A cheap way to measure pH is by using organic based pH indicator paper. Although this is cheap, it is often single use and can be inaccurate at both low and high pH. Here we present a new inorganic pH indicator based on ferricyanide intercalated zinc-aluminum layered double hydroxide (LDH). The LDH indicator has a deep orange color at pH 1, and turns yellow with increasing pH up to pH 8 where is has a very deep yellow color. The dark yellow color lightens with increasing pH until it turns completely white at pH 13. Color changes from are rapid and reversible and the LDH maintains its brucite-type crystal structure. Indicator response time is 90 seconds at 25<sup>o</sup>C and decreases with increasing temperature. The white phase obtained at pH 13 is amorphous with a very small rate constant for the reverse reaction which regenerates original layered double hydroxide. The color changes are not affected by nitrates, phosphates, carbonates, halides and sulphates and the LDH does not undergo anion exchange with any of these ions.

Ferricyanide intercalated zinc-aluminum layered double hydroxide pH indicator

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