School Tensiometer
K6
User manual
V03
© KRÜSS GmbH, Hamburg 2002-2011
Art.-Nr.15009
School tensiometer K6 page 1
Inhaltsverzeichnis
1 The K6 Tensiometer..........................................................................3
1.1 Design of the apparatus ............................................................................................. 3
1.2 Basic rules for surface tension measurements.............................................................. 3
1.3 Thermostat vessel K 801 ............................................................................................. 3
2 Handling, cleaning and safekeeping the ring ................................4
2.1 Handling .................................................................................................................... 4
2.2 Cleaning .................................................................................................................... 4
2.3 Checking the ring ....................................................................................................... 4
2.4 Safekeeping after use ................................................................................................. 5
3 Preparations for the instrument .....................................................5
4 Measurement ...................................................................................6
4.1 Measuring interfacial tension at the phase border liquid/gas ....................................... 6
4.2 Measurement of interfacial tension at the phase border liquid/liquid........................... 6
4.3 Calibration ................................................................................................................. 6
4.4 Temperature Dependence........................................................................................... 7
5 Changing the balance system .........................................................8
5.1 Removing the old balance system ............................................................................... 8
5.2 Building in the new balance system ............................................................................ 8
6 Knowledge base.............................................................................11
6.1 Measurement principle............................................................................................. 11
6.2 Measurement value correction.................................................................................. 12
7 Appendix ........................................................................................13
7.1 Correction factors..................................................................................................... 13
7.2 Technical data of the KRÜSS standard ring................................................................ 14
7.3 Questions and Problems / Contact ............................................................................ 15
School tensiometer K6 page 3
1 The K6 Tensiometer
1.1 Design of the apparatus
A torsion wire is tensioned between the two clamping chucks (Æ pages 9 and 10). Using the
two handwheels (No. 2 on page 9) a torsion may then be imposed on the wire. The
tensiometer is adjusted with the handwheel at the back end of the wire. The handwheel in the
front part of the wire is firmly connected to a pointer. The pointer glides over a scale on which
the value of the interfacial tension is indicated in mN/m. The balance beam (9) is clamped onto
the torsion wire in an angle of 90°. The measuring device is suspended at the end of the
balance beam.
The measuring device must be absolutely level so that all parts of it are subjected to the same
forces at the same time when passing through the surface. When turning the torsion wire the
balance beam leaves the zero-position: the ring is lifted or lowered. This vertical movement is
registered by projecting the balance beam on mark (7). The measuring platform (6) used to
support the glass vessel, filled with the liquid to be analysed, can be raised and lowered. The
precise adjustment has to be performed with handwheel (5).
1.2 Basic rules for surface tension measurements
Similar to other analyzing methods the measurement of interfacial tension is subject to a
number of effects leading - unless they are correctly observed - to incorrect or even misleading
results.
All parts of the apparatus coming into contact with the liquid to be measured have to be kept
meticulously clean, since the interfacial tension reacts in a very sensitive way to all kinds of
contamination.
The enrichment of molecules active in the surface or the interface generally takes place very
slowly, that is to say, the parameters age and temperature influence the measurement signifi-
cantly. During any measuring series it is therefore essential to maintain constant test
conditions. Temperature stability of the liquid can be obtained with the thermostat vessel K
801 (5).
1.3 Thermostat vessel K 801
If measurements have to be performed at a constant temperature, the flat table, delivered with
the standard equipment has to be replaced by the thermostat vessel K 801 that is connected
to a thermostat.
page 4 School tensiometer K6
2 Handling, cleaning and safekeeping the ring
The Du Noüy ring is a highly sensitive probe which gets useless when deformed. Each standard
ring is delivered in a wooden protecting box and checked for proper shape before delivery.
2.1 Handling
To take the ring from the box remove the lid by slightly turning it. Take the opening between
your fingers so that the ring can fall into the palm of your hand. Take the ring by the shaft
with the other hand.
Don’t ever touch the ring itself with your fingers. It can easily
get deformed by even low forces.
!
