Automatic gravimetric apparatus for recording the adsorption isotherms of gases or vapours onto solids

The assembly described was built with the aim of obtaining high resolution and precision, together with automaticity. It was designed around a symmetrical commercial vacuum microbalance (Setaram), its main features being the following:(1) Controlled “residual” vacuum during outgassing of the sample.(2) Two possible procedures for the introduction of the adsorbate: either a conventional — but automated — point by point procedure (with discontinuous introduction of gas or vapour) or a continuous procedure, specially suited for high resolution work.(3) Measurement of pressure (0–1 atm) by a silica Bourdon gague (Texas Instruments) of 10^?5 accuracy.(4) Direct recording of the isotherms on a XY recorder.(5) Constant level (± 0.5 mm) cryogenic bath (liquid nitrogen or argon) of low consumption (4 liters per 15 hours).(6) Liquid thermostat for adsorption temperatures between ? 30 and + 100°C.(7) Temperature control (up to 50°C) of the whole assembly (manifolds, balance, gauges) allowing to handle condensable vapours.Several examples are given, illustrating the versatility and resolution of the systems. For instance, it allowed to detect, for the first time, a distinct substep at the top of the nitrogen/graphite and argon/graphite isotherms at 77 K. This is now explained by a sudden localization or “freezing” of the monolayer near its completion.