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Measurements of dielectric constants
MEASUREMENTS
OF DIELECTRIC
CONSTANTS
1)
b y P. C O H E N H E N R I Q U E Z Laboratory for Technical Physics. Technical University Delft, Holland.
Summary A description is given of an apparatus for measuring dielectric constants Compactness of construction is combined with great ease of manipulation, highest accuracy and use of minute quantities of material. Attention is called to the fact t h a t the measurement of dielectric constants is waiting for application in a large field of pure as well as applied chemistry. I wish to express my best thanks to Prof. Z w i k k e r for the generous hospitality in his laboratory, and to the firm of K i p p and S o n s for their very agreeable collaboration. T h e dielectric c o n s t a n t (D. C.) of a s u b s t a n c e is g i v e n b y the e q u a t i o n ~ ---- C/CL w h e r e C = c a p a c i t y of a condenser filled w i t h the s u b s t a n c e a n d CL = c a p a c i t y of the s a m e c o n d e n s e r in vacuo. A c c u r a t e d e t e r m i n a t i o n s of small differences in D. C. are of g r e a t i m p o r t a n c e in m a n y cases, e.g. as a t e s t for t h e p u r i t y of substances, for d i p o l e m e a s u r e m e n t s etc. These d e t e r m i n a t i o n s involve the a c c u r a t e m e a s u r e m e n t of small differences in c a p a c i t y : A ¢ = A C/CL. I f CL is k n o w n A C h a s to be m e a s u r e d . I n the preceding p a p e r s o m e c o n v e n i e n t w a y s of p r o c e e d i n g were e x p l a i n e d for m e a s u r i n g v e r y small c a p a c i t y differences. If we w a n t to d e t e r m i n e A ~ b e t w e e n t w o substances, we m u s t replace the first s u b s t a n c e in t h e c o n d e n s e r (L) b y the second one a n d c o m p e n s a t e the change in c a p a c i t y w i t h a precision-condenser P (see fig. 1 in the p r e c e d i n g p a p e r ,,Accurate c a p a c i t y m e a s u r e ments").
Precision condenser. T h e precision c o n d e n s e r used, is of the linear t y p e . I t is s c h e m a t i c a l l y g i v e n in fig. 1. T h e c o n s t r u c t i o n is such as to m a k e it possible to r e m o v e t h e last - - 434 - -
MEASUREMENTS OF DIELECTRIC CONSTANTS
435
bit of ,,play" of the hollow screw S. If the condenser is constructed with the utmost care, it is strictly linear to within 0,001 cm, at least in a part of the scale. It can be read to 10-4cm. In total a difference of 5 cm, can be determined with it.
A C B
2
Fig. 1.
Fig. 2.
In using the condenser not the slightest axial, nor vertical, stress should be exerted upon it; the tangential stress for the rotating movement has to be transmitted b y a special mechanism, which is also shown in fig. 1.
