INTRODUCTION Metal soaps are used as stabilizers for nylon threads, PVC molding composition, polyester, as an adhesive for steel cord and rubber in radial tiers 1-4 . The structure and properties 5-7 of these soaps play and important role of their applications in different fields. Ultrasonic velocity have been found wide applications owing to their ability to characterise the physico-chemical behaviour of solutions. The velocity of ultrasonic waves in aqueous and non aqueous solution of electrolytes have been reported by several workers 8-10 . In the present communication attempts have been made to compute various acoustic parameters from ultrasonic velocity measurements of thorium soap (butyrate, valerate, cproate and caprylate) solutions. EXPERIMENTAL All the chemicals used for the preparation of thorium soaps were of BDH/AR grade and were purified by standard methods. Thorium soaps were prepared by direct metathesis of the corresponding soap with the required amount of aqueous solution of thorium nitrate with constant stirring, the precipitated soap was filtered off and wahsed first with distilled water and finally with alcohol. The soaps were first dried in an air-oven at 50-60°C and then Acoustic behaviour of thorium soap solution VIDHYA SAGAR and MEERA SHARMA* Department of Chemistry, Agra College, Agra - 282 002 (India). (Received: March 05, 2009; Accepted: April 20, 2009) ABSTRACT The ultrasonic velocity of thorium soaps has been measured in benzene-methanol mixture (50-50%) at a constant temperature (40±0.05°C). The ultrasonic velocity and density data have been used to evaluate the adiabatic compressibility, intermolecular free length, molar sound velocity, specific acoustic impedance and other acoustic parameters. Thorium soap solutions obey Bachem's relation. The results confirm that thorium soaps act as weak electrolyte in dilute solution below the CMC. The values of CMC are in agreement with the values obtained from other properties. Key words: Thorium, butyrate, valerate, caproate and caprylate. Oriental Journal of Chemistry Vol. 25(2), 345-348 (2009) under reduced pressure and further purified by recrystallisation. The solutions of thorium soaps were prepared by dissolving the weighed amount of soaps in the required volume of benzene- methanol mixture (50-50%). The solutions were kept at a constant temperature for two hours in a thermostat. The ultrasonic velocity measurements were recorded on an ultrasonic interferometer (M- 83, Mittal Enterprises, New Delhi) at a frequency of 1 MHz at a constant temperature (40±0.05°C). The probable error in velocity results was 0.2%. The various acoustic parameters such as adiabatic compressibility, β, intermoluecular free length, L f specific acoustic impedance, Z and molar sound vlocity, R have been calculated by using the following relationships. ...(1) ...(2) ...(3) ...(4) ...(5) where n 1 , n 2 and M 1 , M 2 are the number of moles and molecular weights of solvent and solutions respectively. K is Jacobson's constant¹¹.