Construcllon of the Cell Fabrication of a Wooden Jig The jig is used to hold the cell window and guide the saw hlade. A 25-mm-diameter cavity, 8 to 10 mm deep, is drilled into a small pieceofpine, about 5 X 6 X 2 em. Thedepth should be such that, when the cell window is inserted in theeavity, some wood, 3 or 4 mm perhaps, extends above the top face of the cell window. The block is sawed in half directly through the center of the cavity, and the two halves are joined together with wood screws. If the size of the cavity is just right, the cell may be placed in the cavity, and the screws tightened so that the jig firmly grim the cell and prevents it from moving during the sawing operation. Next, using the scroll saw hlsde, a guide groove is sawed in the wood parallel to the first cut and 4 mm off to the side. The depth should extend down to about 2 mm from the bottom of the cavity. This completes construction of the jig. Cuning the Groove in the Cell Window I t is particularly important to use the right hlade for sawing the groove. The scroll saw blade, (Rockwell International part No. 40-198, obtainable from The Gage Co., 3000 Liberty Ave, Pittsburgh, PA 15238, pkg. of 6 for $3.70) is a special saw blade designated as g w d for cutting leather. There is practcally no set to the teeth, a factor that was found to be especially desirable. Several other blades were tried, such as an ordinary coping saw blade, hut these all proved unsatisfactory due to the large set to the teeth, which caused the hlade to undercut the swface and cause chipping. With the Rwkwell blade, chipping problems were eliminated. The hlade may be held in a jeweler's saw, but a perfectly satisfactory groove can be cut by holding the blade by hand and applyinguniform pressure over the face of the salt plate. A standard unpolished 25-mm-diameter, 5-mm-thick NaCl disk (Harshaw, 6801 Cochran Road, Solon, OH 44139, Stock Number 02505, $5.00 each) is placed in the jig, the screws are tightened to hold the disk firmly, and a groove is sawed completely across the cell using the scroll saw blade and the guide groove in the jig. The groove in the cell window is made to a depth such that a 24-gauge needle will just fit completely into the groove. This can be determined while the cell window is still in the jig. A light touch on the saw is desirable, as too much pressure may cause chipping or fracture. only one of the windows needs a groove. The thickness of the saw blade is such that a needle lareer than a 24 zauze - - will not fit the groove. Pollshing the Cell Windows Both eell windows are oolished usinzstandard techniques using a medium grit with 154 water in alcohol for roughgrinding ~~~
~~
and various mixtures of water and alcohol for polishing. Water itself works well if polishing is carried out quickly.
Fabrication of the Window Spacer A 25-mm diameter circle is drawn on a piece of 0.004-in. (0.1-mm) brass shim (McMaster-Carr Supply Co., P.O. Box 440, New Brunswiek, NJ 08903-0440, $3.48 for a 6- x 60-in. roll, various thicknesses are available) and cut out with scissors. A rectangle, 17 X 10 mm, is drawn on the circle, startingat the edge ofthe circle and then cut out with scissors, figure. The edges of the various cuts are tapped lightly with a small mallet to make sure that the edges are flat and are not turned over in any way.
being careful not to let it drop. The cell may he dried by flushing with air using a syringe as an air pump. For dauhle-beam dispersive instruments two e l l s are required, the reference eell, and the sample cell. Although the cells are reasonably well matched, they are not perfectly matched, and so bands due to the solvent may be evident. Using the double beam Perkin Elmer 1310 snectroohotometer. a weak uncompensated dc14 b k d at 1540'cm-I is observed, indicating only a slight cell mismatch. With FT-IR instruments both the solvent background and the sample are run using the same sample cell; hence, unmatched cells are not a problem. However, if several cells are in use, students must recognize that, if cells are switched, a new background of pure solvent should be run in order to compensate perfectly for the spectrum of the solvent. In our hands the cells are matched well enough so that using different cells for background and sample gives good results.
