Bromothymol Blue Paper

Quick Answer: Bromothymol Blue Paper is indicator paper impregnated with bromothymol blue, which is yellow below about pH 6.0, blue above about pH 7.6 and green in between, so it shows at a glance whether a solution is slightly acidic, close to neutral or slightly alkaline.

An Indicator Centred on Neutral

Most single indicators change colour well away from pH 7: methyl orange on the acid side, phenolphthalein on the alkaline side. Bromothymol blue is one of the few whose transition sits across neutral, which suits questions such as ‘is this water neutral?’ or ‘has this solution become slightly acidic?’

In biology teaching, bromothymol blue solution is familiar from carbon dioxide experiments: exhaled breath or respiring organisms turn it from blue through green to yellow as carbonic acid forms, and photosynthesising water plants turn it back. The paper brings the same indicator into strip form for spot checks of samples, such as water from those experiments, aquarium water or neutralised solutions.

Its value lies in that narrow band. Outside it, the strip stays fully yellow or fully blue however far the pH moves, so strongly and mildly acidic samples look alike. Pair it with full-range paper, such as the pH Indicator Paper (1 to 14), when the pH could be anywhere.

Applications

• Checking whether drinking, aquarium or pond water samples are near neutral.
• Confirming that acid or alkali waste has been neutralised to around pH 7.
• Biology practicals on respiration and photosynthesis that follow carbon dioxide through pH.
• Comparing indicators with different transition ranges in chemistry lessons.

Specifications

Care & Handling

• Store the strips sealed and dry; carbon dioxide and other acidic gases in the air can shift the colour of exposed paper.
• Read the colour immediately after wetting, under white light.
• Transfer a drop of sample onto the strip with a glass rod.

Why Choose LabEquip

Biology and chemistry teachers, aquarium and water-testing clubs, and labs that neutralise waste before disposal choose bromothymol blue paper for its near-neutral band. LabEquip supplies it among the test papers in General Lab Products; a term’s supply can be arranged via our contact page.

Frequently Asked Questions

What colour is bromothymol blue paper at pH 7?

Around neutral it shows green, the midpoint between its yellow acid colour and its blue alkaline colour.

Why use it instead of universal paper?

Its colour change is concentrated around neutral, so it shows small shifts between slightly acidic and slightly alkaline more clearly than a full-range chart, where neutral is a single colour block.

Can the paper show carbon dioxide?

Indirectly. Carbon dioxide dissolves in water to form weak carbonic acid, which lowers the pH. A water sample exposed to carbon dioxide can shift the strip from blue towards green or yellow.

What does a fully yellow result mean?

The sample is below about pH 6. It could be mildly or strongly acidic, so test with a full-range paper to find out how acidic.

Is it suitable for swimming pool water?

Pool test kits usually use phenol red, whose range suits pool water targets. This paper will show whether the water is roughly neutral, but a dedicated pool kit is better suited to routine pool control.

How should the paper be stored?

In a closed container, dry, cool and dark, away from acid and ammonia vapours.

Last Updated: September 2026