In accordance with ANALYTICAL CHEMISTRY’S policy of publishing surveys on timely subjects, two contributed reviews on new topics, Automatic and Recording Balances and Thermometric Titration, are being presented together with the regular invited reviews ~~
Review of Fundamental DeveloDments in Analysis
Automatic and Recording Balances Saul Gordon and Clement Campbell Picatinny Arsenal, Dover, N. J.
in laboratory rcsearch investigations have been towards the automatic and continuous recording of data which could previously be obtained only by tedious and time-consuming manual measurements involving point by point readings and subsequent plotting of the results. I n recent years, commercially available instrumentation has been very widely applied to electroanalytical techniques (potentiometry, amperometry, polarography, conductometry, coulometry), absorption spectrophotometry, chromatography, gas analysis (thermal conductivity, combustion analyzers), the measurement of physical properties such 3 a . density, viscosity, resistivity, and of conditions such as temperature, pressure, and relative humidity. However, despite these rapid and profound advances in laboratory automation, the fundamental operation of precision weighing has been relatively neglected until recent years (94). Although a number of automatic and recording balances were developedmd used by individual research investigators, most laboratory investigations involving a study of weight changes as a function of time, temperature, or any other variable, were accomplished by means of point by point measurements. Despite these many balances reported in the literature, it was not until the Chevenard thermobalance was marketed in France about 1950 that a recording laboratory balance became commercially available. With the publication of Duval’s monograph (53) describing thermogravimetric studies of inorganic analytical precipitates, using the Chevenard thermobalance, the application of automatic recording balances was brought to the attention of chemists in many fields for whom such equipment could be of inestimable value. The past 2 years have seen increased activity among instrument manufacODERN TRENDS
turers both here and abroad, witnessed by the present choice of 18 commercially available recording balances: six domestically manufactured, one each in England and Switzerland, three in France and seven in Western Germany. In addition to the use of a n automatic recording balance for thermogravimetric analysis and the evaluation of precipitates proposed for gravimetric analysis, there are a great many other applications for which a recording balance may be used (Table I) in fields such as metallurgy, textiles, ceramics, paint and coal technologies, ‘ biochemistry, biological sciences, mineralogy, and most of the other fields of physicochemical endeavor. This review describes the types of automatic recording balances, the principles of their operation, and the areas in which they have been applied. The authors have tried to include descriptions of all the balances that are automatically recording, or obviously lend themselves to such operation. In addition, there is a comprehensive survey of the commercially available recording balances manufactured both here and abroad. PRINCIPLES OF OPERATION
The primary requirements of a suitable automatic and continuously recording balance are essentially those for an analytical balance-accuracy, sensitivity, reproducibility, capacity, rugged construction, and insensitivity t o ambient temperature changes. I n addition, the recording balance should have a n adequate adjustable range of weight change, a high degree of electronic and mechanical stability, be able to respond rapidly t o changes in weight, be relatively unaffected by vibration, and be of sufficiently simple construction t o minimize the initial cost and need for maintenance. From a prac tical point of view, the balance should
Table 1. Applications of Automatic and Recording Balances
Corrosion of metals Thermal decomposition of inorganic and organic compounds Solid state reactions Roasting and calcination of minerals Thermochemical reactions of ceramics and cermets Pyrolysis of coals, petroleum, and wood Determination of moisture, volatiles, and ash Absorption, adsorption, and desorption properties of materials Rates of evaporation (drying curves) and sublimation; latent heats Dehydration and hygroscopicity studies Effect of load stress on fibers Sedimentation of aerosols and powdered materials Rates of crystallization and diffusion Studies of permeability and osmotic pressure changes Changes in specific gravity or gas density Specific surface as determined by weight of adsorbed gas Metabolic rates of small plants and animals Variations in weights of Emall laboratory animals Automatic thermogravimetric analysis Effect of radiation on various substances Thermal and oxidative degradation of p o1y m er s Growth of living organisms in controlled environments Weight of effluent from chromatographic or other separatory system Development of gravimetric analytical procedures
be simple to operate and versatile in that it dan be used for varied applications by the addition of auxiliary equipment, such as sample holders, heating and cooling devices, and vacuum or controlled atmosphere jackets. The recording balances reported in the literature or commercially available may be grouped into two types on a basis of their mode of operation. They Itre the null-point and the deflection types of instruments, schematically illustrated in Figures l and 2. VOL. 32. NO. 5, APRIL 1960
271 R
