Determination of chemical oxygen demand by a flow injection method based on microwave digestion and chromium speciation coupled to inductively coupled plasma optical emission spectrometry

The present paper evaluates the applicability of a new FIA system for COD determination. The new system, flow injection microwave solid phase extraction by means of activated carbon (FI-MW-SPE), consists of a digestion circuit, placed in a home MW oven, coupled to an ICP-OES spectrophotometer. Doehlert experimental design was used to speed up the optimization of different experimental variables studied for assisted digestion methods. The method provided a high throughput of about 18 samples h−1. To assess the accuracy of analytical methods linear regression, elliptic joint confidence region (EJCR) was used. A large linear range of 2.78–850 mg O2 L−1 with an excellent detection limit of 0.94 mg O2 L−1 was obtained. The interference by high chloride concentration was studied, and values below 3000 mg Cl− ions L−1, allowed the estimation of COD load without any masking agents. COD values for various types of wastewater samples were correlated with those obtained by standard manual methods. Moreover, interferences due to matrix nature are absent; since matrix is washed out of the column before Cr (III) is eluted. This method reduces the time, reagent volume, hazardous emission, external contamination, with a good reproducibility and accuracy.