A Semimicro-Kjeldahl Distillation Apparatus C. E. REDEMANN, California Institute of Technology, Pasadena, Calif.
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distillation flask. KO ammonia is liberated until all the air has been swept out of the apparatus with steam. None of the standard acid in the receiver is sucked back into the distillation flask. The entire apparatus may be assembled on a single small-sized ring stand, thus making the unit both rigid and easily portable. A large number of determinations can be made with a single piece of apparatus in a short time, since cleaning is extremely rapid and distillation of the ammonia requires only a few minutes. I n addition, the apparatus is built in three closely coupled units, any one of which may be easily removed for repair in the event of breakage.
UMEROUS designs have been published for both
macro- and micro-Kjeldahl distillation units. Some of these are very satisfactory in their performance, but all show some ,tendency to suck back the standard acid unless considerable care is exercised by the operator. A few require for fabrication a degree of skill in glass blowing seldom attained by anyone other than a professional glassblower. Various sources of error in both the macro- and microKjeldahl determination of nitrogen have been discussed by Schulek and Vastagh ( 2 ) . The use of rubber connections, to which these authors strongly object, is less objectionable than was formerly the case because of the high grade of amber-colored pure gum tubing and stoppers now available, and the small area of rubber in contact with the steam which suitable design readily reduces to less than 1 sq. em. The loss in ammonia resulting from the large volume of air diluting the ammonia in the early part of the usual Kjeldahl distillation, t o which Miller (1) has called attention, is best prevented by sweeping out the air in the apparatus with steam before liberating the ammonia with alkali. The semimicro-Kjeldahl distillation unit herein described possesses the following advantages: Distillation is carried out from the same flask in which the digestion is performed, which in this case is a standard 100-ml. Pyrex Kjeldahl
Construction The construction of the apparatus is adequately explained by Figure 1. The capillary tip below the stopcock should have a bore between 2 and 3 mm. If the bore is smaller than 2 mm. the sodium hydroxide runs in too slowly, while if the bore is much larger than 3 mm. a solid column of liquid fails to form and steam may be expelled through the funnel top above the stopcock. The inner tube in the condenser must be of thin-walled glass to give adequate heat transfer. The small trap on the filler tube in the steam-generating flask prevents water from spurting out of it during distillation, a t the same time leaving the flask open to atmospheric pressure. As a consequence, if the standard acid starts to suck back out of the receiver, it rises only a few centimeters in the delivery tube before a few bubbles of air are drawn through the filler tube into the apparatus. These bubbles having restored atmospheric pressure within the apparatus, the distillation then continues without interruption. Water may also be run into the flask without removing the filler tube from the apparatus. The tube below the small trap in the filler tube should be a t least 25 cm. in length and should extend nearly to the bottom of the large flask. The style of trap used here has been found very efficient and is desirably compact.
Operation
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FIGURE 1. APPARATUS
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A 20- to 60-mg. sample is weighed into a 100-ml. Pyrex Kjeldahl distilling flask, and 2 to 5 ml. of c. P. concentrated sulfuric acid and any one of the numerous catalysts that have been suggested are added, the seleniummercury-potassium sulfate mixture (3) being one of the best. Digestion is then completed in the usual manner. The flask and contents are allowed to cool to room temperature, after which 15 to 20 ml. of distilled water are added and the contents of the flask are well mixed. During the digestion of the sample the large flask in the distillation unit is filled approximately two-thirds full of distilled water and heated to boiling with an efficient burner. A blank Kjeldahl flask is fitted in place and the apparatus is well steamed. The blank Kjeldahl flask is now removed and replaced by one containing the diluted digested sam le A second burner is placed under the KjelLhI flask and the two burners are so regulated that a moderate current of steam passes into the Kjeldahl flask. In the meantime a 200-ml. conical flask containing an appropriate amount of standard acid is placed under the delivery tube with the tip of the delivery tube about 1 cm. below the surface of the acid. After the apparatus is completely filled with steam, the burner is temporarily removed from under the Kjeldahl flask and 10 t o 15 ml. of 18N sodium hydroxide solution, which has been allowed t o stand until carbonate-free, are added in
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Table I show the excellent performance of this apparatus, TABLEI. TESTRUNSON KJELDAHL DISTILLATION APPARATUS even with much shorter periods of distillation than those Time of Nitrogen Deviation from recommended and with samples containing as little as 0.6 Distillation Found Present Error Mean of Group mg. of nitrogen in the form of ammonium sulfate. This Min. Mg. M Q. % % series of determinations represents a complete group of test 12.42 +0.4 +0.2 -0.08 +o.os distillations. No results were omitted because of inaccuracy. ... 0.00 -0.16 To make sure that the standard acid could not be sucked ... -0.16 0.00 ... +o. 08 -0.08 back, in several determinations the burners were turned off +0.2 +0.4 i:iio -0.5 -0.2 while the apparatus was in the midst of a distillation. No ..* -0.1 +o. 1 sucking back occurred in any part of the apparatus. -0.2 -0.06 .