The liquid condenser. A condenser devised for measuring liquids is shown in fig. 2. It consists of two glass tubes concentrically adjusted; the inner tube is inburned platinated on the outside, the outer tube on the inside. The two thin platinum layers form the coatings of the condenser. The distance of the coatings is 0,5 ram. The capacity of the condenser is about 10 cm, the volume only 0,5 cm. The smallness of this volume offers the considerable advantage that measurements require only very small quantities of material. Taking into account that the differences in capacity can be computed with an extreme
436
P. COHEN HENRIQUEZ
accuracy, the relatively small capacity of the condenser is sufficient. It is not sure that the choice of a larger condensor would increase the accuracy of the measurements. We observed that the reproducibility of the filling decreases (measured in capacity units), if we take a larger condenser. Besides, in most cases it serves no useful purpose to determine the D. C. with an accuracy greater than 10-4, the slightest impurity (humidity! !) making the pretended accuracy illusory. The liquid is poured in at A (see fig. 2), while B is kept closed with the thumb. By blowing at C, the liquid is removed at B. After the liquid is blown out, the condenser is rinsed twice with low-boiling ligroin (b. p. 30°--40°C). Drying is accomplished by a current of thoroughly dried air (it takes less than three minutes). The liquid-condenser is mounted in a thermostat, consisting of a D e w a r-vessel, containing about 400 cm 3 of the bath-liquid (oil, water, or mercury). For measurements at ordinary temperatures the liquid need not be heated nor stirred. It is true that after each rinsing, followed by evaporation of the ligroin, the temperature suffers a small decrease. This, however, does not exceed 0,03°C. The small error, introduced by this fall of the temperature, is compensated in the following way: for each measurement we have three readings : L filled with the first liquid, L filled with the second liquid, and L, filled with the first liquid again; from these readings we calculate two values for A ~, one of which is too high, and the other too low. Of course, the rinsing wiht ligroin, with subsequent drying, is not necessary if sufficient quantities of both liquids are available for rinsing. For measurements at temperatures, higher than room-temperature, the liquid in the thermostat is electrically heated by Nichrome resistance-wire, wound round a glass tube. Stirring is accomplished in this case by passing a weak current of air through the bath-liquid. For low temperatures an U-shaped tube is inserted in the thermostat through which cooling liquid is passed (for very low temperatures liquid air). In the last case hexane or low-boiling ligroin is very suitable as bath-liquid. For determining CL, (the capacity of the condenser in vacuo), two liquids of precisely known D. C. difference have to be measured only o n ce.
MEASUREMENTS OF DIELECTRIC CONSTANTS
437
For the computation of the differences between two liquids of high D. C. the total capacity of the circuit m a y become too large. In that case it is possible to use an other liquid-condenser, with a still smaller capacity, but it is easier to place a capacity in series with (P + I.). The thermostat with the liquid-condenser, the precisioncondenser, the indicating system and the other accessories (see fig. 9 of the preceding paper ,,Accurate capacity-measurements") are mounted together in a box of about 30 × 30 × 30 cm. The box is mounted on four tennisballs, which are excellent for deadening all vibrations and shocks of the table upon which it is placed 1). Besides the condenser for liquids we constructed also a condenser for gases and vapours, of very small size, and a capacity of 180 cm. In addition to this a very simple device is constructed for automatical regulation of the vapour pressure, demanding only minute quantities of material. A description will shortly be published. Measurements of the D. C. are gaining steadily in importance. They are of considerable interest in the investigation of purely theoretical problems (it is hardly necessary to mention here t h e dipole-moments), b u t they may as well be applied with success to industrial purposes 3). The D. C. being a very goad check for the purity of materials and the composition of mixtures, it may provide a convenient method of controling and regulating chemical plants. As an example of its use in industrial laboratories, we call attention to the determination of the water-content of materials, b y extracting the substance with dioxane and measuring the increase of the D. C. of the extracting liquid 3). Particulars concerning the use of the instrument for measuring dipoles are communicated in ,,Rec. Trav. Chim. Pays Bas" 4). Received February 1)
16, 1935.
T h e a p p a r a t u s , d e s c r i b e d in t h i s p a p e r c a n be o b t a i n e d f r o m
January 1935. Kipp
en
Zonen,
Delft, Holland. 2) H a a r d t u n d Co. ( R e i c h s s t r . 37/39. D i i s s e l d o r f , G e r m a n y ) , r e c e n t l y b r o u g h t o n t h e m a r k e t t h e , , D i ( q k o m e t e r " , a n a p p a r a t u s , d e s i g n e d b y P r o f . L. E b e r t a n d D r . E. W a I d s c h m i d t. I t is d e s c r i b e d in , , C h e m i s c h e F a b r i k " 7. 180. 1934. An apparatus, designed by Prof. Wolf, is c o n s t r u c t e d b y S p i n d l e r and H o y e r ( r e p r e s e n t a t i v e in H o l l a n d : I-I 8 f e 1 t, T h e H a g u e ) . B o t h a p p a r a t i a r e s u i t a b l e if h i g h a c c u r a c y is n o t n e c e s s a r y . 3) E b e r t . A n g e w . C h e m i c . 47. 305. 1934. 4) R e c . T r a v . C h i m . 15 M a r c h . 1935. 327.
