Of acid-base indicators

Harvey, D. T. Statistical Evaluation of Acid/Base Indicators, /. Chem. Educ. 1991, 68, 329-331. 

A less accurate but more colorful way to measure pH uses a universal indicator, which is a mixture of acid-base indicators that shows changes in color at different pH values (Figure 13.5, p. 359). A similar principle is used with pH paper. Strips of this paper are coated with a mixture of pH-sensitive dyes these strips are widely used to test the pH of biological fluids,... 

Because the pH changes rapidly near the stoichiometric point, an indicator is suitable for a titration if the pH at the stoichiometric point is within one unit of p K pHstoichiometric point P In 1 Table 18-2 lists a selection of acid-base indicators, and Example shows how to select an appropriate indicator. 

Conductometric titration rests on the marked changes that occur near the titration endpoint in the relation between conductivity and the amount of titrant added (an extreme or inflection point). It is used in particular for the titration of acids with base (and vice versa) in colored and turbid solutions or solutions containing reducing and oxidizing agents (i.e., in those cases where the usual color change of acid-base indicators cannot be seen). 

While the role of acid-base indicators in acidimetric titrations has been surpassed by more advanced, automatic, potentiometric methods various metallochromic indicators are still in use in complexometric analysis. Here we shall deal only with acid-base indicators. 

Litmus was probably the most popular choice of acid-base indicator, but it is not a good choice for colour-blind chemists. The use of phenolphthalein as an acid-base indicator comes a close second. Phenolphthalein (XIII) is another weak organic acid. It is not particularly water soluble, so we generally dissolve it in aqueous ethanol. The ethanol explains the pleasant, sweet smell of phenolphthalein solutions. 

Experimental Results. Experimental values of pan have been obtained with the potentiometric method as well as originally with the indicator method using colorimetry of acid-base indicators. 

To be useful in a titration, the pH range of an indicator must lie within the vertical section of the graph. The pH range of a variety of acid-base indicators is given on p. 20 of the SQA Data Booklet. 

Place bottle of hexamethylene diamine in a pan of hot water to melt a small amount of the solid (m. p. 42-45°C). Dissolve 2.0 g of sodium hydroxide pellets in 100 mL of water. Add 5.8 g of hexamethylene diamine and stir. Add a few drops of acid-base indicator or food coloring if desired. Store in a capped bottle. 

Bromothymol blue and cabbage juice work in much the same way as phenolphthalein. As the acidity of the solution they re in changes, their molecular structure changes as well. These different structures, three kinds in each case, re-radiate different colors of light. Of course, there are many more kinds of acid-base indicators, and each one is either a weak, low-concentration acid or a weak, low-concentration base. That s why they interact with the acids and bases... 

The charge types of the acid and alkaline forms of the indicators significantly affect salt and solvent behavior of acid-base indicators. A brief summary of the major effects is as follows. 

Azo indicators A group of acid/base indicators that have in common the stmcture R—N=N—R. ... 

A series of acid-base indicators can be used to estimate the pH of an unknown solution. Use the values given in Table 19-4 to determine the possible range of pH values of the following solution. The solution was colorless with phe-nolphthalein, yellow with methyl orange, and yellow with methyl red. 

The manner in which various solvents affect the properties of acid-base indicators has not as yet been formulated quantitatively. A more exact treatment of these properties in ethyl alcohol and in alcohol-water mixtures will be attempted in the next two sections. First, however, the literature bearing on this subject will be reviewed. 

The influence of other solvents on the properties of acid-base indicators. 

W. Szbms, Handbook of acid-base indicators, CRC Press, Boca Raton, Florida, 2007. ISBN 9780849382185. ION EXCHANGE... 

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