A nalvtical
Edition
J
OCTOBER 15, 1931
Volume 3
Number 4
Improved Technic for Microgravimetric Analyses' Paul L. Kirk and Roderick Craig DIVISION OF
F ALL t h e v a r i o u s
BIOCHEMISTRY, UXIVERSITY OF CALIFORNIA MEDICAL SCHOOL, BERKELEY, CALIH
An apparatus is described for microgravimetric was found to be a very suittypes of microchemianalyses, such as sulfate, halide, and phosphate deable lubricant since it is infic a l a n a l y s e s , those terminations, on a very small sample. The technic nitely miscible with water and for use of the apparatus is described. The accuracy is a l c o h o l , b u t dissolves so basedondeterminationof the weight of a precipitate seem to found to be chiefly limited by the possible accuracy slowly that the ground glass offer the a r e a t e s t technical of weighing rather than connections easilv retain it - and method of precipitation difficultiei, This is largely by the apparatus itself. almost indefiniteli when the owing to the necessity of parts are pressed together. transferring the precipitate quantitatively from a reaction A filter pad of asbestos is necessary. This must be of a vessel to a filter, an operation in which small losses are very apt very good quality and in a proper state of division. Asbestos to occur. Such losses are of little importance in macroanalyses, prepared as for use in the micro calcium method of Kirk and but on a micro scale they may represent a large percentage error. Schmidt (2) is satisfactory. Here a good grade of asbestos Obviously there is need for a type of apparatus which will washed in acid and ignited was thoroughly dried and ground combine the reaction vessel with the filter in such a manner either in a ball mill, or, more laboriously, by a mortar and that the transfer of precipitate is either eliminated or rendered pestle, until it was quite fine. If the ground product is suseasy. It is the purpose of the authors to describe in this paper pended in water, the fineness will be found easy to grade by a practical apparatus of this nature and a technic for its use the differential rate of settling. in such determinations as those of sulfate, halide, phosphate, As used in an ordinary gravimetric analysis, such as sulfate etc., all of which are used constantly in both chemical and or halide, the procedure is as follows: The disconnected filter biological work, tube B is placed in a short stopper in a suction flask. Very mild suction may be produced by use of a pinchcock on the Description of Apparatus rubber tube to the vacuum line. This may be opened slightly Figure 1 shows diagrammatically the design of the ap- from time to time. I n order to make the filter mat properly, paratus. A , the reaction chamber, is made from a portion of the flask of asbestos suspension is shaken and a little of it a test tube attached at the bottom to a heavier tapered tube removed with a pipet having a large orifice, and is run into of smaller diameter, suitable for grinding. In A is a rod, D, the filter tube. This gives a thin layer of comparatively hooked a t the top and ground into the tapering end of A . coarse asbestos which is sucked down firmly. After a few The outside of this tapering end is, in turn, ground into the moments, the coarse asbestos in the flask has settled and top of filter tube, B. About a centimeter from the bottom of some of the fine asbestos is removed with the pipet and run on this ground connection is a platinum plate made from foil to the filter until a thin but quite tight filter pad is obtained. about 0.005 inch (0.127 mm.) in thickness. This plate has After the usual washing with water several times, it is washed several small punched holes of 0.5 mm. or less. It fits with a small amount of alcohol delivered from a small pipet, tightly inside the tube, and after it is in place it is fused to and finally in the same way with a little ether. The tube is the inside of the glass tube by use of a fine blast flame. The withdrawn from the stopper, wiped with a soft cloth, and apparatus can be constructed by a glassblower of moderate placed in a copper block to reach equilibrium with respect to skill without any serious difficulty. A second rod, E , should moisture and temperature. It is weighed on a microbe constructed with a hook on the end for removal of the balance, and then replaced on the suction flask and moistened ground rod, D,from its seat. The hooks on both rods must with a little water which is sucked slowly through. If the be of sufficient strength to withstand a moderate pull, and for filter pad has been properly made and washed, this water will this reason, the size of rod in the hook should be drawn down filter through absolutely clear. If it carries through a little little if any from the size of the remainder of the rod. asbestos, it should be thoroughly washed as before, redried, and reweighed, This will occasionally save repeating a Experimental Procedure determination entirely, since loss of any asbestos when filterIn using this type of filter, a few points of technic are impor- ing practically invalidates the result. tant. Dry ground-glass joints nearly always leak a little, and In the meantime, the reaction chamber is prepared for the the use of grease in this apparatus is not desirable. Glycerol precipitation. The plug D is moistened with glycerol and pressed firmly into its seat in the end of A . I n working with 1 Received Marrh 16, 1931
0
ANALYTICAL EDITION
346
