Methyl orange

Appearance under ordinary conditions: orange-yellow leaves or powder, flakes. Methyl orange is soluble in water of 0.2 g per 100 g, preferably in hot.

In solutions with pH 2, the absorption maximum is at a wavelength of 505 nm (λmax = 505 nm). Isobestic point corresponds to a wavelength of 460 nm ^[2] .

The transition of color in aqueous solutions from red to orange-yellow is observed in the pH range 3.1-4.4 (in an acidic medium red, in an alkaline medium yellow).

The color transition interval is affected by: temperature, the presence in the solution of salts, organic solvents , protein substances and others. The effect of temperature is most significant for indicators that are weak bases : for example, for methyl orange at room temperature, the color changes within pH 3.1-4.4, and at 100 ° C within pH 2.5-3.7.

Toxic ^[3]

It is used as an acid-base indicator, titrant in the determination of strong oxidizing agents, spectrophotometric determination of oxidizing agents ( chromium , bromine ).

A 0.1% aqueous solution is used in analytical chemistry as an indicator .

It changes color from red in an acidic environment (pH 3.1 to 4.4) to orange in neutral and yellow in alkaline.

Laboratory synthesis method:

Methyl orange is obtained by diazotizing sulfanilic acid , and then combining the resulting substance with dimethylaniline .

Execution technique:

A portion of sulfanilic acid is dissolved in 25 ml of a 2 M sodium hydroxide solution (2 g of NaOH in 25 ml of solution). A sample of sodium nitrite weighing 4 g is then dissolved in the same solution. After this, the solution is cooled with ice and poured into 25 ml of a 2 M hydrochloric acid solution cooled by ice (in addition to external cooling, pieces of ice can be placed in the solution). A portion of dimethylaniline is dissolved in 5 ml of 1 M hydrochloric acid, cooled with ice, and the diazobenzenesulfonic acid solution obtained above is poured into the cooled solution. The formation of dye. Another caustic soda solution is poured to a strongly alkaline reaction. The sodium salt of the dye is released from the solution in the form of orange-brown petal-like crystals. After a few hours, the dye is filtered off with suction and washed with a funnel of 25 ml of water. Then it is carefully squeezed on filter paper and dried in a porcelain cup in a water bath.

To memorize the color of the indicator of methyl orange in alkalis and acids, a mnemonic poem is used:

From alkali I am yellow in a fever
I am pink from acids, as from shame.
And I rush into the water without looking back
Here I am almost always orange.

Another way to remember:

The environments are in order: acidic, neutral and alkaline. The poem for remembering the colors of the rainbow goes the same way. "Every Hunter Desires ..."

Acidic medium (pH <7) - Each

Neutral Medium (pH = 7) - Hunter

Alkaline environment (pH> 7) - Desires

• basic properties of methyl orange
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