R. L. GARMAN, W ashington Square College, New York U niversity, New York, N . Y.
Fi g u r e 1 . Ty p i c a l Cu r v e
S
EVERAL investigators have discussed continuous-read- ing conductance meters. Thus Jander and Schorstein (3) use a galvanometer with a bridge, while Sand and Griffin (1) use a dry rectifier with a bridge network. Treadwell (4) and Callan and Horrobin (2) supply the voltage to the cell and read relative resistance with a vacuum-tube device.M any of these methods are not very sensitive or lack general portability and require the
use of calibration curves.
A vacuum-tube i n s t r u ment has been constructed which i n c o r p o r a t e s a n oscillator supplying audio frequency voltage to a self- c o n t a i n e d b r i d g e , a de
tector, and direct current meter which indicates the r e s i s t a n c e of a conduct
a n c e cell. These func
tions are performed with a single 6A6 tube whose cir
cuit elements are properly a r r a n g e d so t h a t meter readings are a linear func
tion of the resistance of the
conductance cell. The instrum ent derives all power from alternating or direct current mains.
T h e o r y
The output voltage of a bridge is a parabolic function of the resistance of any one of its arms. A typical example is shown in Figure 1. The plate current-grid volts curve of a vacuum tube is not a linear function in the region of high negative values of grid voltage. A typical curve for a high mu triode is shown in Figure 2. (Direct current voltages were used in these calculations for simplicity and are permissible since the curves are used as illustrations only.) If these curves can be completely superimposed, an instrument m ay be con
structed in which the plate current of an electron tube is a linear function of the re
s i s t a n c e of one arm of a bridge. Complete su
perimposition is difficult to attain in practice, but m ay be a p p r o x i m a t e d sufficiently to satisfy the requirements of conduct
ance titrations.
O p e r a tio n The upper section of the t r io d e 6A6 (Figure 3) is used as an oscillator and the alternating v o l t a g e developed across resistance R i is applied to the bridge through condensers Cj and 0«. The voltage from the unbalanced bridge is applied to the primary of the step-up transformer T The stepped-up volt
age is applied to the grid of the lower section of the triode which is negatively biased to zero plate current b y adjustment o f /?s. A sensitivity control, R,, is used to attenuate the alter
Fi g u r e 2 . Ty p ic a l Cu r v ef o r I Ii g u M u Tr io d e
nating current voltage from the bridge while adjusting R3 and to reduce the sensitivity, preventing the meter from reading off- scale in the performance of titrations where large changes in resistance are anticipated at the end point. Under these con
ditions of operation the meter will read the “off-balance” voltage of the bridge, the readings increasing as the bridge is progres
sively unbalanced.
As the resistance of the conductance cell increases, the shunt resistance between the plate side of choke L\ and cathode also increases. This has the effect of increasing the voltage across the bridge terminals and distorts the grid volts-plate current curve shown in Figure 2. This distortion is desirable, since the resulting plate current will be more nearly a linear func
tion of the resistance of the conductance cell.
The degree of linearity and sensitivity attained with this set-up is shown in Figure 4. Since the meter readings are linear with resistance over the range of plate current from 100 to 500 microamperes, the bridge m ust be unbalanced slightly before readings are taken. This setting may be performed after the titration has progressed to a certain extent, since the first readings are usually excluded in the final determination of the end point. This has a further advantage because al
most full scale of the meter may be used in the region near the end point for greater accuracy.
-o o
-I -I 0 V - 4 C - D C
Fig u r e 3 . Dia g r a m
Ri. Hi.R..
R,.Ri.
Ri.Ri.
Ri.R,.
Rm.
C u
5 t o 7 m egohm s, 1 w a tt 1-m egohm v o lu m e co n tro l 50,000-ohm v o lu m e co n tro l V o ltag e d iv id e r, 1000 ohm a, 25 w a tts V oltage d iv id e r, 20,000 ohm s, 25 w a tts V olum e co n tro l, 15,000 ohm s
G en eral ra d io p o te n tio m e te r, 1000 ohm s G en eral ra d io p o te n tio m e te r, 100 ohm s 15 ohm s, 100 w a tts
140 ohm s, 100 w a tts C o n d en ser, 0.01 m fd., m ica
Ct,C»,Ct. C ondensers, p ap er, 0.5 m fd ., 200 v o lts C%. E le c tro ly tic condenser, 8 m fd.
