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Ref. No.: SOIR 304- 1963 (E)
IS0
IN TERN AT I ON AL ORGAN I Z AT1 O N FOR STAN DARD I Z AT I O N
IS0 RECO M M EN DATION
R 304
SURFACE ACTIVE AGENTS
DETERMINATION OF SURFACE TENSION
AND INTERFACIAL TENSION
1st ED IT I ON
[May 1963
COPYRIGHT RESERVED
The copyright of IS0 Recommendations and IS0 Standards
belongs to IS0 Member Bodies. Reproduction of these
documents, in any country, may be authorized therefore only
by the national standards organization of that country, being
a member of ISO.
only valid standard is the national standard of that country.
For each individual country the
Printed in Switzerland
Also issued in French and Russian.
Copies to be obtained through the national standards organizations.
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BRIEF HISTORY
The IS0 Recommendation R 304, Surface Active Agents-Determination of Surface Tension
and Interfacial Tension, was drawn up by Technical Committee ISO/TC 91, Surface Active
Agents, the Secretariat of which is held by the Association Française de Normalisation (AFNOR).
Work on this question by the Technical Committee began in 1960 and led in the same year
to the adoption of a Draft IS0 Recommendation.
In March 1961, this Draft IS0 Recommendation (No. 438) was circulated to all the IS0
Member Bodies for enquiry. It was approved by the following Member Bodies:
Australia France Mexico
Austria Germ an y Poland
Belgium Hungary Portugal
Canada Iran Romania
Chile Ireland Spain
Colombia Israel
United Kingdom
Czechoslovakia Italy U.S.S.R.
Yugoslavia
No Member Body opposed the approval of the Draft.
The Draft IS0 Recommendation was then submitted by correspondence to the IS0 Council,
which decided, in May 1963, to accept it as an IS0 RECOMMENDATION.
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ISO/R304 - 1963 (E)
IS0 Recommendation R 304 May 1963
SURFACE ACTIVE AGENTS
DETERMINATION OF SURFACE TENSION
AND INTERFACIAL TENSION
FOREWORD
Surface and interfacial tensions are fundamental properties of solutions of surface active agents.
The measurement of these characteristics, however, does not allow any suppositions as to their
detergency, wetting, foaming activities, etc.
Indeed, no connection can be established between the performance properties of the surface
active agents and the surface or interfacial tensions of their solutions.
.
1. SCOPE
The purpose of this IS0 Recommendation is to set out a method for the determination of:
(1) the surface tension of aqueous or organic solutions of surface active agents and mixtures
containing one or more surface active agents;
(2) the interfacial tension of systems of two immiscible phases containing surface active agents.
These determinations also apply to the measurement of surface tensions of pure liquids or solu-
tions, and to that of the interfacial tension of binary liquid systems other than those mentioned
above.
2. PRINCIPLE
The method consists in measuring the force which should be exerted vertically on a plate or a
stirrup in contact with the surface of the solution being studied in order to draw up the film
that is formed.
3. APPARATUS
3.1 Constitution of apparatus
The apparatus consists of the following:
3.1.1 Tensiometer
The tensiometer, which should be capable of being fitted with plates or stirrups, consists
essentially of three parts:
(a) horizontal platform which can be moved vertically downwards or upwards by
means of a micrometer screw;
(b) a balance to give a continuous measure of the force applied either to the plate
or to the stirrup, and at least capable of a precision of 0.05 dyn/cm;*
a device for indicating the force expressed in dynes per centimetre of the peri-
(c)
or the stirrup, transmitted to the balance.
meter of the plate
Dyne pcr centimetre (dynlcm is the unit of the CGS system. The unit of the International System (SI) is the newton per metre (Njm):
1 N/m= IOa dynjcm.
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ISO/R 304 - 1963 (E)
~
I
3.1.2 Plate or stirrup
PLATE consists of sheets of platinum of about 30 mm
x 20 mm x 0.1 mm; it is roughened by sand
blasting or emery papering in the direction of the
longest dimension; it is fixed to a suspending rod,
20 to 30 mm
at a short distance from one of the shorter sides,
and in the axis of symmetry.
STIRRUP (see figure) is made of platinum wire of
diameter not exceeding 0.5 mm; the length of the
horizontal arm is 20 to 30 mm and the length of the
3
I
two branches is 10 mm; the lower ends of these
Platinum bolls
branches terminate in two small platinum balls which
FIGURE. - Stirrup
act as counterweights.
3.1.3 Sample vessel
The sample vessel for the solution on which the measurement is to be made is a small
glass vessel containing a fairly large quantity of liquid (watch glasses should not be
The ideal is a small tank with the form of a parallelepiped having sides of at
used).
least 8 cm. Only the form of a parallelepiped enables the liquid surface to be cleaned
before measuring the surface tensions of pure liquids. In the case of solutions of sur-
face active agents, and especially in the case of dilute solutions, the surface should not
be cleaned, and cylindrical containers can be used provided that the diameter is at
least 8 cm.
This vessel can be raised or lowered by the screw control, in order to alter itsposition
in a vertical plane.
3.2 Preparation of apparatus
There are two possible cases represented
3.2.1
If the solution or one of the phases of the binary system wets the plate perfectly, the
plate should be preferred to the stirrup.
3.2.2 If the solution does not wet the plate perfectly or, in case of a measurement of inter-
facial tension, if one of the two phases does not wet the plate preferentially, a stirrup
should be used.
Wash the glassware carefully with hot sulphochromic mixture, rinse in double-distilled
water until neutral and then dry. (The double-distilled water should be prepared in an
apparatus fitted throughout with ground-glass joints and kept in a container with ground-
glass stopper, the ground-glass joints being not greased; rubber should not be used in any
circumstances).
Clean the platinum plate or stirrup with hot sulphuric acid (mass density at 20 OC p =
1.83 g/ml), and then rinse with double-distilled water until neutral.
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IÇO/R304 - 1963 (E)
3.3 Calibration of apparatus
3.3.1 Principle
The calibration of the apparatus consists principally in adjusting it so that the readings,
such as the displacement of the spot of light from an optical device, are expressed directly
in dynes per centimetre.
When making the measurement, the roughened platinum
(a) WHEN USING A PLATE.
plate is surrounded by an adsorption layer, and everything happens as if the liquid
were making tangential contact with a theoretical plate consisting of the plate
surrounded by its wetting sheath. It is thus easy to dry the plate properly
because, thanks to inevitable lack of symmetry, when the plate is removed from
the liquid, one end of its base comes away and leaves one corner immersed. If
withdrawal is continued very slowly, the plate comes out in required drainage
conditions, without bringing away any droplets. In practice, in order to make
the measurement, a certain depth of the plate is wetted, it is drained as indicated
above, and only then, with the plate c
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