ANALYTICAL CHEMISTRY
1058 /
,Table V.
Comparison of Results by Power-Cheanut and Semimicromethods
Cai~eine‘ PowerCaffeine &miChwnut Method, % >oromeihod,* % Sample Range Ao. Range Av. 1.01-1.07 0.99-1.07 1.03 t.06 Green Santos ooffee 1.11-1.1s 1.18 1.08-1.13 1.11 Roanted Santoa ooffee 1.66 1.65-1.72 1.68 1.66-1.66 Soluble coffee product 0.034.os 0.04 Soluble deoaffrnated coffw 0.03-0.05 0.04 2.79-2.86 2.76 2.71-9.79 2.82 Tea leaves 2.13-2.21 2.14-2.20 ’ 2.17 2.17 Soluble tea product Repreaentu 8 determinations eaoh sample.
’
~
~~
Table VI.
Comparison of Results .by Bailey-Andrew and Semimicromethods Caffeine BaileyCaffeine,&miAndrew dethod. % micromethod*, % Range Av. Range Av. 3.22-3.27 3.21 . 3.31-3.36 3.34
Sample Soluble ooffee 1.74-1.80 1.77 Soluble ooffw product 1.14 ROMMooffee 1.13-1.16 Deoaffdmted ooffea 0.03-0.04 0.036 a Repreaenta 3 determinations eaoh sample.
1.81-1.82 1.23-1.24 0.04-0.04
1.82 1.23 0.04
(equal parte by weight). Wash the caffeine from the column with hot water under reduced pressure (300 to 400 mm. of meraury) and collect about 150 ml. of filtrate. Do not allow the column to become dr durin the extraction process. Add 5 ml. of sulfuric acid ( 1 0 g b y vokume) to the filtrate and reduce the Cool, extract with five 15volume to about 50 ml. by boilin ml ortions of chloroform, and co7lect the chloroform extract in a d n ar digestion flask. In the case of tea only, collect the ohlorokrm extract in a se aratory funnel and wash with 2 ml. of 1% potaasium hydroxi&. Transfer the chloroform to the
.
Hengar di &ion flaplk and wash the alkali with two 5-ml. port i o of ~ cdoroform. Di$iIl off the chloroform by heatin m a water bath. Determine mtrogen by the Hengar technique k r the K‘eldahl rocedure. $h.e ca%einein coffee brew or coffee extract may be deteAned by ueing a suitable aliquot. Time required is 2 hours for ssmples in duplicate. ,COMPARISON OF METHODS
A variety of samples have been analyzed by the semimicromethod .for comparison with the Power-Chesnut and Bailey-Andrew methods. The results in Tables V and VI show good agreement between the methods. The precision of the semimicromethod waa aa good aa or better than any other method used in this laboratory. LlTERATURE CITED
(1) h o . Offic. Agr. Chemiete, J . Assoc. OBc. Agr. C h i e t a , 30,70-1 (1947). (2) Aseoo. Offic. Am. Chemieta, “Official and Tentative Methods of Analysis,” 6th ed., 18.14, p. 217, 1945. (3) Crowell, a. K., J . Assoc. Ofic.A n . ChemZste, 29, 38 (1946). (4) Hadorn, H., and J u n g h , R., Mitt. L e b m . Hyu., 40,190-201 (1949). (5) Hengar Co., rhiladelphia, Pa., “Hengar Technique for Kjeldahl Procedure. (6) Ishler, N. H., Finucane, T. P., and Borker, Emanuel, ANAL. 20, 1162 (1948). CELEM., (7) Taylor, A., and Taylor, D. J., Analyst, 74, 463 (1949). R ~ C X V EOctober D 8, 1949. Presentad before the Diviaion of Agricultud and Food Chemistry at the 117th Meeting of the AMERICAN CHEMICAL SOCIETY,Philadelphia, Pa.
Accurate Control and Vaporizing System for Small liquid Flows HARTWELL F. CALCOTE,‘Experiment Incorporated,Richmond 2, Va.
A system has been developed for accurately metering very small liquid flows of the order of 0.005 ml. per second, and a chamber constructed for vaporizing the liquid and mixing it with air. Any liquid may be used without individually calibrating the apparatus.
