Instrument Division Fisher Scientific Company 71 1 Forbes Ave. Pittsburgh, Pennsylvania 1521 9 Gl 7 5672 Y ~ L

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FISHER Surface Tensiometer Model 20

CQNTENTS

THE INSTRUMENT UNPACKING ASSEMBLY CALIBRATION OPERATION

Measuring Surface Tension Measuring Interfacial Tension Converting Scale Readings

MAINTENANCE Annual Application of Oil Replacement of Torsion Wire

REPLACEMENT PARTS

ILLUSTRATIONS

Figure

1 Mechanical System 2 Measuring Surface Tension 3 Measuring Interfacial Tension 4 Correction Factor Char t

THE INSTRUMENT

The FISHER Surface Tensiometer, Model 20, i s used to determine the apparent surface tension and interfacial tension of liquids. Employing the ring method to achieve speed and accuracy, the instrument i s applicable to many educational, biological, medical and industrial determinations in connec- tion with water, petroleum products, detergents, pharmaceu- tical mater ia ls and other substances.

The Model 20 utilizes the principles of operation originally devised by Dr. P i e r r e Lecomte du Nouy, the noted biochemist, for studies of blood se rum and other biological fluids. Essen- tially a torsion-type balance, it i s the kind of instrument currently specified by the American Society for Testing Materials in Methods D-971 (interfacial tension of oil against water) and D-1331 (surface and interfacial tensions of detergents).

In the du Nouy method, a platinum-iridium ring of precisely known dimensions i s suspended f rom a counter-balanced lever a rm. The a r m i s held horizontal by torsion applied to a taut stainless steel wire, to which it i s clamped. Increasing the torsion in the wire r a i ses the a r m and the ring, which c a r r i e s with i t a film of the liquid in which i t i s immersed. The force necessary to pull the t e s t ring f r e e f rom this surface film i s measured.

The Surface Tensiometer shows this "apparent" surface o r interfacial tension (the la t ter measured at the interface between two immiscible liquids) on a calibrated dial. The dial readings can be used directly for comparative studies o r converted to "true1' values by using a correction factor chart.

UNPACKING

The FISHER Surface Tensiometer i s shipped in a single carton. Carefully unpack the contents, and check each i tem against the packing l i s t below. If any i tems a r e damaged, save al l packing mater ia l and file claim with the c a r r i e r ; i f any i tems a r e missing, notify your neares t F i she r Branch.

Quantity Item Number

1 Surface Tensiometer 14-812 1 Platinum -Iridium Ring 14-812-5 2 Torsion Wire Assembly 12208 1 Instruction Manual 15041

ASSEMBLY The Surface Tensiometer consists of a torsion wire balance with a dial calibrated in dynes pe r centimeter, a weighed platinum- iridium ring, and a movable table. The torsion wire balance

and movable table a r e mounted on the stand within the case. The ring i s shipped in a wooden container which i s secured to the main pedestal of the stand.

P r i o r to use, the instrument should be placed on a solid support (table o r bench) and leveled. To do this, simply open the door and, using the two knobs near the left edge of the base, adjust the feet until the spi r i t level indicates that the cabinet i s level.

F o r shipping purposes, the tension has been relieved f rom the torsion wire. In o rde r to r e s t o r e the tension, back off the screw which i s accessible through the smal l hole in the upper center portion of the r e a r of the cabinet. Loosening this screw allows the tension spring (Item 4 of Figure 1) to exer t p r e s s u r e against the bar (Item 2), thus restoring the tension on the wire. The packing mater ia l which supports the junction of the torsion wi re and level a r m should be removed, a s should the packing mater ia l on the level a r m near the index m i r r o r and a r r e s t mechanism (Items 12 and 23 of Figure 1). The Surface Tensiometer i s now ready for calibration.

CALIBRATION

The calibration of the torsion wire and hence of the Surface Tensiometer has been carefully tested a t the factory but should be checked before use and adjusted i f necessary. The calibration i s c a r r i e d out s o that the dial will read directly in dynes pe r centimeter.

