Methods and apparatus for measuring the concentration of decomposable substances, such as urea, are disclosed. The disclosed methods include adding a gaseous buffer, such as CO2, to the solution containing the decomposable compound, measuring the conductivity of the solution, decomposing the decomposable compound, measuring the conductivity of the thus-decomposed compound solution, and calculating the differential conductivities between the two measured solutions. The apparatus for carrying out these methods are also disclosed.