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Solubility and dissociation of humic acids
 Kratochvílová, Romana ; Jaroslav,, Záhora (referee) ; Klučáková, Martina (advisor)
The behaviour of humic acids in aqueous solutions is closely connected to the content of acid functional groups and their dissociation abilities. The titration methods are the most often used for the research of acid-basic characteristics of humic acids, their results should lead to the determination of the content of functional groups and their pKa. However the determined values depend on many factors including the titration speed. The aim of this bachelor’s thesis is to verify the possibility of UV/VIS spectrophotometry´s use as alternative method for the determination of pKa of humic acids and to compare the gained results with so far used method according to Henderson-Hasselbach. The principle of newly used spectrophotometry method is the preparation of humic acids solutions in three different media with different pH values. Acid groups of humic acids are practically completely dissociated in intensely basic environment and the measured absorbance is caused by anion of „humate“. On the contrary their dissociation in acid environment is suppressed so much that we measure the absorbance of non-dissociated molecules of humic acids. It was found out that after the modification it was possible to use the spectrophotometry method for the determination of pKa of these substances, while the use of method was verified within the three different samples of humic acids. The optimal ratio humic acids – water and the extent of wavelength for measuring UV/VIS spectra were determined. It was proved that the values of pKa determinated by these methods better describe the real dissociation behaviour of humic acids in water than traditional Henderson-Hasselbach method. The UV/VIS spectra gained in combination with the results of pH measuring and the conductivity next showed the differences in the behaviour of differently prepared samples of humic acids.
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