, as well as espousing many humanistic, educational and cultural causes. 

Bassam Z. Shakhashiri, "Colorful Add-Base Indicators," Chemical Demonstrations A Handbook for Teachers of Chemistry, Vol. 3 (The University of Wisconsin Press, Madison, 1989) pp. 33-40. 

Figure 7.17 The reaction of CO2 with water, (a) Water has been made slightly basic and contains a few drops of bromthymd blu an add-base indicator that is blue in basic solution, (b) When Dry Ice , C02(s), is added, the color changes to yellow, indicating an acidic solution. The mist is due to water droplets condensed from the air by the cold CO2 gas. 

Robert C. Mebane and Thomas R. Rybolt, "Edible Add-Base Indicators," y. Chem. Educ, Vol. 62,1985, 285. 

Methyl orange is a dye and add-base indicator. Locate the structure and classify this dye on the basis of the systems presented in this chapter. 

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