Miniature Helical Stirrer David S. Bright Analytical Chemistry Division, National Bureau of Standards, Washington,D.C.20234
Application: Chemical Kinetics a n d Spectrophotometry. It is often necessary to stir small volumes of liquids in an optical rectangular cuvette (10 X 10 X 40 mm). Magnetic stirring devices are available that are placed in the bottom of such a cuvette; however, appreciable delay in mixing is observed near the top of the long and narrow space. This delay in mixing the upper portion of the solution with the total solution is noted at the faster stirring speeds which tend to make the magnetic stirring device operate unevenly in the cuvette. Further, in many instruments, the cuvette is housed in a chamber having heavy metal parts which cause the magnetically induced eddy currents to prevent the driving magnet from turning at the required speeds. Although developed specifically for the above application, this helical stirrer should be useful wherever volumes of severa1 milliliters are to be stirred, where it is desirable to have a flexible stirrer, and where one wishes to avoid sliding or touching parts that are in contact with the stirred liquid. If made in large quantities, the stirrer might be inexpensive enough to be disposable. Stirrer Design. The stirrer consists of a helix cut from a length of chemically inert, stiff but flexible tubing such as Teflon (Figure 1).A small end portion of the tubing is left intact to fit over the shaft of a miniature motor. The helix should be made nearly the length of the stirred volume. The helix flexes when spun in air, but stabilizes along its axis when spun in liquids over a wide range of varying viscosities, e.g., acetone (0.33 cP),water (1cP),and glycerine (1000 cP).The stirring rate, for spectrophotometric purposes, is limited by the formation of air bubbles drawn in by the vortex (Figure 2). If the helix is vertical and spun “upward” (e.g., clockwise and if it extends looking downward upon a right-handed
FLEXIBLE HELICAL STIRRER
,
‘CY LlNORlCAL SIDE TUBE
SMALL ARROWS: CIRCULATION OF -LE
Flgure 1. The helical stirrer is made of Teflon Tubing of about 2-mm o.d., thin walled, and about 40 mm long. The helix retains about half of the tubing material. The helix could be made of any tubing of sufficient
chemical inertness and stiffness
0.8
06
y1
v
z 2=
s1 LL1
< 02
-
0
20
TIME, a
Flgure 2. The left and right absorbance vs. time curves, respectively, reflect the rapid mixing of approximately 0.1 and 0.5 ml of dye solution injected into the top of the cuvette (Figure 1) which holds approximately 7 ml of water. The peaks represent the bolus of dye solution passing the light beam; the level portions to the right of the peaks represent achievement of complete mixing
above the liquid surface, the downward pull of the vortex is slightly compensated by the upward flow of liquid, thus allowing greater stirring speeds. At stirring speeds just below bubble formation, the helix is quite stable and mixing is very rapid. The major advantages of the helical stirrer are that it is easily made, that it is readily aligned to avoid all contact with the cuvette walls, and that it is nearly unbreakable and harmless to the cuvette, should it accidentally contact its walls or bottom. Sample Container Design. The helical stirrer can be used in conjunction with a side tube as shown in Figure 1. At the expense of custom glass work and of doubling the sample volume, this container decreases the mixing time by allowing the sample to circulate around the stirrer. Further, it prevents any obstruction of the light beam passing through the cuvette. Note that the container is open, and that rapid mixing is obtai.ned without pressurized flows or special seals.
RECEIVEDfor review September 15,1976. Accepted October 4, 1976.
ANALYTICAL CHEMISTRY, VOL. 49, NO. 1, JANUARY 1977
191
