DECEMBERI15, 1937
591
ANALYTICAL EDITION
evidence that it may be unnecessary with heavier and denser precipitates, and this possibility is being investigated. Any ,pycnometric method is obviously dependent upon accurate knowledge of precipitate density. The values of precipitate density here reported differ in some degree from those reported elsewhere. It is proposed, therefore, to study the factors which particularly affect precipitate density, in order to ascertain in which parts of a pycnometric analysis particular care is needed. Such a study should also throw light upon certain fundamental characteristics of gravimetric precipitates. The precision and accuracy obtained in the analyses here reported indicate that the present method of pycnometric analysis should compare favorably with good macrogravimetric procedures. I n cases in which a precipitate acts
badly during drying or ignition, the pycnometric method may offer considerable advantages. It is proposed to extend this study to other conditions, including especially the effects of foreign ions, and to other precipitates, including organic compounds, in order to evaluate better the possibilities of this method of analysis.
Literature Cited (1) Chatelet, L.,Chimie & Industrie, Special No. 199 (April, 1934). (2) Gruntfest, I. J., unpublished results, Brown University, 1937. (3) Johnston, J., and Adams, L. H., J. Am. Chem. Soc., 34, 66684 (1912). (4) Oknin, I. V.,Zavodskaya Lab., 4,420-1 (1935). (5) Popper, B., 2.anal. Chem., 16, 157-71 (1877); 18,14-38 (1879). RECDIVED October 25, 1937.
Microgasometric Analysis with the Dilatometer Determination of the Carbonate Radical BEVERLY L. CLARKE AND H. W. HERMANCE, Bell Telephone Laboratories, New York, N. Y.
G
ASOMETRIC methods are particularly desirable in microanalysis because of the relatively large volume of the measured product, compared to the mass of the sample. Several procedures are available. The gas may be evolved against the pressure of a column of mercury and its volume determined from the measured increase in pressure. Such a manometric method requires that the free space in the apparatus be precisely known. To ensure accuracy of reading, the measured pressure difference must be reasonably great and the reaction must therefore be such as to proceed to completion a t pressures above atmospheric. I n addition to the difficulties impor;ed by such conditions, it is obvious that there is danger of leakage through rubber or ground-glass connections from a system under pressure. Further, the solubility of the gas in any liquid present increases with pressure. Another procedure is to evolve the gas under constant pressure and measure the increase in volume. This is most commonly done by displacing a suitable liquid from a calibrated tube. The necessary apparatus is rather cumbersome for microchemical purposes. When the volume of gas is small, errom due to its solubility in the displaced liquid are likely to be serious. Nernst (8) used the mercury bead dilatometer to measure small volume increases in vapor density determinations by the Victor Meyer method. The dilatometer consists of a graduated hori~ontaltube of small bore, closed by a droplet of mercury which is free to move until the internal and external pres-
FIGURE1. DILATOMETER
sures are equaliaed. This movement, over the graduated scale, indicates the sense and the magnitude of the volume change. Applied to micromethods, such an arrangement possesses particular merit. I t s construction is simple, permitting a reduction of the apparatus capacity to a point commensurate with the small volumes dealt with. Errors attendant upon measurement of the evolved gas by liquid displacement are not present in the dilatometer method. The authors have applied such a dilatometer to the determination of carbon dioxide in samples of corrosion product weighing only a few milligrams. Because of the high solubility of carbon dioxide in most liquids, low results were obtained when acids, either dilute or concentrated, were used to liberate the gas, even when the volume of acid did not exceed 0.2 cc. Release of dissolved carbon dioxide by boiling the acid introduced complications in the construction of the apparatus and was not completely effective in the closed system because of the re-solution of the carbon dioxide by the condensed film on the walls. A method was therefore devised in which the carbon dioxide is liberated by a fused acid salt, potassium pyrosulfate, in which its solubility is negligible. The need for condensation chambers, moisture traps, and special arrangements for introducing the sample is thus eliminated. A simply constructed microretort for carrying out the fusion and the dilatometer constitute the assembly.
