Determination of Chemical Oxygen Demand in Spent Caustic by Potentiometric Determination

Measurement of the COD of a spent caustic solution involves firstly digestion of a test sample with dichromate solution and secondly measurement of dichromate remained by titration by ferrous ammonium sulfate [FAS] to an end point. In this paper we study by a potentiometric end point with Ag/AgCl reference electrode and gold rode electrode. The potentiometric end point is sharp and easily identified especially for the samples with high turbidity and color that other methods such as colorimetric in this type of sample do not result in high precision. Because interim of titration responds quickly to potential changes within the [Cr+6/Cr+3& Fe+2/Fe+3] solution producing stable readings that is lead to accurate COD measurement. Finally results are compared with data determined using colorimetric method for standard samples. It is shown that the potentiometric end point titration with gold rode electrode can be used with equal or better facility

• Hamed Harrafi
• Masoumeh Khedri
• Karim Karaminejad

• chemical oxygen demand
• spent caustic and potentiometric determination

[1] Clair N. Sawyer, Perry L. McCarty, Gene F. Parkin (2003). Chemistry
for Environmental Engineering and Science (5th ed.). New York:
McGraw-Hill. ISBN 0-07-248066-1
[2] Suarez, F. "Pluses and Minuses of Caustic Treating", Hydrocarbon
Processing, pp 117-123, Oct 1996.
[3] Maugans, C.; Ellis, C. "Age Old Solution for Today's SO2 and NOx",
Pollution Engineering, April 2004.
[4] Carlos T.; Maugans, C. "Wet Air Oxidation of Refinery Spent Caustic:
A Refinery Case Study", NPRA Conference, San Antonio, TX,
September 2000. WAO for Refinery Spent Caustic
[5] Kumfer, B.; Felch, C.; Maugans, C. "Wet Air Oxidation of Spent
Caustic in Petroleum Refineries", NPRA, March 2010, San Antonio,
TX. WAO for Spent Caustic in Petroleum Refineries
[6] Standing Committee of Analysts, Chemical Oxygen Demand
(Dichromate Value) of Polluted and Waste Waters (second edition)
1986, Methods for the Examination of Waters and Associated Materials,
ISBN 0117519154, in this series
[7] Ballinger, D., Lloyd, A., Morrish, A., 1982. Determination of chemical
oxygendemand of wastewaters without the use of mercury salts. Analyst
107, 1047-1053.
[8] Lenore S. Clescerl, Arnold E. Greenberg, Andrew D. Eaton. Standard
Methods for Examination of Water & Wastewater (20th Ed.).
Washington, DC: American Public Health Association. ISBN 0-87553-
235-7.
[9] Smith,FG., 1951. Determination of formal oxidation potentials of ferricferrous
and dichromate-chromic systems. Anal. Chem. 23(6) 925-927.