Differential Sound Absorption Technique and Effect of Ion-Pairing and Pressure on Sound Absorption in Seawater and Aqueous Mixtures of Magnesium Sulfate and sodium Chloride

摘要

Sound absorption in the ocean between 5 kHz and 350 kHz is principally due to the relaxation process associated with a pressure dependent chemical reaction of MgSO4. This research has resolved two conflicts regarding the effect of pressure on sound absorption and relaxation frequency due to MgSO4 in sea water and the reduction of sound absorption by the addition of up to 0.6 M NaCl to low concentration (0.02M) solution of NaCl. The reduction of absorption in MgSO4 solution upon the addition of NaCl can be accounted for theoretically by Debye-Huckel theory and by formation of MgCl(+) and NaSO4 ion-pairs. The measured reduction is less than that observed by Kurtze and Tamm, however. At this concentration of MgSO4, the effects of ionic strength and ion-pairing are about equal. The results are consistent with disassociation constants of 0.0062, 0.1, 0.178 for the MgSO4, NaSO4(-) and MgCl(+) ion-pairs, respectively up to 0.12 M NaCl addition. For 0.6M NaCl additions to 0.02 M MgSO4, the absorption results cannot be explained by either simple Debye-Huckel theory, the Johnson and Pytkowicz model of ion-pairing nor the Pitzer approach. The values of maximum absorption per wavelength were determined to an absolute accuracy of about 1% from .000080 to .000016. For the differential absorption measurements in MgSO4 and NaCl mixtures, the measurement accuracy is sufficient for quantitative studies of ion-pairing in electrolyte mixtures, particularly for unsymmetrical salts.