Methyl orange is a

pH indicator 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. Hence, ...

frequently used in

titration Titration (also known as titrimetry and volumetric analysis) is a common laboratory method of quantitative chemical analysis to determine the concentration of an identified analyte (a substance to be analyzed). A reagent, termed the ''titrant'' ...

because of its clear and distinct color variance at different pH values. Methyl orange shows red color in acidic medium and yellow color in basic medium. Because it changes color at the p''K''_a of a mid strength acid, it is usually used in titration of strong acids in weak bases that reach the equivalence point at a pH of 3.1-4.4. Unlike a universal indicator, methyl orange does not have a full spectrum of color change, but it has a sharp end point. In a solution becoming less acidic, methyl orange changes from red to orange and, finally, to yellow—with the reverse process occurring in a solution of increasing acidity.

Indicator colors

In a solution that decreases in acidity, methyl orange moves from the color red to orange and finally to yellow with the opposite occurring for a solution increasing in acidity. This color change from yellow to red occurs because the protons in the acidic solution react with the N=N bond of the molecule, protonating one of them and changing the visible light absorption of the molecule to reflect more red light than orange/yellow. In an acid, it is reddish and in alkali, it is yellow. Methyl orange has a p''K''_a of 3.47 in water at .

Other indicators

Modified (or screened) methyl orange, an indicator consisting of a solution of methyl orange and

xylene cyanol Xylene cyanol can be used as an electrophoretic color marker, or tracking dye, to monitor the process of agarose gel electrophoresis and polyacrylamide gel electrophoresis. Bromophenol blue Bromophenol blue (3′,3″,5′,5″-tetrabromophen ...

, changes from grey-violet to green as the solution becomes more basic.

Safety

Methyl orange has

mutagen In genetics, a mutagen is a physical or chemical agent that permanently changes genetic material, usually DNA, in an organism and thus increases the frequency of mutations above the natural background level. As many mutations can cause cancer i ...

ic properties.MSDS
from ScienceLab.com, Inc. Retrieved 2011-09-24 When methyl orange is put under

oxidative stress Oxidative stress reflects an imbalance between the systemic manifestation of reactive oxygen species and a biological system's ability to readily detoxify the reactive intermediates or to repair the resulting damage. Disturbances in the normal ...

, one of the double-bonded nitrogen atoms that connects the aromatic rings gets radicalized and can further break down into

reactive oxygen species In chemistry, reactive oxygen species (ROS) are highly reactive chemicals formed from diatomic oxygen (). Examples of ROS include peroxides, superoxide, hydroxyl radical, singlet oxygen, and alpha-oxygen. The reduction of molecular oxygen () p ...

aniline Aniline is an organic compound with the formula C6 H5 NH2. Consisting of a phenyl group attached to an amino group, aniline is the simplest aromatic amine. It is an industrially significant commodity chemical, as well as a versatile starti ...

s, which are

carcinogen A carcinogen is any substance, radionuclide, or radiation that promotes carcinogenesis (the formation of cancer). This may be due to the ability to damage the genome or to the disruption of cellular metabolic processes. Several radioactive sub ...

ic and can mutate DNA. Various bacteria and enzymes can also cause this breakdown to occur.

Synthesis

Methyl Orange is an azobenzene derivative that can be formed from dimethylaniline and

sulfanilic acid Sulfanilic acid (4-aminobenzenesulfonic acid) is an organic compound with the formula H3NC6H4SO3. It is an off-white solid. It is a zwitterion, which explains its high melting point. It is a common building block in organic chemistry."Sulphan ...

through first, a diazonium salt formation with the sulfanilic acid, followed by a nucleophilic attack from the dimethylaniline and rearomatization.

UV/Vis Spectrum

The absorption of methyl orange on the UV/vis spectrum is between 350-550 nm, with its peak at 464 nm. This is in the green-purple visible light range and explains why methyl orange is, in fact, orange.

References

External links

{{Commons category, Methyl orange
Informative page on different titration indicators, including methyl orange
Azo dyes PH indicators Benzenesulfonates Anilines Organic sodium salts Dimethylamino compounds Acid dyes