Apparatus for Continuous Delivery of liquid at Constant Rate A. R. RICHARDS', Trinidad Leaseholds, Ltd., Pointe-a-Pierre, Trinidad, B. W . I .
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amount, provided that gas is escaping slowly from the bubbler. Tube L should be of the same diameter as K ; if the pressure in E can be maintained constant by any external device, the closed tube, L, and the connection to E are unnecessary. The gas from J is admitted to either of reservoirs A or B, or to both, through the common 3-way cock, M . Cocks N a n d 0 communicate with the capillary leaks, P. These are unnecessary if the pressure in E is constant. When delivery from A is almost complete, B is filled and cocks G and S are closed. Anv increase of Dressure in E raises the liauid level in K and the pressure in J is-increased an equal a m o h t . Gas passes through cock M and bubbles through the charge and so increases the pressure in A . Thus the li uid head across D is maintained. If the pressure in E is reduce1 the pressure in!J is reduced, but unless gas leaks through N the gas pressure above the charge in A will not be reduced. Therefore the leak should be adjusted so that the rate of pressure drop in the empty reservoir with M closed is a t least equal to the maximum rate of pressure drop in E. I n practice this is slow-rapid pressure fluctuations in E do not reach J,as they are damped by the capillary, &. To change reservoirs M is slowly turned, so that J is in communication with both A and B and 0 is opened. There is a slight surge of liquid from A if M is not turned slowly, and it is advisable to locate the cock well above the bottom of the reservoir as shown. Cis then slowly turned, so that both reservoirs are in communicstion with D. At this stage although the liquid levels in A and B are different and J and D are in communication with both reservoirs, there is no relative change in liquid levels. Delivery will take place from both reservoirs if required. Cocks C, M , and N are turned to isolate reservoir A and complete the change-over. The volume of liquid in B should be read before the chenge-over and the volume in A after. A may now be refilled, the gas being vented through R.
HEN a large quantity of liquid is to be delivered continuously it is common practice to provide two containers, refilling one while liquid is being delivered from the other. However, there is a possibility that the operation of the system will be disturbed while the delivery rate from the refilled reservoir is being adjusted. This is particularly irksome when the delivery rate is low or the setting of the control cock is difficult, because it requires some time t o detect and then'to correct the changed rate. The simple apparatus shown in Figure 1 overcomes this difficulty by maintaining a constant liquid head across the control cock. It has been applied successfully to continuous fractionating columns and to continuous reactors. "
F
Y
Y G
Figure 1
The advantages offered over two independent constant feed reservoirs are: maintenance of feed rate without resetting the control cock; independence of pressure changes on the delivery side of the device; reduction of the reservoir size with consequent reduction of fire hazard if inflammable liquids are being used. As a result, the reservoirs may be thermostated economically.
The two calibrated reservoirs, A and B, discharge through the common 3-way cock, C, and control cock D into the drop counter and sight glass, E. The reservoirs are filled through cocks F and G.( ,:Air or an inert gas is admitted through cock H into tube J, where it is maintained a t a pressure determined by the depth of immersion of the tube in the water or mercury bubbler, K . This is, in turn, dependent on the pressure in L and in the sight glass, E, so that the pressure in J exceeds that in E by a constant 1
ACKNOWLEDGMENT
Thanks are due to the management of Trinidad Leaseholds, Ltd., for permission to publish the details of this apparatus.
Present address, Caribbean Development Co., Port-of-Spain, Trinidad,
B. W. I.
Micro-Kjeldahl Determination of Nitrogen Use of Potassium Biiodate in Iodometric Titration of Ammonia ROBERT'BALLENTINE~ AND JOHN R. G R E G G ~ Department of Zoology, Columbia University, New York, N . Y . tion of the particular procedure employed. However, the Kjeldah1 method, like any other, is of real value only when performed under carefully standardized conditions which f i s t must be determined if dependable results are to be obtained. These conditions necessarily vary with the apparatus and reagents used, with the material to be analyzed, and with the various manipulstiona in the whole analytical process (5). The data reported here have been obtained in working out a modification of the Kjeldahl method for use in a study of the nitrogen metabolism of amphibian embryos (1). The chief fea-
H E Kjeldahl method for the determination of nitrogen has Tbeen widely used in biological chemistry, and a great many modifications have been described. I n spite of the fact that it is largely empirical, its almost universal acceptance has caused its accuracy tqo often to be taken for granted. Work whose final validity has depended on the accuracy of nitrogen determinations often hss been published without any apparent critical examina-
* Present addresa, California Institute of Technology, Pasadena, Calif.' Present address, Department of Biology, The Johns Hopkins University, Baltimore 18, Md. I
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