The automatic null-point balance, Figure 1, incorporates a sensing element which detects a deviation of the balance beam from its null position; horizontal for beam balances and vertical for the electromagnetic suspension type. Through appropriate electronic or mechanical servo-linkage, a restoring force-electrical or mechanical weight loading-is then applied to the beam, returning it to the null point. This restoring force, proportional to the change in weight, is recorded either directly or through an electromechanical transducer. Deflection balances, Figure 2, involve the conversion of balance beam deflections about the fulcrum into p h o b graphically recorded traces; recorded electrical signals generated by appropriate displacement measuring transducers; or electromechanically drawn curves. Related types of deflection balances are: the helical spring wherein changes in weight are manifested in contractions or elongations which may be automatically recorded by suitable transducers; the cantilevered beam, constructed so that one end is &xed and the other end, from which the sample is suspended, is free to undergo deflection; the suspension of. a sample by an appropriately mounted strain gage that stretches and contracts in proportion to the weight changes; and the attachment of a beam to a taut wire which serves as the fulcrum and is rigidly &xed at one or both ends so that deflections are proportional to the changes in =eight and the torsional characteristics of the wire. A number of electronic and electromechanical principles may be employed for detecting deviations of the balance beam from its horizontal or vertical rest point in the null-point recording balance. These methods are summarized in Table 11. Photoelectric and photomechanical detectors make use of the varying intensities of a light source impinging upon a phototube as a result of balance beam fluctuations about the null position. This is accomplished by a light source, intermediate shutter or mirror, and either single or dual phototubes. The displacement of the shutter attached to the balance beam intercepts the light beam so as to either increase or decrease the intensity of light acting upon the phototube due to the change in weight. The resulting change in the magnitude of the current from the phototube is used to electronically or electromechanically restore the balance to its null point. It is also possible to use a mirror mounted on the balance beam to reflect the light upon a dual phototube, so that fluctuations of the beam result in a deflection of the light to one photocathode or the other aa changes in weght occur. Other
N. D.
RESTORING
WEIGHT CHANGE
‘ FORCE
1I
’
~
!
Figure 1.
>
’
Mechanical Electrical
Null-type balance
Reatohng force proportional to change in weight. Null detector, N.D., may b e optical (electronic), mechanical, electromechanical, electn‘cal, or radiation detector CANTILEVER
BEAM
.-.q I
A t
---
- - *;
Figure 2. Deflection-type balance
+
]---e--
Deflection proportional to change in weight
TO RSlON WIRE
SPRING
1
methods for electronic null detection include the change in: capacity of a condenser as a function of balance beam displacement: inductive coupling between plates and/or coils; magnetic coupling; the output of a diflerenthl transformer as a function of armature displacement; nuclear radiation flux; or output of a strain gage circuit. In each of these cases, one element of the attached to ‘the transducing system balance beam in close proximity to the other element which is maintained in a &xed position. Mechanical methods, although less satisfactory, are also feasible for this purpose. These involve “feelers” which operate servomotors through appropriate relays, or are modifications of the older type mechanical galvanometric stripchart recording potentiometers. Having detected the departure of the balance beam from its rest point, any one of several methods summarized in Table I11 may be used to restore the position of the balance to its null point. The appropriate restoring force can be applied by either electronic or electromechanical means. Electronic restoring devices may be solenoid coils operating upon a magnetic armature attached to the balance beam and freely suspended within the coil. A current of su5cient magnitude is a p plied to the coil so that the balance beam remains a t its null point. This current can be obtained either directly from the null-detecting circuit or through the operation of a servodriven potentiometer. The restoring force can also be supplied by the repulsion of a coil fixed in position below an opposing short-circuited coil, which is attached to the balance beam: the necessary current may be supplied in the same
+
manner as for the afore-mentioned solenoid. .In additional niohfication of a restoring force coil involves energizing a coil attached to the beam and located in the field of a permanent magnet. A completely mechanical form of weight loading uses a sermmechanically driven chain on a conventional type of chainomatic balance. To restore the balance beam after a gain or loss in weight, it is necessary to use either two motors or a reversing motor with the appropriate electronic circuits for operation by the null detector. Completely mechanical linkage between a chain, balance beam, and an older type of mechanically recording potentiometer can also be utilized for automatically restoring the balance to its rest point The restoring force can also be applied by the electrolytic dissolution or deposition of an electrode suspended from the balance beam. Recording the changes in vieight, methods for which are shown in Table IV,is best accomplished by measuring the current or voltage applied to the null-point restoring device, such as the solenoid or the inductive coil. Alternatively, precision potentiometers may be coupled to the servomotoroperated chain drive, or to the mechanical device used to energize the restoring force coil. This precision potentiometer is made a part of a Wheatstone bridge circuit or energized by a battery with suitable limiting resistors and then connected to a recording instrument. A balance consisting of a beam, chain, and old mechanical recorder can be modified to produce a record directly as a result of the mechanical linkages involved. The clsssical method of recording with deflection type balances is that of
