Oliveira, Sara M.Lopes, Teresa I. M. S.Tóth, Ildikó V.Rangel, António O. S. S.2010-10-112010-10-112007"Analytica chimica acta". ISSN 0003-2670. 600 (2007) 29–34http://hdl.handle.net/10400.14/2819A flow system with a multi-channel peristaltic pump placed before the solenoid valves is proposedto overcome some limitations attributed to multi-commuted flow injection systems:the negative pressure can lead to the formation of unwanted air bubbles and limits the useof devices for separation processes (gas diffusion,dialysis or ion-exchange). The proposed approach was applied to the colorimetric determination of ammonium nitrogen. In alkaline medium, ammonium is converted into ammonia, which diffuses over the membrane,causing a pH change and subsequently a colour change in the acceptor stream (bromothymol blue solution). The system allowed the re-circulation of the acceptor solution and was applied to ammonium determination in surface and tap water, providing relative standard deviations lower than 1.5%. A stopped flow approach in the acceptor stream was adopted to attain a low quantification limit(42ugL−1) and a linear dynamic range of 50–1000ugL−1 with a determination rate of 20h−1.engMulti-commuted flow systemGas diffusionWatersTrace levels of ammoSpectrophotometryjournal article10.1016/j.aca.2007.01.019