Automatic Instrumental Methods for Determination of Critical Solution Temperatures

Abstract

Several instrumental means for the determination of critical solution temperatures using photoelectric detection and control are described. Heater control is effected either through a Schmitt trigger circuit or with limit switches mounted on standard recorders. Temperature is recorded on standard recording potentiometers. Precision of measurement to 1!=0.01' C. is obtainable although limitations in volume measurement and reagent purity may easily Iimit useful results to about f0.05" C.PPARENTLY, Brown ( 2 ) was the first to use photoelectric A methods for the automatic determination of critical solution temperatures-specifically, the aniline point of petroleum products. In his application a simple photoelectric relay wzm used to control the heating or cooling of the test sample and cause it to attain temperature equilibrium automatically. The pre.sent work was undertaken for two reasons. First, the authors believe that Brown's technique can find analytically useful extensions to many other chemical systems and, second, so many commercially recording techniques and instruments are now available that several new and original approaches are possible. This brief report affords the analyst several choices for the automatic recording of critical solution temperatures and for analytical information which such systems can provide. Each method reveals its own advantages as well as limitations and can best be adapted to the particular instrument which is at hand. When a binary liquid-liquid system which exhibits the critical solution phenomenon is heated to a certain critical temperature, it passes from a state of miscibility to immiscibility, or vice versa, depending upon the particular system under study. This critical temperature depends not only on the components of the system, but also on their relative concentrations. Within certain concentration limits, depending on the system in question, the determination of the critical solution temperature constitutesasensitive quantitative analysis of the system. Conversely, when one component and the concentrations are given, a qualitative analysis may be made for the other component. This analysis is the basis for the ASTM ( 1 ) aniline point determination. Both methods require a previous calibration in which components and concentrations are known, but the literature contains many examples and even empirical equations for some series of compounds.If the system is stirred while in its immiscible state, a turbid mixture results; when the miscible state is reached, this turbidity disappears.Simple photoelectric methods can be used to detect this transition and to initiate a variety of control functions. EXPERIMENTALGlass Sample Container. A cylindrical shape was selected in order to use ordinary standard-taper glassware. The dimensions were adjusted so that a vessel using a 55/50 standard-taper top had a total volume of about 300 ml., and a 200-ml. sample was used each time. The upper portion of the vessel was fitted with a reflux condenser, two wells for heating coil outlets, ...

Automatic Operations in Analytical Chemistry

“…Mosley, Luchesi, and Müller (172) described automatic instrumental methods for the determination of critical solution temperatures. Several means were described using photoelectric detection and control.…”