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employing the high resolution ultrasonic spectroscopy (HRUS). The behaviour of sodium
salts of humic and fulvic acids (NaHA, NaFA) was investigated from very low concentration
(0.001 g L –1 ) up to 3.5 g L –1 .
2. MATERIALS AND METHODS
HS standards (Table 1) were purchased from IHSS, titrated to pH 7.5 with
0.1 M NaOH in automatic titrator (Schott, TitroLine Alpha Plus). After a constant value was
reached for 60 min, the sodium humate solution was filtered and freeze-dried. The product
was milled in an agate mortar and stored at room temperature in a sealed container.
Table 1. Examined IHSS humic and fulvic standards
1S104H-5 2S102F 2S101H 1S103H
Leonardite Humic
Acid Standard
Elliott Soil Fulvic
Acid Standard II
Suwannee River
Humic Acid Standard II
Florida Peat Humic
Acid Standard
For monitoring ultrasonic velocity and attenuation a HRUS102 device (Ultrasonic-
Scientific) was employed. It consists of two independent quartz cells tempered by a water
bath; cell 1 serves as sample cell and cell 2 as reference. All experiments were carried out
at 25.00 ± 0.02°C, under constant stirring (600 rpm) and at 8 different frequencies. The
resolution of the spectrometer is down to 10 –5 % for ultrasonic velocity and 0.2% for
attenuation measurement. NaHA and NaFA were dissolved in deionised water to obtain
concentrations of 1, 5 and 10 g L –1 , respectively. In order to assess mechanism of
aggregation, 1 mL of deionised water was added to each cell. The solution in cell 1 was
added stepwise every 10 min (i.e. when constant ultrasound velocity values were
achieved) with prepared NaHA or NaFA solutions.
The concentration increment in the ultrasonic velocity A was calculated from the
equation (6):
U1
− U2
A ≅
U ⋅ c ⋅ ρ
2
0
where U 1 and U 2 are the values of ultrasonic velocity in solution and solvent, respectively,
c is the weight concentration of the solute and ρ 0 is the density of the solvent. A is
determined by the compressibility of water in the hydration shell of the solute molecules
and, in the case of molecular aggregates, by their intrinsic compressibility (7).
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