February 15, 1942
ANALYTICAL EDITION
below 0.02 mg. of arsenic are reduced in less than 30 minutes, whereas 0.1 mg. of arsenic must stand 4 to 5 hours. There is no harm in letting the solutions stand overnight if the reagent blank for setting the photometer is allowed to stand in a volumetric flask for the same length of time. The blank for 2.5 ml. of perchloric acid in 15 ml. of water increases according to the time it stands in the flask. The presence of 0.4 gram of sodium bisulfite is inconsequential in this connection. But when the molybdate and sulfonic acid reagents are added to a dilute acid solution that has stood for two days in a volumetric flask, a milky-blue tinge appears. It is imperative, therefore, that the reagent blank be prepared and determined for the same time interval as that used for the sample. Since the straight colorimetric readings had been made on these samples, and it was known that none of them was high enough to approach a combined value of 0.1 mg. of arsenic, a 3-hour reducing period was adopted to be sure that all the arsenic which might be present was in the arsenite form. PROCEDURE FOR ELIMINATION OF ARSENATES. An aliquot of the perchloric acid extract of the soil is placed in a 25-ml. VOJUmetric flask and enough acid is added t o bring its concentration up t o 2.5 ml. An appropriate-sized glass scoop is used to add 0.4 gram of solid sodium bisulfite. Since the bisulfite does not affect the blank, such a measurement is close enough. The neck of the flask is washed with water and the volume brought t o 20 ml. The flask is shaken a little t o dissolve the solid,, and then is allowed t o stand on the bench for 3 hours. A blank IS prepared at the same time and in the same manner. After standing, the procedure for the development of the blue color is continued as described above. If a lower second reading is obtained, it indicates that arsenic is present in the sample and that the lower reading is the true value for the phosphorus. The values obtained with this modification agreed with those of the straight determination and proved conclusively that no arsenic was present in the soils used in this study. The perchloric acid method was also run on six samples of soil that were very kindly supplied by R. D. Chisholm of the Moorestomn, N. J., laboratory of the Bureau of Entomology and Plant Quarantine. These soils had been treated with lead arsenate for the control of Japanese beetles and contained 0.017 to 0.059 per cent of arsenic. After the arsenates had been converted to arsenites by the procedure just described, the phosphorus was easily determined. The instrument used, however, gives only arbitrary readings not simply related to transmission values and has no spectral dispersion. It would be necessary, therefore, to measure true spectral transmissions in order to determine arsenic in the presence of phosphorus.
(7) King, E. J., Biochem. J . , 26, 292 (1932). (8) Martland, M., and Robison, R., Ibid., 20, 847 (1926).
CHEM.,ANAL. ED., (9) Milner, R. T., and Sherman, M. S.. IND.ENCI. 8, 427 (1936). (IO) Pett, L. B., Biochem. J., 27. 1672 (1933). (11) Pinck, L. A., Sherman, M. S., and Allison, F. E., Soil Sci., 51, 351 (1941). (12) Rae, J. J., and Eastcott, E. V., J . Biol. Chem.. 129, 255 (1939); see p. 257. (13) Robinson, W. O., U. S. Dept. Agr., Circ., 139, 13 (Oct., 1930). (14) Shelton, W. R. and Harper, Horace J., Iowa State Coll. J . Sci., 15, 403 (1935). (15) Smith, G. Frederick, Chemical Co., “Perchloric Acid”, (Nov., 1934) : “Mixed Perchloric, Sulfuric, and Phosphoric Acids and Their Applications in Analysis” (1935). (16) Volk, G . W., and Jones, Randall., Soil Sci. SOC.Amet. Proc. 2, 197 (1937). (17) Willard, H. H., and Cake, W. E., J . Am. Chem. Soc., 42, 2208 (1920). before t h e Division of Analytical and Micro Chemistry a t the PRESENTED SOCIETY, Atlantic City, N. J. 102nd Meeting of the AMERICANCHEXICAL
