An Improved Weight Azotometer C. W. KOCH, T. R. SIMONSON,
AND V. H. TASHINIAN University of California, Berkeley, Calif.
A simplified weight azotometer for micro-Dumas determinations uses the Clarke-Winans principle of weighing displaced mercury for measuring nitrogen volume, but disassembly between determinations is unnecessary. It permits greater precision than the conventional Pregl volumetric azotometer. Calculations have been simplified by introducing a nomogram which gives a single factor for multiplication of mercury weight to get nitrogen weight.
AN
AZOTOMETER for micro-Dumas analyses which permits determining nitrogen by weighing an equal volume of mercury has several major advantages over the Pregl volumetric azotometer ( 2 , 3 ) . Errors from caustic adsorption on the surface of the glass and change of calibration from corrosion are completely eliminated. Errors due to the volume estimation are reduced to a minimum because 1 microliter of nitrogen corresponds to approximately 13.5 mg. of mercury. Potassium hydroside does not come in contact with the stopcock lubricant. A mercury azotometer has been developed in this laboratory which is similar to that proposed by Clarke and Rinans ( I ) , but more simple in design. A more important advantage lies in the fact that the portion of the apparatus containing mercury is not disconnected a t any time, and so there is no danger of trapping air bubbles in the mercury. In addition to causing a possible error in the amount of mercury weighed, an air bubble also increases the difficulty in adjusting the potassium hydroxide level in the absorption tower. APPARATUS
The apparatus shown in Figure 1 consists of two units, the carbon dioxide absorption tower and the mercury displacement unit. The absorption tower is constructed of 12-mm. Pyrex tubing and
E -A
1-mm. capillary tubing and is about 45 cm. (18 inches) high. A ring, A , is etched on the capillary tubing attached to the exit of the absorption tower. The volume of the capillary tubing between etched mark A and stopcock E shou!d be small, SO that any temperature or pressure change during the period of combustion will not affect appreciably the volume of nitrogen gas collected. The mercury displacement unit is attached to the absorption tower by means of a 7/25 ground joint which can be sealed with wax. The sole function of the joint is to facilitate cleaning of the apparatus. The body of the mercury displaoement unit is constructed of 8-mm. Pyrex tubing. This is drawn down to join the microstopcocks, B and C, giving an over-all length of about 12.5 em. (5 inches). The volume of this part of the unit is approximately 3 ml. The side arm of the unit is made of 5-mm. tubing. 4 length of 0.5-mm. capillary tubing is attached to C and drawn out to give an easily controlled rate of mercury flow. PROCEDURE
Mercury is added to the bottom of the absorption tower to a height approximately halfway between the gas entrance tube and the leveling bulb connection. Sufficirnt 50% potassium hydroxide solution is addpd through the leveling bulb to permit the tower to be filled by raising the leveling bulb. The mercury displacement unit is filled through reservoir D with clean, dry mercury; care is taken to remove all trapped air bubbles. The combustion tube is swept out in the usual manner. The level of the potassium hydroxide solution is adjusted to 9 in the capillary portion of the absorption tower with stopcocks B and E open. This can be done easily by mounting the leveling bulb support on a rack and pinion attachrd to a ring stand. When the level of the potassium hydroxide solution is adjusted, E is closed, and the location of the leveling bulb is marked so that it can be returned to the same position. The bulb is then lowered, and the sample is burned in the usual way. After the combustion and sweeping procedures have been completed, the leveling bulb is returned to its previous position, and B and F are closed. E is then opened to the mercury displacement unit, and mercury is drained through C into a tared container, until the level of the potassium hydroxide solution has just returned to mark A . The amount of mercury transferred to the weighing vessel is then weighed to the nearest 5 mg. CALCULATIOR S
The analytical results are ralculated fiom the weight of mercury displaced. The usual calculation is accomplished by determining the volume of mercury from its weight and density at the experimental temperature. The amount of nitrogen may then be determined by ieduring the volume to standard conditions, using the experimental temperature and barometric pressure (corrected for the vapor pressure of water over 50% potassium hydroxide). The development of a nomogram for the major part of the work has greatly simplified the calculations. The data in the International Critical Tables for the density of mercury and the vapor pressure of water over 507, potassium hydroxide as functions of temperature have been fitted by empirical equations within the temperature range 18" to 34' C.:
t3
drig = 13.576,
Figure 1. Apparatus
PH*O
1133
- 0.00102T
(mm.) = -0.45
+ 0.15T
ANALYTICAL CHEMISTRY
1134
--740
-_
-
REDUCTION DIAGRAM FOR N I T R O G E N C O L L E C T E D OVER 50% KOH
(MEASURED BY WEIGHING MERCURY
OF
EQUAL VOLUME)
--745
-
(mg. N2) = F (9.Hg.1
-
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LT
3
cn cn
f8
0'
h
---750
a,
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W
J
t I
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-
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0 -
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CL -IW
30
I --755
0 CL Q m
---
34
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-*.079 O8OI
--765 Figure 2
1135
V O L U M E 21, NO. 9, S E P T E M B E R 1 9 4 9 Table I.
Factors for Reducing Grams of Mercury to Milligrams of Nitrogen
Temperature,
c.
