Short communication Application of the Walkley–Black titration for the organic carbon quantification in organic rich sedimentary rocks Faina Gelman ⇑ , Ruth Binstock, Ludwik Halicz Geological Survey of Israel, Malkhei Israel 30, Jerusalem 95501, Israel article info Article history: Received 18 October 2011 Received in revised form 28 December 2011 Accepted 29 December 2011 Available online 11 January 2012 Keywords: Organic carbon Oil shale Walkley–Black titration Dry combustion abstract Forty-three samples of Israeli organic-rich sedimentary rocks (oil shales) have been analyzed for organic carbon content by a modified Walkley–Black titration method and dry combustion method. A good cor- relation (R 2 = 0.92) between the OC results obtained by two methods is observed, but a correction factor is suggested to be applied to OC results obtained by the Walkley–Black method. Considering the simplic- ity of the modified Walkley–Black titration method its general applicability to the analysis of total organic carbon content in the oil shale deposits may be preferable in many cases. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction The Walkley–Black (WB) titration method is one of the classical methods for rapid analysis of organic carbon (OC) in soils and sed- iments. The method is based on the oxidation of organic matter by potassium dichromate (K 2 Cr 2 O 7 )-sulfuric acid mixture followed by back titration of the excessive dichromate by ferrous ammonium sulfate (Fe(NH 4 ) 2 (SO 4 ) 2  6H 2 O). The average oxidation number for organic carbon is considered as zero and the reactions involved in the WB titration method are as following: C 0 þ 2H 2 O ! CO 2 þ 4H þ þ 4e  Cr 2 O 2 7 þ 14H þ þ 6e  ! 2Cr 3þ þ 7H 2 O Fe 2þ ! Fe 3þ þ e  Although the method is widely used for OC analysis in soils and sediments, the accuracy of the method remains an issue. One of the questionable points of the method is the extent of the oxidation by dichromate. In the early variations of the method [1,2], the soil samples were not heated during the oxidation step and the mean recovery of OC was found to be about 76%. Therefore it was pro- posed to introduce a correction factor of 1.32 for the quantification of the OC content in the soil samples [1]. Afterwards it was pro- posed to apply extensive heating of the sample during oxidation [3,4] to overcome the concern of incomplete digestion of the or- ganic matter. In that case, no correction factor was required. How- ever, the temperature of the oxidation must be strictly controlled due to the possible decomposition of acidic dichromate above 150 °C. This complicates appreciably the procedure and does not always lead to satisfactory results [5]. In order to evaluate the accuracy of the WB titration, it is nec- essary to compare the results with those obtained by other meth- ods. One of the most reliable methods for the OC analysis is a dry combustion based on the high-temperature oxidation of a sample, appropriately pretreated in order to completely remove inorganic carbon. Numerous studies comparing the OC results obtained by these two methods have been conducted previously. However the de- duced conclusions were not decisive. Thus, for example, a good agreement between the results was found for some types of Aus- tralian soils [6], whereas some underestimation in soil organic car- bon content by WB titration for Indian soils was reported [7]. In the study comparing TOC in recent marine sediments, Leong and Tan- ner found that the values obtained by the dichromate method were too high for anoxic sediments and too low for oxic sediments [8]. Despite the numerous studies concerning the use of the dichro- mate titration for TOC analysis, the practical applications of the method are mainly limited to analysis of soils and recent marine sediments. In the present study we examined the use of the mod- ified WB titration method for organic carbon analysis of oil shales. Exploitation of oil shale reservoirs for fuel production has resumed its importance due to the increasing demand for fuel in the last decade. Organic carbon content is a primary decisive characteristic for the determination of the economic potential of the deposits. Nowadays, OC analysis of the oil shales is performed mostly by a 0016-2361/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2011.12.053 ⇑ Corresponding author. E-mail address: faina@gsi.gov.il (F. Gelman). Fuel 96 (2012) 608–610 Contents lists available at SciVerse ScienceDirect Fuel journal homepage: www.elsevier.com/locate/fuel