Comparison of potentiometric titration,IR spectrophotometry and segmented micro-flow analysis to determine inorganic C in alkaline solutions

A segmented micro-flow analysis procedure was described for the determination of total carbonates in alkaline solutions. The method is based on CO(2) diffusion from an acidified sample through a silicone membrane. The gas is then collected in a coloured acceptor solution. The decolouration of the solution due to the trapped CO(2) is then determined by spectrophotometry at a wavelength of 550 nm. The method was evaluated for the range 0-1000 mg C CO(2) L(-1) at a sampling frequency of 60 h(-1). The method was repeatable (RSD better than 1%), accurate (1.5 and 2.5% for standards) and the limit of detection was close to 6 mg C CO(2) L(-1). The results were also compared with those obtained by the potentiometric reference method (PT) and an infrared spectrometric method (IRS). The measurements obtained showed good agreement with those obtained using PT and IRS methods, the PT method being the most accurate. The SFA method appeared to be a really efficient and a more suitable technique for routine C CO(2) determination, although some results showed that for standard solutions, the measured concentrations were significantly different from those obtained with the PT method for concentrations higher than or equal to 800 mg C CO(2) L(-1). For 0.25 M NaOH samples, the three methods tested gave similar results in the range 0-600 mg C CO(2) L(-1).