Screw-Cap Test Tubes as Spectrometer Cuvets G. C. Llsensky and E. Llmon Beloit College, Beloit. WI 5351 1
W e have been using screw-cap test tubes a s spectrometer cuvets i n our introductory chemistry course. Screw-cap test tubes' have several advaitages withrespect t o normal round cuvets for use in Spec 20 o r similar spectrometers. (1) The cap encloses vapors from the sample solution. For example, our intraduetory chemistry class doing a spectrophotometric analysis for copper with concentrated ammonia and filling the samole tubes in a fume hood. manaeed to keeo the ammonia concentrabon in the room below &e odo;thresholci. (2) The cap protects the sample from the atmosphere. For example, an orange mercury dithizonate solution turns blue under illumination hy sunlight or projeeto~.~ The solution then reverts to orange, and the rate can be followed in a spectrometer. This rate is dependent on the water concentration of the xylene, so the solution must be kept sealed. We have used the same mercury dithizonate solutions sealed in screw-cap test tuhes for over 18 months. (Incidentally, the xylene for this experiment can be dried over CaClz quite readilv.) , (31 The price of the screw-rap test tubes is one third of the price of the spertrorneter cuvets normally used. The spectrometer round cuvetscome witha whitelrne painted on the tube for alignment. The
by up to0.02 absorbance units. Thus, as with the normal spectrometer round cuvets, careful work requires that the screw-cap tubes be inserted with the same orientation each time.
shape on the Kimax tuhes serves the same purpose. Given these advantages, we were curious about the path length uniformity of the screw-cap tubes. An acetate-buffered bromothymol blue solution was examined at 430 nm in a Spectronic 20 interfaced3 to an Aoole .. IIe comouter. The 12-hit resolution of the A D conversion was used to record the absorbance more precisely than could be read from the meter of the spectrometer. Figure 1shows the absorbance values obtained for this solution using 50 different sample tubes. Different tubes varied over a range of 0.02 absorbance units. Thus, as with the normal spectrometer round cuvets, careful work requires either that a single screw-cap tube be used for both blank and sample or that matched pairs of tuhes be found. Figure 2 shows the absorbance values for 10 tubes selected at random when each was rotated bv 45' increments. Some tuhes are rounder than others,asevidencedSbythe range in absorbance values for a given tube. Rotational position changed the ahsorhance value
Flgwe 1. Absarbance of aam, solution measwed in 50 differenttubes.
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Kimax 13- X 100-mm borosilicate glass culture tube with PTFEfaced rubber liner screw can. Fisher No. 14-930-10A. Petersen, R. L.; ~aylon,'i.H. J. Chem. Educ. 1985, 62 802-603: Meriwether, L. S.: Breitner, E. C.: Sloan, C. L. J. Am. Chem. Soc. 1965,87,4441-4446. GraphSPEC and DATACARD interface boardfrom Anadata, 355 N. Ashland Ave., Chicago. IL 60607.
0.354 . 0 10
2
20 30 40 Tube Number
4 6 8 Tube Number
1
I
50
0
Figwe 2. Absabance of same solutlon measured in >O differemNDss with mch tube rotated 0 . 4 5 , 9 0 and 135'.
Volume 66
Number 10
October 1989
889
