Adjustable Automatic Pipet Boris Nebesar Mineral Sciences Division, Mines Branch, Department of Energy, Mines and Resources, Ottawa, Canada
In a number of combustion methods for the determination of various elements which employ a titrimetric readout, it is necessary to use a given volume of an absorption solution to absorb the evolved elements or compounds. The technique is employed, in particular, for the determination of sulfur after its separation as oxide(s) at high temperatures and is of current interest in the international stapdardization of the determination of sulfur in iron ores ( 1 , 2).
There are two kinds of analytical trains used for this purpose: those operated with the carrier gas under pressure, the flow being interrupted after each determination, and those in which the carrier gas is continuously pulled through the system by pumping. The absorption solution is measured either into a beaker (2-4) or directly into a graduated cylinder which also acts as the absorption vessel, as for example, in (I). In addition, it can be measured or delivered into a special absorption vessel up to a pre-measured mark while the carrier gas is either flowing (5, 7) or not flowing ( 4 , 6, 8). In the former case, the volume depends on the gas flow and can be only approximately known because of the interfering bubbles; the operator must watch the level carefully during each addition. In a continuously vacuum-pumped analytical train, the conditions are even more complex, because the system cannot be opened for the addition of the absorption solution. A technique of approximale.volume addition had to be used (9). T o better control the volume of the absorption solution added in similar situations, a versatile, adjustable automatic pipet has been constructed and is described herein.
CONSTRUCTION Assembly. The pipet has been assembled from components and materials that are readily available in most laboratories. Figure 1 is a schematic diagram of the device. A polypropylene graduated cylinder (D) of suitable volume is cut horizontally a t a mark close to the lower value of the desired adjustable range. In our case, a 100-ml graduated cylinder was cut at the 70-ml mark. With a cork borer, a hole is cut through the center of the bottom of the cylinder. The hole should fit the tubing of the glass stopcock (G) tightly. To strengthen this joint mechanically, a suitable neoprene rubber stopper (F) is cut horizontally to fit into the depression of the cylinder support, and a hole is bored through its center to fit the stopcock tubing tightly. The stopcock (G) is mounted with sheet-metal spring clips, protected with plastic tubing, on to a wooden block (I). The glass tubing of the stopcock is shortened to such a length as not to protrude too far into the cylinder, because its height determines the volurne of the absorption solution held back by the pipet. T o further reduce this volume, a silicone rubber ring (E) may be inserted into the cylinder. The block, (I) is bored to accommodate a standard laboratory aluminum rod (H) which is fixed by a small machine screw (not shown) a t the back. The rod is held in a standard laboratory clamp. This assembly is firm enough to allow singlehanded operation of the stopcock. At the top of the cylinder, the component for adjusting the volume delivered by the pipet is made from a disposable plastic syringe (B) of suitable volume (in this case, 20 ml). The flange a t the back, and the Luer syringe joint in-
side the front of the syringe body are cut away to allow for easy insertion into the cylinder and for a faster flow of the solution. The seal-ring (C) is made from a stopper by horizontally cutting away the excess material at the appropriate diameter, and boring the hole by a cork borer. Silicone rubber is preferred because of its permanent softness, facilitating the sealing, also for its resistance to the organic solvents used to dry the pipet. The syringe is joined by a plastic or silicone-rubber tubing to the glass tubing of a commercially available overflow unit for conventional automatic pipets (A). This unit also provides the second support point by clamping into a three-prong clamp. Letters designating other items on Figure 1 are self-explanatory. Operation. The operation is exactly the same as with conventional single-volume automatic pipets, but the design described allows for a considerable and continuously adjustable range of volumes between 60 and 90 ml to be delivered. T o change the volume, the seal-ring is temporarily loosened and the syringe is pushed through it until its upper edge is aligned with the desired graduation mark. The actual volume delivered must be determined by a preliminary calibration and is reproducible for a given setting. There is an air space between the cylinder and syringe walls because the level of the solution in the cylinder is determined by the position of the syringe. The pipet is filled with the absorption solution from a storage bottle, using compressed air from the laboratory line, the air flow being set a t 200 ml/min. Calibration. Neither the values on the graduated cylinder nor those on the syringe represent the true outflow,
I
”*
Figure 1. Adjustable automatic pipet (A) Commercially available overflow unit for conventional automatic pipet. ( B ) 20-ml disposable plastic syringe: 21.6-mm 0.d. (C) Silicone rubber seal-ring (from stopper No. 5): 21.6-mm i.d. X 10 mm high. (0) 100-ml polypropylene graduated cyllnder; 25.6-mm i.d. ( E ) Silicone-rubber ring: 25.3-mm o.d., 10.3-mm i.d. (F)Rubber stopper support. (G)Three-way stopcock No. 4 with Teflon plug; tubing 10.3-mm 0.d. (Hj Standard laboratory aluminum rod; 180 mm long X 12.7-mm 0.d. (0 Wooden block: 40 mm wide X 80 mm high X 60 mm long; with spring clips. (J)Titrant inflow. ( K ) Titrant outflow. (L)Titrant overflow to waste.
