Monograph pubs.acs.org/doi/book/10.1021/acsreagents
Methyl Red (2-[4-(Dimethylamino)phenylazo]benzoic Acid; C.I. Acid Red 2) Part 4, Monographs for Reagent Chemicals: General Descriptions, Specifications, and Tests eISBN: 9780841230460 Tom Tyner Chair, ACS Committee on Analytical Reagents James Francis Secretary, ACS Committee on Analytical Reagents
ABSTRACT This monograph for Methyl Red provides, in addition to common physical constants, a general description including typical appearance and applications. The monograph also details the following specifications and corresponding tests for verifying that a substance meets ACS Reagent Grade specifications including: Melting Point, Clarity of Alcohol, Aqueous Solutions, and Visual Transition Interval.
C15H15N3O2 (free acid) C15H14N3O2Na (sodium salt) C15H15N3O2·HCl (hydrochloride)
Formula Wt 269.30 Formula Wt 291.29
CAS No. 493-52-7 CAS No. 845-10-3
Formula Wt 305.76
CAS No. 63451-28-5
Note: Three forms of this indicator are available: the compound named above (I), the sodium salt of the compound (II), and the hydrochloride of the compound (III). Compound I must meet the specifications for melting point (range), clarity of alcohol solution, and visual transition interval. This form is recommended for nonaqueous titrations, particularly when an aprotic solvent is used. Compound II must meet the specifications for clarity of alcohol solution, clarity of aqueous solution, and visual transition interval. This form is the choice for titrations in aqueous media and is also suitable for nonaqueous titrations where the medium is an amphiprotic solvent. Compound III must meet the specifications for clarity of alcohol solution and visual transition interval. This form can be used as an indicator for titrations in aqueous media and amphiprotic solvents.
© 2017 American Chemical Society
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DOI:10.1021/acsreagents.4228 ACS Reagent Chemicals, Part 4
ACS Reagent Chemicals
Monograph
pubs.acs.org/doi/book/10.1021/acsreagents
GENERAL DESCRIPTION Typical appearance . . . . . . . . . . . . . . . . . . . red to brown solid Applications . . . . . . . . . . . . . . . . . . . . . . . . pH indicator
SPECIFICATIONS Melting point . . . . . . . . . Clarity of alcohol solution . Clarity of aqueous solution Visual transition interval . .
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179–182 °C (I) Passes test (I, II, III) Passes test (II) From pH 4.2 (pink) to pH 6.2 (yellow) (I,II,III)
TESTS Melting Point [Part 2: Measurement of Physical Properties; Melting Point].
Clarity of Alcohol and Aqueous Solutions Dissolve 0.1 g of II in 100 mL of water. For all three forms (I, II, and III), dissolve 0.1 g in 100 mL of alcohol. Not more than a faint trace of turbidity or insoluble matter should remain. Reserve the alcohol solutions for the test for visual transition interval.
Visual Transition Interval Dissolve 1 g of potassium chloride in 100 mL of water. Adjust the pH of the solution to 4.2 (using a pH meter) with 0.01 N hydrochloric acid. Add 0.15 mL of the 0.1% solution reserved from the test for clarity of alcohol and aqueous solutions. The color of the solution should be pink. Titrate the solution with 0.01 N sodium hydroxide to a pH of 5.5 (using a pH meter). The solution should be orange. Continue the titration to a pH of 6.2. The solution should be yellow.
© 2017 American Chemical Society
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DOI:10.1021/acsreagents.4228 ACS Reagent Chemicals, Part 4