Acknowledgment The contribution of Donald Feathers, Senior Scientific Instrument Technician, Department of Chemistry, to the design and construction of the jig and cell, is greatly appreciated.
Assembly of the CeN A small amount of a two-component fastdrying epoxy cement is mixed, and then, using a toothpick, a thin coating of epoxy is aoolied . . to one side of the snacer. The soacer is placed on a flat surface, and the cell window, thcone witnout the gnrove,rs placedon the spacer with pressure tu spread out the cement evenly and to squeeze out air huhhles. When the cement has set, 5-10 min, the window is turned over, some more epoxy is mixed, and another thin coating is applied to the other side of the soacer. The second window is placed on top oE the spacer so that the groove is parallel to the Long side of the window and is within the window. Pressure is applied as before until the glue sets. Since both ends of the groove are now open, the one under the spacer must he closed by forcing in a small amount of epoxy with a toothpick. The cell assembly is allowed to dry for an hour or two before use.
'The use of pure llqulds Is not recommended sin- me spacer thickness may cause lhe absorbances of some peaks to be so great that Ihey bottom out badly.
Construction and Use of an Inexpensive Microburet Mono M. Slngh, Zvl Szafran, and Ronald M. Pike Merrimack College Nonh Andover, MA 01845
The introduction of microscale laboratories in general chemistry requires the use of microscale titrations using microburets and pipeta. The cost of buying microburets for the large number of students enrolled in general chemistry may be prohibitive. In our general and analytical laboratories at Merrimack College, we have been using mierohurets made from readily available, inexUsing the Cell pensive, materials. This m&roburet has advantages over other micra-devices because Thecell is filled usineasvrinee fitted with it allows the formation of more uniform a q 2.1 nwdle or smallFr. he &Ishould be filled withadutiun' just prior to running r h ~ drops at its micro-tip, its accuracy is much greater, and the manipulations during spectrum; otheruide, excessive evaporation transfer of solution are much easier. may occur. After the spectrum has been run, The construction of the microburet is the cell is flushed with the solvent, usually shown in the figure. The followingmaterials carbon tetrachloride, and allowed to dry. are needed for its construction: a 1.00-mL The needle does not fit tightly enough to graduated pipet, a 34-cm-long piece of rubwithdraw the samole from the cell usine the syringe, and so, i i t h e cell is to be emptied (Continued on page A126) and dried, a good way is just to shake it once,
Volume 68
Number 5
May 1991
A125
the microrcale laboratory ber or Tygon tubing, a fine-bore end tip (available commercially-alternatively, the end of a broken buret or the tip of a Pasteur pipet may he used), and asmallglass head of slightly bigger diameter than that of the rubber tubing. First, insert the glass bead into the rubber tubing. Moistening the glass bead with a droo of water will heln in slidiw the head to the'center of the tubing. ~ e x i insert , the buret tip into one end of the rubber tubing, and attach the graduated pipet to the other end. In order to prevent leaks, make sure that the rubber tubing fits tightly to the graduawd plpet and the rnd tip. Tudeliwr n solution from the buret. press the class bead with the thumb and furefineer.-~heoressure will create a nassaee for ;he ~ u l u t i hTu fill the buret, p i s e the tip of the microhuret in thr desired solution. apply suction at the other end of the buret (using a pipet bulb or pipet pump), simultaneously squeezing the glass head. Rinsing of the pipet can be accomplished in the same fashion. We have performed acid-base and redox titrations using these micropipets without any difficulty. Student results obtained by micro methods of titrations are comparable to theoretical values,or to those ohtained by normal methods. For these titrations, anywhere between 100-1000 rL of solution have been used. The microburet has several advantages over the standard buret. It is easy to fill (no overflowing), easy to control, is much less expensive, and the titration requires much less time. The accuracy of such burets is +1 rL. Since the titration time is greatly reduced, it can also be used for transfering dilute solutions of KMnOd or KzCr207.The use of rubber tubing, however, limits its use to aqueous solutions.
A126
Journal of Chemlcal Education
