92168.pdf

74 Liquid-gas and liquid-liquid interfaces

where pl and p 2 are the densities of the lower and upper phases; a =

0.7250 and b = 0.090 75 m"^1 s^2 for all rings; c = 0.045 34-1.679 r/R;

and r is the radius of the wire.

Drop-volume and drop-weight methods

Drops of a liquid are allowed to detach themselves slowly from the tip

of a vertically mounted narrow tube (Figure 4.7) and either they are

weighed or their volume is measured. At the point of detachment

y

2irr 2irr

(4.13)

where m is the mass of the drop, V is the volume of the drop, p is the

density of the liquid, r is the radius of the tube and <f> is a correction

factor.

Figure 4.7 Detachment of a drop from the tip of a narrow tube

The correction factor is required because on detachment (a) the
drop does not completely leave the tip, (b) the surface tension forces
are seldom exactly vertical and (c) there is a pressure difference
across the curved liquid surface^147. $ depends on the ratio r/Vm.
Values of $ have been determined empirically by Harkins and
Brown^148 '^149. It can be seen that values of rlVm between about 0.6
and 1.2 are preferable (Figure 4.8).
A suitable tip which has been carefully ground smooth used in
conjunction with a micrometer syringe burette gives a very convenient