Nitrate can be converted to ammonia in an alkaline medium using zinc or Devarda's alloy reductant [39] and this approach has been reported for determination of nitrate with a gas-sensing probe [40] and a bulk acoustic impedance detector [41]. Zinc has also been used as a heterogeneous reducing agent for conversion of nitrate to nitrite in batch mode [42] and [43] which is optimal under neutral or slightly acidic conditions [44].
In this work, a simple hybrid reagent-injection flow analysis method that utilizes on-line zinc reduction in the spectrophotometric determination of nitrate is described. The consumption of Griess reagent is minimized by the use of reagent-injection, and the use of zinc entirely eliminates the use of cadmium and its associated toxic waste. The method, which involves reduction under weakly acidic conditions, has been shown to be suitable for on-line measurement of nitrate in marine, estuarine and fresh waters, and has been deployed in extended field trials aboard the SV Pelican 1.