MACHEREY-NAGEL Duotest pH Paper

Duotest reads pH from two indicator zones at once. Each zone changes colour over a different part of the range, and the pair is matched against the chart as a combination rather than singly.

Why two zones read better than one

A single indicator gives one colour per pH value, and the eye can only separate so many shades of one hue. Two indicators give a pair of colours, and the number of distinguishable pairs is far larger than the number of distinguishable single shades. The practical result is resolution to roughly 0.3 pH units from a paper strip — close to what a cheap meter delivers, with no calibration, no electrode and no battery.

That is what makes Duotest the paper to reach for when a reading has to be better than about 6 but the work does not justify setting up a meter.

Six ranges

A wide pH 1–12 for orientation, and five narrow windows — 1.0–4.3, 3.5–6.8, 5.0–8.0, 7.0–10.0 and 9.5–14.0 — that overlap to cover the whole scale. Each is available as a reel in a dispenser and as a refill pack for it.

For a still finer reading, Tritest uses three indicator zones on the same principle. For coloured or turbid samples, neither works and PEHANON is the answer.