Make s u r e that the torsion a r m a r r e s t mechanism (Item 23) i s holding the a rm. Hang the platinum-iridium ring on the hook a t the left end of the lever. Cut a smal l s t r ip of paper and fit i t onto the ring to ac t a s a platform for a weight which will be used for the calibration. Release the torsion a r m , and adjust the knob on the right side of the case until the index (Item 11) and i t s image a r e exactly in line with the reference line of the m i r r o r (Item 12). Turn the knob beneath the main dial on the front of the instrument until the vernier reads zero on the outer scale of the dial. A r r e s t the torsion a rm.

Place a known m a s s for calibration on the paper platform (600 mil l igrams i s quite suitable and simplifies calculations). Release the torsion a rm. Turn the knob on the right side of the c a s e in a counterclockwise direction until the index and i t s image a r e again exactly in line with the reference line of the m i r r o r . Record the dial reading to the neares t 1 / 10 scale division (by use of the vernier) .

It i s now necessary to determine the accuracy of the calibration f rom the reading obtained. The apparent surface tension, S, i s given a s follows:

where:

M = the weight expressed in grams g = acceleration of gravity expressed in

cm/sec2 L = mean circumference of the ring in

centimeters ? ,S = dial reading = apparent surface tension

\ in dynes per cm

For example, suppose that a 600 milligram weight was used. The circumference of the ring i s 6. 00 cm, and the value for

2 g i s 980 c m / s e c . Then we find that

If the dial reading differs from the calculated value, then the effective length of the torsion a r m must be adjusted until these two values do agree. This adjustment i s accomplished by turning the knurled knob at the left end of the lever a r m (Item 10) s o a s to move the hanger hook. If the recorded dial reading i s greater than the calculated value, move the hook to shorten the effective length of the arm. Conversely, i f the dial reading i s less than the calculated value, move the hook to lengthen the effective length of the arm. Repeat the calibration procedure until the dial reading and calculated value agree. The dial will read directly in dynes per centimeter.

The mean circumference of rings i s 6. 00 cm in general. Slight deviations may be found, and the circumference of each ring i s noted on i t s container. In the preceding example, 600 milli- g rams were used to simplify calculations.

The value of g, the acceleration of gravity, varies slightly from place to place on the surface of the earth. F o r precise work, the use of the exact value of g for the locality i s recommended. For certain localities, this value may be listed in various hand- books; o r i t may be determined experimentally.

OPERATION

In making a determination of surface tension, careful preparation of the sample and the Surface Tensiometer must precede the actual manipulation of the instrument.

FIGURE 1 - MECHANICAL SYSTEM

The sample should be placed in a g lass beaker o r cyl indrical ves se l with a d iameter of a t l e a s t 45 mi l l imeters . F o r testing oil s amples according to ASTM Method D-971, the g lassware should be cleaned according to a definite procedure. Any res idual oil f r o m the previous sample i s removed with pe t ro- l eum naphtha o r benzene followed by seve ra l washes with methyl ethyl ketone and water , then the g lassware i s i m m e r s e d in a hot cleaning solution of chromic acid. The g lassware a l so should be r in sed thoroughly with tap water , then with dist i l led water . It should be dra ined i n a n inver ted position over a clean cloth, unless i t is to be u s e d immediately.

The plat inum-ir idium ring should be cleaned by r insing i t in pe t ro leum naphtha o r benzene, then by r insing i n methyl ethyl ketone. The r ing should then be heated in the oxidizing portion of a gas f lame.

The preceding instruct ions apply par t icu lary to prepara t ion for determinations on oil samples . A comparable degree of clean- l i nes s should be sought for other samples.

Since surface tension i s dependent upon temperature, considera- tion must be given to this factor. For theoretical work, tem- peratures must be specified; however, 25OC i s the temperature most commonly used. For control work, the operator should use the same temperature for each type of measurement.