E le c tro ly tic condenser, 4 m fd.
Low ra tio au d io tra n sfo rm e r
S in g le -b u tto n c arb o n m icro p h o n e tra n sfo rm e r C h o k e, 30 H .
C hoke, 30 H . 200 ohm s 0-500 m ic ro am raeter Ct.Tu
Tu LuLt.
na.
MARCH 15, 1936 ANALYTICAL E D ITIO N 147
oa
4 CO ►
Fig u r e 5 .
10 e c - o f 0.1049 S SODIUM HÏWWXXCE
Co n d u c t a n c e Tit r a t io n RX3XSTABCX - OHIO
Fig u r e 4 . Li n e a r it y a n d Se n s it i v i t y At t a in e d P la te c u rre n t—re sista n c e of c o n d u ctan ce cell
C o n s tr u c tio n
The oscillating circuit was designed to offer a maximum in stability and yet lend itself to easy construction.
Transformer T\ must be correctly poled to permit oscillations, and resistance Ri should be as high as possible yet allowing suf
ficient energy feed-back to sustain oscillations. A high value of this resistance assures smooth operation and good wave form.
The setting of Rt is not critical and may be adjusted for maximum oscillator output ( — 2 to —3 volts are usually satisfactory).
may be any low-ratio audio transformer, and since many of these units have a natural period of oscillation at approximately 1000 cycles per second, condenser Ci may be omitted.
The bridge may be standard equipment or may consist of large-size wire-wound potentiometers since no calibration is required. Since the ratio of resistance in the bridge arms deter
mines the curvature of the curve shown in Figure 1, the values must be as stated to attain the desired linearity. If, however, the apparatus is to be designed to measure higher resistance values of the conductance cell, the resistance of the bridge arms as well as the impedance of transformer T, should be increased propor
tionately. Meter readings are relative and the proper setting of resistors Ri and Rt may conveniently be determined for a par
ticular range of resistance by substitution of a variable resistance box for the conductance cell. The settings of Rj and Rs are varied until equal increments of resistance produce equal increments of microamperes.
The inclusion of a rectifier and filter system permits satis
factory operation on both alternating and direct current mains.
Figure 5 illustrates a typical titration curve. In each case 10 cc. of 0.1049 N sodium hydroxide subsequently diluted to 200 cc. were titrated with magnesium acetate. The end point for curve 1, taken by the usual bridge methods, is 6.73 cc.; for curve 2 taken with the instrument it is 6.71 cc.
The error m ay be assumed to rest entirely with the inaccura
cies encountered in the extrapolation.
The circuit described here does not represent the only one suitable for the purposes outlined. I t represents rather a simple scheme which m ay be translated into a compact in
strum ent and the cost of the parts, including the meter and cabinet, need not exceed twenty-five dollars. A more com
plicated circuit extending the principles described here could certainly be made in which the conditions of linearity are more closely approximated. I t is the ardent hope of the author th a t such circuits will be devised to further the tech
nic of conductance titrations.
S u m m a r y
A vacuum-tube conductance m eter deriving power from alternating or direct current mains combining an audio oscillator-bridge and a detector-meter system has been de
scribed. The application to conductometric titrations has been illustrated and it has been shown th a t meter readings are linear with resistance over four-fifths of the m eter range.
L ite r a tu r e C ite d
(1) B ritto n , ‘‘C onductom etric Analysis,” London, C hapm an and Hall, 1934.
(2) C allan and H orrobin, J . Soc. Chem. Ind., 47, 329 (1928).
(3) Jander, G., and Schorstein, H ., Z. angeio. Chem., 45, 701 (1932).
(4) Treadwell, Helv. Chim. Acta, 8. 89 (1925).
Re c e i v e d O cto b er 18, 1935. P rese n ted before th e D iv isio n of P h y sical a n d In o rg an ic C h em istry , Sym posium on R e c e n t A d v an ces in M icrochem icai A nalysis, u n d e r th e title ‘‘U se of M u lti-P u rp o se R ad io T u b e s in A n aly tical C h em istry ,” a t th e 8 9 th M eeting of th e A m erican C hem ical Society, N ew Y o rk , N . Y „ A pril 22 to 26. 1935.
o
W h e n Is a Desiccator?