2.2 Cleaning
Aqueous solutions and emulsions: Rinse the ring under warm flowing water and
with distilled water. Then heat the ring to red glow with a gas burner. Using an alcohol burner
could lead to uneven heating and to ring deformation. Also, do not heat the ring to white
glow; this could also destroy the ring.
Organic Samples: After measurements with organic samples the ring must first be rinsed
with an organic solvent. The further procedure is the same as described for aqueous solutions.
2.3 Checking the ring
Fix the clean ring at your K6. Fill a sample vessel with clean water and rise it to a position of
about 1-2 mm under the ring. The mirror image of the ring should be clearly visible and be
absolutely parallel to the surface.
Now rise the surface level until the ring touches it. The ring should not move to the side but
strictly stay in the centre position. When the measurement is finished, the ring also shouldn’t
leave the centre position when the surface is lowered.
School tensiometer K6 page 5
For a further test of the proper shape of the ring, hold it by the shaft and rotate it by turning
the shaft between your thumb and forefinger. The ring should rotate without any recognisable
variation in height and without side-to-side movement. The light reflexes from the surfaces can
help you in your assessment.
Fig. 1: Checking the shape of the ring
In case of a ring deformation it is possible to re-establish the proper shape by yourself. The
necessary ring adjustment tool TO01 can by ordered at KRÜSS.
2.4 Safekeeping after use
Before putting the ring back into the box it must be cleaned as described in section 2.2 .
Hold the open box in one hand and the shaft of the ring in the other hand. Lead the shaft into
the box as far as possible, than let the ring glide into its position..
3 Preparations for the instrument
See pages pages 9 and 10:
The instrument is levelled by the adjustment-screws (3) and the box-level.
Clean an check the ring as described in section 2.2 and 2.3.
According to DIN the sample-vessel is cleaned in an oxidizing acid, boiled in distilled water for
a considerable time and briefly flamed in a gas burner prior to use. However, according to our
experience, intense cleaning with water and a suitable, clean organic solvent is sufficient.
The heigth of the measuring vessel is regulated with handwheel (5).
Caution! The distance shouldn’t be more than 2cm. Otherwise
the guide leader pin will be turned out of the guiding device.
Tightening screw (5) again the thread will be damaged by the
!
guide leader pin and the sample table will tilt.
It is important to take care that the distance from the ring to the vessel walls is equal on all
sides. Prior to each measurement the circuit division (1) has to be set to 0 with handwheel (2).
The lever arm (9) carrying the ring should coincide in its zero-position with the white field of
mark (7). If this is not the case, the zero position has to be readjusted by turning of knob (8).
The instrument is now ready for measurement.
page 6 School tensiometer K6
4 Measurement
See pages 8 and 9:
4.1 Measuring interfacial tension at the phase border liquid/gas
The carrier of table (6) is carefully raised until the ring submerges centrally into the liquid for
some 5 mm and is then adjusted with screws (10) and (12). In order to obtain a plain surface
the liquid should calm down for a while. By turning screw (5) to the left the measuring table is
lowered until the lever arm (9) moves downwards, out of the white field of mark (7), that is to
say, the ring is held fast by the surface-film of the liquid. Actual measurement begins at this
point.
By turning handwheel (2) clockwise the torsion of the ribbon is increased and thus a pull is
applied to the ring. This leads to an upward displacement of the lever arm (9) out of the white
field of mark (7). The lever arm (9) is now brought back to zero-position by lowering the
measuring-vessel (turn screw (5) to the left)). This alternating of careful increase of the pull
and subsequent lowering of the measuring-vessel has to be repeated until the film "breaks",
that means, until the upward pull acting on the ring has completely overcome the interfacial
tension forces and breaks out of the surface in an upward direction.
4.2 Measurement of interfacial tension at the phase border
liquid/liquid
When measuring interfacial tension between two immiscible liquids (e.g. oil and water), the
ring is first submerged into the lower specific heavier phase (in this case water). The specific
lighter phase (in this case oil) is then pipetted above carefully.