Semimicropycnometer for Heaiv Water J
B. J. FONTANA AND MELVIN CALVIN University of California, Berkeley, Calif.
I
N T H E course of investigating some exchange reactions, small samples (about 1 cc.) of heavy water were manipulated and purified by distillation in a high-vacuum line. A simple and convenient pycnometer was devised which took advantage of the use of the high-vacuum line for filling it. The densities determined by means of this pycnometer are reproducible to 1 to 2 parts per 10,000, without thermostatic control. The results can be calculated for any temperature between 15” and 40” C. within this limit of error, provided the difference between the temperature of the actual determination and the temperature at which the density value is desired is not greater than 5”. For the usual determination of d:6 or diOthe pycnometer is then conveniently weighed a t room temperature. TABLEI. TYPICAL PYCNOMETER CALIBRATION (Weight of pycnometer, 3.4616 grams) Weight of Water
W
Acknowledgment The writer wishes to thank Franklin E. Allison for his many helpful suggestions and S. B. Hendricks, M. S. Anderson, W. 0. Robinson, W. L. Hill, E. J. Jones, H. G. Byers, and L. A. Pinck for their constructive criticism of the manuscript. The author is indebted t o K. D. Jacob of this bureau for supplying various phosphates which were used as checks upon the method, and to his staff for making several volumetric phosphorus determinations that were used to check the preliminary trials of the method.
Literature Cited (1) M e n , R. J . L., Biochem. J., 34, 858 (1940). (2) Assoc. Official Agr. Chem , Official and Tentative Methods of Analysis, 4th ed., p. 12 (1035). (3) Fiske, C., and Subbarow, Y . ,J . Bid. Chem., 66, 375 (1925). (4) Giesekina, J. E., Snider, H. J., and Getz, C. A., IND.ENG. CHEM.,ANAL. ED.,7, 185 (1935). (5) Hilbert, G. E., Pinck, L. A., Sherman, M. S., and Tremearne, T. H., Soil Sci.. 46,409 (1938): seep. 411. (6) Kahane, Ernest, “L’Action de l’bcide Perchlorique sur les Matibres Organiques”, p. 124, Paris, Hermann et Cie, 1934.
.
185
0,7935 0.7937 0.7935 0.7936
Temperature
c.
1,2578
20.9 21.2 20.7 21.1
Av. a
D
D/Wa 1.2574 1.2578 1.2576 1.25765
Deviation from Mean X 10-4 f1.5 -2.5 i-1.5 -0.5
tl.5
= density of water a t temperature of determination; D / W then is re-
ciprocal of volume of pycnometer.
The method employed by Gilfillan and Polanyi (3) in filling their flotation micropycnometer suggested the construction of a n ordinary weight pycnometer which could be filled by the same method. Their micropycnometer was evacuated and the sample allowed to draw up into the pycnometer upon re-admitting air to the system. The authors’ pycnometer is illustrated in Figure 1, a. Sizes of from 0.5- to 1-cc. capacity made from ordinary Pyrex tubing have been constructed and used. The pycnometer is lowered into the removable tube, Figure 1, b, with the ground tip resting at the bottom of the bulb. The tube is inserted in a vertical position at a suitable place in the high-vacuum line. When the sample has been suitably purified, it is frozen (carbon dioxide or liquid air), and the system, including the pycnometer arm, is evacuated to 10-6 mm. or better for 15 to 30 minutes. Such a high degree of evacuation is necessary to
186
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 14, No. 2
The pycnometer may be cleaned with chromic acid cleaning solution by alternately filling and expelling, with the use of an ordinary water aspirator.
i
8 mm. 0.d.
+,
I /
tapered point
a
16cm
I
OF PYCNOMETER AXD VACUUM LINE FIGURE 1. DIAGRAM ATTACHMENT
The reproducibility of the density determinations is illustrated by the typical calibration results tabulated in Table I for a pycnometer of about 0.8-cc. capacity, with four separate samples of ordinary water. The average deviation from the mean is seen to be slightly greater than 1 part in 10,000. I n order to calculate the composition of the sample of heavy water, using the formula determined by Longsworth (6,9),the density at 25" C. is required. The density at 25" C. (or at any other temperature, tz, between 15" and 40") can be readily calculated from the determined density at temperature tl. The difference in density, dz - dl, at the respective temperatures, tP and tl, is obtained for ordinary water from any suitable tables (4). This difference is added to the observed value of the density to give the desired density. TABLE11. VARIATION OF DIFFERENCEIN DENSITYOF D20 AND H1O WITH TEMPERATURE" t o c. 5 10 15 20 25 30 35 40 A(Ad:)
prevent the formationof an airbubblewhen thesampleisintroduced into the pycnometer. The sample is then thawed and distilled over into the pycnometer arm at room tem erature b immersing the arm in a cooling bath (carbon dioxi2 or liquig air). The sample is again melted and air is carefully allowed to re-enter the evacuated system. The sample should rise into and completely fill the pycnometer with no visible air bubble at the top of the bulb. Not more than 0.05- to 0.1-cc. excess of water over the capacity of the pycnometer is required by this method. If a very small air bubble appears, it will usually dissolve if the pycnometer is allowed to stand, with tip still immersed, for an hour or more. The weight of the dissolved air is negligible. When the pycnometer is removed from the tube and suspended on the balance pan the sample should preferably be about 5" to 10" below room temperature.
