A simple optical flow injection ammonia sensor has been developed, based on the reflectance changes at 620 nm of an indicator fitted with a gas membrane. The sensor uses a commercially available pH indicator strip for a simple and fast assay. The response time to a 1-10 mM change in ammonia concentration is fast (≤10 seconds) and is fully reversible. The reversal time of the sensor is ≤30 seconds for ammonia assay and it can also be used for fast urea assay within the range 2-10 mM
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