Short Communication A modified method for the determination of chemical oxygen demand (COD) for samples with high salinity and low organics I. Vyrides, D.C. Stuckey * Department of Chemical Engineering and Chemical Technology, Imperial College London, South Kensington Campus, London SW7 2AZ, UK article info Article history: Received 21 February 2008 Received in revised form 13 June 2008 Accepted 17 June 2008 Available online 9 August 2008 Keywords: Chemical oxygen demand Modification of the standard method High salinity Low organics abstract This study proposes a modification to the standard method for the determination of the chemical oxygen demand of samples with a salinity up to 40 g NaCl/L and low organic concentrations (20–230 mg COD/L). The masking of chloride by the use of a HgSO 4 :Cl ratio of 20:1 prior to digestion, and the use of 3 g K 2 Cr 2 O 7 /L in the digestion solution resulted in an error of less than 10% and 12% for samples containing 40 g NaCl/L at 20–190 mg COD/L and 230 mg COD/L, respectively. Comparison of the standard method with the new proposed method using a synthetic sewage highlights the large errors (50–85%) of the stan- dard method in contrast to an error of less than 10% for the proposed modified method. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Chemical oxygen demand (COD) is the main parameter widely used to estimate the organic content of wastewater. The value of COD shows the oxygen equivalent of the organic content that can be oxidized by potassium dichromate (K 2 Cr 2 O 7 ) using silver sulfate (Ag 2 SO 4 ) as a catalyst under acidic conditions (H 2 SO 4 ). The stan- dard method (APHA, 1999) is well established and is used by the majority of researchers and plant operators in the waste–wastewa- ter field to express the polluting ability of a wastewater. However, there are limitations when measuring the organic matter in wastewater samples with chlorides higher than 2000 mg/L using the standard method. This is due to the oxidation of chloride ions expressed by the following equation: Cr 2 O 2 7 þ 6Cl  þ 14H þ ! 3Cl 2 þ 2Cr 3þ þ 7H 2 O ð1Þ The standard method suggests the use of a HgSO 4 :Cl ratio equal to 10:1 when the chloride concentration is up to 2000 mg/L in or- der to mask the excess of chloride by the formation of HgCl 2 . How- ever, many researchers have found that this ratio (10:1) in samples with chlorides more than 2000 mg/L contributed to a significant error at low and moderate CODs (Baumann, 1974; Freire and Sant’anna, 1998). Moreover, Ballinger et al. (1982) tested the stan- dard method by using a ratio of HgSO 4 :Cl of 40:1 for a sewage effluent (60 mg COD/L) at 4 g Cl  /L, and found that even at this high ratio the values were 60% higher compared with the samples without chloride. Canelli et al. (1976) investigated the chloride electrode response in the presence of HgSO 4 by increasing the level of NaCl. As the concentration of chloride increased from 100 mg Cl  /L to 1 g Cl  /L the potential at the chloride electrode decreased from 230 mV to 170 mV. This highlights the effect of free Cl  even in the presence of high HgSO 4 . Moreover, according to Hejzlar and Kopacek (1990), at a sample digestion temperature of 150 °C the ability of HgSO 4 to mask chloride effectively is reduced. Neverthe- less, in several modified methods the masking occurs during the digestion stage (Freire and Sant’anna, 1998), and this may contrib- ute to an error at higher concentrations of salinity. Baumann (1974) employed the open reflux method, and used a HgSO 4 :Cl ratio of 10:1 to mask the chloride; he suggested the use of sample-specific chloride factors in the calculations in order to estimate the correct value. Nevertheless, samples that contain high chloride concentrations show high standard deviations when they are oxidized, so the specific correction factor cannot be applied with accuracy for the analysis of saline wastewater samples. Apart from this, the open reflux method requires relatively large amounts of sample, high amounts of reagents and considerable glassware. An alternative method suggested by standard methods is the use of a closed reflux method that uses less reagents and has a higher detection limit compared with the open reflux meth- od. Gonzalez (1986) employed a modification of the closed reflux standard method and only found a 3.9% error at 3 g Cl  /L for a sam- ple containing 631 mg COD/L (Table 1). Soto et al. (1989) used the closed reflux titrimetric method to examine the effect of different concentrations of HgSO 4 in the digestion solution at different chlo- ride concentrations. Increasing HgSO 4 , even up to 130 g/L in the 0960-8524/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2008.06.038 * Corresponding author. Tel.: +44 207 594 5591; fax: +44 207 5945638. E-mail address: d.stuckey@imperial.ac.uk (D.C. Stuckey). Bioresource Technology 100 (2009) 979–982 Contents lists available at ScienceDirect Bioresource Technology journal homepage: www.elsevier.com/locate/biortech