Highly sensitive determination method for total carbonate in water samplesby flow injection analysis coupled with gas-diffusion separation

A spectrophotometric method for the determination of total carbonate in wat er samples was developed. The method is based on the color change of an aci d-base indicator in relation to the concentration of permeable gas substanc es through a membrane. By using a new portable FIA system equipped with a g as-diffusion unit, a highly sensitive and on-site determination of total ca rbonate in aqueous solutions was investigated. A new color-change system wi th 4-(2',4'-dinitrophenylazo)-1-naphthol-5-sulfonic acid (DNN5S) was develo ped. Absorbance changes of the reagent solution were measured at 450 nm wit h a light-emitting diode (LED) as a light source. A new type of gas-diffusi on unit was used, and was constructed with double tubing: the inner tubing was a micro porous PTFE (polytetrafluoroethylene) tubing (1.0 min inner dia meter and 1.8 min outer diameter, pore size 2 mum, porosity 50%); the outer tubing was made of glass with 2.0 min inner diameter. The optimized system conditions were as follows: the sample size was 200 mul, the temperature o f the air bath for the gas-diffusion unit was 25 degreesC, and the length o f 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 wit h less carbonate was proposed: the carbonate content of the water was decre ased down to 5 x 10(-7) M. The calibration graph was rectilinear from 1 x 1 0(-6) M to 10(-3) M, and the detection limit (corresponding to a signal-to- noise ratio of 3) was 1 X 10(-6) M of carbonate. The relative standard devi ation (RSD) of ten measurements of 2.3 x 10(-5) M Na2CO3 solution was 1.9%. The total carbonate in various kinds of water (such as river, sea, rain, d istilled and ultra purified) was determined.