A method for operating an analytical apparatus for determining concentration of an analyte, especially an oxidizable substance, in a sample liquid, comprises the steps of: placing the analytical apparatus in maintenance operation; operating a metering system, which includes a pump, especially a peristaltic pump, and a supply line, for metering single volume units of a sample liquid from a source via the supply line into a reactor, wherein there exists in the reactor a temperature, which is greater than the boiling temperature of the sample liquid, so that a volume unit of the sample liquid metered into the reactor transforms at least partially into the gas phase following entry into the reactor, especially due to heat transfer from contact with a surface within the reactor, especially directly after contact with the surface within the reactor, and wherein a carrier gas is flowing through the reactor; registering at least one measuring transducer signal for detection of the transforming of the volume unit of the sample liquid into the gas phase, and deriving, with application of the measuring transducer signal, a signal correlated with an instantaneous operating state of the metering system.