different sets of apparatus, a curious fact has come to light. With some reaction chambers, it is a simple matter to clean them well enough so that precipitate does not cling to the malls. With others, this is exceptionally difficult. In attempting to avoid having precipitate cling to the walls, the use of acetone in the solution was found to be remarkably efficient. In the analysis of sulfates, for example, the precipitation is carried out as follows: About 0.5 cc. of pure acetone is used to rinse down the sides of the vessel and the rod D which has alreadv been inserted. This remains in the vessel. Immediately, b little water and a few drops of dilute hydrochloric acid and finally the sample are added. The sample may be added from a weight buret, a carefully calibrated microburet, or a micropipet. The chamber A is then dropped into a cutoff test tube which is of the proper length to just contain it, and with an inside diameter a bit larger than the chamber A , and which is immersed in a boiling water bath. When h e a t e d sufficiently a few drops of barium chloride solution are added slowly with shaking or twirling between the hands. The tube is replaced in the water bath and heated with occas i o n a l t w i r l i n g u n t i l the barium sulfat,e is completely coagulated. In the presence of acetone, the coagulation of barium sulfate is very complete and rapid, and clinging t o the walls is e l i m i n a t e d . With some sets of apparatus it is apparently not necessary to use acetone. The reason for this is still obscure. Figure 1-Diagram of Apparatus In precipitating h a l i d e s , (Actual She) less trouble is experienced from clinging to the walls and usually no difficulty is found in making the precipitation directly in the usual way. When the tube A is heated sufficiently i t is removed and the ground end is moistened with glycerol and inserted into the end of B on the suction flask. The plug is pulled and mild suction applied. The hook E which may have some precipitate adhering, is carefully washed off with the wash bottle, and the hook of D is hung over the edge of A. The precipitate is of small bulk, and consequently a few washings are sufficient. 'Il-ithout allowing the sides of A to dry and deposit any dry precipitate, a small pipet full of alcohol is used to rinse clown the sides. This immediately stops the climbing of the precipitate and rinses it quantitatively into the filter from the walls, providing only that the walls have previously been well cleaned with chromic acid, and that the precipitate has been properly coagulated and has not been allowed to dry on the malls. When the last of the precipitate has been removed from the walls, A is removed from B and laid aside. The material on the filter and on the ground portion of B is washed further with water and alcohol, and finally with a little ether, to remove all traces of glycerol. The filter is dried as before on a copper block, equilibrated, and weighed. Originally, it was feared that a trace of glycerol niight remain after the final washing, and to assure removal of this, the tube was heated in a regenerating block as described by Pregl
.-D
E
C
Vol. 3, No. 4
(3, p. 76) to a temperature of about 200' C. This w a carried out on both the empty tube and that containing the precipitate. Later this procedure was abandoned since it did not increase the accuracy of the method and required considerably more time. The filter apparatus has been tested with a number of points in view. Leakage around the ground connections would be serious, but is absolutely eliminated by the use of glycerol. The largest chance for error would seem to be in removal of all the precipitate from the sides of the reaction chamber to the filter. That part of the precipitate which creeps up the sides is very simply washed down by alcohol. The use of acetone in the mixture prevents quite effectively any tight adherence to the walls. No other agent except acetone was found for this purpose, although alcohol and glycerol were both tried. However, if the particular type of precipitate cannot be removed by alcohol, the entire apparatus may be weighed without exceeding the capacity of the microbalance. I n either case, policing, with its technical difficulties, is avoided. Results
Analyses were run on approximately 0.01 N sulfuric acid solution and 0.01 M potassium chloride solution, and several attempts were made to analyze for minute amounts of sodium according to the method of Barber and Kolthoff(1). The results of some typical analyses of sulfuric acid and potassium chloride are given in Table 1. Table I-Analyses of Sulfuric A d d a n d P o t a s s i u m Chloride W T OF
COXCEN-
ORIGINAL W T .OF MATERIAL OF OF PRECIPI- FOUND SAMPLE SAMPLE TATE LN SAMPLE ERROR REMARKS CC. Mg. dfg. % SULFURIC ACID^ 2 5 $0 95 Weighed tubes on ana 5 5 95 00101 N lytical balance 2 48 $0 1 Weighed tubes on ana00101 N 5 6 90 lytical balance 2 2.375 00101 N 0.998 +o 7 -0 53 1.100 0 442 OO1OON 1 0 9588 0 24 Used acetone 0 0098 AT 2 2 282 0 4832 +O 33 Used acetone 0 0098 N 1 1 150 0 7227 $0 2 Used acetone 15 1 720 0,0098 N TRATION
AMT.
-
POTASSIUM
0.0100 M 0,0100 M
1
144
0.749
CHLORIDE^ +0.4
1.474 -1.2 2 2.835 -1 7 1 1.41 0.733 0,0100 M 0 1 1 435 0.746 0,0100 M b Precipitate, silver chloride. a Precipitate, barium sulfate.
The weighings were carried out in all cases, except the two noted on Table I, on a microbalance with a sensitivity of about 0.005 mg. The weights of the precipitates amounted in most cases to a little over 1 mg. It is seen immediately that a considerable part of the errors observed fell within the error of the balance. The filter tubes average about 2 grams in weight, and in all cases they were weighed with use of a glass tare of practically the same weight. Consequently, it is believed that the limiting factors in the use of this apparatus are, first, the sensitivity of the balance used, and second, small variations due to occlusion of precipitant, solubility of precipitate, and so forth, and are not to any appreciable extent due to inherent difficulties with the apparatus itself. As stated above, various attempts to use this apparatus for analysis of sodium according to the method of Barber and Kolthoff were made, Although their method is essentially micro in its original form, it is not necessary to use a microbalance. When samples containing about 2 mg. of sodium were used exactly as they describe, excellent results were obtained, but on reducing the sample to between 0.05 and 0.06 mg. of sodium, using this apparatus and the microbalance, erratic and usually low results were obtained. Deviations of from 5 to 25 per cent were common, and no way of adapting the method was found.
