Robert J. Hanmhan University of Florida Gainesville
Vacuum Line Buret for Measuring Liquid Sample Volumes
i t is occasionally necessary to prepare a number of duplicate samples of a degassed solution by vacuum line procedures-for example, solutions of methyl iodide in cyclohexaue for radiolysis studies. Work of this type is usually tedious because the samples have to be prepared one a t a time; if a large batch is degassed, it fractionates as succesive samples are removed and the samples do not have the same composition. A solution to this problem has been found by using greaseless, O-ring sealed vacuum stopcocks which have recently been made availab1e.l As shown in the diagram, a vacuum line buret was constructed using an ordinary 5-ml Pyrex Mohr pipet with O-ring stopcocks (A and B) attached above and below the calibrated volume. A bypass tube with a n ordinary vacuum stopcock (C) serves to evacuate the submanifold with the cell which is to be filled. I n operation, the apparatus is evacuated, stopcocks B and C are closed and A is opened. Some glass wool is wrapped around the buret and soaked in liquid nitrogen, condensing the volatile liquid sample which has previously been prepared in another part of the vacuum line (not shown). When transfer of the entire sample is complete, stopcock A is closed and the sample allowed to melt. The liquified sample can be stirred t o promote homogeneity if a glass covered stirring bar has been enclosed in the buret; it can be worked up and down with a magnet. Finally, to prepare a sample of any desired volume, liquid nitrogen is placed around the sample tube D, the initial volume in the buret is noted, stopcock B is opened and the desired quantity of liquid allowed to run out, freezing immediately in tube D. There is one precaution to be observed in using this apparatus. The apparent volume of liquid above stopcock B depends on the tension with which the stopcock is closed, since its Teflon plug tends to compress. Hence a fixed position must be established for closing
"Lab Crest Quiok Opening In Line Valves," rnsnufactured by Fischer snd Porter Co., Wmninster, Pa.
the stopcock, and a corresponding allowance made when judging the level a t which to shut the stopcock. Various modifications of the apparatus are possible. By using two burets instead of one, and filling each with one of the liquids being studied, it is possible to prepare quickly a solution of any desired concentration. For experiments on a smaller or larger scale, a 1-ml M o b pipet or a 25- or 50-ml buret can be used. We have also found the apparatus useful when working with a pure liquid rather than a solution, particularly when a large nnmber of samples must be prepared in succession.
Vosvvm line buret.
The work described here was done in connection with research supported in part by Atomic Energy Commission Contract AT-(40-1)-3106. Mr. B. Mason Hughes and Dr. M. B. Fallgatter constructed and tested the apparatus described.
Volume
41, Number 1 I, November 1964 / 623
