1526
ANALYTICAL CHEMISTRY
the bridge. Thickening the glass slightly a t the tip provides a slight shoulder and permits tighter r k i n g of the pa er pulp. Such an electrode, with one saturate potassium chlorife bridge, has a resistance of less than 400 ohms (measured at 2000 cycles on an alternating current bridge). When chloride ions interfere, a second bridge, filled with saturated alkali nitrate solution, is attached to the bottom of the chloride bridge to form a single unit. This extra bridge increases the total resistance to about 600 to 700 ohms. With the nitrate bridge in place no chloride ions can be detected in the test solution after 24 hours. Even though it may take much longer for chloride ion contamination t o occur, it is a simple matter to clean and replace the nitrate bridge solution daily. The parts of this half-cell are held together with short pieces of gum-rubber tubing used as stoppers. A piece of Chrome1 wire is inserted between the glass and rubber surfaces to vent air during assembly. After removal of the vent wires, the unit is reasonably airtight and leakproof. The electrolysis vessels are cups of 10- to 15-ml. capacity made from borosilicate glass tubing 25 mm. in outside diameter, flanged like test tubes. In use they are supported within the 26mm. holes cut in a polystyrene sheet which is held just above the surface of a constant-temperature water bath. A polystyrene block containing three holes to accommodate the calomel electrode, dropping mercury electrode, and purging gas inlet is bolted to a clamp holder and supported from a vertical rod bolted to the bench top. All the parts which dip into the electrolysis vessel are thus supported from a single steady mounting and may be raised or lowered into the vessel as a unit.
thus employed routinely for the same solution, cleaning is not necessary. A negligibly small volume of reagent which leaks past, and lubricates, the piston, maintains sufficient vapor pressure to check evaporation and thus to prevent sticking of the piston. The pipet is inserted in a 2-hole rubber stopper and kept in the reagent bottle, always ready for immediate use. In cleaning the pipet for use with another reagent, it may be sufficient to fill the bulb with clean water and draw in and expel water through the delivery tube several times. However, little effort and time are required to remove the stop and piston under hot water and scrub the glass parts or soak them in suitable cleaning solution. The stop may be located in relation to the graduated scale before cleaning and replaced in the same position afterward. Prediction of performance and limitations of the pipet with a variety of solutions and solvents is not justified by limited esperience to date. The pipet has been used chiefly to avoid the inaccuracies and mistakes bvhich occur in dispensing reagents from graduated burets or pipets in rapid routine analyses under considerable mental tension. Its use is also suggested for the handling of reagents which are irritating or poisonous when pipetted into the mouth.
The various parts may be made by even an amateur glass blower and because of their simplicity may be easily and thoroughly cleaned. By elimination of troublesome agar or gelatin plugs, the bridges may be used a t higher temperatures if desired.
Continuous Mercury Still. Paul B. Hamilton, Alfred I. du Pont Institute, I-iemours Foundation, Wilmington, Del. P3MM 00
Dispensing Pipet with Glass Piston. Frederic E. Holmes, Clinical Laboratory, Christ Hospital, Cincinnati, Ohio. rapid, and accurate dispensing pipet shown in T thecheap, diagram is presented for its convenience and utility HE
without comparison with previous apparatus available for the same purposes.
A capillary delivery tube, having an inside diameter of 1.5 to 2.5 mm., is fused to the barrel of a syringe as shown. Borosilicate glass was used in both syringe and delivery tube. The piston, P, and stop, S (cross-hatched), are cut from the plunger of the same syringe. The stop is fixed in the cylinder by paraffin having a melting point of approximately 58' C. (Tissuemat, sold by the Fisher Scientific Co.) in such a position that the excursion of the piston draws in and espels the volume of solution desired. The bulb, partly squeezed to provide reduction in pressure for filling, is attached by means of a short piece of glass tubing inserted in a piece of rubber tubing (heavy black lines) which serves as a one-hole stopper. If the zero of the graduated scale on the syringe has been altered in fusing on the deliverv tube. h a 1 adiustment to &act vdume is made by warming the cylinder a t S under the hot tap (70" C . ) , moving the stop as required, and allowing the paraffin to cool and set again. The stop is held in place very firmly by the paraffin. Both 2-ml. and 3-ml. pipets have been in daily service in this laboratory for several months for the addition of 10% sodium tungstate in the preparation of protein-free blood filtrates. When
x m c u m still for contin-
A uous
operation, similar to that described by Jelinek, Huber, and h t l e [J. Cheni. Educatzon, 26,597 (1949)], but of simpler construction is show n in the diagram. Ground joints, stopcocks, or complicated glass blowing are not needed. Elimination of a stopcock for evacuation purposes at D precludes leaking of air through an imperfect seal or freezing after a long period of operation. Gm heating is preferred because of the simplicity and rapidity of installation and the great ease of adjusting the heat required for smooth operation. In operation the still provides a continuous supply of clean dry mercury at the rate of 600 grams (45 ml.) per hour with minimum expenditure of time and unskilled labor. A still of this form has been in continuous daily operation in the writer's laboratory for 3 years, during a-hich time it has been dismantled for cleaning only once. hlercury is filtered through chamois to remove particulate matter and poured into reswvoir A. An oil pump, protected by a trap, is connected with rubber tubing to outlet B and a low flame is turned on. With the pump on, mercury is allowed to distill for an hour to allow maximum removal of air and moisture from the system. The rubber tube connected to the outlet is then clamped and a 70-cm. column of mercury allowed t o collect in the outlet arm. The rubber connection to the outlet is then broken and the still is ready for operation. The flame is adjusted so that distillation of mercury from chamber C to condensing arm D takes place without bumping. Freshly distilled mercury is collected a t the outlet. The introduction of mercury into the still and collection of distilled mercury are thus accomplished continuously a t atmospheric pressure, while distillation takes placp in a completel>- sealed evacuated space. Health hazards from mercury vapor are therehy kept minimal.
