Comparison of the microcolorimetric and macrotitrimetric methods for chemical oxygen demand of oil shale wastewaters

Abstract

Modlficatlons are described for both the mlcrocolorimetric and macrotltrlmetrlc chemical oxygen demand (COD) procedures. For the former, a prolonged coollng period, followed by extensive mixing, minimizes the spectrophotometric Interference from refractive index changes and catalyst-induced preclpltate. For the latter, automated tltration and absorbance monitoring near the chromate-chromic isosbestic point improves accuracy and reproduclbillty. The applicability of these modified methods for quantifying the COD of oil shale wastewaters was demonstrated. For 12 oll shale wastewaters, the microcolorimetric method gave COD values (900-150000 mg/L) that dld not dlffer statlstlcally ( P > 0.10) from those obtained by the macrotltrimetric COD method. The relatlve standard devlatlons for flve repllcates of each wastewater were less than 1.3% for the macrotitrimetric method and generally less than 5.0% for the mlcrocolorimetrlc method; no matrix effects were evident.The chemical oxygen demand (COD) test was originally used as a rapid estimator of the biochemical oxygen demand (BOD) of wastewater organic material (I). The numerical value obtained from a COD test reflects both the organic carbon concentration and the overall oxidative state of the organic material. The COD of a sample is defined by the absolute amount of hexavalent chromium that is reduced during 2 h of digestion by potassium dichromate in a solution of 50% sulfuric acid. Ideally, organic compounds are completely oxidized to carbon dioxide and water with the simultaneous stoichiometric reduction of the orange, hexavalent dichromate ion (Cr") to the green, trivalent chromic ion (Cr3+). The degree of reduction is quantified either by colorimetry (i.e., by determination of the remaining dichromate ion or, alternatively, by determination of the newly produced chromic ion) or by titrimetry (i.e., redox titration of the remaining dichromate ion). This value is then related stoichiometrically to equivalents of oxygen.The original method for determining COD (using titrimetric quantitation) has undergone numerous revisions. The major modifications include (i) the addition of silver as a catalyst to enhance oxidation of certain biodegradable compounds (Le., aliphatic hydrocarbons, straight-chain alcohols, and fatty acids) that are not fully susceptible to the chemical oxidant and (ii) the addition of mercuric sulfate to complex chloride ion so that chloride precipitation of the silver catalyst and chloride oxidation by dichromate are minimized. Although these revisions have improved the "accuracy* of the method, the large scale of the macrotitrimetric method (50 mL of sample and a total waste volume of approximately 300 mL/sample) and the concomitant space, energy, and wasteIPresent address: Department of Public Works, City and County of San Francisco, 750 Phelps St., San Francisco, CA 94124-1091. disposal requirements have prompted a search for alternative means to minimize these costly disadvantages.A micro variation of the COD method, which uses colorimetric d...

A Nafion Film Cover to Enhance the Analytical Performance of the CuO/Cu Electrochemical Sensor for Determination of Chemical Oxygen Demand

“…This is one of the most important parameters to take into consideration when monitoring surface water quality in lakes, rivers or wastewater. COD values represent the amount of oxygen, mg·L −1 , consumed in the oxidation process of organic contaminants by oxidant reagents such as O 2 , Cr 2 O 7 −2 , MnO 4 − and Ce 4+ [1,2]. Reaction kinetics of the oxidant reagents with some organic or inorganic contaminants affecting the COD values of a given water system is quite slow.…”

Determination of Chemical Oxygen Demand Based on Novel Photoelectro‐bifunctional Electrodes

“…Considering the increasing quantities of pollutants being discharged into water bodies, environmental monitoring and control have become issues of global concern 1. Chemical oxygen demand (COD) is one of the most widely used parameters in the assessment of water quality and pollution control as it can comprehensively represent the organic pollution level 2, 3. COD is defined as the number of oxygen equivalents consumed in the oxidation of organic compounds by strong oxidizing agents, such as dichromate or permanganate 4.…”

A high throughput chemiluminescence method for determination of chemical oxygen demand in waters

“…These methods include spectrophotometry [2], flame atomic absorption spectrometry (FAAS) [3][4][5], photocatalytic degradation method [6][7][8][9][10][11] and electrochemical method [12][13][14][15][16][17], etc. Especially, these COD detection methods can be combined with flow injection technique, which could greatly raise analysis efficiency.…”