FIGURE 2 - MEASURING SURFACE TENSION

Measuring Surface Tension (see Figure 2):

The cleaned platinum-iridium ring should f i rs t be attached to the hook at the end of the lever arm. The a r r e s t mechanism should be holding the a r m a t this time.

The liquid to be measured i s t ransferred to the clean glass vessel and placed on the sample table. The sample table i s moved around until i t i s directly beneath the platinum-iridium ring. Raise the sample table until the ring i s immersed in the test liquid. The ring should be in the liquid, beneath the surface so that the entire ring will be wetted. About 118 inch immer- sion i s generally considered sufficient.

The torsion a r m i s now released and the instrument adjusted to a zero reading. Adjust the knob on the right side of the case until the index and i ts image a r e exactly in line with the refer- ence mark on the mi r ror . Be careful to keep the ring in the liquid during this manipulation, raising or lowering the sample table i f necessary be means of the knob adjustment underneath the table. Now turn the knob beneath the main dial on the front of the case until the vernier reads zero on the outer scale of the dial.

Lower the sample table until the ring i s in the surface of the liquid, adjusting the knob on the right side of the case to keep the index lined up with the reference mark on the mi r ror . The surface of the liquid will become distended, but the index must be kept on the reference. Continue the two simultaneous adjust- ments until the distended film a t the surface of the liquid breaks. The scale reading a t the breaking point of the distended film i s the apparent surface tension.

FIGURE 3 - MEASURING INTERFACIAL TENSION

Measuring Interfacial Tension ( see Figure 3 ) :

INTERFACIAL TENSION FROM MORE DENSE LIQUID TO LESS DENSE LIQUID i s made by exerting an upward force on the ring. Procedure i s very similar to surface tension measurements; and cleanliness of ring and sample container is , of course, essential.

The more dense liquid i s t ransferred to the clean glass vessel and placed on the sample table, and the table moved beneath the platinum-iridium ring. The sample table i s raised until the ring i s immersed about 1 / 8 inch and wetted by the heavier liquid. The torsion a r m i s now released and the instrument adjusted to a zero reading. Adjust the knob on the right side of the case until the index and i ts image a r e exactly in line with the reference mark on the mi r ror . Be careful to keep the ring

i n the liquid during this manipulation by raising o r lowering the sample table (if necessa ry ) by means of the knob adjustment beneath the table. Turn the knob beneath the main dial on the front of the c a s e until the ve rn ie r reads ze ro o n t h e outer sca l e of the dial.

Pour the l ighter liquid onto the surface of the heavier liquid to a depth of about 114 to 1 / 2 inch, depending on the two liquids. The layer of l ighter liquid should be deep enough so that the ring will not enter. the upper sur face of the l ighter liquid before the interface f i lm ruptures.

Before proceeding with the actual measurement , i t m a y be advisable to allow the interface to age o r stabilize for a time. F o r example, ASTM D-971 p resc r ibes a n aging period of 30 seconds for an interface between oil and water .

Lower the sample table until the ring i s in the interface between the two liquids, adjusting the knob on the right s ide of the c a s e to keep the index lined up with the reference m a r k on the m i r r o r . The interface between the two liquids will become distended, but the index mus t be kept on the reference. Con- tinue the two simultaneous adjustments until the distended f i lm a t the interface ruptures. The sca le reading a t the breaking point of the interfacial f i lm i s the apparent interfacial tension.

INTERFACIAL TENSION FROM LESS DENSE LIQUID TO MORE DENSE LIQUID i s made by exerting a downward force on the ring. The r ing used on the Surface Tensiometer i s weighted, and i n effect exe r t s i t s own force , the effective downward force being increased by decreasing the upward force f rom the torsion wi re a s exerted through the lever a rm. The procedure differs only slightly f rom that employed in the previously described measuremen t of interfacial tension; and the cleanliness of the r ing and sample container is , of course, essential .

