V O L U M E 2 2 , N O . 4, A P R I L 1 9 5 0
603
filrn. Interference due to charring of the film occurs only in those color reactions which require hot concentrated sulfuric acid. For such tests, cryst,als are carefully removed from the film 80 that there is no contaniination by cellulose particles, because any fitiers that may dissolve in the hot acid will produce a brown coloration which will alter or mask the color of a particular reaction. Crystals must be removed from the paper for optical studies under polarized light. Although the crystalline forms are cle:irly defined when t’he film m0unt.s are observed under crossed Kicols, the paper is birefrarting and the opt,ical constants of the tla are altered accordingly. In carrying out microprecipitation reactions for the production of characteristic crystals, the reagents may be added directly to cut film sections, inasmuch as the transparent film particles floating in the drop on a microscopic slide do not interfere with t,he formation or observation of the crystalline precipitate. Because the film is insoluble in most organic solvents, except such cellulose solvent,s as acetone and ethyl acetate, crystals can be dissolved from the film I>?- immersion in suitahle organic solvent.
In the preparation of derivatives where microcondensationtype reactions are to be carried out in nonaqueous solution, cut film sections are of a decided advantage because organic liquids have much less tendency to creep on the film than on a glass surface: consequently, the reaction mixture remains as concentrated droplets on the film surface. If resublimation is necessary, the sublimate may be transferred to the lower half of the sublimation tube by means of an organic solvent, or the film sections may be used directly. Charring of the film at the higher temperatures inside the heating block will not affect the nature of the sublimate. The fractional sublimation of a fen- drugs commonly encountered is illustrated in Figures 3 to 11. LITERATURE CITED
(1) Clarke, R. L.. and Hermance, H. W., ISD. E s c . ED.,11, 50 (1939). ( 2 ) Hubacher, 31.H., Ibid., 15, 448 (1943).
CHEhi.,
AN~L.
UItramicrodetermination of Nitrogen W. I,. DOYLE
AYn
J. €1. OAIOTO, Unisersity of Chicago, Chicago, I l l .
Over the range of 0.1 to 10 micrograms of nitrogen, the present method is simpler than the Carlsberg micro-Kjeldahl procedures. A two-step digestion procedure is used to avoid bumping and the distillation tubes are coated writh silicone in place of paraffin. The ammonia is received in potassium dihydrogen phosphate and titrated w-ith acid to pII 4.6, using bromocresol green indicator.
T
HE procedure described is based o n Cxrlsberg methods ( 1 , 21, with simplifFing modifications and improved hydrophobic coating for distillation tubes. Although it does not achieve the ultimate accuracy nor lower limits of the Bruel method ( 2 ) , i t is sufficiently simple to warrant general use by technicians m o d w ately skillful in microchemistry. For the past year it has given uniformly consistent results in the range 0.5 to 5 micrograms of nitrogen, providing bet,ter accuracy and fewer lost determinations than have been hitherto obtainable. Bruel et al. ( 2 ) have discussed related micro-Kjeldahl procedures and Glick (3)h:ts described Carlsberg procedures and basic apparatus. A4major difficulty in micro-Kjeldahl destruction of organic material using small volumes ( < 10 pl.) of digestion mixture has been the high probability of loss by sputtering or bumping. This was overcome a t Carlsberg by the use of a hot sulfuric acid bath or sealed ampoules. The present method employs two heated metal blocks as digestion racks; one serves to eliminate most of the water, the second to complete destruction a t 250” C. APPARATUS AND REAGENTS
Destruction tubes, 6 X 50 mm. (Kimble 45060). Coated distillation tubes, 6 X 25 mm. (Arthur H., Thomas Company 2463-hI). The tubes are coated by filling wlth a 570 solution of G.E. Dri-film 9987 in chloroform and then inverting to drain off the excess. The hydrophobic film is set by heating in a drying oven a t 110O C. for 2 hours or overnight. The tubes are boiled in three changes of redistilled water to remove residual acid, after which they are dried in the oven and stored until used. Such coatings are inert to enzymes and appear to resist the strong alkali used in the distillation for a t least three determinations. Digestion Blocks. Aluminum blocks 5 cm. thick and 7.5 em. square are cut from 2 X 3 inch (5 X 7.5 cm.) atock with 25 holes 7 mm. in diameter and 15 mm. deep to receive the destruction tubes. A hole is drilled horizontally to receive a thermometer and the block is heated with a gas micro burner or an electric hot plate. Teflon Transfer Plate. Du Pont polytetrafluoroethylene polymer, 5 cm. in diameter and 5 mm. thick with a polished upper surface, is ruled into several segments. Alternatively, a piece of
l/lt;-inch Teflon sheet or a piere of glass covered with 0.002-inch Teflon tape may be used. The tape is translucent. Micropipets, Carlsberg model and specification (S), 2 of 5, 3 of 1 0 , l of 20, 1 of 50, and 1 of 100 pl. Microburet, of 0.05- or 0.1-ml. capacity, or alternatively a colorimeter with microcuvettes (
