| Abstract: | The fragmentation of one synthetic and four soil humic acids was studied in solutions of sodium hydroxide (pH 13) at different temperature between 0 and 90°C. The dissolved parts of the initial amounts of humic acids decreased to a minimum with the increase in temperature from 0 to 10°C. The fragmentation in this region was a function of both; temperature and concentration. At 15–20°C all of the initial amounts of humic acids were dissolved. The rate of fragmentation increased rapidly between 15 and 20°C and later slowed down between 20 and 40°C. This was due to the reaggregation of the particles of the produced hymatomelanic acids into particles of similar solubilities as those of the residual nonfragmented parts of the humic acids. This reaggregation took place probably through the formation of hydrophobic bonds. The fragmentation increased at temperatures above 40°C, and reached a maximum between 50 and 70°C depending on the type of the humic acid. The decline in fragmentation at temperatures higher than those corresponding to the maxima is most probably due to chemical reactions, in which the fragmentation products especially fulvic acids recombine together at about 60°C. Importance of these results in analytical methods of extraction and characterisation of humic acids is discussed. |