T
mits operation for longer periods of time. This is possible beH E control and measurement of small liquid flows are an ever-occurring problem in many investigations-e.g., catacause the feed tube can be made longer without complicating the lytic studies and the, determination of burning v e l o p h In mechanical system as would be necessary with a pulley mangeaddition, it is frequently necessary to transform the liquid inh@e m e n t i t is only necessary to increase the,pressure on the u p vapor state without interrupting the steady flow, An apparatus‘ stream Bide of the orifice. has been constructed which fulfills these needs wjth the added advantage that the rate of liquid feed is independent of the maLIQUID FEED SYSTEM terial used. It is thus only necessary to calibrate the system once. Even lower flows than those used in this work, 0.006 to “he liquid feed system is ahown in Figure 1. An increaae in 0.06 ml. per second, are readily possible with the arrangement. preesure in the air chamber causes the mercury in the liquid feed tube to rise, forcing the liquid into the mixing chamber. Because Hogg, Verheus, and Zuiderweg (2)suggested a system in which the liquid waa forced out of a feed tube by a mercury column, the the flow through a sonic orifice flowmeter is independent of the mercury being raised by a synchronous motor and pulley mangedownstream preasure ( 2 ) and linearly dependent on the upstream ment. Although this gives very constant flows if proper precau- .- p r y u r e , the increase in preesure in the air chamber, and thus the tions are taken to prevent slippage in the pulley system, the flow liquid feed rate, is determined by the setting of the upstream presrate can be varied only by changing the gear ratio connecting the sure. The rate of flow through a sonic orifice is given by Equamotor to the pulley or by changing the size of the pulley wheel. tion l : . The application of sonic orifice flowmeters ( 2 ) to the problem permits the mercury column to be r a i d a t a linear rate which is continuously variable by changing the pressure on the sonic orifice. The sonic orifice system is also superior to the synchm where V volumetrio $ow rate, C = dieoharge coefficient, A = nous motor and pulley arrangement &I that it conveniently per-
-
1059
V O L U M E 22, NO. 8, A U G U S T 1 9 5 0
where g acceleration of gravity, d = diameter of liquid feed diameter of mercury reservoir, p~~ = density sf mertube, D cury, pila. = density of liquid, PI = pressure a t which V is measured (atmospheric), and V . = volume of air chamber and associated leads. In differentiating the equation for p r e a m , it is assumed that the mixing chamber pressure does not change with time. Because differentiation with respect to time of the equation for flow through a constriction gives zero, the pressure drop due to the flowing liquid has no effect on the rate of flow. The calibration is, therefore, independent of liquid viscosity. Any pressure drop through a small constriction will, of course, have to be balanced by the total pressure on the left of the mercury reservoir (Figure 1), Although usual laboratory temperature variations, or the rate of such changes, are insufficient to cause errors, it may be necessary for extremely accurate meaaurements to insulate the air chamber thermally, Variations in liquid density will, in general, make only a small change in flow rate (Equation 2), because the density of mercury is roughly fifteen times the density of most organic liquids and these densities vary over a relatively narrow range. When the density of the material is sufficiently different from the calibrating liquid, a correction can be easily made by applying Equation 2. To determine the weight of liquid or the number of moles delivered per unit time, it is only necessary to know the liquid density. 6
5
-4
ATMOSPHERE TO
-
449 RESERVOIR
(9 AIR CHAMBER
Figure 1. Liquid Flow Control System
orifice cross-sectional area, P = upstream pressure, p = gas density, and K = a function of y ( y = C,/C,). An expression for the volumetric rate of liquid flow can be readily derived by equating the pressure in the mercury reservoir at any given instant of time to the preasure in the liquid feed tube -that is, the pressure in the air chamber plus the hydrostatic pressure of the mercury in the mercury reservoir is balanced by the sum of the hydrostatic pressure of the mercury in the liquid feed tube, the hydrostatic preasure of the liquid, the pressure drop due to the flowing liquid, and the static prassure in the mixing chamber. Substituting in the expressions for pressure, and differentiating with respect to time, one obtains the formula:
8 I-
W
ol
4
IL
,019
.006 90
40 ORIFICE
60 80 PRESSURE, Cat! I
100
I
Figure 3. Calibration Curve for 0.06-Mm. Jewel
In the particular experimental setup that has been used, the sonic orifice jewel (synthetic ruby beanng jewel) diameters varied from 0.06 to 0.12 mm., the volume of the air chamber was roughly 35 liters, the diameter of the mercury reservoir was 2.8 cm., and the li uid feed tube was 60 om. long and had a diameter of 1.270 cm. ( I c e Trubore round Pyrex tube). Liquid flow ratea in the range of 0 . 0 5 to 0.05 ml. per second have been used. This ran e can be easily extended with orifices of different sizes. Tfe method is ca able of producing even lower flow rates by increasing the air chamger volume, Ve,or by decreasin the diameter of the fuel feed tube, d, or the mercury reservoir,
I8
5.
