Constant-Flow Buret Based on Principle of Mariotte Flask JOHN KEENAN TAYLOR AND ENRIQUE ESCUDERO-MOLINS' National Bureau of Standards, Washington, D . C .
N MASY experimental procedures it is necessary or desirable to use a device for introducing or metering a liquid a t a constant rate of flow. Of the arrangements that may be used for this purpose, one based upon the principle of the Mariotte flask is advantageous because a model capable of high precision can be constructed by even a relatively unskilled technician. Furthermore, a wide variety of rates of flow can be obtained by simple adjustment of the effective working head and/or the rate-regulating tip
Table I.
h Cm. 4.97 4.38 3.83 2.62 2.00 1.67 1.28 0.86
F
Rates of Flow Using Air Inlet T u b e w i t h Radius of 3 Rlm. &Ianometer Reading,
Bubble Pressure,
hm
hb
Cm.
Cm.
R a t e of Flow G./min
5.50 4.87 4.40 3.30 2.48 2.20 1.75 1.41
0.53 0.49 0.57 0.68 0.48 0.62 0.47 0.65
0,2637 0,2649 0,2634 0.2630 0.2688 0,2625 0.2608 0.2599
SIuntil air bubbles issue freely from the tip, T . The flask is now ready to deliver liquid a t a constant rate of flow, by opening &pcock Sa. The rate of flow is determined by the effective pressure head, H , the dimensions of the rate-restricting tip, R, and the viscosity of the liquid. Inasmuch as the viscosit of aqueous solutions has a temperature coefficient of about 1or 2 5 per degree, it is necessary to place the flask in a bath, the temperature of which is maintained constant to about 0.1 ' C. to ensure precise results. The rest of the delivery tube need not be thermostated. From an inspection of Figure 1, it is evident that the effective hydrostatic head, H , is given by
H = h'
- k,
where h, is the reading of the manometer. Because h, = h
+ hb
where hb is the excess pressure required to form a bubble, and h' = h
+ h"
H
- ha
then
I I
D
V I
Figure 1. B u r e t Apparatus of this type has been reported by Zentner ( 2 ) . In the present paper, the factors affecting the precision of operation of such devices are discussed and experimentally verified and an improved design capable of high precision is described. EXPERIMENTAL
A diagram of the apparatus is shown in Figure 1. A 34/45
6
male joint is sealed to a 500-ml. Erlenmeyer flask. The delivery tube and solution-air inlet tube are ring-sealed into the cap, which also has sealed to it a stopcock to facilitate the partial exhaustion of the flask. (An open-end manometer, filled with the same liquid as the flask, was sealed to the apparatus used for study of the be+ havior of the buret.) In o eration, the flask is filled with the liquid and, with sto cock a closed, suction is applied through stopcock SZuntil theyiquid rises into the horizontal portion of the tube. The delivery tube, D,is then inserted into a beaker containing the liquid, and suction is applied again a t Sg until the tube i s filled completely. With S2 and SSclosed, suction is applied to
8
1
Present address. Indipendencia 30, Zaragosa, Spain.
h"
The bubble pressure, hb, is at a maximum when the bubble 1s approximately hemispherical. Any further diminution of the pressure above the liquid cause9 the bubble to become unstable and it is detached from the tip. The introduction of this air causes the pressure, h,, t o decrease and the liquid may rise into T. As more liquid leaves the flask the pressure in the vessel is reduced and a new bubble of air forms a t T.
T HERMO S T A T ~
J,
=i
811 this theory was verified by an experimental study in which tips of various diameters were installed a t T and several values of h" were employed. The flow rates were measured by weighing the effluent from D while measurements of h and h, were made with a cathetometer. Results found when using a tip, T, of radius 3 mm., and a head, h", of 14 cm. are given in Table I. The first column gives the height of liquid above the air inlet tip, and the second gives the maximum value for the manometer reading. T h e n a bubble entered the air space above the liquid the manometer reading decreased by several millimeters and then rose again to a maximum value a t the time the next bubble was detached. The difference of columns 1 and 2 is the maximum bubble pressure, h,, tabulated in column 3. Column 4 gives the rate of flow observed for each value of h. The average deviation of these values from the mean is *O.S%. Another series of experiments with a tip of radius 3 mm., but with a head, h", of 85 cm. gave values for the rate of flow that showed an average deviation from the mean of *0.3%. The rates of flow given in Table I are not instantaneous values but averages obtained by collecting drops over a period of several minutes. The rate at any moment varies, owing to the fluctus-
1576
V O L U M E 21, NO. 12, D E C E M B E R 1 9 4 9 rahle 11.
IS77
Rates of Flow with Fine Capillary Air Inlet
Tube llauometer Heading,
Bubble Pressure,
h
hm
hb
Cm.
Cm.
Cm.
R a t e of Flow G./min.
