A Simplified Micro-Kjeldahl Apparatus JOSEPH E. SCOTT AND EDWARD S. WEST University of Oregon Medical School, Portland, Ore.
T
turns in passing through the trap. Two Pyrex bulbs are placed above the inverted tube to hold it in place and also to serve as additional baffles. The water condenser may be omitted if desired, though the writers have experienced less trouble with sucking back when the condenser is used. The distillation may also be carried out more rapidly with the condenser.
HE apparatus for the micro-Kjeldahl determination of nitrogen devised by Parnas and Wagner (4) is complicated and fragile as well as cumbersome to set u p and manipulate. Recently Kirk (3) and Fife ( I ) have described different types of apparatus for the same purpose, which seem to represent improvements over that of Parnas and Wagner. For some time the writers have used a n apparatus which may be of interest because of its relative simplicity, compactness, strength, and accuracy. Figure 1 shows the details of construction.
The sample t o be analyzed is. digested in the distilling flask according to the procedure of Harte (8). After cooling, 25 ml. of water are added and the alkali tube is placed in the flask. The contents of the flask are further cooled if necessary and then 15 ml. of 65 per cent sodium hydroxide are carefully added from a pipet through the tube, forming a layer under the acid solution. The glass joint is moistened with water and the distilling head firmly set in place with the tip from the condenser just under the standard acid (10 ml. of 0.0143 N hydrochloric and 10 cc. of water) in the receiving tube. A vessel of cold water is placed around the distilling flask, which is then gradually shaken to mix the alkali and acid. The clamps (rubber-covered) holding the apparatus should be well tightened to hold the glass joint in place. The condensing water is turned on slowly and the flask is heated with a microburner. The alkali tube remaining in the flask serves as a boiling tube. A small U-shaped boiling tube may also be advantageously used, in addition. Distillation is continued until solid sodium sulfate crystallizes out and the volume in the distilling flask has been reduced to about one-half. During the distillation the receiving tube should be moved down at intervals, so that the condenser tip never extends much below the surface of the acid. The last part of the distillation is carried out with the condenser tip just above the receiving acid. There is generally some bumping as the sodium sulfate separates, but with care this causes no trouble. After the distillation is complete, the tip of the condenser is washed down with a few milliliters of water and the excess acid in the receiving tube is titrated with 0.0143 N sodium hydroxide, using four drops of Tashiro’s indicator (2 ml. of 1.0 per cent aqueous methylene blue mixed with 100 ml. of 0.04 per cent methyl red in 50 per cent alcohol). Blank determinations on the reagents gave about 0.07 ml. titration, correction for which was made in the calculations.
a
I; I
I I I I I
I I
Zoom$ I
I I
The titration is most efficiently carried out with the aid of a mechanical stirrer and buret which have been previously described (7). The stirring can be satisfactorily done, however, with a loop stirrer having a right-angle bend to serve as a handle.
I I
I I
I
I I I
TABLE I. ANALYSISOF NITROGEN-CONTAINING SOLUTIONS
lOOm1.--
FIGURE 1. DIAGRAMOF APPARATUS
Substance
Nitrogen Present
Nitrogen Found
(NHSzSO4
Mg 0.997
Urea.
0.991
Tyrosine
0.595
Creatinine
0.955 0.961
Mg 0.987 0.993 0.993 0.991 0.992 0.985 0.989 0.599 0.595 0.591 0.948 0.967
.
Error
% -1.0 -0.4
-0.4 -0.6
+0.1 -0.6 -0.2 10.7 0.0 -0.7 -0.7
-0.4
Table I gives a series of unselected results obtained on ammonium sulfate, urea, tyrosine, and creatinine.
It consists of a 100-ml. digestion and distillation flask (a 100-ml. KjeldahI flask) attached by a ground-glass joint to a distilling head composed of a West distilling trap (6) and a West condenser ( 6 ) . The only part of the construction which may give trouble to the amateur glass blower is making the trap. As can be seen from the diagram, the trap consists essentially of a small sealed-in tube (about 6 mm.) with open end over which a small test tube, with a hole near the bottom, is inverted. There is a small water-return hole (about 2 mm.) in the bottom of the 6-mm. tube. The rim of the inverted test tube should have a few indentations in it (not shown) to facilitate return of wate: to the distilling flask. As can be seen, the vapor makes two 180
Literature Cited (1) (2) (3) (4) (5) (6) (71
Fife, IND.ENG. CHEM.,Anal. Ed., 8, 316 (1936). Harte, Ibid., 7, 432 (1935). Kirk, Ibid., 8, 223 (1936). Parnas and Wagner, Biochem. Z.,125, 263 (1921). West, IXD.ENG.CHEM.,Anal. Ed., 2, 199 (1930). Ibid., 4, 445 (1932). I b i d . , 8, 62 (1936).
R ~ C F J V BAugust D 19, 1936.
50