I8
0
PO0
400 TIME
600 800 , SECONOS
1000
1900
Figure 2. Calibration of Liquid Feed System for 0 . 0 6 M m . Jewel
In practioe the system ia calibrated by determining the change in mercury height in the feed tube aa a function of time for various pressures upstream from the sonic orifice. Thia gives a straight line (Figure 2), the slope of which, in combination with the feed tube diameter, yields the volumetric rate of feed. A plot of the
ANALYTICAL CHEMISTRY
1060 volumetric rate of liquid feed against the orifice prmure also gives a straight line, aa it should (Figure 3). When only a small quantity of sample is available, or it is desired to operate for a short length of time, the by-paea valve is opened and the mercury height in the feed tube raised until the liquid enters the mixing chamber. At the end of a run, the valve to the atmosphere is opened, relesaing the pressure in the system so that liquid from the “feed supply” reservoir may be fed into the “liquid feed tube,” preparatory for the next run. VAPORIZATION AND MIXING
If it is necessary to vaporize the liquid steadily and mix it with a gas, this can be accomplished with the mixing chamber shown in Figure 4. The glass nozzle extends just a fraction above the */,-inch hple, so that no aapirator action is obtained which would ve oscillations if present. The rapid passage .of air over the fquid dro lets, formed a t the tip of the nozzle, gwes a very fine spray. &e chamber and associated tubing are all stainless steel exce t for the lass tube from the liquid control system; thus any Tiquid can %e used which does not attack glass or stainless steel. Because the tube enters the chamber at a position where only air is present in the writer’s setup, the glass-metal seal is made with de Khotinsky cement. In a later setup hypodermic stainless steel tubin was substituted for the glass nozzle. By means of the controf valve on the gas inlet side, a pressure of approximately 10 to 15 pounds per square inch gage is maintained across the small annular air opening around the nozzle; some of the air is by-passed into the top of the chamber by the contrd valve. The heat of vaporization is furnished by heating the mixing chamber and the by-pass inlet gas line with an electric furnace.
Of the several types of mixing and vaporization methods tried at these very small flow rates, this is the only one which has performed satisfactorily under routine operation without oscillations. ACKNOWLEDGMENT
The author wishes to acknowledge the assistance of Moreland R. Irby, Jr., and Charles M. Barnett, who calibrated the system.
CONTROL FROM LIO!?STEM I
Figure. 4.
Vaporization and Mixing Chamber
The work described in thia paper was done in connection with Contract NOrd 9750 for the Naval Bureau of Ordnance, U. 5. Navy, as part of Project Bumblebee. LITERATURE CITED
(1) Anderson, J. W., and Friedman, R., Res. Sci. Instruments, 20,81 (1949). (2) Hogg, H., Verheua, J., and Zuiderweg, F. J., Trans. Faraday SOC., 35,999 (1939). REWEIV~D December 10, 1949.
f recipitation of Oxalates from Homogeneous Solution Separation and Volumetric Estimation of Zinc EARLE R. CALEY, LOUIS GORDON’, AND GEORGE A. SIMMONS, JR.
The Ohio State University, Columbw, Ohio an earlier paper ( 8 ) it was shown that magnesium could be INprecipitated in an easily filtrable form from 85% acetic acid
solution by the slow decomposition of ethyl oxalate, thus avoiding the experimental difficulties formerly encountered in the separsr tion of magnesium oxalate for the indirect determination of magnesium with permanganate. This paper summarizes the results of experiments on the precipitation of zinc by this =me technique. With slight modification, the procedure for the precipitation of magnesium is also suitable for zinc. If wed without modification, the ainc oxalate is precipitated in very large crystals that are not only inconvenient to filter but apperently impure. Such crystals are formed when the precipitation from homogeneous solution occurs a t too slow a rate. By using more ethyl oxalate Preaent a d d r w , Department of Chemistry, Byraou~eUnivenity, Byracum. N. Y. 1
initially the rate is increased, crystals of desirable physical properties and purity are obtained, and the time of precipitation is shortened. PROCEDURE
Concentrate the neutral zinc solution in a 25Gml. beaker to a volume of 10 to 12 ml. or dissolve the reeidue of dried salts containing the zinc in 11 ml. of water. Add 85 ml. of glacial acetic acid in which 1 gram of ammonium acetate has been dissolved. Then add 4 ml. of ethyl oxalate, stir well, and heat ra idly to a proximately 100’ C. Cover the beaker with a watch gram and pram on a hot plate eo regulated that the eolution ie maintained at approximatel this same temperature. Allow 1.6 hours for precipitation. there is any doubt as to maintenance of the recommended temperature durine; this riod, add 5 minutes before filtration, 5 d.of 85% acetic a c i f h a t .has been saturated wath ammouum oxalate at mom temperature. E’llter and wash
6
