Abstract: A colorimeter for continuously monitoring the concentration of an ionic species in an aqueous solution comprises a source 30 of optical radiation, a flow cell 10 including three flow chambers 11, 12 and 13 interconnected so that a sample of the solution can pass through each flow chamber in succession, and photodetector devices 41, 42 and 43 positioned to respond to radiation transmitted through the solution in the flow chambers 11, 12 and 13, respectively. The lengths of the three flow cells are different from each other to provide a continuous real-time calibration check based upon photodetector responses to radiation transmitted through different quantities of the solution. Electronic circuitry interconnecting the radiation source 30 and the photodetector devices 41, 42 and 43 maintains the intensity of the radiation emitted by the source 30 at a substantially constant value.

Abstract: An apparatus for monitoring ion concentrations in an aqueous solution comprises a plurality of electrodes insertable into the solution. At least two of the electrodes are ion-selective for particular ions. Each ion-selective electrode is electrically connected to a corresponding impedance buffer, which generates a low-impedance output signal having the same voltage as the electode. A plurality of operational amplifiers in full differential configuration is provided, each operational amplifier receiving a particular pair of output signals generated by the impedance buffers. Each differential amplifier generates an output signal indicating the potential difference between the impedance buffer output signals received as inputs by the differential amplifier.