A
ing Automatic Bal PHILIP EWALD see Valley Authority, Wilson
HIS paper describes the design ot a recording, automatic of an balance, The instrument, composed essentj&. analytical and an Old R. & NorthruP automatic recording mechanism, employs a motor-driven chain for restoring equilibrium of the balance arm when a change in weight A recording balance employing a motor-driven chain, hut using an electronic halancing system, has been described by Muller and Carman (1). Possible advantages of the balance described in this paper over the electronicbalance are of construction, simplicity of and stability. The sensitivity of the balance could be made equal to that of the electronic balance by use of a more modern balancina- mechanism, such as the Leeds & Xorthruu Micromax. The balance mas constructed for use in rock phosphate sedimentation studies, but other applications suggested by the nature of the instrument involve studies of weight changes, such as dehydration, hygroscopicity, rapid corrosion, and other chemical transitions, and changes in specific gravity.
Dam, Ala.
matically in Figure 2. The mittenal to be weighed is suspended from one end of the balance ann and a chain is suspended either directly from the other end or from the auxiliary weight pan, m shown in Figure 1. The chain is wound on a drum connected to the restoring mechanism. A light-weight A-frame mounted on top of the balance arm engages the galvanometer needle of the recorder. When the balance arm is tilted by m increase of weight, the galvanometer needle is moved t o one side, the chopper bar sets on the restoring mechanism, and the drum unwinds the chain to restore equilibrium. When the x.eight decreaes, the needle moves in the opposite direction, the restoring action is brought into play, and the drum winds the chain until
-
MOTOR- DR BAf
Lu
.COUPLING ... . .
TAKE-UP DRUM
Figure 1 shows the assembled apparatus. The balance is mounted on a shelf, the recorder mechanism is placed above it, and the sedimentation tube and temperature control are mounted below. The mechanism of the instrument is shown diagram-
FIGURE2. AUTOMATIC BALANCING MECHANILII
WIRE TOGETHER NEEDLE RIBBON CRIMPED AT ANGLE FOR RlDlGlTY
0 OOSXI/~'ANALYTICAL OR PhOTOGRAPHERS MAGNESLM RlBBON
/
TO BALANCE ARM
\
AGATE BEdRlNB
FIGWE1. ASSEMBLEDAPPARATUS
FI
66
".
_..I
____I_.._(abov, BEARING MOUNTING (below) -.lllll
-__.I-
January 15, 1942
A N A L Y T I C A L EDITION
equilibrium is restored, The recorder mechanism is designed so that restoring action is proportional to displacement, which eliminates hunting. When constructin the apparatus, no permanent change is made in either the bafance or the recorder. Both instruments are restored easily to their original condition. The slide-wire of the recorder mechanism is replaced by a drum with flanges. The brass drum has a diameter of 16.9 cm. (6.78 inches) and a width of 2.5 cm. (1 inch), and is provided with 90 em. (3 feet) of chain made bv splicing a cheap necklace chain. The irregularities of the chain are offset by its length. The A-frame is made of magnesium ribbon, crimped into a V shape for rigiditv, and 15 attached t o the balance aiiii with fine copper wire. Maximum sensitivity is obtained by making the slot that engages the alvanometer needle as narrow as possible. .Jarring of the bayance arm, which is caused by the chopper-bar action, is eliminated by coupling the balance arm with the galvanometer, as shown in Figure 2. The galvanometer coil is short-circuited to produce the necessary damping effect. The recorder mechanism is mounted above the balance in a position where the galvanometer needle is directly above the center line of the balance. Because the chain and recorder mechanism form a loop around the chart roll, it is necessary to mount a chart take-up within the chain loo to prevent the unrolling chart froin fouling the chain. An orgnary Bindow-shade roller I < used for thi- purpose. The roller spring supplies the new-arv tension. Laboratory vibration, cause the agate pan bearings of the balance arm to rotate on the flat agate plates when equilibrium is maintained for a long period. This trouble is eliminated by substituting glass bearings made of sections of Pyrex tubing for
67
the flat agate plates (Figure 3). The instrument is enclosed in glass to avoid the effects of air currents.
The weight-change range and sensitivity of the balance depend upon the drum size and chain used. In the model described, this range is 1 gram over a chart width of 25 cm. (10 inches). With this spread, a 1-mg. weight change can be detected. The sensitivity of the instrument can be increased up to about half the limit of sensitivity of the balance by moring the attachment point of the chain on the balance arm toward the center. Thus, if the analytical balance is sensitive to 0.1 mg., the instrument can be adjusted to record weight changes of 0.25 t o 0.20 mg. Initial balancing is accomplished by adding weights t o the auxiliary pan. Keights also must be added from time to time when the weight change to be measured exceeds the automatic range of the instrument.
Acknowledgment The writer wishes to acknowledge the interest of J. W. Lefforge, who suggested the possibility of using a Leeds Bi Northrup recording mechanism in the construction of the recording automatic balance.
Literature Cited (1) Muller, R.H., and Garman, R . L., IND. ENG.CHEM.. ANAL.ED., 10, 436 (1938).
Glass Electrode as a Reference Electrode in Electrornetric Titrations LOUIS LYKKEN AND F. D. TUE~I;1II,ER Shell Development Company, Emeryville, Calif.
T
HE use of the glass electrode as a reference electrode nas suggested by Heintze ( 1 ) in 1934. I n the following year, Wynd (6) discussed the use of the glass ieference electlode in determining the oxidation-reduction potential of a solution. Later, Steuart and Carruth (4)found that a glais tube containing mercury was a satisfactory reference electrode for redox titrations. Several years ago, while noiking with the argentimetric method of Tamele and Ryland ( 5 ) ,the authors tried using t h e glass electrode ab a reference electrode in precipitation titrations. This application was very successful and i t was subsequently estended to other precipitation and redox titrations in which the hydrogen-ion concentration was not a function of the titrating reagent. I n these trials, the older fragile electrode as well as the modern durable and shielded electrode was used. The glass electrode serves as a satisfactory reference in a n y potentiometric titration in which the hydrogen-ion activity remains practically constant throughout the titration. Therefore, for most titrations, the glass electrode will function as a useful reference electrode in solutions t h a t contain excess base, excess acid, or a sufficient amount of a n effective buffer, especially when t h e titrant is chemically inert so far as changing the hydrogen-ion activity is concerned. If the true hydrogen-ion activity, or pH, of the solution is known or can be readily determined, the glass electrode may also be used as a reference when measuring accurate half-cell potentials. The glass reference electrode has three important advantages over conventional reference electrodes. (I) It requires very little preparation and is therefore readily available for use. (2) It is chemically inert and therefore does not contaminate a solution t h a t can be safely handled in glass. The glass electrode is especially useful in argentimetric titrations, as it
FIGURE 1.
ARGENTIMETRIC POTENTIAL-VOLE~IE TITRATION CURVES
TYPICAL
Obtained by using a shielded glass reference electrode
