Neutral Litmus Paper

A qualitative indicator paper that shows acid or alkaline on one paper, starting from a neutral purple. It gives a yes or a no, not a value.

Why it is measured

Neutral litmus removes the need to carry both papers: it starts purple and moves red for acid or blue for base, so a single strip answers the question in either direction.

Where it is used

• general laboratory use
• field spot checks
• teaching demonstrations
• unknown sample screening

What it answers

A qualitative paper does one thing and does it without ambiguity. There is no scale to interpolate and no lighting to argue about: the colour changes or it does not.

That makes it the right tool when the decision is binary — proceed or do not — and the wrong tool when anyone needs to know how far past the threshold the sample is. For a value, use an indicator paper with a scale or a meter.

Practical note

The starting colour is the reference; compare against an unused strip rather than from memory, since the paper darkens with age.

Supplied in 3 presentations covering different measuring ranges. The range is chosen by model: 8.0.