A simple photometric detector is described which, because of the high stability of the light source, permits determinations of metal ions at the parts per 10⁹ level with 4-(2-pyridylazo)resorcinol as the spectrophotometric reagent. By virtue of the design of the transducer it also functions as a refractometer capable of determinations of solutions of organic and inorganic compounds down to a lower limit of approximately 0.01%m/m. The theory of this function is discussed.

A gallium phosphide light-emitting diode and a silicon phototransistor act as light source and sensor, respectively. The output current from the phototransistor is converted into voltage by the current to voltage converter described.

The transducer is designed as a flow-through cell which, when used in conjunction with standard unsegmented continuous-flow apparatus, is capable of sampling rates of up to 300 per hour with a relative standard deviation of the result of 1.5%. At slower flow-rates, with a sampling rate of 160 per hour, the relative standard deviation is less than 1%.