A Graduated Pvcnometer J
G. ROSS ROBERTSON University of California at Los Angeles, Calif.
IN
Economy may be attained by sacrificing only one pipet, from which two graduated sections are cut. The resulting irregularity in numerical markings detracts from the appearance of the finished Pycnometer, but not from its usefulness. The 45" angles at the top facilitate drainage, in contrast to certain older Sprengeltype pycnometers, but make filling more difficult where a rubber connecting tube is not permissible. I n the latter case a ground-on glass angle tube may be prepared to fit in place of one of the glass ca s. &he pycnometer is suspended in a thermostat with side window or equivalent, and is read with complete immersion of the filled portion. I n calibration it is filled with the usual air-free distilled water approximately to mid-positions on the graduated stems. The true volume, presumably at 20' or 25' C., is then calculated for the contents represented by the two stem readings. It makes no difference whether these stem readings are at the same horizontal level. I n 1-mm. capillary tubes very slight irregularities cause decided differences in position of meniscufi.
AN article concerned primarily with another topic Shedlovsky and Brown (8) incidentally described a new quartz pycnometer featuring two graduated stems. Apparently this item has not been recorded in abstract indexes under "pycnometer" or like caption, and has thus probably escaped deserved attention. The technique of tube graduation needed with the quartz device requires, however, facilities not commonly at hand. The substitution of a standard commercial graduated tube for the quartz tube, as described below, readily permits the construction of a glass pycnometer of the type mentioned. Only ordinary glass-blowing skill is required. As suggested in the accompanying figure, the upper parts of two graduated pipets are incorporated in a U-tube assembly of 20- to 25-ml. capacity. The current Kimble blue-line Kahn serological pipet, total delivery 0.2 ml., graduated in 0.001-ml. divisions spaced about 1 mm. apart, is used. Probably the most substantial construction is effected by sealing the two segments of pipets to the intervening piece of large tubing first in a straight line. The U-bend is next made, and finally the 45' bends. For workers of limited glass-blowing skill it will probably be easier to prepare the body of the pycnometer in two equal sections connected by a U-tube of sma!l caliper, as illustrated in a commercial Sprengel pycnometer now on the market ( 1 ) . Such a modification is not quite so easily filled, however.
When the pycnometer is ready to receive the liquid under investigation, i t is not necessary to fill to the same marks recorded for water. Even should the level on each side be as far as 0.010 ml., or 10 scale divisions, from the position of calibration, and the temperature of the thermostat be 5' from the standard 20" a t which the pipet was graduated, the error is negligible. It is convenient to post a reference value for the volume of the pycnometer filled on both sides to the zero marks. In each subsequent determination one need only subtract the sum of the two new scale readings from the posted constant. CALIBRATION.Pycnometer holds 20.4692 m!. at readings 0.0245 and 0.0270 (sum 0.0515). Posted reference value is 20.4692 0.0515 ,= 20.5207 ml. DETERMINATION. Pycnometer is filled to readings 0.0305 and 0.0322 (sum0.0627). Volumeof contentsis20.5207 - 0.0627 = 20.4580 ml.
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If the pycnometer carries one stem with larger numbers, as in the "economy" model described, no new trouble is encountered. A posted reference value for zero reading may still be computed exactly as in the example given above. Such zero reading has only arithmetical significance, however, since it is physically impossible to fill the device to zero on both sides. If this pycnometer should be produced by a manufacturer, who obviously does his own tube graduation, the two zero marks should be a t the lower ends, with scales reversed. This change makes the arithmetic involved in density determination more straightforward and more quickly understood. The device may then be likened to a precision graduated cylinder to be calibrated for contents. Since the scales are read while the pycnometer is immersed in a thermostatic bath, the blue-line feature is especially desirable. This pycnometer has the conveniences of flushing and filling long known in the Sprengel models. Furthermore the new device shows directly and continuously, without call for adjustment of any kind, when the contained liquid is at the constant temperature of the bath. The inaccuracy involved in bringing a meniscus to a mark, as in models requiring application of filter paper, is not involved. Since there is no fitting of a tapered ground stopper, the errors due to uncertainty of seating of such joints are not present.
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Literature Cited (1) Central Scientific Co., Chicago, Ill., Catalog item 15,775. ( 2 ) Shedlovsky, T., and Brown, A. S., J : Am. Chem. SOC.,56, 1066 (1934).
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