ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-8-5997-2008 How many carboxyl groups does an average molecule of humic-like substances contain? Salma I. 1 Láng G. G. 1 Eötvös University, Institute of Chemistry, Budapest, Hungary 17 10 2008 8 20 5997 6002 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/8/5997/2008/acp-8-5997-2008.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/8/5997/2008/acp-8-5997-2008.pdf

The carboxyl groups of atmospheric humic-like substances (HULIS) are of special interest because they influence the solubility in water, affect the water activity and surface tension of droplets in the air, and allow formation of chelates with biologically active elements. Experimentally determined abundances of the carboxyl group within HULIS by functional group analysis are consistent with our knowledge on the average molecular mass of HULIS if the number of dissociable carboxyl groups is assumed to be rather small. The best agreement between the average molecular mass derived from the existing abundance data and the average molecular mass published earlier occurs for assuming approximately one dissociable carboxyl group only. This implies that HULIS can not be regarded as polycarboxilic acid in diluted solutions. The average molecular mass of HULIS derived from our electrochemical measurements with the assumption of one dissociable carboxyl group or equivalently, one dissociable sulphate ester per molecule ranges from 250 to 310 Da. It was concluded that HULIS are a moderately strong/weak acid with a dissociation constant of about p<I>K</I>=3.4, which fits well into the interval represented by fulvic and humic acids. The mean number of dissociable hydrogen (i.e. of carboxyl groups and sulphate esters jointly) in HULIS molecules was refined to be between 1.1 and 1.4 in acidic solutions.

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