Apparatus The apparatus shown in Figure 1 has two essential parts, the gas measuring buret, A, and the retort, B. The buret is constructed of thermometer tubing having a bore of approximately 2 mm. and a capacity of 2 cc., graduated in 0.01-cc. divisions. By means of a reading lens, tenths of a division may be estimated. A funnel-shaped opening at the outer end facilitates cleaning, and also serves t o receive the mercury droplet when the gas volume accidentallyexceeds the capacity of theburet. Aninterchangeable (No. 5 ) ground joint connects the retort with the dilatometer. The droplet of mercury used in the buret should not be too large. Its length should be about two and one-half times the bore of the tube. A tapper, C, made from an electric bell, helps to overcome the resistance opposing the movement of the mercury. The retort unit,, constructed of quartz glass, consists of a heavy-walled, conical bulb of about
598
INDUSTRIAL AND EN(XNEERING CHEMISTRY
FIGURE 2. DETAILSOF RETORTASSEMBLY 1-cc. capacity joined to which are two tubes: e, for the introduction of the sample, and d, communicating with the dilatometer. These tubes make an angle of about 45" with the axis of the retort bulb. Tube d has a capillary bore, 1 to 1.5 mm. in diameter, and is about 10 om. long. Close to the retort, two or three small dilations in the tube bore serve to entrap any sulfur trioxide that may be accidentally given off or any pyrosulfate that may be spattered. The narrow bore of the tube reduces the dead space in the apparatus and its length decreases the heat transmission to the buret. The tube terminates in the male member of the ground joint connecting the retort unit with the dilatometer. Tube e is approximately 4 cm. long and has a bore of about 5 mm. It is provided with a ground-in sto per terminating in a solid process which extends downward an! substantially fills the space within the tube, thus reducing unnecessary volume. A mica disk, about 4 cm. in diameter, perforated a t the center, fits over the retort bulb (B, Figure 2) and protects tubes d and e from excespive heating. It is held in place by wires, looped over the tubes as shown. The apparatus is mounted on a wooden anel which forms the back of a box, as shown in Figure 1. The %ox, the front panel of which is a detachable glass window, constitutes an air thermostat. The heater is a Nichrome wire, J, strung around the interior of the box as indicated. The thermoregulator, F , employs a glass vessel, h, blown with extremely thin walls, containing air as the expansion fluid. This type of regulator was found most satisfactory for the purpose.
Reagent POTASSIUM PYROSULFATE. In a porcelain evaporating dish, heat gently 50 grams of c. P. potassium pyrosulfate until it no longer effervesces. Then slowly raise the temperature until sulfur trioxide fumes just commence to be evolved. Discontinue the heating, and pour the molten pyrosulfate out onto a porcelain plate, previously heated in an oven. Break up the solidified cake immediately and grind to about 10-mesh in a porcelain mortar. Preserve in a tightly stoppered bottle.
Procedure Adjust the thermoregulator to maintain the temperature, in the closed box, 2" or 3" above that of the room. Thoroughly clean the retort unit with hot chromic-sulfuric acid mixture, rinse with water and acetone, and dry in an oven. Introduce sufficient pynosulfate reagent to fill about three-fourths of the retort bulb, and heat gently until it has melted and the air bubbles have been completely expelled. After cooling, attach this part of the apparatus to the microburet, taking care that the p d joint is lubricated with a minimum quantity of a heavyodied grease. With the cap removed from the sample introduction tube, gently force the mercury bead to the initial point on the buret scale, this operation being best accomplished by attaching a rubber tube to the open end of the buret and carefully blowing into it. Replace the cap, also lubricated, on the sample tube; re lace the front panel of the box and allow the apparatus to stanfundisturbed for 5 minutes to bring about temperature equilibrium. Finally, bring the mercury bead to its position of rest by means of the tapper and take the initial reading on the scale as well as the temperature of the box. Before the actual determination, make a trial of the a paratus a8 a check on the tightness of the joints, the absence orvolatile impurities in the pyrosulfate, and the ability of the operator to carry out the fusion without softening the wall of the retort. Gently heat the retort bulb until the pyrosulfate again fwes, taking care in this operation not to concentrate the flame at one
