Congo Red Paper – Packs & Rolls

Quick Answer: Congo Red Paper is test paper dyed with the indicator Congo red, which is red above about pH 5 and turns blue to blue-violet below about pH 3; it is used mainly to show the presence of free strong acid, and is supplied in packs and rolls.

Detecting Free Mineral Acid

Congo red changes colour in the acidic part of the scale, between roughly pH 3.0 and 5.2. That is well below the change point of litmus, so a solution of a weak acid such as acetic acid may turn blue litmus red but only partly changes the Congo red strip, while a strong mineral acid such as hydrochloric or sulfuric acid turns it clearly blue.

This selectivity is why chemists use it to test for ‘free mineral acid’. In organic preparations, for example when the salt of a carboxylic acid is acidified to precipitate the free acid, hydrochloric acid is added until a drop of the mixture turns the paper blue, showing that enough strong acid is present. In analytical and inorganic work the paper helps distinguish strongly acidic solutions from mildly acidic ones.

The colour change is reversible: a blue strip turns red again with alkali. Like other dyed reagent papers, it is for laboratory use only, so strips should be handled with dry hands or forceps.

Specifications

Applications

• Checking that enough hydrochloric acid has been added when liberating an organic acid from its salt.
• Distinguishing strong mineral acids from weak acids in teaching practicals.
• Testing for free acid in pickling or etching solutions during technical training.
• Comparing indicator ranges alongside litmus, methyl orange and universal paper.

Care & Handling

• Store the paper sealed and away from acid fumes, which turn unused strips blue.
• Transfer a drop of the mixture with a glass rod rather than dipping a strip into a reaction flask.
• Lay an unused strip beside the test strip when the colour change is only partial.

Why Choose LabEquip

Organic chemistry teaching labs, analytical training centres and industrial training workshops are the usual buyers of Congo red paper. LabEquip lists it alongside Bromocresol Green Paper and the other indicator papers under General Lab Products; pack or roll quantities can be requested on the contact page.

Frequently Asked Questions

What colour does Congo red paper turn in acid?

It turns blue to blue-violet in strongly acidic solutions, below about pH 3. Above about pH 5 it stays red, and in between it shows an intermediate purple.

Why use it instead of litmus for acids?

Litmus changes near neutral and reacts to both weak and strong acids. Congo red changes much lower on the scale, so a clear blue indicates a strongly acidic solution, which is what free mineral acid tests need.

How is the paper used in organic preparations?

When acidifying a carboxylate or phenoxide salt to release the free compound, acid is added until a drop of the mixture turns the paper blue. That shows the mixture is strongly acidic and the conversion is complete.

Can the blue colour change back?

Yes. Adding alkali to a blue strip turns it red again, because the colour depends on pH.

Does Congo red paper give a pH value?

No, it only shows whether the pH is below or above its transition range. Use a pH indicator paper or a meter for a number.

How should the paper be stored?

Keep it in a closed container, dry, and away from acid vapours and direct light, and use a fresh strip for every test.

Last Updated: September 2026