This paper reports on the optical detection system of a prototype nitrite sensor for seawater analysis. The sensor uses a colorimetric Griess reaction that produces an Azo dye with an absorption peak at 540 nm. A simple low-cost double beam spectrophotometer using a green light emitting diode (LED) light source, two integrated photo detectors and lock-in detection is described. A log-ratio amplifier is used to produce an output proportional to absorbance. The optical devices were mounted in a cast resin block to prevent shock and vibration. Experimental results show the detection system has a limit of detection of 0.1 muM.