Page 17 - AN INTRODUCTION TO SURFACE CHEMISTRY By ERIC KEIGHTLEY RIDEAL

12 SURFACE TENSION OF LIQUIDS

Modifications of the method.
Instead of a capillary dipping into a large vessel of liquid, we
may reduce the quantity of fluid needed and simplify the measure-
ment of the height h by using a U-tube of which one branch is of
capillary and the other of wider bore. The difference h of height
between the liquid in the two branches can then be easily read by
means of a cathetometer or found by a slight modification of
Ferguson's method.
Again the case of two parallel plates dipping vertically or in-
clined at a slight angle at a short distance apart in the liquid
closely resembles that of the capillary tube and has been used by
Quincke and Volkmann.
For the case of the parallel plates the equation for the surface
tension is approximately
\ ,_dhgG.-P»)
2
where d is the distance between the plates. This method compares
unfavourably with the preceding ones, owing to greater difficulties
of adjustment, the necessity for using optically plane glass, and
larger quantities of liquid.
5. The drop weight method.
The method of measuring the surface tension of a liq~id from
the weight of a drop formed slowly and allowed to drop from a tip
ground plane which is with a sharp circular edge, has frequently been
employed owing to the simplicity of the method. The experimental
difficulties to be overcome in order to obtain reproducible results
consist in great measure of ensuring that the drop shall be allowed
to form on the tip, which the liquid must wet uniformly, so slowly
that it is practically in static equilibrium at the moment before
it is detached. Loss by evaporation from the receiving vessel may
thus be an important factor for which correction must be applied.
Again for many solutions the rate at which the gas-liquid interface
attains equilibrium is extremely slow, this is especially true for
many solutions of biological importance which contain materials
of low diffusivity. Thus it may be necessary to permit a drop to
hang under conditions approaching its maximum size for several
minutes to ensure the attainment of such equilibrium.