AIDS FOR ANALYTICAL CHEMISTS Use of the Syringe as a “Shrinking Bottle”
Robert A. Silverman and Gilbert Gordon Department of Chemistry, University of lowa, lowa City, lowa 52242
A common problem in chemistry is the quantitative handling of solutions containing volatile components. These solutions cannot be exposed to a gaseous phase without changing their concentrations. When such a solution is contained in a bottle, removal of a sample of the liquid draws an equal volume of air (or other gas) into the bottle. The subsequent saturation of this gaseous phase causes the concentrations in the remaining liquid to change. In addition, small temperature changes may cause large concentration changes in the stored liquid because of the redistribution of the volatile component between t h e liquid and gas phases. These problems can be avoided with a “shrinking bottle” t h a t contracts in order to expel each sample, and never admits a gas phase. We have modified a syringe and find t h a t it is well suited for this purpose. Aqueous chlorine solutions have been successfully stored in 50-ml “shrinking bottles” a t 5 “C for several months. Occasional samples can readily be withdrawn for use in kinetic or analytical studies, b u t air is not allowed into t h e syringe, and the delivery port can be stoppered after each sampling. The chlorine titer changed less t h a n 1%per month. The contact with air at the air-barrel-plunger intersection need not pose any danger to air-sensitive solutions, because the evaporation a t t h a t intersection slowly draws liquid from the body of the syringe to t h a t point. This one-way flow effectively isolates the liquid in the syringe, but may draw a bubble of air into the tip. The first por-
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Figure 1. Screw-controlled s h r i n k i n g bottle A . Screw; B, brass frame; C, guide for screw: D. indicator wheel, 100 marks per rotation (indicator pointer not shown); E , 50-ml glass syringe: F, glass water jacket; G, retaining springs: H , Luer tip
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tion of the liquid from the tip is discarded shortly before taking a sample. By adding precise volume control, the “shrinking hottle” concept may be adapted t o the transferring of known volumes and the mixing with known volumes of other solutions, without contact with t h e air. The device shown in Figure 1 was designed for kinetic studies of air-sensitive solutions. It consists of a water-jacketed 50-ml glass syringe with a metal frame and screw to control t h e position of the plunger. T o prevent binding, the screw, plunger, and barrel must all share the same axis, and sidewise pressure on the top of the plunger must be avoided. T o accomplish this, a metal guide is provided for the end of the screw, and a simple thrust bearing for t h e screw-plunger contact. The flange of the barrel is clamped in place, or glued with epoxy cement into a close-fitting hole in the metal frame. The water jacket is affixed to the barrel with a waterproof cement, sealing wax, or putty. Calibration can be carried out by expelling small samples which are measured either by weight or by acid-base titration. Fitted with a n ordinary machine screw, the 50-ml size is capable of delivering small samples with a reproducibility of f0.005 ml, but a calibration curve is advisable, as the volume delivered per turn of the screw varies slightly along the length of the screw. For experiments involving the mixing of two solutions inside the syringe, a magnetic stirring bar is placed in t h e syringe. and the total volume contained a t a given screw setting is determined by acid-base titration. In the mixing operation, the air-sensitive solution is contained in the syringe. By turning the screw, small samples are expelled for analysis. Finally, a known volume of the second solution is drawn into the syringe by turning t h e screw backwards a given number of turns, and the mixture is stirred magnetically. The details of the mixing procedure and the calculations need take into account only the total volume, the backlash of the screw, and the “dead” volume in the tip. In kinetic experiments, samples of t h e mixed solution can be expelled periodically for analysis. Another procedure can be useful for mixing in other vessels, such as a spectrophotometer cell. The cell must have a ground-glass stopper. The air-sensitive solution in the syringe is cooled, if necessary, for it must he more dense than the solution with which it is to be mixed. A small stirring bar is placed in the cell and the cell is filled with the initial solution. By turning the screw, a measured volume of t h e air-sensitive solution is introduced as a n unmixed layer a t the bottom of the cell. The overflowing cell is immediately stoppered without any air bubble. A simple test of the liquid t h a t overflows during this operation can be used to show that none of t h e air-sensitive solution was lost or exposed to the air. Finally. the cell contents can be magnetically stirred and t h e dilution ratio is calculated from the temperature-corrected volume delivered and the total volume contained in the cell. Received for review May 23. 1973. Accepted July 30. 1973.
A N A L Y T I C A L CHEMISTRY, VOL. 46, NO. 1, J A N U A R Y 1974
