Pipet for Titrating Dark-Colored Liquids in Two Phases MILDRED M. HICKS-BRUUN
AND
LAWRENCE W. CLAFFEY, Sun Oil Company Research Laboratory, Norwood, Pa.
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THE petroleum and other industries it is often necessary to determine t h e free acidity as well as saponification numbers of darkcolored substances. As m a n y of t h e s e s u b stances have comparatively high melting points and are very little soluble in alcohol, in recent years it has become t h e p r a c t i c e to use a b e n z ene- alcohol solution, both for titrations with aqueous alkali (1, 4 , and as a saponification medium with caustic potash and subs e q u e n t titration with a q u e o u s acid ( 5 ) . In either case the addition of an aqueous solution t o t 11 e benzene-alcohol inixt u r e precipitates a second laver: the dnrkc o l o r e d “substance remains in the upper phase (the benzene-alcohol layer), while the acid.ic constituents and excess alkali remain in the aqueous alcoholic layer. Coburn (4) has pointed out that sodium chloride may be added to render the two-phase separation sharper. Methods in which the benzene-alcohol solution is used are often helpful in the determination of the end point, and moreL
over a top benzene layer serves to protect caustic alkali solutions from atmospheric carbon dioxide during titration. However, the top layer is often dense and black and refuses to remain intact during the necessary swirling of the flask in titration. Furthermore, after the two layers are broken it not only requires several minutes for them to separate again, but also it is difficult to determine the exact end point of a yellowish or reddish solution beneath a heavy black layer. To meet this need a pipet was devised in this laboratory which materially aided the operator’s ability to determine the end point. The pipet was made a convenient length to remain in the titration flask throughout the titration, and may be of clear glass or with a white background. From time to time, the stopcock was opened and a convenient depth of the liquid was sucked into its stem by means of the rubber bulb, after which the stopcock was closed. If a portion of the benzene layer entered the pipet, it rose to the top of the liquid. The color of the indicator could then be closely observed and the liquid returned to the flask by opening the stopcock and applying pressure to the bulb. In this manner the progress of the titration could be noted until a satisfactory end point had been reached. Although there are several titration flasks with forms of sealed-in tubes, such as those of Bezzenberger (3) and Baader (2), the pipet here described was found not only to be simpler and more effective, but also better adapted for the technic of saponification numbers. Many uncertainties in the end points of dark-colored solutions have been avoided by its use.
Literature Cited (1) Albert, K., Albertschrift, No. 15, p. 55. (2) Baader, Chem.-Ztg., 50, 891 (1926). (3) Bezzenberger, F. K., J. Am. Chem. SOC.,39, 1321 (1917). (4) Coburn, H. H., IND.ENQ.CEEM.,Anal. Ed., 2, 181 (1930). (5) Schreiber, H., J. Am. Chem. SOC.,29, 74-5 (1907).
RECEIVED February 12, 1936.
Errors in Microweighing Due to the Use of Lead Shot in Tares ROGER J. WILLIAMS, Oregon State College, Corvallis, Ore.