HAROLD SIMMONS BOOTH A N D LUCILLE MCINTYRE, W estern Reserve U niversity, Cleveland, Oliio
W
H E N the authors completed the study establishing porous barium oxide (1) as a drying agent comparable with phosphorus pentoxide, it became the practice in this laboratory to use barium oxide (furnished through the kindness of M. J. Rentschler of the J. H. R. Products Co., Wil
loughby, Ohio) as a reagent in desiccators, on account of its cheapness and its efficacy. I t occurred to them th a t it would be interesting to learn the rate of drying a space, such as a desiccator, with barium oxide and the other common drying agents.
A p p a r a tu s
The method consisted of bringing air saturated with mois
ture into contact with the drying agent under the conditions which exist in a desiccator, and of observing the rate of de
crease in the am ount of moisture present in th a t air, as indi
cated by the decrease in pressure observed. I t was decided th a t a manometer, on which the pressure could be read di
rectly owing to a Toricellian vacuum on one side, could be used to measure the drop in vapor pressure sufficiently ac
curately for this purpose.
In preparation for a run, moist air was drawn through the gas- washing bottles through stopcocks 4 and 5, while 3 (Figure I) was closed, into the gas holder, D, by applying suction to the outlet at the bottom of D. Water was then run into D from vessel C through stopcock 6 until the pressure inside D equaled that of the outside atmosphere. All stopcocks were closed and the moist air was allowed to stand overnight in D.
One hundred grams of the fresh drying agent were put into a Petri dish which just fitted the bottom of the desiccator. The lid on the desiccator was greased with a stopcock grease of proved low vapor pressure, and the cross piece of the wooden frame hold
ing the desiccator in place was fastened down. A rubber stopper of a size near that of the handle of the desiccator lid was fastened to the wooden cross piece so that when the latter was in place, the pressure upon the handle of the cover of the desiccator was relieved by the elasticity of the stopper. The desiccator was sealed to the apparatus near stopcock 1 by DeKhotinsky cement.
Stopcock 2 was opened, and the manometer, B, and the desic
cator were evacuated. Stopcock 2 was then closed and the ap
paratus allowed to stand overnight to be sure that no slow leaks would develop. The next day stopcocks 3 and 5 were opened and the water-saturated air was forced into the desiccator by allowing water to run into bottle D from vessel C through stop
cock 6 until the pressure in the desiccator was about 760 mm.
As soon as the stream of air was shut off, the vapor pressure was read on the manometer and the time recorded. The pressure was read at intervals of 1 minute until the fall in pressure was slow
and thereafter was read at intervals of 0.5-mm. change on the manometer. This was the approximate decrease during the 1-minute intervals first recorded. Readings were taken until the pressure change was so slow that it would not affect the curve materially. The apparatus was allowed to stand until the next morning, when the final reading was taken. At least six runs were made with each drying agent, but only the representative ones were plotted. This method, while rough, yielded some in
teresting information.
R e s u lts
Figure 2 shows representative curves for each drying agent used, the fall in vapor pressure being plotted against the time elapsed. While the results show little significant differentia
tion between the rates a t which the air in a desiccator is dried by different desiccants, the uselessness of a desiccator as commonly used is clearly revealed when the curves are
exam-Fi q u b e 2 . Re p r e s e n t a t i v e Dr y i n g Cu r v e s
ined. The average analyst has a holy respect for the power of a desiccator to keep his ignited crucibles dry during the cooling process, yet is careless about keeping the desiccator closed and about opening for only the briefest time. The writers have seen a student open a desiccator, go to the oven or elec
tric crucible furnace, get his crucible, return, and p u t it in the desiccator to “cool in a nice dry atmosphere.” Let us as
sume th a t half of the air in the desiccator has been displaced, and th a t the air let in is half saturated, or roughly th a t a par
tial pressure of 5 mm. of water vapor is now in the desiccator.
The crucible will attain practically room temperature in 10 minutes, in which time the drying agent will have lowered the partial pressure of the water vapor in the air in the desic
cator only 2 mm., while the crucible is simultaneously ad
sorbing the moisture from the air in the desiccator. Gen
erally the air in the analytical laboratory is nearly saturated with moisture, so th a t the probable moisture content in the desiccator would be much higher.
Even some textbooks advise the student not to place the cover on the desiccator until the crucible has cooled somewhat or else the expansion of air will blow the cover off and break it!