X 10-4
..
$7.0 +4.9 + 2 . 6 +1.2 -0.8
-2.7
-2.6
The recent data of Chang and Chien (f 8) indicate that these values are very probably in error. The effect of the n e b data is to make A( Ad) slight1 lar er (approximately 1.5 X 10-4 part larger) in the intervals of 15-20° and 20-25' C. only. The newer data, of course, should be used t o estimate corrections as suggested herein. a
c?:
That this transformation can ordinarily be done with negligible error is seen from the data of Lewis and MacDonald (6) as recalculated by Farkas (2). Table I1 lists the differences between the difference in density of heavy water and water at various temperatures calculated from the data of Farkas (2). The difference in density between ordinary water and pure deuterium oxide is seen t o vary by less than 4 parts per thousand from the 25" C. value over the range from 15" to 40" C.-that is, in this range the largest error possible in a density value so transformed would be 4 parts in 10,000 when interpolating from a value at 15" to one at 25" C. Ordinarily the error will be much less than this-for example, transforming densities over the 5" ranges from 20" to 25" or 25" to 30" results in a n error of 1 to 2 parts in 10,000. I n more dilute solutions of deuterium oxide these errors should be of the same order of magnitude or less. For greater accuracy throughout the whole range of temperatures given, the data of Farkas (2) may be used to estimate small corrections for the transformations. The nonlinearity of density with mole fraction (7) must, however, be borne in mind when making estimates of the corrections for mixtures of deuterium oxide and water. Though especially suited to the determination of the density of heavy water, this pycnometer might be used for other pure liquids or volatile mixtures. In such cases, however, temperature control would probably be desirable.
3 3 9 0 20
40
60
80
100
120
Time in Minutes
FIGCRE 2. TYPICAL PYCNOMETER WEIGHING
The pycnometer is carefully wiped and then suspended with ground tip up. The balance case is then closed (thermometer in balance case) and the pycnometer is allowed to stand for about one hour before weighing. As the sample warms up to room temperature the water expands through the capillary tip and evaporates at the tip as fast as it appears. Constant weight is reached in less than an hour and is maintained for a long period after. This is illustrated in Figure 2, where a typical example of a series of weighings of a filled pycnometer is given, starting 20 minutes after the sample was put into the balance case. To empty, the pycnometer is replaced in its tube with the tip resting on a small metal coil a short distance above the bottom of the bulb. The sample usually will not pump out directly upon evacuating, but the water slowly evaporates along the capillary until a small bubble appears at the top of the capillary. On readmitting air to the system and tapping the pycnometer, the air bubble will rise to the top of the bulb; evacuating now readily expels the sample.
Literature Cited (1) Cheng, T. L., and Chien, T. Y., J. Am. Chem. Soc., 63, 1709 (1941). (2) Farkas, "Light and Heavy Hydrogen", p. 173, Cambridge University Press, 1935. (3) Gilfillan and Polanyi, 2.phvsik. Chem., 166A,254 (1933). (4) Handbook of Chemistry and Physics, 24th ed., p. 1648, Cleveland, Chemical Rubber Publishing Co., 1940. (5) Lewis and -MacDonald, J. Am. Chem. Soc., 55, 3057 (1933). (6) Longsworth, Ibid., 59, 1483 (1937). (7) Luten, Phys. Rev., 45, 161 (1934). (8) Stokland, Ronaess, and Tronstad, Trans. Faradag SOC.,35, 312 (1939). (9) Swift, J . Am. Chem. Soc., 61,198 (1939). CONTRIBUTION from the Radiation Laboratory and the Depsrtment of Chemistry. Work supported by funds from the foundation of the late A. B. Miller.