Lion of the pressure above the liquid. In the experiment where the head was 85 cm., the rate of bubbling a t tip T , and hence the fluctuations in h,, varied from 2 to 20 seconds per bubble for values of h between 5 and 0.6 em., respectively. In the esperiment where the head was 14 cm., the rate of bubbling was approximately four times these values. Consequently, the rates of flow lor short periods of time probably differed significantly from the tabulated values. The most satisfactory design makes use of a tube drawn out to a very fine capillary at T and has a relatively large value of h". The resiilts found with such an apparatus, given in Table 11, show that the rate of flow is constant within the esperiniental error of measurement. The fluctuations of the manometer reading were not greater than 0.1 mm.; for h = 5.5 em. the rate of bubble formation was 0.70 second per bubble, nhereas for h = 0.36 cm. the rate was 0.97 second per bubble. Because of the rapiNi rate of hubhling, the small variation in this rate, and the
small fluctuation of h,, it is believed that the constancy of the rate of flow over short periods of time did not differ significantly from the values measured over the longer interval. The precision of apparatus built according to the above description is attested by its use in automatic titrations in which the reagent is added a t a constant rate and the amount of reaction is determined by the elapsed time from the start to the end point ( 1 ) . The results found in such service are comparable with those obtained using the best volumetric analytical technique. The disadvantage of the requirement that the apparatus be thermostated is offset by the advantages of flexibility of rate of delivery and simplicity of construction. For metering large volumes of liquids a t precise rates, modifications similar to those of Zentner ( 2 ) may be used, provided the restrictions discussed in the present paper are followed. SUM>.I4RY
4 constant-flow buret of high precision is based on the priilc*iplr of the Mariotte flask. By placing the rate-restricting tip in the liquid to be delivered, thermostating the flask, and using a fine capillary tip for the air inlet, rates of flow constant to better than 0.1% over hoth short and long periods of time have been obtained LITERATURE CITED
(1) Barredo, J. M. G., and Taylor, J. K., Trans. Electrochem. Soc., 92, 303-10 (1947) ; Preprint 92-26. (2) Zentner, E. T., ISD. ENQ.CHEM., ANAL.ED.,16, 474-5 (1944)
RECEIVEDDecember 21,
1948.
Modified Thyratron Thermoregulator Circuit D. F. SWINEIIiRT, University of Oregon, Eugene, Ore. THE simple on and off thyratron thermoregulator circuit such out by the tube and so stops running, while at the same time ( 1 ) is very reliable because the thyratron the heater is activated. The fan is directed a t the water surface d l carry enough current to operate a 250-watt heater directly of the thermostat and serves to cool it. By this means the a d thus eliminate the usual relay. However, the circuit dethermostat may be operated several degrees below room temjcribed incorporates two small batteries which have a limited life neiature (1). of service, especially if they become overheated by close proxThiq circuit has given satisfactory service for more than a yea1 imity to the thyratron or otherwise. for the control of two large water thermo. These batteries may be eliminated by stats (approximately 40-gallon capacity) .he exceedingly simple modification ol Schaenk ( 2 ) has recently proposed a chis circui~shoirn in Figure 1. Here the somewhat similar circuit, but has used circuit is entirel) alternating currenta much higher voltage and lower resistoperated. The current through the therance in the thermoregulator circuit and moregulator cannot be more than 3 ui 4 the thermoregulator is connected directly microamperes, nliich is low enough to across the pori-er line. The latter is unprevent appreciable corrosion of the merdesirable for safety's sake and it is d e wry contact. sirable to use as lorn a potential and m The circuit is self-explanatory except high a resistance as possible in this part for one or two details. The two connecof the circuit to minimize corrosion at tions marked B and C must be made so the mercury contact. This circuit is b e that the grid and anode are opposite in ing used with thermoregulators which phase with respect to the anode return. are open to the air. Undoubtedly this [f, on first trial, the tube refuses to show consideration is much less important if control when the thermoregulator leads closed thermoregulators are used, out of are shorted together, leads B and C must contact n-ith the air. The present cirbe interchanged. cuit uses one thousand times less current On first glance it appears that current through the thermoregulator than does 16 always flowing through both the heater Schwenk's circuit. Figure 1. Jlodified Circuit and the fan. This is true, but it must Tube FG57 be remembered that the impedance of a A . Anode (cap) LITERATURE CITED B , C. Connections small fan motor is much larger than that D . Grid and anode return E. %volt transformer of a heater. When the tube is not firing, (1) Garrett, A. B., IND. ENG.CHEM., ANAL F. Fan ED.,10, 324 (1938). the fan runs by means of current through G. Grid (2) Schwenk, H. X., J . Phys. & CoZlob3 H . Thermostat heater the heater. This current is too small to R I . 309,008 ohms Chem., 52, 761 (1948). Rz. 1 megohm dissipate appreciable heat in the heater. T. Thermoregulator When the tube fires, the fan is shorted Y. 120-volt power line RECEIVEDFpbruary 14, 1949.
'[ as that of Garrett
I*
l v l