Q li
Table II. Transducers for Null-Point Detection and for Measuring Changes in Weightwith Deflection-TypeBalances
OPTICAL Light source-mirror-photographic paper Light source-shutter-photocell ELECTRONIC Capacitance bridge Mutual inductance: coil-plate, coil-coil DitIerential transformer or variable permeance transducer Radiation detector (Geiger tube) Strain gage MECHANICAL Pen electromechanically linked to balance beamor coulometer
!L
L
8 - B A L A N C E BEAM C-COUNTERPOISE M-MIRROR 0-OIL POTS R- SUPPORT ROD S-SAMPLE CRUCIBLE
Table 111. Methods of Applying a Restoring Force to Null-Type Balances
W
Figure 3. Chevenard photographically recording deflection thermobo Ia nce
measuring the balance beam deflections photographically by means of a light beam reflected from a mirror mounted on the balance beam; relatively small deflections are magnified by the use of a sufficiently long optical path. This technique involves recording the deflections directly on a photographic paper wrapped around a motor-driven drum. The deflections may be measured electronically by a shutter attached to the balance beam and operating so as t o intercept a beam of light impinging upon a phototube; the light intensity is a measure of the balance beam displacement, and therefore of the change in weight. A capacity-sensitive device, inductive-sensing element, radiation detector, or linear variable differential transformer can also be used t o measure beam deflections electronically. The first three methods require that a condenser element, induction coil, or radioactive material be suspended from the beam; last uses a n armature freely suspended from the beam into the ditrerentia1 transformer coil. Strain gages connecting the beam to the base of the balance have also been used. Appropriate circuits are employed with these devices to convert the changes in electrical properties t o signals which may then be recorded. The selection ‘of the type of automatic recording balance t o be purchased or constructed depends largely upon the application and a Consideration of the physical and chemical parameters involved. Among these are the load, range of weight change, accuracy, sensitivity, speed of response, environmental conditions of atmosphere, tem-
MECHANICAL Addition or removal of discrete Feights; or beam-rider positioning Incremental or continuous application of torsional or helical spring force Incremental or continuous chainomatic operation Incremental addition or withdrawal of liquid (buoyancy) Incremental increase or decrease of pressure (hydraulic) ELECTROM.ICNETIC ISTERACTION Coil-armature Coil-magnet Coil-coil ELECTROCHEMICAL
Coulometric dissolution or deposition of metal at electrode suspended from balance beam or coulometer
perature and pressure, type of recording system, and constructional complexity of the apparatus. Although a nulltype balance has a n inherently larger load-to-range ratio than a deflection balance, their accuracy, sensitivity, and speed of response are comparable as they are primarily a function of the electronic or electromechanical servosystems required for automatic graphic recording. This limitation is inherent in any of the electronic recorders which have a practical accuracy no better than &0.2%, even though other components of the balance may be capable of greater accuracy. With automatic weight loading and multirange recording over several full-scale excursions, a greater accuracy can be achieved. However, the null balances are also more complex because of the null detector and restoring-force system which are not required in deflectiontype instruments. The choice of recording technique must be made among the photographic, mechanical-pen linkage, and electronic graphic methods. With the photographic techniques, which are confined to the deflection
Table IV. Recording Techniques for Automatic Balances
hlECH.4SICAL Pen linked to potentiometer slider Pen linked to chain-restoring drum Pen or electric arcing-point on end of beam Pen(s) linked to servo-driven photoelectric beam-deflection follower PHOTOGRAPHIC Light source-mirror-photographic paper Drum: time base Flatbed: temperature base-mirror galvanometer ELECTROSIC Current generated in a transducin circuit-, g., photocell, different iaf transformer, variable permeance bansducer, strain gage, bridge, radiation detector, capacitor, inductor Current passing through the coil of an electromagnet
balances. extremely accurate recordings can be obtained a t the expense of the inconvenient handling and processing involved with light-sensitive papers. Electronic recording is more versatile and convenient than a mechanically linked system because of the many transducers that can be used to obtain a n electrical signal proportional to the change in weight as measured by either a null or deflection balance. Furthermore, the continuously recorded analog data from the primary curve can be simultaneously translated into other useful forms such as derivatives, integrals, logarithms, or any other desired function, many of which lend themselves to the digital okerations associated with automatic computation and automated processes. AUTOMATIC A N D RECORDING BALANCES
