A Modified Buret for Microanalysis of Gases DAVID C. GRAHAME University of California, Berkeley, Calif.
I
N USING the method for the microanalysis of gases de-
.
scribed by Blacet and Leighton ( I ) , the author experienced considerable difficulty because of the tendency of the mercury levels to drift up or down the capillary buret during the time required to take the readings. A method of avoiding this difficulty has been described by Blacet, MacDonald, and Leighton ( 2 ) ,but an alternative method, used in this laboratory, has proved to have valuable additional advantages. The drift in the mercury levels is evidently due to the fact that a small displacement of the upper level downward, for example, causes a marked increase in the pressure of the gas and of the pressure of the mercury in the rubber pouch a t the base of the buret. This increased pressure makes the gas contract and the rubber pouch distend, both of which cause the upper mercury level to move downvard again. If the rubber is not sufficiently stiff, the sample of gas may even be lost into the base of the buret in this manner.
lax errors. Two lines, A and B in Figure 1, are marked on the horizontal portion of the capillary, one close to the bend, the other close to the end of the water jacket. I t is convenient to choose positions for these marks such that the volume of the capillary between the marks and the zero of the buret is an integral number of the units of volume employed. This may be readily accomplished if the apparatus is calibrated after assembly but before the horizontal arm of the water jacket is sealed on at C-C’. Readings are taken by drawing the gas into the buret in the manner described by Blacet and Leighton and adjusting the upper level to coincide with A or B . The lower mercury level is read from the graduations on the buret. Water from a large vessel is circulated through the water jacket. Since its temperature never deviates largely from room temperature, no appreciable error arises from the fact that a small fraction of the gas extends beyond the end of the water jacket when B i s used. This arrangement has been adopted t o relieve the operator from eyestrain during the adjustments, For measuring very small volumes of gas (less than about 20 cu. mm.) A must, of course, be used. The temperature of the water in the water jacket is conveniently read from the thermometer arranged as in Figure 1. I n a buret of this type the mercury shows no tendency whatsoever to drift in the manner described. If the mercury in the movable cup (shown by dotted lines) is always brought to a fixed level when a reading is to be made, the pressure on the sample of gas will be the same in consecutive readings unless large barometric changes have occurred. The time required for most analyses is short enough to permit one to ignore entirely all pressure corrections, which is a considerable simplification over the procedure ordinarily followed. A marked advantage of this type of buret is that it can be cleaned without being dismantled. The “scum” carried into the buret is carried by the upper mercury thread, and since this is not ordinarily drawn into the vertical portion of the capillary tube, only the horizontal portion requires cleaning. To clean the buret the mercury cup is removed and a beaker containing a cleaning solution of potassium dichromate and concentrated sulfuric acid is put in its place. Before the tip of the buret is submerged in the solution, however, the mercury in the buret is drawn back until the level of the thread is about 10 cm. below the bend, D. The cleaning solution is then drawn into the buret up to D but no farther. After several hours the acid is expelled and distilled water is drawn up in its place. The buret is dried after several such rinsings by evacuating the capillary, an operation which must be done with some care to avoid drawing mercury into the horizontal tube. Minor advantages of the buret here described are that only one reading is taken from the calibration curve for each measurement and that the volume may be recorded directly without taking differences.
W
l!
FIGURE 1. DIAGRAM OF BURET
This difficulty may be readily overcome by maintaining the upper mercury level in a horizontal rather than a vertical capillary tube. The same principle has been employed in a different manner in an apparatus described by Swearingen, Gerbes, and Ellis ( 3 ) .
Summary A modification of the Blacet-Leighton gas microburet is described. The mercury threads are easily adjusted and do not drift from their set positions. Corrections for pressure are not ordinarily required. The buret may be cleaned without dismantling.
A simple method which does not occupy an undue amount of space is shown in Figure 1, which is similar t o the buret described by Blacet and Leighton except that the water jacket now follows the capillary tube up and over the bend at the top. The water jacket is made from a Pyrex buret sealed at the t w o ends with picene wax (not with de Khotinsky cement, since this slowly
Literature Cited
disintegrates under water). The capillary tube which runs through the buret is placed near the front surface of the buret, Tather than centered, so that the lower mercury level can be sead from the buret graduations without danger of serious paral-
(1) Blacet and Leighton, IND. ENQ.CHEM., Anal Ed., 3, 266 (1931). (2) Blacet, MacDonald, and Leighton, Ibid., 5, 272 (1933). (3) Swearingen, Gerbes, and Ellis, Ibid., 5, 369 (1933).
351
