Use of the magnetic suspension balance for the study of liquid mixtures

Previous contributions to the 9th and 12th V.M.T. Conferences have described the use of microbalances to study the absorption of hydrocarbon vapours by essentially involatile liquid absorbents to determine solution thermodynamic properties. The quartz spring balance is useful over the greater part of the relative vapour pressure range but lacks precision in the lower part of the range as the absorption decreases. In this lower pressure region the greater sensitivity and greater load capacity of the commercial beam vacuum microbalance can be utilised and the measurements extrapolated with confidence to give the activity coefficient for the absorbed vapour at infinite dilution in the liquid absorbent. However, interaction of the volatile hydrocarbons studied with the components of the balance limits the use of the latter balance to the lower pressure region.The Sartorius magnetic suspension balance combines the attributes of these two balances in that it is not affected by the organic vapours studied and provides the load-to-precision ratio that the determination of accurate activity coefficients requires. This enables liquid mixtures with one volatile and one or more involatile components to be studied over the entire concentration range on the one apparatus. Results are reported for the absorption of hexane by squalane (hexamethyl tetracosane) and compared with those previously determined using a quartz spring balance and a Sartorius electronic microbalance. The modifications found necessary to the magnetic suspension balance for this work are described.