A robust microbalance of high sensitivity. Suitable for weighing sorbed films

Abstract

The sorption balance of McBain and Baker may be made of high degree of sensitivity, but the total weight which it will then support is correspondingly diminished. For weighing a monomolecular film on a comparatively heavy plane surface it is necessary to use a balance of beam type. A balance sensitive to 4 x 10^-9 grams can be made sufficiently compact to be placed in a horizontal tube so that it may be used over a wide range of temperatures and pressures including the conditions necessary for freeing from sorbed material the surfaces to be weighed. such a balance may be so designed that the volumes of the two sides of the balance are equal and unaffected by buoyancy. If, likewise, there be placed upon the two sides of the balance equal volumes of the same substance but of different area of surface, the differential change of weight caused by adsorption will be measured directly. A final desideratum is that the balance should be of null point type and free from any bind of hysteresis. Steele and Grant described a microbalance made of fused quartz which was sensitive to 4 x 10^-9 grams and was capable of weighing 0.1 gram with an accuracy of 1 x 10^-7 gram, added improvements were described by Gray and Ramsay .Aston simplified the design of the beam and made a balance specially adapted to the determination of gas densities. Stock and Ritter made a further modification by replacing the fused charts bulks-edge with a pair of needle points. They also described a null-point balance in which a magnetised steel needle was sealed horizontally within the beam. The magnetic held from a bar magnet held vertically over the centre of the balance was used to compensate changes in level of the beam. The intensity of this controlling magnetic field was varied either by altering the distance between the bar magnet and the balance, or by increasing the held of the bar magnet electrically.