Change color

These coloration changes are given by most compounds which possess the -CHj-CO— group for further examples, see Tests 4 (a) and (b) for Ketones, p. 344. 

Murexide test. Place about o-i g. of uric acid in a small evaporating-basin and moisten with 2 3 drops of cone. HNO3. Heat very gently to dryness, and then add i drop of aqueous NHj from a glass rod a purple coloration is produced due to the formation of ammonium purpurate or murexide. Now add a drop of NaOH solution the coloration changes to blue. 

Indicator Chemical name pH range pK Color change... 

Mixed indicators give sharp color changes and are especially useful in titrating to a given titration exponent (p7). 

Violet Rose at pH 9.6, violet at 10 sharp color change... 

Name pH Range Color Change Acid to Base Indicator Solution... 

Name Reduction Potential (30°C) in Volts at Suitable pH Range Color Change Upon Oxidation... 

A selected list of redox indicators will be found in Table 8.26. A redox indicator should be selected so that its if" is approximately equal to the electrode potential at the equivalent point, or so that the color change will occur at an appropriate part of the titration curve. If n is the number of electrons involved in the transition from the reduced to the oxidized form of the indicator, the range in which the color change occurs is approximately given by if" 0.06/n volt (V) for a two-color indicator whose forms are equally intensely colored. Since hydrogen ions are involved in the redox equilibria of many indicators, it must be recognized that the color change interval of such an indicator will vary with pH. 

In Table 8.26, E° represents the redox potential at which the color change of the indicator would normally be perceived in a solution containing approximately 1A7H+. Lor a one-color indicator this is the potential at which the concentration of the colored form is just large enough to impart a visible color to the solution and depends on the total concentration of indicator added to the solution. If it is the reduced form of the indicator that is colorless, the potential at which the first visible color... 

Methyl green, hexamethylpararosaniline hydroxymethylate (component of mixed indicator) dissolve 0.1 g in 100 mL alcohol when used with equal parts of hexamethoxytriphenyl carbinol gives color change from violet to green at a titration exponent (pi) of 4.0. 

Methyl yellow, p-dimethylaminoazobenzene, benzeneazodimethylaniline (indicator) dissolve 0.1 g in 200 mL alcohol pH range red 2.9-4.0 yellow. The color change from yellow to orange can be perceived somewhat more sharply than the change of methyl orange from orange to rose, so that methyl yellow seems to deserve preference in many cases. See also under methyl orange. 

A list of several common acid-base indicators, along with their piQs, color changes, and pH ranges, is provided in the top portion of Table 9.4. In some cases. 

The photo in Colorplate 8b shows the indicator s color change for this titration. 

A few substances indicate the presence of a specific oxidized or reduced species. Starch, for example, forms a dark blue complex with 13 and can be used to signal the presence of excess 13 (color change colorless to blue), or the completion of a reaction in which 13 is consumed (color change blue to colorless). Another example of a specific indicator is thiocyanate, which forms a soluble red-colored complex, Fe(SCN) +, with Fe +. 

Dilute solutions of nominally 0.001 M NaOH and HGl are used to demonstrate the effect of an indicator s color transition range on titration error. Potentiometric titration curves are measured, and the indicator s color transition range is noted. Titration errors are calculated using the volume of titrant needed to effect the first color change and for a complete color change. 

Color Changes in Quartz. AH quart2 contains small amounts of substitutional Al, typically 0.01%, as well as similar amounts of interstitial hydrogen or alkah metal ions. When an Al " replaces a one positive charge is missing and electroneutraUty is maintained by or Na", etc. Irradiation ejects one of a pair of electrons on an O adjacent to an Al ... 

The indicator method is especially convenient when the pH of a weU-buffered colorless solution must be measured at room temperature with an accuracy no greater than 0.5 pH unit. Under optimum conditions an accuracy of 0.2 pH unit is obtainable. A Hst of representative acid—base indicators is given in Table 2 with the corresponding transformation ranges. A more complete listing, including the theory of the indicator color change and of the salt effect, is also available (1). 

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