Measurement is performed analogue to 4.1. The interfacial tension value has been reached
when the lever arm (9) can no longer be brought into zero-position by slowly lowering the
measuring-table (over-elongation of the film). After each measurement the adjustment of the
circuit division of the instrument has to be brought back into its zero-position.
4.3 Calibration
The interfacial tensiometer is calibrated by KRÜSS with double distilled water (interfacial
tension at 20°C = 72.8 mN/m). If another value should be obtained in the course of time, in
particular when using a new ring, it is best to operate with a correction factor. If the value
obtained for water at 20°C is not 72.8 mN/m but 73.2 mN/m, the correction factor will be
equal to the quotient of the theoretical and indicated value:
72.8
= 0.995
73.2
The indicated values must then be multiplied by this correction factor.
School tensiometer K6 page 7
4.4 Temperature Dependence
The interfacial tension of water depends on the temperature. If the water used for calibration
is not at a temperature of 20°C, the value for interfacial tension should be obtained from Fig.
2, where empirically determined values of interfacial tension are plotted against temperature.
Fig. 2: Temperature dependence of interfacial tension of double distilled water
page 8 School tensiometer K6
5 Changing the balance system
See pages pages 9 and 10:
5.1 Removing the old balance system
Bring stretcher (1) into horizontal position by turning the handwheel (1 a) . Bring stretcher (2
with adjustment screw (2a) in horizontal position. While pushing pressure pin (3) you can
remove the old balance system.
5.2 Building in the new balance system
Introduce wire holder (4) with balance beam on top into the wire stretcher (1). Press pressure
pin (3) and insert wire holder (4a) in wire stretcher (2) with a pair of tweezers. Shift wire
holders (4) and (4a) to the middle of the boring. Bring the balance beam below the limit stop
(5). Hang on the platinum ring. Turn handwheel (1 a) to zero position. Bring the balance beam
in the middle of the white mark (7) by turning adjustment screw (2a).
The tensiometer is now ready for measurement.
School tensiometer K6 page 9
Construction of the K6
(l) Scale in mN/m; (2) handwheel with pointer; (3) screws for level regulation; (4) bubble level;
(5) micrometer screw; (6) sample table; (7) mark; (8) handwheel for zero adjustment;
(9) balance beam; (10) and (12) handwheels for fixing the crossbar; (11) sample table carrier
page 10 School tensiometer K6
Schematic drawing
(l) and (2) wire tensioning device; (la) handwheel; (2a) Adjustment screw;
(3) pressure pin; (4) and (4a) wire holders; (5) stop for balance beam
School tensiometer K6 page 11
6 Knowledge base
6.1 Measurement principle
The ring method is known since the 19th century and became a widely used method for
surface tension measurement since Lecomte Du Noüy described the first ring tensiometer in
1919. As measurement probe a horizontally hanging platin ring with known dimensions is
used. KRÜSS uses a standard ring with an average ring radius of R =9,545 mm and a wire
radius of r = 0,185 mm. These ring dimensions result in a wetted length of lb = 119.95 mm.
The platin ring is immersed in the sample liquid and pulled out again (Fig. 3).
Fig. 3: The three phases of a ring measurement
The value obtained from the measurement is the maximum force needed to pull the ring
through the surface. The following equation describes this process:
P
σ =σ* ⋅F = ⋅F with lb = 2π ⋅ ( Ri + Ra )
lb
(σ = actual surface tension value; σ* = measured surface tension value; F = correction factor;
P = maximum force at the ring; Ri = inner ring radius; Ra = outer ring radius)
The correction factor is needed to take the weight of the lifted liquid volume into
consideration.
page 12 School tensiometer K6
6.2 Measurement value correction
When the ring is lifted, not only the force resulting from surface tension but also the
gravitational force of the lifted liquid volume Vh is measured and thus falsifies the
measurement.
Fig. 4: Ring with lamella
For this reason, a correction factor with which the measured value must be multiplied is
necessary.
Correction according to Harkins & Jordan
The correction values for a range of density differences between the phases were obtained
experimentally and published in a tablework by HARKINS and JORDAN. Instructions and a
table for the correction with K6 are given on p. 13.
