NOTES ON ANALYTICAL PROCEDURES Catalytic Activity of Selenates in the Kjeldahl Method for Determination of Nitrogen ROBERT S. DALRYMPLE
AND
G. BROOKS KING
Department of Chemistry, State College of Washington, Pullman, Wash.
I
Although selenates have been found to b e more effective catalysts in the Kjeldahl digestion for protein nitrogen than elemental selenium, prolonged digestion gives low results.
I
c 1.5
S 1931, Lauro ( A ) discovered that small amounts of seleniunl
f
0) Ul
were effective in catalyzing the decomposition of proteins i n Kjeldahl' digestions. Although the action of selenium, selenium oxychloride, and certain selenites in catalyzing these decompositions has been fairly extensively investigated since that time, there appears to be no study of the effect of selenates on digestion time.
2
+.
J 1.0
Sreenivasan and Sadasivan ( 7 ) employed copper selenate in a study of the mechanism of selenium catalysis, but did not report the effect of the salt on digestion time. They proposed a mechanism for the catalysis in which selenium is alternately oxidized to selenious acid and reduced to elemental selenium. The selenium, after complete oxidation of protein, is present as selenious acid. However, in the presence of mercuric oxide, all the selenium is presumably oxidized to selenic acid. Osborn and Krasnitz ( 5 ) report that a combination of mercuric oxide and selenium acts much more effectively than either mercuric oxide or selenium alone in catalyzing the decomposition of proteins. In view of this fact and in the event that selenic acid does play a role in selenium catalysis in the presence of mercuric oxide, possibly a selenate would he more effective as a catalyst than either selenium or a selenite.
0.5
0 Figure 1.
EXPERIMENTAL
The determinations were carried out with the conventional Kjeldahl apparatus in the usual manner, using a gas flame as a source of heat. Elemental selenium used was a preparation of Eimer and Amend. The selenates were prepared by treating analytical reagent carbonates of the metals with selenic acid, the preparation of xhich has been described ( 3 ) . The salts were twice recrystallized from water. The selenates in hydrated form were weighed out in amounts such that the selenium content of each sample was 0.10 to 0.15 gram. This was added directly to the weighed sample of protein. Bradstreet ( 2 ) reports that more than 0.25 gram of selenium gives low results. The digestions
Table
+ KnSO4 N
I1
1
% 2.59
1.67
1.
0.5 0.73 1 1.25 1.5 2
2.5 3
0.37 0.84 1.20 1.44 1.46 1.51 1.63 1.67
CuSeOI Time N Hours % 2 2.59 2.5 2.63 3 2.60 6 2.30 0.5 0.98 0.75 1.37 1 1.55 1.25 1.66 1.5 1.67 2 1.66 2.5 1.59 3 1.50 4 1.36
3.0
4.0
I t is evident from the data that the three selenates Rere markedly more effective in catalyzing the digestion than selenium in the elemental form. Although the relative effectiveness of the selenates in general is not pronounced, copper selenate and cadmium selenate proved somewhat more effective than cadmium selenate in all the determinations carried out. It was noted CdSeOi that clearing of the digestion mass is no criterion Time N as to completeness of digestion, a fact previously Hours % reported by Ashton ( 1 ) . The time of digestion 2 2.49 3 2.61 is an important factor in the accuracy of the results. Reference to Figure 1 shows that the 0.5 0.57 nitrogen obtained, except when elemental 0.75 0.97 selenium is the catalyst, rises to a maximum and 1 1.32 1.25 1.47 then falls off. The danger of loss of nitrogen 1.5 1.55 on prolonged digestions with selenium catalysts 2 1.60 2.5 1.66 has been reported by Sandstedt (6). It would not 3 1.61 appear practicable, therefore, to employ selenates as catalysts, since the digestion time for maximum I
CaSeOc Time Iu Hours 70 2 2.56 3 2.61 4 2.51 0.5 0.75 1 1.33 1.5 2 2.5 3
9.0
Time in Hours Effect of Selenates on Digestion Time
DISCUSSION
Effect of Selenium Catalysts
Se Time N Hours % 6 2.63
I.o
were carried out for varying lengths of time in the presence of the four catalysts: selenium, copper selenate, calcium selenate, and cadmium selenate. All analyses represent the average of at least two determinations. The results of duplicate deterrninations were in error by no more than 0.2%. While the data recorded in Table I are only a portion of those obtained in the study, they are fairly representative. Sample I \?as pea meal, very difficult to decompose completely. Six hours were required for the decomposition using metallic selenium as a catalyst; with copper selenate the time was cut to 2.5 hours; with calcium selenate to 3 hours; and with cadmium selenate to 3.5 hours. Sitrogen obtained by the official method (KjeldahlGunning-Arnold) is somewhat lower than the maximum obtained using selenium or selenates as catalysts. However, in three other samples, data for one only of which are included here, the nitrogen content by the official method agreed well with the maximum values obtained with the selenium or selenate catalysts. Data for sample S o . I1 are shown graphically in Figure 1.
The purpose of the prwent investigation was to Jeternline the relative catalytic action of several selenates in thr Kjeldahl determination and compare their effectiveness to elemental selenium in this redpert.
Protein Sample HgO No. Time Hours I 1
-1
0.94 1.39 1.59 1.62 1.67 1.64 1.60 1.57
403
404
INDUSTRIAL AND ENGINEERING CHEMISTRY
yield of nitrogen would have to be rather accurately determined and controlled for each type of protein sample. LITERATURE CITED
(1) Ashton, J. Agr. Sci., 26, 239 (1936). (2) Bradstreet, R. B., IND. ESG. CHEM., ANAL. ED., 12, 657 (1940).