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the course of 1 to 2 minutes, slowly enough t o avoid excessive heating. (If a mercury catalyst has been used 1 to 2 ml. of 1 M sodium sulfide solution should be added immediately after the sodium h droxide.) The burner is then replaced under the Kjeldahl iask and the two burners are so adjusted that 40 to 50 ml. of distillate collect in 10 t o 12 minutes. If bumping starts, the burner under the Kjeldahl flask is turned out and the other one is increased. After 8 to 10 minutes the receiver is lowered and 5 to 10 ml. of additional distillate are collected, the lower end of the condenser being above the surface of the solution in the flask. The acid in the receiver is then titrated in the usual manner.
Performance To test the performance of the apparatus a standard solution of ammonium sulfate was used, so that no error due to incomplete digestion would be introduced. The data in
Summary Directions are given for constructing and operating a semimicro-Kj eldahl distillation apparatus embodying the following advantages: one flask only for digestion and distillation; standard acid not subject to sucking back; apparatus compact and portable; and ammonia not liberated until apparatus is air-free. The precision on samples containing as little as 0.6 mg. of nitrogen indicates that this apparatus may be used even on a micro scale.
Literature Cited (1) Miller, H.S., IND. ENG.CHEJI.,Anal. Ed., 8,50 (1936). (2) Schulek, E., and Vastagh, G . , 2. anal. Chem., 92,352 (1933). (3) Sreenivasan, A., and Sadasivan, IT., IND. ENG.CHEM.,Anal. Ed., 11, 314 (1939). CONTRIBUTION 719, California Institute of Teahnolopy.
Constant-Level Still for Redistillation of Water D. A. WILSON AND H. S. STRICKLER Endocrine Laboratory, Elizabeth Steel Magee Hospital, Pittsburgh, Penna.
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tities of various elements] such as iodine or lead, water redistilled from alkali or alkaline permanganate must be routinely used for the final washing of all glassware. Of the various setups that have been tried over a period of 3 years in this laboratory for the continuous redistillation of water, the one described here is the best. It has also been used successfully in two other laboratories in this district. The figure is drawn to scale, as indicated. Water is boiled in a modified 1-liter Claisen flask, D, with two or three beads to re vent bumping, and is condensed in the double condenser, When the level of water in D drops below any point predetermined by the height to which G is set, air passes up through the water in the outer column to the safety bulb, B, and thence through the capillary, K , to the reservoir bottle, S. This starts the flow of water through the siphon tube, H . The flow continues until the end of G is again covered with water and the water in G is in hydrostatic equilibrium with that in reservoir S. The purpose of capillar K is to introduce an appreciable lag in the flow of water into &sk D, so that in no cme does the water cease boiling. The condenser bore must not be appreciably smaller than the outlet, M . Thus, with a coil-type condenser the constant-level device did not function. Although any condenser meeting the above requirement may be used, the compact double condenser] C , C, is found t o give adequate condensation. To keep out dust, it is well to cover L with Pyrex glass wool. M may be closed entirely. The apparatus may be mounted by using a rod frame against a laboratory shelf on which S sits, or by constructing a wooden stand.
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