W hy use a desiccator a t all if th a t is the criterion? How can students following such advice ever get constant weight on a
MARCH 15, 1936 A NA LYTICAL E D IT IO N 149 gravimetric calcium determination? The authors tell stu
dents to hold the cover on and let some of the heated air es
cape if it will. The rather facetious title of this paper is in
spired by these observations, in the hope th a t it will serve to arouse analysts to this unsuspected source of error.
However, in view of the moisture always inadvertently ad
mitted, no m atter how rapidly the desiccator is opened and closed, it is useless to use a desiccant of the highest absolute drying power such as phosphorus pentoxide, since the object cooling in the desiccator will ordinarily be removed for weigh
ing before the space can be dried completely. Save for the danger of spilling, sulfuric acid should be satisfactory. If a neutral desiccant is desired, calcium chloride free from cal
cium hydroxide is useful. Where an alkaline drying agent is permissible, porous barium oxide is excellent, and is particu
larly valuable in determinations affected by carbon dioxide, such as gravimetric calcium. Since barium oxide swells con
siderably on absorbing moisture, the bottom of the desiccator should not be more than half full.
Porous barium oxide is an industrial product, being pro
duced in the first step in manufacturing barium peroxide, and is available more cheaply than anhydrous calcium chloride, which is difficult to prepare. The exhausted barium oxide can be used for the preparation of standard barium hydroxide solutions for alkalimetry. definite quantity of a vapor, and the determination of vapor pressure, temperature, composition diagrams, etc.
A number of investigators have described thermostats which may be operated a t low temperatures (1, 2 ,
4
-6
) . Someare rather elaborate pieces of apparatus, while others are relatively simple. The therm ostat described here is easily made from materials found in any well-equipped laboratory, and has been used to maintain a constant temperature in the range + 2 5 ° to —75° C. The fluid used in the bath m ay be acetone, alcohol, or kerosene, depending on the temperature to be maintained. Acetone freezes a t —94.6° and retains its mobility a t the tem perature of solid carbon dioxide. Alcohol becomes viscous below —40°. Kerosene may be used for temperatures to about — 40° C.
The container for the bath is a 3-liter Pyrex beaker, A, which is set in a 4-liter beaker, B. The air space between the two beak
ers serves to prevent the bath from cooling too rapidly. The formation of ice between the beakers may be prevented by seal
ing the crack with rubber cement used to fill cuts in the tread of automobile tires. The nested beakers are placed in a can, C, whose diameter is 5 cm. (2 inches) greater than that of the larger beaker, and centered by a ring made from rubber tubing of appropriate size. The can containing the beakers is placed in a wooden box, D, fitted with cork fining, E, built around the square tin can, F. If a constant temperature is wanted for several hours only, the space between the outer beaker and can C is filled with a slush of well-crushed dry ice and alcohol. If a constant temperature is wanted for a considerable time, the space, O, is also filled with crushed dry ice.
The bath is stirred with a turbine paddle attached to a Bakelite or metal rod, the other end of which is connected to the shaft of a small induction motor. An induction motor is desirable to avoid igniting the vapors from the bath.
Since the bath is constantly being cooled, it is necessary to supply heat to maintain a constant temperature. This is done by means of a heating coil made from 10 cm. of No. 24 resistance wire welded or clamped to leads made of No. 18 iron wire, which are connected to the secondary terminals of a 25-watt toy trans
former. The current in the primary circuit of the transformer is made and broken by a relay as the temperature varies. The proper amount of heat necessary to offset the cooling is obtained by adjusting the voltage.
The thermostatic element shown in Figure 2 is a bimetallic strip, A, made of brass and invar and consisting of four bows with the open end of the bows directed toward the long axis. The distance between the open ends of the bow will increase or de
crease on warming, depending on whether the invar is on the inside or outside of the bow. A brass or preferably glass or Bakelite rod, B, rests on the lower end of the last bow. The upper end of the rod fits into a cup attached to a screw, E, threaded onto a piece of spring brass, C, one end of which has a fixed position.
Screw E is adjusted so as to place a slight tension on the spring arm, C. The other end rises and falls with a change of tem
perature in the bath, making and breaking the grid current of a radio relay circuit (3). A thermionic relay control of some sort is required, since no sparking should occur between the contacts.
To start the therm ostat, the bath is
ing agent. The gas bubbles have only a short distance to go before reaching temperature is reached the screws are adjusted so th a t the heat is discon
(6) Ubbelohde, Trans. Faraday Soc., 26, 236 (1930).
Re c e i v e d D ecem ber 23, 1935.