The m o r e dense liquid i s t r ans fe r red to the clean g lass vesse l , to a depth of 114 to 112 inch, depending on the two liquids. The layer should be deep enough so that the ring will not con- tac t the bottom of the vesse l before the interface f i lm ruptures.

The l ighter liquid i s now poured onto the surface of the heavier liquid a l ready in the vesse l , and the vesse l on the sample table moved beneath the platinum-iridium ring. The sample table i s then r a i sed by the g r o s s adjustment until the ring i s immersed in the l ighter liquid and wetted by it (about 118 inch above the liquid interface). The tors ion a r m i s now released, and the ins t rument adjusted to a z e r o reading. Adjust the knob on the r ight s ide of the c a s e until the index and i t s image a r e exactly in line with the r e fe rence m a r k on the m i r r o r . Be careful to keep the ring in the l ighter liquid during this manipulation by

raising or lowering the sample table (if necessary) by means of the knob adjustment beneath the table. Turn the knob beneath the main dial on the front of the case until the vernier reads zero on the inner scale of the dial.

Raise the sample table until the ring i s in the interface between the two liquids, adjusting the knob on the right side of the case to keep the index lined up with the reference mark on the mi r ror . The interface between the two liquids will become distended, but the index must be kept on the reference. Continue the two simultaneous adjustments until the distended film a t the inter- face ruptures. The scale reading a t the breaking point of the interfacial film i s the apparent interfacial tension.

Converting Scale Readings:

Readings of the Surface Tensiometer give apparent surface tension and apparent interfacial tension. In order to obtain the t rue surface tension or t rue interfacial tension, the relationship

i s used, where S i s the t rue value, P the apparent value a s indicated by the dial reading, and F a correction factor. The correction factor F i s dependent on the size of the ring and the s ize of wire used in the ring, the-apparent surface or inter- facial tension, and the densities of the two phases. The re - lationships a r e expressed by the following two forms, either one of whichmay be used in preparing a correction factor chart (like the one shown in Figure 4).

where:

F = the correction factor R = the radius of the ring r = the radius of the wire of the ring ', P = the apparent value or dial reading D = the density of the lower phase d = the density of the upper phase K = 0.04534 - 1. 679 r / R C = the circumference of the ring , a = 0.725 b = 0.0009075

The a, b, and the numerical par t of K a r e universal constants for all rings.

In order to use the correction factor chart, the operator must initially calculate the value of the expression P / ( D - d) for each sample, and know the value of R / r . For the Surface Tensio- meter , the circumference of the ring i s 6 centimeters and the radius of the wire i s . 007 inches. There may be some slight variation in these dimensions, but the value of R / r i s usually 53. 2. Curves a r e shown for R / r = 50 and R / r = 60, so that correction factors may be found for any value of the ratio between 50 and 60.

Should a ring of different diameter be employed, the user can prepare a curve f rom the above equations. All quantities used in these equations a r e expressed in the centimeter-gram- second system, with densities expressed in grams per cubic centimeter; P (apparent surface o r interfacial tension) expressed in dynes per centimeter; while R, r, and C a r e expressed in centimeters.

Use of the correction factor i s usually limited to theoretical work. Fo r control purposes, reproducibility within certain tolerances i s sufficient.

FIGURE 4 - CORRECTION FACTOR CHART

Maintenance on the Surface Tensiometer will normally be limited to an annual application of oil and occasional replacement of the torsion wire.

Annual Application of Oil:

The zero adjustment shaft and bearing (see Items 14 and 17 in Figure 1 ) should be oiled once every twelve months; two drops of light oil should be sufficient. Putting a drop or two of oil on the post and adjusting nut of the sample table i s also advisable once a year.