OF DIELECTRIC
CONSTANTS
1)
b y P. C O H E N H E N R I Q U E Z Laboratory for Technical Physics. Technical University Delft, Holland.
Summary A description is given of an apparatus for measuring dielectric constants Compactness of construction is combined with great ease of manipulation, highest accuracy and use of minute quantities of material. Attention is called to the fact t h a t the measurement of dielectric constants is waiting for application in a large field of pure as well as applied chemistry. I wish to express my best thanks to Prof. Z w i k k e r for the generous hospitality in his laboratory, and to the firm of K i p p and S o n s for their very agreeable collaboration. T h e dielectric c o n s t a n t (D. C.) of a s u b s t a n c e is g i v e n b y the e q u a t i o n ~ ---- C/CL w h e r e C = c a p a c i t y of a condenser filled w i t h the s u b s t a n c e a n d CL = c a p a c i t y of the s a m e c o n d e n s e r in vacuo. A c c u r a t e d e t e r m i n a t i o n s of small differences in D. C. are of g r e a t i m p o r t a n c e in m a n y cases, e.g. as a t e s t for t h e p u r i t y of substances, for d i p o l e m e a s u r e m e n t s etc. These d e t e r m i n a t i o n s involve the a c c u r a t e m e a s u r e m e n t of small differences in c a p a c i t y : A ¢ = A C/CL. I f CL is k n o w n A C h a s to be m e a s u r e d . I n the preceding p a p e r s o m e c o n v e n i e n t w a y s of p r o c e e d i n g were e x p l a i n e d for m e a s u r i n g v e r y small c a p a c i t y differences. If we w a n t to d e t e r m i n e A ~ b e t w e e n t w o substances, we m u s t replace the first s u b s t a n c e in t h e c o n d e n s e r (L) b y the second one a n d c o m p e n s a t e the change in c a p a c i t y w i t h a precision-condenser P (see fig. 1 in the p r e c e d i n g p a p e r ,,Accurate c a p a c i t y m e a s u r e ments").
Precision condenser. T h e precision c o n d e n s e r used, is of the linear t y p e . I t is s c h e m a t i c a l l y g i v e n in fig. 1. T h e c o n s t r u c t i o n is such as to m a k e it possible to r e m o v e t h e last - - 434 - -
MEASUREMENTS OF DIELECTRIC CONSTANTS
435
bit of ,,play" of the hollow screw S. If the condenser is constructed with the utmost care, it is strictly linear to within 0,001 cm, at least in a part of the scale. It can be read to 10-4cm. In total a difference of 5 cm, can be determined with it.
A C B
2
Fig. 1.
Fig. 2.
In using the condenser not the slightest axial, nor vertical, stress should be exerted upon it; the tangential stress for the rotating movement has to be transmitted b y a special mechanism, which is also shown in fig. 1.
The liquid condenser. A condenser devised for measuring liquids is shown in fig. 2. It consists of two glass tubes concentrically adjusted; the inner tube is inburned platinated on the outside, the outer tube on the inside. The two thin platinum layers form the coatings of the condenser. The distance of the coatings is 0,5 ram. The capacity of the condenser is about 10 cm, the volume only 0,5 cm. The smallness of this volume offers the considerable advantage that measurements require only very small quantities of material. Taking into account that the differences in capacity can be computed with an extreme
436
P. COHEN HENRIQUEZ
accuracy, the relatively small capacity of the condenser is sufficient. It is not sure that the choice of a larger condensor would increase the accuracy of the measurements. We observed that the reproducibility of the filling decreases (measured in capacity units), if we take a larger condenser. Besides, in most cases it serves no useful purpose to determine the D. C. with an accuracy greater than 10-4, the slightest impurity (humidity! !) making the pretended accuracy illusory. The liquid is poured in at A (see fig. 2), while B is kept closed with the thumb. By blowing at C, the liquid is removed at B. After the liquid is blown out, the condenser is rinsed twice with low-boiling ligroin (b. p. 30°--40°C). Drying is accomplished by a current of thoroughly dried air (it takes less than three minutes). The liquid-condenser is mounted in a thermostat, consisting of a D e w a r-vessel, containing