Clyde P. Brocketl
University of Toronto Toronto, Canada
Constant Flow in Titration
The ingenious simplicity in a LIConstantflow Device for Use in Titrimetric Analysis," by van Swaay and Lolley,' has inspired alternatives in the arrangement, for the sake of even greater constancy and range of flow rate. The first contribution is a replacement of the overflow delivery which often fails to maintain constancy within really fine limits. This is simply a down-poinb ing vacuum probe, adjustable in height, which sucks water out a t the level of its tip as fast as it enters the container, thus keeping the level exceedingly constant. I n practice, water enters at a controllable rate; and the probe is connected to a vacuum aspirator. The rate at which the water is admitted is adjusted to somewhat less than the rate at which the vacuum probe can remove it. When the two are correctly balanced, the probe will remove water mixed with air, which appears as a succession of small bubbles in the water stream, each one representing a correction to the water levelhundreds of them per second. The adjustable height of the probe allows a wide variation in the depth of the water surrounding the sleeving, as may be seen in Figure 1. The jacket container now has no side arm. The second contribution, a variant of the capillary restriction,' virtually eliminates all clogging difficulties so likely to afflict fine-bore discharges. This introduces resistance to flow in the form of a column of glass microbeads, supported by glass wool just above an on-off burette valve. Since a column of glass beads, as shown, offers a myriad of flow exits to the titrant, a particle large enough to seriously obstruct a capillary tube VAN
SWAAY,M.,
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LOLLEY, R. F., J. CHEM.EDUC., 42,381
(1965).
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lodges harmlessly among the micro-beads and causes no amreciable alteration in the flow rate. Choice of bead &e and bed depth permits additional range in the rate of ... flnw~ ....... Good uniiortuity oi bend size and perfection i t 1 yhc ricity nrcc+rntid if day-to&y rcpnalu~~ihili~y i;dr..iwd.
/ lournol of Chemical Education
Figure
1
Burette assembly
since otherwise a slow process of bead classification is likely to vary the flow rate progressively in time. Very good constancy has been obtained at a fixed temperature by using beads such as "Ballotini Solid Glass Balls, clear lead glass," which are available in diameters ranging from 0.12 to 0.40 mm (Grade KO. 13 to Grade No. 9, respecti~ely).~ The data shown in Figure 2 illustrate how constant the rate of flow of water from this burette can he. Indeed, t,he constancy is probably even better than indicated, since measurements of flow were timed by hand-operated stopwatch rather than automatically. This tabulation constitutes a time-base calibration. Only distilled water mas measured; tap water was avoided because of the dissolved air it would release in 2 Obtaiuahle from English Glass Cornpan); Limited, Scudamore Rd., K.P.E., Leirester, England.
0 Figure 2.
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5
10
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15 20 Volume delivered l m l l
Constancy of flow rote
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the length of its passage through the burette. Non-aqueous titrations may be performed with this burette using glacial acetic acid or ethylene diamine, providing the collapsible sleeve is butyl rubber, polyethylene, chlorinated polyethylene or polypropylene. But only polyethylene will be satisfactory with benzene and then only if it is no warmer than room temperature. Most rubber used a3 sleeve material, while reasonably satisfactory for many inorganic titrants, cannot be expected to withstand either glacial acetic acid or benzene. I n more general terms, the user of this burette must exercise reasonable chemical judgment by considering the extent to which a given titrant may react chemically with the two materials concerned, viz., glass and the plastic membrane. I n this context, strong alkaline solutions should not be used because of the glass. Indeed, even wit,h distilled water alone, a re-calibration should be performed periodically if quantitative results are desired from day to day, and flow rates of one ml/ min or less are required. The success of the sleeve in maintaining a constant liquid level within it is dependent upon easy collapsibility. Hence thin and non-rigid material is essential, whatever its composition may be. Since this uncomplicated gravity burette is devoid of electro-mechanical upkeep, it serves as an inexpensive and instrnctive aid in obtaining potentiometric titration curves for student viewing on a millivolt recorder. To the extent that one aspect of a potentiometric titration curve may he to observe whether it is symmetrical in the area of maximum interest, is it better served by a flow of titrant that is highly consta.nt. This is easily achieved with the gravity burette herein described.
Volume 43, Number 4, April 1966
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