Abstract: An apparatus for in-line monitoring of nitrite content in a metalworking fluid is provided, the apparatus comprising a sample inlet for receiving a sample of a metalworking fluid, a dilution inlet for receiving a dilution fluid, a reagent inlet for receiving a photoactive reagent, a reaction volume for containing a sample mixture in fluid communication with the sample inlet, dilution inlet and reagent inlet, a photometer for monitoring the sample mixture, and a flow control system for controlling fluid flow in the apparatus, to: selectively introduce the sample, the dilution fluid and/or the photoactive reagent from the respective inlets to the reaction volume to form the sample mixture, retain the sample mixture in the reaction volume and discharge the sample mixture from the reaction volume.