Highly Sensitive Determination Method for Total Carbonate in Water Samples by Flow Injection Analysis Coupled with Gas-Diffusion Separation

A spectrophotometric method for the determination of total carbonate in water samples was developed. The method is based on the color change of an acid-base indicator in relation to the concentration of permeable gas substances through a membrane. By using a new portable FIA system equipped with a gas-diffusion unit, a highly sensitive and on-site determination of total carbonate in aqueous solutions was investigated. A new color-change system with 4-(2′,4′-dinitrophenylazo)-1-naphthol-5-sulfonic acid (DNN5S) was developed. Absorbance changes of the reagent solution were measured at 450 nm with a light-emitting diode (LED) as a light source. A new type of gas-diffusion unit was used, and was constructed with double tubing: the inner tubing was a micro porous PTFE (polytetrafluoroethylene) tubing (1.0 mm inner diameter and 1.8 mm outer diameter, pore size 2 µm, porosity 50%); the outer tubing was made of glass with 2.0 mm inner diameter. The optimized system conditions were as follows: the sample size was 200 µl, the temperature of the air bath for the gas-diffusion unit was 25°C, and the length of the gas-diffusion unit was 15 cm; each flow rate was 0.3 ml min−1. For measuring carbonate at low concentrations, a method for preparing water with less carbonate was proposed: the carbonate content of the water was decreased down to 5 × 10−7 M. The calibration graph was rectilinear from 1 × 10−6 M to 10−3 M, and the detection limit (corresponding to a signal-to-noise ratio of 3) was 1 × 102212;6 M of carbonate. The relative standard deviation (RSD) of ten measurements of 2.3 × 10−5 M Na2CO3 solution was 1.9%. The total carbonate in various kinds of water (such as river, sea, rain, distilled and ultra purified) was determined.