1
October 15, 1931
347
INDUSTRIAL A N D ENGINEERING CHEMISTRY
The advantages of this apparatus over that of Pregl are several. This one can be used for any method in which he uses the glass filter (3, p. 136) as in halogen and phosphate determinations. It can also be used in place of the microNeubauer crucible in the determination of sulfate. An accuracy as great as Pregl reports for sulfate determinations is not claimed for this method, for his method of ignition and rewashing is not easily adapted to this apparatus. However, i t is felt that an accuracy of *0.1 per cent is rarely necessary on such small samples as are here used, and the saving in time and labor is very considerable over his method. I n the determination of sulfur by combustion as described by Pregl (3, p. 152), difficulty due to a large volume might be encountered. Since it is only necessary to weight the filter tube B, it is possible to make the reaction chamber A as large as desired so that this difficulty is readily overcome. No loss of precipitated barium sulfate through the filter is necessary if it is properly coagulated by heating and the asbestos pad is made from fine enough fiber. This apparatus is decidedly less costly than the micro-Neubauer crucibles and much more simply
Determination
Of
manipulated. A much smaller amount of glass is weighed than with Pregl's glass filter tubes, and here again the manipulation is simpler, Likewise, the speed of filtration is decidedly greater, since there are several holes in the platinum disk, each one of about the same diameter as the single hole in the glass filter tube. The amount of asbestos is also smaller. Many other uses in gravimetric analysis may suggest themselves. It should, for example, be entirely feasible to carry out gravimetric calcium analyses by this procedure, if the suggestion of Willard and Boldyreff (4) of igniting the oxalate to carbonate at 460' C. is followed. In biological work, it is frequently desirable to use gravimetric methods for small amounts of organic materials, and here the determination might be simplified by use of this apparatus. Literature Cited (1) Barber, H H I and Kolthoff, I. M., J A m Chem. Soc , 60, 1626 (1928) (2) Kirk, P L , and Schmidt, C.L. A , J . Bzol. Chem , 83, 311 (1929). (3) Pregl, F., "Quantitative Organische Mikroanalyse," 3rd ed , Springer , 1930 (4) Willard, H H I and Boldyreff, A W , J Am Chem. Soc , 6 2 , 1 8 8 8 (1920)
Smoking Point
Fats'
John M. McCoy MEATINSPEC~ION LABORATORY, BUREAUOF ANIMAL INDUSTRY, WASHINGTON, D C.
HE smoking point of a fat is the lowest temperature at
T
which sufficient decomposition takes place to produce visible smoke. As is well known, all fats decompose when heated to sufficiently high temperatures, with the formation of a variety of volatile substances, the most characteristic and familiar of which is acrolein. When observed under appropriate conditions, these volatile decomposition products appear as visible smoke. The significance of the smoking point has been recognized for some time. Blunt and Feeney (1) studied the factors affecting the smoking point, its bearing on the utilization of fats, and described a method for its determination. Although this method is suitable for the comparison of different fats, at the same time it does not take into account all of the factors which may cause variation in the smoking point observed, and on that account is not satisfactory for use by various observers working in different l a b o r a t o r i e s and at different times. A more exactly standardized method is therefore required. I n the work of this laboratory, it has been found desirable to determine the smoking point on a number of samples of fats submitted for examination. As soon as the work was undertaken, the need of a standard method became evident. Accordingly, the method described was developed. The apparatus used, pictured in Figure 1, is simple, the time required for making determinations is short, and the degree of accuracy is sufficient for the purpose. The method consists in heating the fat under uniform c o n d i t i o n s until smoke amears, and -noting the temperature. In preliminary experiments it was found that the smoking point is affected by the rate of heating and conditions of observation. Possibly the area of the surface exposed to air is also a factor. In order to attain uniform results, it is necessary, therefore, to heat a definite quantity of fat in a container of standard size
and dimensions and observe the smoke under standard conditions. Electric heaters of the type commonly used for Kjeldahl digestions (Gilmer heaters) were selected for heating. A rheostat is connected in series with the heaters so that the rate of heating can be reduced as the anticipated smoking point is approached Two heaters are connected in series with the rheostat. The top of one heater is covered with an asbestos plate with a 2'/4-inch (5.71-em.) circular opening in the center. the other bya similar plate with a 7/8-incli(2.22-em.) opening.
Figure 1-View
of Apparatus
I
1 Received
June 22,1931.
The containers used are standard 100-cc. round-bottom flasks, G om. in diameter with 6-em. neck, and made of Pyrex glass. To make the smoke more readily distinguishable, a black screen 11 X 14 inches (27.94 x 35.56 cm.) in size and made of dull black cardboard, such as is used to protect photographic paper from light, is placed directly behind the heaters.