Correction according to Zuidema & Waters
For measurement values <25 mN/m and density differences > 1g/cm3 the equation by
ZUIDEMA and WATERS should be applied (see p. 14).
School tensiometer K6 page 13
7 Appendix
7.1 Correction factors
Harkins and Jordan
The K6 tensiometer is linear compensated so that the correction factor for a measurement of
pure water at 20°C is 1; this corresponds to a literature value for the surface tension of water
of 72.8 mN/m.
Under all other conditions the measurement value must be multiplied by a correction factor.
Determine the difference between the densities (for phase border liquid/gas just take the liquid
density) in find the corresponding column. Then find the measured value in the line and read
the value from the table entry
Density 0.65 0.8 1.0 1.2 1.4
difference
σ [mN/m] Factor Factor Factor Factor Factor
100 1.070 1.049 1.026 1.011 0.998
97 1.065 1.043 1.023 1.008 0.995
94 1.063 1.040 1.020 1.005 0.993
91 1.058 1.037 1.018 1.003 0.991
88 1.054 1.034 1.014 1.000 0.988
85 1.052 1.031 1.012 0.998 0.986
82 1.049 1.029 1.009 0.995 0.984
79 1.043 1.024 1.006 0.991 0.980
76 1.040 1.023 1.003 0.988 0.977
73 1.037 1.018 1.000 0.986 0.974
70 1.033 1.014 0.996 0.982 0.972
67 1.028 1.011 0.993 0.979 0.968
64 1.023 1.006 0.990 0.976 0.964
61 1.019 1.003 0.985 0.973 0.960
58 1.015 0.999 0.982 0.969 0.956
55 1.012 0.996 0.978 0.964 0.953
52 1.007 0.990 0.974 0.959 0.950
50 1.004 0.988 0.972 0.957 0.945
48 1.001 0.985 0.967 0.954 0.943
46 0.998 0.980 0.964 0.951 0.941
44 0.994 0.978 0.960 0.948 0.938
42 0.990 0.975 0.957 0.944 0.935
40 0.987 0.972 0.954 0.941 0.931
38 0.981 0.968 0.951 0.938 0.929
36 0.979 0.963 0.946 0.935 0.923
34 0.975 0.958 0.942 0.931 0.919
32 0.970 0.954 0.940 0.926 0.915
30 0.964 0.950 0.935 0.921 0.911
28 0.959 0.944 0.930 0.916 0.906
26 0.954 0.940 0.924 0.911 0.904
24 0.949 0.935 - - -
22 0.942 0.929 - - •-
20 0.937 0.921 - - -
19 0.933 0.918 - - -
18 0.930 0.914 - - -
17 0.924 0.911 - - -
16 0.920 0.906 - - -
15 0.915 0.902 - - -
page 14 School tensiometer K6
Zuidema and Waters
For measurement values <25 mN/m and density differences > 1g/cm3 the following equation
by ZUIDEMA and WATERS should be applied to calculate the correction factor:
0.01452 ⋅ σ * 1.679
F = 0,725 + + 0.04534 − .
1l
4 b
2
⋅ ( D − d ) R
r
When the known values of the KRÜSS standard ring are put in this equation, it is simplified to:
σ*
0.4036 ⋅ 10 −3 ⋅ + 0.0128 + 0.725
(D−d )
The uncompensated value for σ * must be inserted in the equation and not the read-off linear
compensated value (see page 13). Multiply the read-off value by 1.062 and insert the result in
the equation.
7.2 Technical data of the KRÜSS standard ring
material: Platinum-Iridium
wetted length 119.95 mm
Circumference: 59.97 mm
Ring radius R: 9.545 mm
Wire radius r 0.185 mm
R/r: 51.6
School tensiometer K6 page 15
7.3 Questions and Problems / Contact
For further questions about the K6 or other KRÜSS products, please contact KRÜSS:
KRÜSS GmbH
Technical Support
Borsteler Chaussee 85 – 99a
22453 Hamburg
Tel.: +49/040/514401-33
Fax.: +49/040/514401-98
E-Mail: service@kruss.de
Please also visit us at our webpage:
http://www.kruss.de