Replacement of Torsion Wire:

Replacement of the torsion wire may be necessary f rom time to time due to accidental breakage. By referring to Figure 1, the replacement procedure can be readily visualized. The torsion a r m (Item 5) should be held by the arres t ing mechanism (Item 23) , and the clamping block (Item 6) supported by a suitable wood o r cardboard block between itself and the casting. The cover of the vernier pointer on the front of the case should be removed by loosening the s e t screw. Now the screw a t the back of the case which controls the tension spring (Item 4) should be turned in so a s to release a l l tension on the wire. The front wire holder (Item 3) should be loosened so a s to release the front end of the wire. Now the screws on the clamping block (Item 6) of the torsion a r m should be loosened. The torsion wire should now be entirely free. If i t i s not broken, i t can be removed by pulling toward the r ea r of the instrument.

The replacement wire has a bar (Item 2 ) attached for holding in the tension spring (Item 4). The wire should be installed by working the front end forward f rom the rear , being careful not to bend o r kink the wire. Do not fasten the wire in the clamping block (Item 6) a t this stage. It should be brought forward through the sleeve (Item 14) to the front wire holder (Item 3) . A scribed line shows the position in which the wire should be placed in order to center it. Hold the wire taut with pliers and tighten the two screws of the front wire holder. Excess wire may now be cut off and the cover of the vernier pointer assembly replaced.

Tension should now be applied to the wire by slacking off on the screw that holds the tension spring. This screw i s accessible through the r ea r of the case. The procedure i s the same a s when the instrument was originally unpacked and readied for use.

The torsion a r m must now be attached to the wire. This i s best accomplished by removing the left screw from the clamping block (Item 6) and loosening the right screw. The wire must r e s t in the groove in the block, and when so fitted, the two screws a r e tightened. Upon removing the support f rom under the clamping

block, check and if necessary adjust the diameter of the new torsion wire a s follows:

1. Level the instrument.

2. Cut a small s t r ip of paper and place i t on the platinum-iridium ring to fo rm a platform.

3. Place a 1 :000 gram weight on the platform.

4. Release the torsion arm.

5. Adjust knob on right side of instrument case until index pointer on a r m coincides with reference line on mirror .

6 . Turn knob to right of main dial on front of instrument until vernier reads zero on outer scale of dial.

7. Remove 1.000 gram weight f rom platform.

8. Place a 1.800 gram weight on platform.

9. Adjust knob on right side of instrument case until index pointer on a r m coincides with reference line on mirror.

NOTE: The reading obtained should be 65.7 to 66.0. If the reading i s higher, the diameter of the torsion wire between the a r m and the r ea r of the instrument must be reduced. Conversely, i f the reading i s lower, the diameter of the wire between the a r m and the front of the instrument must be reduced. In each case, Crocus cloth should be used. The wire must be kept syrnrnet- r ical and round with uniform diameter along i ts length.

10. Calibrate instrument as directed on pages 2 and 3 under CALIBRATION.

REPLACEMENT PARTS

The following i s a l i s t of parts which may need to be replaced over a period of time. To a s s i s t in identifying each item, the tabulation includes c ross reference numbers to the mechanical system drawing (Figure 1).

Visual Reference - Item (Figure 1)

Torsion Wire As.sembly Clamp Bar Tension Spring Torsion Arm Arm Clamping Block Index Assembly Hanger Spring Hanger Assembly Knurled Nut Mirror Vernier Pointer Dial Shaft Worm Gear Dial Hub Support Casting Bushing Assembly Worm Worm Gear Shaft Large Bearing Small Bearing Inner Clamp Outer Clamp Cam Cam Support Lever Grommet Pedestal Clamp Assembly Table Table Spring Table Screw Table Nut Table Stop Level Counter Weight Grommet Leveling Foot Assy Knob, Front Panel Knob/w Crank Dial Large Bevel Gear Small Bevel Gear Handle

Number

Fisher Scientific Co. Instruments, Apparatus, Furniture and Chemicals for Laboratories.

Manual No. 15041 Revised and Published 6-73 Effective with Serial No. 1 17 Seventh Issue

073-006 Litho in U.S.A.