VOL. 9, NO. 12
spot on the glass, but rather to use an even rotary motion, heating the upper part of the bulb as well as the lower. The temperature should not reach a oint beyond that at which a faint red glow may just be detectexin a darkened room. Failure to,observe these precautions may result in the softening of the bulb with alteration of its shape and a change in the volume of the apparatus. Maintain the pyrosulfate in a fused condition for 5 minutes, then close the box and allow the apparatus to cool to the temperature of the thermostat. Now take readings of the position of the mercury bead a t 1-minute intervals until three successive readings agree. If the details of the procedure have been carefully followed out and the ap aratus was correctly constructed, the change in the volume ofthe gas in the apparatus should not exceed 0.005 cc. When it has been established that the apparatus is working properly, weigh from 5 to 50 mg. of the finely ground sample, depending on the amount of carbon dioxide expected, and introduce into the retort bulb. This operation is most conveniently carried out by means of a small metal spoon with a detachable handle long enough to permit its introduction into the retort (A, Figure 2). Weigh the spoon, together with a small capsule containing the previously dried and finely ground sample, attach the handle and scoop up a portion of the sample, the excess material clinging to the bottom and edges of the spoon being jarred off by tapping against the edge of the capsule. Then transfer contents of the spoon to the surface of the pyrosulfate in the retort by rotating the handle. Detach the spoon, return to the balance pan, and obtain the sample taken as the loss in weight. Adjust the mercury bead to the initial position in the buret and place the cap tightly on the sample introduction tube. Make three readings of the position of the bead at 1-minute intervals, using the vibrator each time. If there is no change, commence the heating of the retort, slowly bringing the pyrosulfate to a completely fused condition and maintaining it thus until there is no further evolution of gas and all the bubbles have escaped. (Often the heat of the hands in manipulating the apparatus preparatory to making the determination will raise the temperature of the air within by 3" or 4". Hence i t is not safe to start immediately without observing the precaution mentioned.) If particles of sample remain clinging to the upper wall of the retort chamber and have not Come into contact with the pyrosulfate, this can usually be remedied by gently shaking the apparatus. Replace the front panel and allow the apparatus to return to the temperature of the thermostat. Read the position of the bead a t 1-minute intervals until 3 consecutive readings agree.
Calculation The volume of carbon dioxide evolved is given by the difference in the positions of the bead. 1 mg. - of GOz at 0" C. and 760 mm. ==0.509 cc.
760 t + 273 7 X 273 where VI - Ti2is the observed volume increase; P,the barometric
):;A.'(
Hence, weight of COn =
X
pressure in mm.; and t, the temperature of the thermostat in O C.
TABLE I. DETERMINATION OF CARBON DIOXIDEIN CERUSSIT~ Equivalent Sample Volume of Taken COz at 25' C.
Me. n 4
n n
5.70
... ...
... o:.iie
0.622 0.472 2.275 1.118 Control experiments.
6.85 5.20 25.15 12.23 4
cc.
Buret Reading Initial Final
cc.
0.075 0.010 0.0130 0.090
0.150 0.048 0.014 0.172 0.260
cc.
Volume of COB at 25' C. Found
0.072 0.010 0.0135 0.091 0.661 0.668 0.491 2.450 1.370
cc.
Error
cc.
-0.003 0.000 +0.0005
o:&i
0.620 0.477 2.278 1.110
+0.001
-0,005 -0.002
+0.005
+0.003 -0.008
Obviously the method has applications to many other determinations. C, Figure 2, shows a retort assembly designed for liquid reagents. The small cup at the right is a weighing vessel. Booth and Jones (1) list sixteen determinations t o be done gasometrically, stating that their list is incomplete.
Literature Cited (1) Booth and Jones, IND ENG.CHUM., Anal. Ed., 2, 237 (1930). (2) Nernst, 2.Elektrochem., 9, 622 (1903). R ~ C E I V EJuly D 19, 1937. Presented before the Microchemioal Section a t the 92nd Meeting of the Amerioan Chemical Society, Pittsburgh, Pa., September 7 t o 11, 1936.
