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Phenolphthalein's common use is as an indicator in acid-base titrations. It also serves as a component of universal indicator, together with methyl red, bromothymol blue, and thymol blue. [3] Phenolphthalein adopts different forms in aqueous solution depending on the pH of the solution.
Upon reduction, the very intense pink color of the cationic form of phenolphthalein fades to a faint yellow color. It is this form of phenolphthalein that is present in Kastle–Meyer test kits. In order to generate the intense pink color indicative of a positive test, the reduced phenolphthalein must be oxidized back to its normal, colored form.
Phenolphthalein can be synthesized by the condensation of phthalic anhydride with two equivalents of phenol under acidic conditions (hence the name). It was discovered in 1871 by Adolf von Baeyer. [8] [9] [10] Synthesis of phenolphthalein [11]
Its availability was announced in the Federal Register, Vol. 68, No. 231 on December 2, 2003 and is listed under "Notices." Presumptive testing has found widespread use by employers and public entities. Most people who take a drug test take a presumptive test, cheaper and faster than other methods of testing.
A universal indicator is usually composed of water, 1-propanol, phenolphthalein, sodium hydroxide, methyl red, bromothymol blue, sodium bisulfite, and thymol blue. [5] The colours that indicate the pH of a solution, after adding a universal indicator, are:
Animation of a strong acid–strong base neutralization titration (using phenolphthalein).The equivalence point is marked in red. In chemistry, neutralization or neutralisation (see spelling differences) is a chemical reaction in which acid and a base react with an equivalent quantity of each other.
Gas phase titrations are titrations done in the gas phase, specifically as methods for determining reactive species by reaction with an excess of some other gas, acting as the titrant. In one common gas phase titration, gaseous ozone is titrated with nitrogen oxide according to the reaction O 3 + NO → O 2 + NO 2. [34] [35]
pH indicators: a graphic view. A pH indicator is a halochromic chemical compound added in small amounts to a solution so the pH (acidity or basicity) of the solution can be determined visually or spectroscopically by changes in absorption and/or emission properties. [1]