about 400 cm 3 of the bath-liquid (oil, water, or mercury). For measurements at ordinary temperatures the liquid need not be heated nor stirred. It is true that after each rinsing, followed by evaporation of the ligroin, the temperature suffers a small decrease. This, however, does not exceed 0,03°C. The small error, introduced by this fall of the temperature, is compensated in the following way: for each measurement we have three readings : L filled with the first liquid, L filled with the second liquid, and L, filled with the first liquid again; from these readings we calculate two values for A ~, one of which is too high, and the other too low. Of course, the rinsing wiht ligroin, with subsequent drying, is not necessary if sufficient quantities of both liquids are available for rinsing. For measurements at temperatures, higher than room-temperature, the liquid in the thermostat is electrically heated by Nichrome resistance-wire, wound round a glass tube. Stirring is accomplished in this case by passing a weak current of air through the bath-liquid. For low temperatures an U-shaped tube is inserted in the thermostat through which cooling liquid is passed (for very low temperatures liquid air). In the last case hexane or low-boiling ligroin is very suitable as bath-liquid. For determining CL, (the capacity of the condenser in vacuo), two liquids of precisely known D. C. difference have to be measured only o n ce.
MEASUREMENTS OF DIELECTRIC CONSTANTS
437
For the computation of the differences between two liquids of high D. C. the total capacity of the circuit m a y become too large. In that case it is possible to use an other liquid-condenser, with a still smaller capacity, but it is easier to place a capacity in series with (P + I.). The thermostat with the liquid-condenser, the precisioncondenser, the indicating system and the other accessories (see fig. 9 of the preceding paper ,,Accurate capacity-measurements") are mounted together in a box of about 30 × 30 × 30 cm. The box is mounted on four tennisballs, which are excellent for deadening all vibrations and shocks of the table upon which it is placed 1). Besides the condenser for liquids we constructed also a condenser for gases and vapours, of very small size, and a capacity of 180 cm. In addition to this a very simple device is constructed for automatical regulation of the vapour pressure, demanding only minute quantities of material. A description will shortly be published. Measurements of the D. C. are gaining steadily in importance. They are of considerable interest in the investigation of purely theoretical problems (it is hardly necessary to mention here t h e dipole-moments), b u t they may as well be applied with success to industrial purposes 3). The D. C. being a very goad check for the purity of materials and the composition of mixtures, it may provide a convenient method of controling and regulating chemical plants. As an example of its use in industrial laboratories, we call attention to the determination of the water-content of materials, b y extracting the substance with dioxane and measuring the increase of the D. C. of the extracting liquid 3). Particulars concerning the use of the instrument for measuring dipoles are communicated in ,,Rec. Trav. Chim. Pays Bas" 4). Received February 1)
16, 1935.
T h e a p p a r a t u s , d e s c r i b e d in t h i s p a p e r c a n be o b t a i n e d f r o m
January 1935. Kipp
en
Zonen,
Delft, Holland. 2) H a a r d t u n d Co. ( R e i c h s s t r . 37/39. D i i s s e l d o r f , G e r m a n y ) , r e c e n t l y b r o u g h t o n t h e m a r k e t t h e , , D i ( q k o m e t e r " , a n a p p a r a t u s , d e s i g n e d b y P r o f . L. E b e r t a n d D r . E. W a I d s c h m i d t. I t is d e s c r i b e d in , , C h e m i s c h e F a b r i k " 7. 180. 1934. An apparatus, designed by Prof. Wolf, is c o n s t r u c t e d b y S p i n d l e r and H o y e r ( r e p r e s e n t a t i v e in H o l l a n d : I-I 8 f e 1 t, T h e H a g u e ) . B o t h a p p a r a t i a r e s u i t a b l e if h i g h a c c u r a c y is n o t n e c e s s a r y . 3) E b e r t . A n g e w . C h e m i c . 47. 305. 1934. 4) R e c . T r a v . C h i m . 15 M a r c h . 1935. 327.