Abstract: An electronic apparatus and method for detecting a quantity of a predetermined substance disposed in a fluid medium. The apparatus includes a probe for contacting the fluid medium and for sensing an electrical condition, such as solution resistance, of the fluid medium caused at least in part by the substance to be detected. There is also included electronic control devices connected to the probe means for generating a signal which is proportional to the quantity of the substance to be detected. The signal is fed to some utilization means, such as a meter, for indicating the amount of substance in solution.