Vcl. 17, No. 6
(3) Gilbertson, L. I., and King. G. B., J. Am. Chem. SOC., 58, 180 (1936). (4) Lauro, M. F., IND.EXG.CHEM.,ASAL. ED.,3, 401 (1931). (5) Osborn, R. A, and Krasnitz, A , J . Assoc. Oficial Agr. Chem., 16, 110 (1933). (6) Sandstedt, R. M., Cereal Chem., 9, 156 (1932). (7) Sreenivasan, A., and Sadasiran, V., ISD. ENG.CHEM.,A N ~ L . ED.,11, 314 (1939).
A n Automatic G a s Circulating Pump School
J. H. SIMONS, T. J. BRICE, A N D W. H. PEARLSON of Chemistry and Physics, The Pennsylvania State College, State College, Pa.
A
i\' .1UTOhIATIC gas circulating pump for use on a vacuum
system has been devised and used with satisfactory results in this laboratory. The design of the pump and the controls is shown in the diagram, The pump consists of two small one-way valves and two 50-cc. bulbs arranged as shown. Gas is drawn in through valve A by lowering the mercury in C, thus reducing the pressure inside the pump below that of the system; the lowering of the mercury is accomplished by applying a vacuum on bulb D. I n this step B acts as a check valve. When the mercury has been lowered sufficiently so that C is empty, the cycle is reversed by letting air into D and allowing the mercury to flow back into C. Gas is forced out through B while A acts as the check valve. The pump used by the authors made a complete cycle once a minute a t 200-mm. pressure. This period could be varied by changes in the dimensions of the various parts of the apparatus. The levels of the mercury in A and B are adjusted by means of the reservoirs below them. The pump can be made to operate a t pressures as loiv as 3 to 4 mm. by suitably adjusting these levels. This is the pressure required to overcome the resistance of the mercury check valves. If other check valves were used this minimum might be reduced. Residual gas in the pump can be let out,into the system by drawing the mercury down into the reservoirs. The pump was designed to operate with a continually varying pressure inside the system. This is accomplished by constructing the right arm of the U-tube of small-bore tubing; pressure variations register on this arm while the maximum height in C' remains practically unchanged. The height between the bottom of this small-bore tubing and the top of C should exceed the mercury equivalent of one atmosphere pressure for safe operation a t pressures down to a few millimeters. The minimum operating pressure in millimeters of mercury is equal to or greater than the height of the bottom of C above the upper contact on 8, plus the resistance of one check valve, and the lowest pressure reached by the operating vacuum pump. The maximum operating pressure is equal to or less than the atmospheric pressure, minus the resistance of one check valve, minus the height of the top of C above the lower contact on D. The operating range of pressures for any set of fixed dimensions or level of bulb D is the difference between the maximum and minimum operating pressures and is equal to atmospheric pressure minus the sum of the distance between the top and bottom of C, the distance between the top and bottom contacts on D, the lowest pressure of the operating pump, and twice the resistance of one check valve. For safety of operation, so that mercury from C does not enter the valve chambers, the minimum pressure encountered in millimeters of mercury plus the height of the top of C above the rest level of the mercury in the small-borc tubing should be equal to or greater than atmospheric pressure. For operating a t pressures down to a few millimeters the upper contact on D must be below the bottom of C by a t least the lowest pressure of the operating pump plus the resistance of one check valve. The maximum operating pressure under these conditions is equal to atmospheric pressure minus the height between the lower contact on D and the top of C. Of course, the lower D is the shorter the period of operation but the lower the maximum pressure. For operating a t pressures above t,his maximum D must be raised relative to C. I n this case the minimum pressure a t which the pump will operate will increase by the amount that the upper contact of D is raised above the highest level that it could have for operating down to a few millimeters. The height of D can be made adjustable by connecting it to the glass system by a flexible rubber tubing.
The operation of the pump is made automatic by an electrical device based on the fact that it requires less force to hold an iron core in a solenoid than to lift it into this position against the force of gravity, particularly as the vacuum below the air leak provides an additional resistance to be overcome in lifting the Folenoid core.
PUPlP
IlOVDC
When the mercury starts rising in the right arm of the C-tube, it completes a circuit through the sealed-in contact a t the bottom of bulb D. The current through the solenoid is sufficient to support the weight of the rod, but not sufficient to lift the rod and overcome the vacuum force, so the air leak remains closed. The mercury continues to rise until it makes contact with the lead a t the top of D. A larger current then flows through the solenoid, and the rod is pulled off the air leak. This contact is immediately broken, since air rushes into D and the mercury starts to fall. The air leak does not close, since the current through the lower contact is sufficient to hold the rod in the solenoid. The mercury continues to fall until the lower contact is broken. A t this point no current flows through the solenoid, the rod falls closing the air leak, and the cycle is repeated. In the or:ginal apparatus an ordinary aspirator supplied the vacuum and an air leak of 7-mm. tubing was used. The stop on the air leak was made of 0.6-cm. (0.25-inch) iron rod and weighed 15 grams. The open end of the glass tube was ground flat. A cork was fitted to the lower end of the plunger rod by means of a centrally bored hole. The rod entered this hole partway through the cork. The hole wm enlarged a t the lower end, and a rubber disk cemented to the bottom face. An air cushion was thus provided above the rubber disk. A direct current solenoid of approximately 400 turns was used, which required a current of 2.3 amperes to overcome both forces on the rod and 1.0 ampere to maintain the weight of the rod. Resistances E and P were approximately 50 and 30 ohms. Alternating current could be used by using a n alternating current solenoid; the design could be further modified by using relays to cut down the current through the mercury.