Barometric Pressure 750 mm. 0.08514 0.08395 0.08278 0.08165 0.08055
740 mm. 0.08401 0.08283 0.08168 0.08056 0.07947
18 22 26 30 34
765 mm. 0.08685 0.08563 0.08445 0.08329 0.08217
Table 11. Typical Analyses Sample Weight, Compound Dihydrocodeine
Mg.
4.732 5.688 5.755 6.579 2.381 3.186 3,958 4.771
Bexamethylenetetramine Dinitrophenol
% N 4.63 4.69 4.69 4.68 40.31 40.24 15.13 15.12
Theoretical, % N 4.65
40.01
15.22
where T is in degrees centigrade. A factor, F , may be defined by the equation (mg. of Nz) = F (grams of Hg). The empirical equations above, combined with the usual gas law calculations, result in the foIlowing expression for F :
F =
8250.3 (1
P + 0.45 - 0.152’ + 0.0035867’ + 0.00000028T2)
P is the barometric pressure in millimeters. This e uation for F is the basis for the nomogram shown in Figure 2. Zalculation of the weight of nitrogen in a sample is reduced to a single multiplication of the weight of mercury displaced by factor F , nomographically determined. The percentage of nitrogen in the sample is then calculated as follows:
%N = [F(weight of Hg in grams)
- (weight of N2blank in mg.) ] X 100 (sample weight in mg.)
The nomogram may be duplicated with the aid of Table I. Lay off parallel, linear scales for barometric pressure and factor F of the desired size and spacing. Locate five points on the temperature scale by the intersection of lines joining corresponding points on the P and F scales. Draw the support for the T scale and subdivide it by linear interpolation. RESULTS
Analyses for only three compounds are reported in this paper, as no changes were made in the standard combustion procedure. The results shown in Table I1 are indicative of the usual precision of routine analyses. A number of benzoic acid and dextrose samples were burned by the standard Dumas method in an effort to determine the consistency of the nitrogen blank. A very limited amount of data is available in the literature on the size of this correction. Only the usua I sweeping precautions were observed. The amount of gas collected when 50 ml. of carbon dioxide were passed through the apparatus corresponded to 0.007 mg. of nitrogen. Ten samples of benzoic acid, varying in size from 3 to 7 mg., were burned, sweeping with the same volume of carbon dioxide. The average nitrogen equivalent was 0.020 mg., with an average deviation of 0.003 mg. Four samples of dextrose (4.8 to 5.2 mg.) were treated in the same way. The blank was slightly lower, averaging 0.018 mg. of nitrogen, with an average deviation of 0.001 mg. The value of the blank used in calculating the analyses of Table I1 was 0.020 mg. of nitrogen in each case. These two compounds were chosen for blank estimation because of their great difference in physical and chemical characteristics. The small difference in magnitude of the two blanks perhaps may be explained by the greater occlusion of air in the voluminous benzoic acid. LITERATURE CITED
(1) Clarke and Winans, IXD. ENG.CHEM., ANAL.ED., 14,522 (1942).
(2) Niederl and Niederl, “Organic Quantitative Microanalysis,” pp. 60-79, New York, John Wiley & Sons, 1938. (3) Pregl-Grant, “Quantitative Organic Microanalysis,”4th ed., pp. 63-78, Philadelphia, Blakiston Co., 1945. RECEIVED December 18, 1948.
Purification of Di-beta-napthylthiocarbazone Molecular Extinction Coeficient in Chloroform at 650 Millimicrons STANCIL S. COOPER AND VERNON K. KOFRONI Saint Louis University, Saint Louis, Mo.
D
I-6-naphthylthiocarbazone, an analog of Xithizone, shows promise of becoming a very important reagent for the determination of microquantities of certain metals (g-4, 6). For the estimation of mercury, Hubbard (6) reports the naphthyl analog is superior to dithizone. Suprunovich ( 9 ) prepared the compound by a method originally employed by Fischer ( 5 )for the synthesis of dithizone and investigated a number of its properties. Hubbard and Scott ( 7 ) found that the method of Suprunovich gave poor yields and modified the dithizone synthesis of Bamberger, Padova, and Ormerod ( 1 ) to obtain the naphthyl analog. Both Suprunovich (9) and Hubbard (6) studied its properties and also those of some of its metal complexes in organic solvents. The compound is found to react like dithizone with one notable exception: It cannot be stripped from a chloroform solution with dilute aqueous ammonia; thus one is limited in chloro1
Present address, Vickers Electric Division, Vickers, Ino., Saint Louis,
Mo:
form to a mixed color procedure. More serious, however, is the fact that the general method of purifying dithizone cannot be employed in purifying the naphthyl derivative from chloroform solution. Because the commercially available material (Eastman Kodak Company product), or that obtained by known methods of preparation, is impure, purification becomes a problem of first importance. A modification of the method of Hubbard and Scott ( 7 ) eliminates the extraction with water and hastens the precipitation of di-P-naphthylthiocarbazone (DN) through chilling the chloroform-absolute alcohol solution in an acetone-solid carbon dioxide bath. By considering the “molecular extinction coefficient” in chloroform a t 650 mM as a measure of “purity” it was found that the material obtained by the modified procedure was “purer” than that reported by Cholak and Hubbard ( 8 ) . Di-8-naphthylthiocarbazone can be extracted from carbon tetra-
