What is Methyl Orange?

by Lucy Bell-Young

Methyl orange is one of the most common indicators used in analytical chemistry to determine pH and titration thresholds. Analytical chemistry requires both qualitative and quantitative data to accurately describe the composition, chemical properties, and molecular structures of substances. 

What is the Chemical Name for Methyl Orange?

Methyl orange is commonly used as a qualitative pH indicator. Its preferred IUPAC (International Union of Pure & Applied Chemistry) name is Sodium 4-{[4-(dimethylamino)phenyl]diazenyl}benzene-1-sulfonate. Its alternative chemical name is Sodium 4-[(4-dimethylamino)phenylazo]benzenesulfonate. These are technical names that indicate the composition and molecular structure of the substance.

Methyl orange is an organic compound that doesn’t permanently react with acids or bases, but instead changes colour from red to yellow in an aqueous acidic solution. Its effective pH range as an indicator in aqueous solutions is from pH 3.1 to pH 4.4. 

The gradual change in colour is only effective in aqueous acidic solutions. Beyond the range, it’s impossible to know if the solution is still acidic, neutral, or basic. Other indicators are used to determine the pH at different ranges:Table of pH colour indicatorsThe accuracy of methyl orange varies depending on several factors, such as temperature and any solvents that are present. For instance, this indicator has a pKa of 3.47 in water at 25°C. Temperature is a crucial factor in determining the accuracy because the speed of the molecules in a solution is dependent on the temperature. Acidic solutions tend to dissociate better as ions in solutions have higher temperature.


What is the Chemical Formula for Methyl Orange?

Methyl orange is an organic compound composed of carbon, hydrogen, nitrogen, sodium, oxygen, and sulphur. Its precise chemical formula is C14H14N3NaO3S. It’s one of the common chemical compounds used as a pH indicator, particularly in titrations, because the colour transitions are easy to distinguish at various pH values. Nonetheless, there are lower and upper limits to this. 

Methyl orange isn’t the same as a universal indicator like litmus paper because it only has a very narrow range of sensitivity. This means it’s only used as a test for acidity within the previously mentioned range (pH 3.1 to pH 4.4). It doesn’t have a full spectrum of colours and cannot be used to test for the alkalinity of a substance. 

When the acidity of a solution decreases, this indicator changes from red to orange and finally to yellow. The reverse is true if the solution becomes more acidic. In a titration experiment, you’ll know that you’ve reached a certain threshold based on the colour change.

Is Methyl Orange a Natural Indicator?

Methyl orange is not a natural indicator. It’s actually synthesised from sodium nitrate, dimethylaniline, and sulfanilic acid.

Firstly, diazonium salt is produced, as shown in the illustration below:

Structural reaction of diazonium salt with dimethylaniline

Diazonium salts are organic compounds that have a functional group that fits the generalised form R−N+2X.

The R represents any organic group, such as an alkyl or an aryl. Meanwhile, the X represents an organic or inorganic anion. Methyl orange is an azo compound, which is a dye. In fact, most azo compounds are used as dyes.

Secondly, the diazonium salt is then coupled with dimethylaniline, as shown in the structural reaction below:

Structural reaction of diazonium salt with dimethylaniline

How Does Methyl Orange Work?

  1. The Ostwald’s Theory

Basically, Ostwald’s theory states that the colour change in methyl orange is because of the ionisation of the indicator. The unionised form has a distinct colour from the ionised form. The indicator itself is either a weak acid or a weak base. The drawback of this theory, however, is the fact that it doesn’t explain the structural changes.

This indicator is a relatively complex organic compound, which changes colours based on its ionisation levels in an acidic solution. To understand what is happening when the dye changes colours, we can simplify the chemical formula of the dye as MeOH.

The hydronium ion (H+) in an aqueous acidic solution is the one that iodises the MeOH into Me(+) and OH(-). On one hand, the hydroxyl ion combines with the hydronium ion, forming water. On the other hand, the ionised methyl ion is the one that gives the colour. The concentration of ions in the solution determines the colour change and the pH threshold within the effective range. The ionic reactions are reversible, depending on the pH level.

  1. The Modern Benzenoid Theory

According to Modern Benzenoid theory, pH indicators like methyl orange exist in interconvertible forms, namely, Benzenoid and Quinonoid structures. The theory states that one form predominates in one medium. The Quinonoid form of the indicator has deeper colour than the Benzenoid form. Methyl orange can exist in either forms depending on the pH level.

Methyl Orange colors in aqueous media with different pH

Why is Methyl Orange Used in Titration?

In titration, methyl orange is used with acidic solutions. It’s used in titration because it has such a narrow range, meaning that it can be used for a secondary indicator after a certain threshold is reached by the primary indicator.

The clear and distinct colour transitions of methyl orange also make it a good indicator for titration experiments. You can adjust the titration knob to allow barely any drops of the titrant into the analyte until the right colour is achieved, indicating the end of titration.

Methyl orange is relatively simple to prepare and use in a laboratory setting. The use of an indicator is useful if you want to determine the equivalence point between a known titrant and an analyte.

It’s also useful in titrating most mineral acids – but not organic acids. It’s suitable for titrating an acid with a moderately weak base, such as sodium carbonate. As an analytical tool, methyl orange is fairly precise when used in titration.

As a final word, you should be careful when storing powder methyl orange because it has the risk of detonating. All azo compounds can potentially detonate, similar to diazo and azido compounds.


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