Deflection Balances. Most of the automatic and recording balances reported in the literature were designed for specific studies and then used only in the laboratories of the respective investigators. The eailiest automatically recording balance n-as constructed by Kuhlmann (90) in 1910. His apparatus, using the deflection principle, consisted of a n analytical balance with a mirror mounted on the center of the beam. h light focused on this mirror was reflected onto a piete of photographic paper wrapped around a clockmotor-driven drum. The relative deflection on the paper was proportional to the change in weight. When the light beam reached the edge of the photographic paper, a photoelectric relay actuated a mechanism which arrested the balance while p weight was dropped onto the pan. Upon release the balance beam returned to its original position to begin a new deflection. Abderhalden (I, 2) described a simVOL 32, NO. 5, APRIL 1960
273 R
ilar completely automatic photographically recording deflection balance for studies of animal metabolism which included a means of extending the range by the automatic electromechanical addition and subtraction of weights. Another single range general purpose deflection balance of this type was developed by Kohler (89). The first automatically recording thermobalance was reported by Dubois (51), who made use of the afore-mentioned photographic recording technique. Samples under investigation were suspended into a furnace from the Dalance beam. Jouin (82), Zagorski (105), and Vytasil et al. (159) constructed similar equipment to study mineralogical specimens, the pyrolysis of analytical precipitates, and the drying of enamels, respectively. Chevenard, Wach6, and de la Tullaye (38) developed a recording thermobalance in which the balance beam w a s suspended by wires and the light sourcemirror-photographic drum recording technique was used. This instrument n-as the prototype for the present commercially available Chevenard thermobalance (3) shown schematically in Figure 3 and described h t e r in this review. Longchambon (97) applied the photographic principle to a unique method of recording changes in weight directly as a function of temperature for studies of the dehydration of minerals. This deflection balance had a mirror mounted on the beam so that the vertical components of light deflections on the photographic paper were proportional to the change in weight. A galvanometer mirror was also placed in the light path so that the horizontal component of the light deflection was proportional to the temperature of the furnace into which the sample n-as suspended. Sinlilar recording thermobalances were later constructed by Orosco (111) for studying the dehydration and decomposition of mineralogical samples and by Spinedi (139) for investigating the oxidation of metals and the decomposition of metallic oxides. Tryhorn and Wyatt (155) utilized the hydrostatic principle for automatically recording weight changes in connection with their studies of adsorption and in measurements of diffusion coefficients. Their recording hydrostatic balance, made from an ordinary analytical balance, was a deflection-type instrument with a rod suspended from one end of the balance beam and partially immersed in a nonvolatile liquid. "he deflection of the beam from its equilibrium position wad a function of the change in weight, and the linear range was determined by the radius of the rod and the density of the liquid. A mirror, mechanically linked t o the balance beam, was used to de274R
ANALYTICAL CHEMISTRY
-
SAMPLE HOLDER
Figure 4. Cantilever deflectiontype load-cell with differentiai transformer as transducer for measuring weight changes (Kertz-
SUPPORT
l---+ Figure 5.
+r'-t
Strain gage-type deflection balance (Bartlett and Williams)
flect a beam of light onto a drum camera and thereby record the change in weight as a function of balance beam displacement. This type of hydrostatic balance was adapted by Campbell and Gordon (33) for graphic electronic recording by using a linear variable differential transformer to measure the balance beam deflections. Another automatic recording balance based on the hydrostatic principle was developed by Dervichian (60)for recording vanations in surface tension as a function of the surface and of time. This deflection balance had a microscoFe cover glass suspended from one end of the beam into the liquid whose surface tension was to be measured, and a rod suspended from the other end into an oil bath which determined the range and acted as a damper. A mirror fixed to the beam reflected a light spot onto photographic paper for automatic recording. By using a torsion wire balance with magnetic damping, Andersson and the Stenhsgens (10) improved upon the Dervichian-type surface tension recording balance. Spinedi (138) studied the anodic aqueous dissolution of metals by means of a photograph-
ically recording deflection balance having a mirror mounted on the beam pointer to deflect the beam of light. Brefort ($7) in his studies of the dehydration of salts employed a photographically recording thermobalance which made use of two optical systems. The change in current of a photocell, upon which was focused a light intercepted by a shutter attached to the deflection balance beam, was proportional to the beam deflections. This current was recorded by means of a mirror galvanometer reflecting a beam of light onto a drum-wound photographic paper. A simple torsion-type sedimentation balance for particle size analysis was devised by Bostock (24) for reading directly the beam deflections on a frosted glass scale in a manner also used by Peters and Wiedemann (115) in their manual beam deflection thermobalance. 'I'hese balances are of interest because of the ease with which they could be converted to automatic recording instruments. An electrical setup which relates deflection of the analytical balance beam to electrical
'GALVANOMETER COIL AND NEEDLE
MOTOR-DRIVEN BALANCING MECHANISM \ PRECISION
I
1
TAKE-UP DRUM
/ SPRINGS
91
LINEAR VARIABLE DIFFERENTIAL TRANSFORMER ARMATURE
' I
I
I I 1
CALIBRATOR
(&?WEIGHT
CHAIN-& \
'\.
