Syringe Buret Adaptor D. D. Siemer', S. D. Reeder, and M. A. Wade Westinghouse ldaho Nuclear Company, Inc., P. 0.Box 4000, ldaho Falls, ID 83403 The widespread availability of sensitive (l-mg or better readability) top-loading electronic balances in modern instructional laboratories makes the use of weight titrations in lieu of classical volumetric titration procedures especially attractive today. A good deal of money can he saved because, since all solutions are made up and dispensed by mass, inexpensive lightweight plastic containers can he substituted for fragile. calibrated glassware. Additionally, the excellent "readability" inherent in making mass measurements of the titrants usually allows experiments to he scaled down by a factor of five or so without loss of analytical precision. Doing so reduces both reagent and waste disposal costs. Other factors favoring the general adaptation of weight titrations include the elimination of both huret-reading biases and volumetric expansion errors caused when titrants prepared at one temperature are dispensed a t another. This note describes a simple adaptor that allows inexpensive (about $0.20 apiece) small plastic syringes fitted with the stainless steel needles that come with them to he conveniently used as weight burets. The adaptor (see Fig. 1) twists on (with one-quarter turn) to the hack of the syringe and is held in place by flexible metal tahs that grip the lozenge-shaped hase flanges of the syringe. A bolt (e.g., SAE 1/4-20 or 1/4-28) threaded through the base of the adaptor is turned to advance the "free3'piston of the syringe and dispense the titrant. Since a complete turn of the bolt dispenses only on the order of 100 rL/mg of titrant (the exact amount depending upon the pitch of the thread
SYRINGE
s
SHEETMETAL
Figwe 1. Adaptor wilh plastic syringe showing method of assembly.
Flgure 2. Use at a larger syringe to filllrinse the small buret-syringe manually.
and the internal diameter of the syringe), it is easy to control reagent addition accurately near endpoints. The adaptor consists of a rectangular piece of thin stainless steel sheet metal glued (with "quick set2'-typeepoxy glue) to an aluminum hase that is drilled and tapped to accommodate a common bolt. The base itself is machined from 1-in.-diameter rod stock and is about 318 in thick. An important part of the hase is a "nose" fitting through the central hole in the sheet metal into the bottom of the syringe. The nose centers the adaptor when it is being twisted onto the base of the syringe. The sheet metal is slotted so that, after the glue has set, two tahs can be readily bent over to grip the base flanges of the syringe. The dimensions of the adaptor are not critical hut should be customized to fit the particular lot of syringes to be used with it. To prepare a syringe for use as a huret, the plunger is removed and cut off immediately behind the rubber "pis-
' Corresponding author. Volume 65 Number 5
Mav 1988
467
ton". The niston is then reinserted into the barrel of the syringe. A; pressure applied to (or withdrawn from) the hase of the buret-syringe with another larger (10- or 15-cc) syringe is used to move the "free" piston for rapid filling or rinsing (see Fig. 2). A piece of gum rubber tubing fitted over the nose of the largersyringe Berves as the air Gal between the two. The majority of users will find that the most convenient way to hold the buret-adaptor assembly during a titration is with the rine and middle fineers of the dominant hand under and surrou~dingthe flangeof the syringe while the thumb and index fineer turn the bolt. The other hand swirls the " titration vessel. Of course, if a magnetic stirrer is used, twohanded manipulation of the buret assembly is more convenient. The following points should be kept in mind when using these burets: 1. The ldancr must be equipped with a draft shield. The shield
should be highenough toweigh thesyringewiththedispensing needle in place when it is plnred upright (seated 01, its base flanpe) on the balance pan. 2. Perform weighing operations with the adaptor uff the syringe. hlany of these top-loading balances are deixned so that 1-mg readabilitv i~ uovsible only if the total massofthe ihject being weighed is less than 20 or30 g. 3. In scaling down experiments, reduce the volumes of solutions, not their concentrations. Good "readability" of titrant quantities is irrelevant if endpoints are rendered "fuzzy" hy excessive
dilution. Reasonable volumes are &10 mL of "unknown" and 2-4 mL of titrant. Most of the classical volumetric determinations used in educational laboratories can be performed with these burets. Compatible titrants include dilute bases, most oxidants, non-halide acids, and aqueous solutions of most chelating agents. In cases where material incompatibilities might be expected, rather modest changes in procedures will usually permit the use of the syringes. For example, nitric acid should be substituted for hvdrochloric acid in acid-base titrations because the stainlkss steel needle corrodes a t a detectable rate in even 0.1 M HC1. Other conceivable objections to small-scale experiments in general and to the use of these syringes as burets in particular, (e.g., the submilligram changes in mass caused by handling the syringes with normally clean hands) do not, in oractice. cause analvtical errors of the maenitude usuallv accepted as inevitable with conventional volumetric equipment. For examde. vlastic svrinae barrels grip the piston sufficiently tightly