T
HE: Kuhlman microbalance which is most highly recom-
mended by Pregl and his followers is provided by the manufacturer, in accordance with Pregl’s recommendations, with a numbered series of small tare bottles of various sizes, and about 50 grams of fine lead shot with which to prepare tares for the different vessels in which material may be weighed. The writer had occasion to weigh small samples of material in relatively large glass weighing bottles (weighing about 6 grams) and, although the balance seemed to be working perfectly and the temperature fluctuations were not excessive, constant values could not be obtained. In one case a weighing differed from that of the previous day by nearly 50y. The cause of the excessive variations was traced to the use of the lead shot which had been provided. As soon as the lead was discarded and broken glass substituted for it in the tares, the difficulty promptly disppeared and weighings became constant. Calculations show that if 5 grams (0.37 cc.) of lead are used to balance a glass vessel (this quantity is not excessive since the balances have a capacity of 20 grams), the difference in the volumes of air displaced by the lead and the glass (taking
2.7 as the specific gravity of glass) is about 1.5 CG. This air, if dry a t standard pressure and 25” C., would weigh 1.78 mg. Under actual conditions, however, the weight of this excess air displaced by the glass vessel will vary 6y per degree of temperature variation, 23y per cm. of barometric variation, and about 21y for the difference between dry and saturated air, Users of microbalances have always attempted to work a t as constant a temperature as possible. It is evident, however, that the errors due to temperature variation are greatly magnified by the use of lead shot. More serious is the barometric variation because changes of a centimeter or more in the barometric reading, which readily occur, may introduce a serious error if lead shot is used. The error due to humidity variation may easily be considerable under some conditions. If one insists on using lead shot, he can record temperatures, barometric readings, and humidities and make suitable corrections, but it is vastly simpler to render all these corrections entirely unnecessary by discarding the lead shot and using tares made of the same material as the vessel to be weighed. It is clear to the writer that manufacturers should supply somewhat larger tare bottles and glass beads (or possibly 229
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INDUSTRIAL AND ENGINEERING CHEMISTRY
aluminum shot) to use in them. I n case heavier objects are being weighed, care should be taken to have the vessel and the tare made of the same material, since t,he difference even between quartz and glass is sufficient to introduce appreciable errors in case the vessels are relatively heavy. Though the writer has had no experience in this connection, it appears that the weighing of the absorption tubes used in carbon and hy-
VOL. 8, NO. 3
drogen determinations must be subject to especially large variations when lead shot are used in the tares, because of the relatively heavy objects weighed. The use of weights instead of tares leads to the same needless errors unless the weights have the same density as the objects to be weighed. R E C E I V April ~ D 3, 1938.
New Apparatus for Analytical Laboratories Crucible Support for Desiccators, Graduated Cylinder, and Flask Cover F. E. TUTTLE University of Kentucky, Lexington, Ky.
A
S A RULE desiccators are supplied without plate or other device for support of crucibles and, on account of the inflexibility and expense of porcelain plate supports, the time and ingenuity of the user are often consumed in the fabrication of makeshift substitutes of glass, pipestem, etc. The crucible support, Figure 1, described in this article is designed for use in the regular 15-om. (6-inch) Scheibler desiccator and will hold six No. 0 or 00 porcelain crucibles with covers or the same number of platinum crucibles of corresponding sizes.
J
FIGURE1
The support is made of pure nickel wire with all intersections securely brazed. Long and continuous use of the support justifies the following claims: By adjusting the legs it is readily adapted to variations in the shapes of desiccators, is held firmly in position during any legitimate use of the apparatus, holds the crucibles securely, may be deformed slightly to accommodate crucibles of different sizes, allows unrestricted circulation of air, and is kept bright with a minimum expenditure of time and the use of a little fine steel wool. GRADUATED CYLINDER. I n those chemical operations where one has occasion to measure rapidly volumes of liquid reagents, solvents, etc., there is great need of a handy measuring cylinder of moderate capacity, one that is sufficiently accurate, not easily upset, sturdy in construction, and not readily broken. The cylinder illustrated in Figure 2 has all these characteristics and has been used in the different laboratories with great satisfaction, particularly in quantitative analytical work. Whereas the tall, narrow, 100-ml. cylinder in regular use a t present is characterized by great instability, generally resulting in breakage when tipped over unless protected by
some sort of shock absorber, this I, qOn,m 4 short, wide cylinder stands more firmly on the table, is more readily filled and emptied, has two lips for convenient pouring, has never b r o k e n when t i p p e d over, is graduated both up and down, and allows yolumes to be measured as accurately as in any other cylinder of the same diameter. It is not recommended for work that should be done only by an accurately calibrated pipet, buret, or certified tall narrow cylinder, but when made according to the d i m e n s i o n s g i v e n it stands s u p r e m e as a general-purpose , r----------6smM,-------~ mea s u r i n g cylinder of 100-ml. capacity. FLASK C O V E R .Many times, FIGURE2 p a r t i c u l a r l y when c o r r o s i v e liquids are being boiled in flasks, it is desirable to resort to some device for control of spattering, evaporation, prevention of free access of air, dust, etc. I n the past small funnels, inverted porcelain crucible covers, and glass bulbs have been employed for this purpose and generally with unpleasant results, because the center of gravity of the miscellaneo covers is above the rim of the vessel being covered. The cover as illustrated (Figure 3) is designed upon a new
FIGURE3