\
OIL DAMPER
THERMOCOUPLE ,TARE FURNACE
CRUCIBLE
-SAMPLE
Figure 7. Null-type recording balance constructed from mechanical galvanometric recording potentiometer (Ewald)
CRUCIBLE
Figure 6. Spring-type deflection balance using differential transformer as transducer (Campbell and Gordon)
current, has been described by Derysgin et al. (51). Binnington and Geddes (18) reported an automatic recording mechanical balance of the deflection t y r e which they used to conduct drying studies of grain products. A record of the beam displacement was obtained by B timed spark emitted from a beammounted stylus to the metallic druni of a variable-speed kymograph through a chart paper wrapped around the drum. At the point of maximum beam deflection, a wire attached to the beam contacted a pool of mercury and actuated a device which sequentially dropped steel balls onto the balance pan, thus restoring the beam to its starting position. Rogers and Earle (1,!?4), in their studies on macaroni drying, adapted a solution balance for automatic recording using a n incremental weight loading system and recording technique similar to that of Binningtm and Geddes. Ahmad (4) devised an automatic recording deflection balance for measuring the moisture content of hygroscopic materials. The range of beam deflection was controlled by a spring, with the pen connected to the beam by a system of levers. For a study of the evaporation of solvents, Couleru (4.55) built a deflection thermobalance with
a n off-center-pivoted beam. The sample n-as suspended into a furnace from the center of the long arm, at the end of which was a pen used to record changes in weight of several grams on a rotating cylinder. Linear variable dlfferential transformers have been recently employed as transducers for several deflectiontype automatic recording balances. By suspending the transformer armature from one end of the balance beam and recording the weight as a function of beam deflections, Peterson (116) was able to convert an ordinary analytical balance to a recording micrcF. balance. Campbell and Gordon (33) used this transducer and the hydrostatic principle of Tryhorn and Wyatt (155) for a simple conversion of analytical balances to the automatic recording of weight changes from micrograms to grams. They (69) also modified a photographically recording Chevenard thermobalsnce for graphic pen-andink recording by using the differential transformer to transduce the balance beam deflections. satava (13Oa) used a ditrerential transformer to obtain meter readings proportional to the deflections of a thermobalance beam. Projection-type single pan constant load balances have also been modified for automatic recording over a 1Wmg.
range by means of this type of transducer as reported by both DeLong (@) and Williamson (164). For absorption analyses Claesson (40) used an instrument which automatically recorded n-eight as a function of refractive index. -1 current proportional to the change in refractive index was obtained by means of 3 Fhotocell arrangement and fed into a mirror-galvanometer. The balince mechanism was the cantilever type nith a mirror mounted on the saml;le end of the beam to deflect a light beam a distance proportional to the chafige i n weight. By properly positioning the mirrors on the balance and galvanometer, a direct plot of weight us. refractive index was obtained on photographic paper in a manner similar to that used by Longchambon (97). -1 cantilevered load cell with a diflerential transformer for measuring deflections, illustrated in Figure 4, has been prcposed by Kertzman (85) as an automatic recording balance. Blau and Carlin (20)described the application of radioactivity to the detection and measurement of balance beam displacement by coupling a piece of radioactive foil to the balance arm near a double ionization chamber. A nonrecording version of this microbalance was subsequently exhibited by Arthur S. LaPine %I Co. (92). Feuer (66) developed a similar sensitive radioactive electronic detector that F a s used in a commercially available SeederKohlbusch microbalance to obtain a reported sensitivity of a t least 1 y per ml.
A strain gage was used to weigh elutriant solutions in a precision reVOL. 32,
NO. 5,
APRIL 1960
* 275 R
FIE L D C 0 IL$.(F IX E D )
Lp,”’ PHOTOCELLS
r-
E G L A N C E BEAM
COUNTER WEIGHT
- E A L G N C E BEAM COIL(MOVAELE1
/