ANALYTICALCHEMISTRY, VOL. 47, NO. 8, JULY 1975
1495
The pipet has proved useful for the addition of the absorption solution without breaking the vacuum in either the visually or spectrophotometrically controlled iodometric titrations and in alkalimetric titrations, all of which are performed in the determination of sulfur in a continuously vacuum-pumped, open analytical system (10). In conjunction with other improvements, this device contributes toward more expedient, less strenuous operation, toward increased throughput by reduction of the analytical times, and toward increased over-all precision by providing a more precise control on otherwise uncontrolled variable. A CYLINDER 60
1
I
IO
I5
,
1 I I35
Figure 2.
Calibration
of
the adjustable automatic pipet
which must be determined by measurement. In Figure 2 , we see that calibration values for either the syringe or the cylinder are linearly related to the outflow; the use of the former is preferred because its narrower diameter affords better precision.
CONCLUSIONS The adjustable automatic pipet is simple, versatile, and allows the reproducible delivery of any intermediate volume within its adjustable range unlike the fixed-volume, conventional automatic pipets. Here it must be pointed out that the word "automatic" is used in the same sense as in the past, to indicate an automatic zeroing and not the automation of the whole operation. The pipet is inexpensive and can be assembled from commercial laboratory supplies; the design concept permits easy adaption to a range of volumes.
1498
LITERATURE CITED
20
SYRINGE SETTING, ML
ANALYTICAL CHEMISTRY, VOL. 47, NO. 8, JULY 1975
(1) International Organization for Standardization, Document ISO/TC-IOP/ SC-2 (U.K.4) 202 E, February, 1970. (2) International Organization for Standardization, Document ISO/TC-102/ SC-2 (France-22) 172 F, July, 1969. (3) International Organization for Standardization, IS0 Recommendation R 671, 1st Ed., February, 1968. (4) Handbuch fuer das Eisenhuettenlaboratorium, "Die Untersuchung der metallischen Stoffe", 2nd ed., Bd.2, Verlag Stahl Eisen. Duesseldorf, 1966, pp 39-45. (5)Instruction Manual for Operation of Leco Sulfur Determinators, Models 517, 518, and 532, Form No. 133A, Laboratory Equipment Corporation, St. Joseph, MI. (8) International Organization for Standardization, IS0 Recommendation R 351, 1st Ed., December, 1963. (7) ASTM Standards, E 350-73, E 351-73, E352-73, E 353-73, E 395-70; American Society for Testing and Materials, Philadelphia, PA, 1973, Part 32. (8) J. A. Maxwell, "Rock and Mineral Analysis", lnterscience Publishers, New York, NY. 1968, p 443. (9) R. F. Jones, P. Gale, P. Hopkins, and L. N. Powell, J. lron Steel lnst., 204, 505 (1966). (IO) 8 . Nebesar, "Vacuum-pumped, Open Analytical System for the Determination of Sulfur by the Combustion Method, Using a Resistance Furnace", Department of Energy, Mines and Resources; Mines Branch, TB 159. Ottawa, Canada, submitted for publication in Mines Branch.
RECEIVEDfor review October 23, 1973. Accepted January 27, 1975.
