An apparatus and methods for measuring the concentration of an additive are disclosed. The apparatus comprises a treatment stream (1) and a dosing stream (2). An additive is added to the dosing stream using a metering device (3). In some embodiments, the dosing stream is mixed after adding the additive using a first mixing device (4). Downstream from the metering device and the mixing device, the concentration of the additive in the dosing stream is measured using a monitor flow cell (5). In some embodiments, the dosing stream and treatment stream are combined (6) and mixed using a second mixing device (7). The concentration of the additive in the treatment stream can be calculated as a function of the volumetric flow rate ratio of the dosing stream to the treatment stream and the measured concentration of the additive in the dosing stream.