Hypodermic Syringes in Quantitative D. A. Davenport
Elementary Chemistry Experiments
Purdue University Lafayette, Indiana
I
I.
The gas laws
In view of their wide availability, high precision, and relatively low cost, it is surprising that hypodermic syringes have not been more widely used in elementary chemistry. We shaU outline a number of experiments and demonstrations involving their use. Many others are feasible and might well exercise the imagination of the ingenious student. A 30-ml syringe with interchangeable parts (in theory this would cut breakage losses by 50%) is used throughout. The syringe is sometimes used without lubrication, but usually glycerol (around room temperature) or hydrocarbon greases (at higher temperatures) serve to lower friction. Fluorocarbon and silicone oils are also found to work well. I n order to change the pressure on a gas trapped in the syringe, identical textbooks (each weighing 904 g in our case) are either piled on, or suspended from, the piston. There are two main advantages to this procedure besides simplicity, avoidance of mercury, and the desire to show that a textbook can on occasion be useful. F i s t , students have some feeling for the "heft" of their textbook whereas they have little idea of the "heft" of a column of mercury. Second, pressures from about 0.25 atm to over 3 atm are readily attainable. Any available gas may be used to fill the syringe and hence the generality of the gas laws is more convincingly demonstrated than when air alone is used. Perhaps the easiest way of withdrawing gases from tanks, Kipp generators, and gas lines is to flush out
and then tie off a length of rubber tubing. Gases may then be sampled in a manner similar to that used in drawing blood. Boyle's Law
To study Boyle's Law a t pressures greater than atmospheric, about 30 ml of any gas are placed in the syringe and the needle is thrust into a solid rubber stopper. The syringe is slipped through a hole in a piece of masonite resting on a ring clamp. Books are piled onto the piston and the appropriate volume readings taken. The data in Figure 1 were obtained in about ten minutes. A plot of 1/V versus the number of texts added is linear (Fig. 1). For pressures less than atmospheric the syringe is inverted and texts are suspended from the piston by means of a string harness. Typical data are given in Figure 2.
Number
Of
Texts t
Figure 2. Boyle's Low plot for air, methane, ond o x y g e n a t prersures less than atmospheric.
I n the interests of accuracy it is preferable to perform the above experiments on different initial samples of gas-the first about 30 ml, the second about 5 ml. If a common sample is used the results are a little less satisfactory and there is a slight discontinuity
Number a1 Texts
Figure 1.
252
/
+
Boyle's Low plot for air ot pressures greater than atmospheric.
Journal o f Chemical Education
at "zero texts" because of the piston weight (35 g). Again it should he emphasized that any gas may be used in the syringe. The syringe may be surrounded by ice water or warm water, thus demonstrating the wide applicability of Boyle's Law. The method is both quick and versatile and a single class may conveniently assemble a large amount of data. A useful take-home exercise is to calculate the weight of the text assuming Boyle's Law and being given atmospheric pressure, the density of mercury, and the distance in cm between zer* and 30-ml marks on the syringe. The values so obtained are within 2y0 of that obtained by direct weighing. A complete analysis of this experiment including allowance for the finite weight of the piston makes a fine student exercise. Charles' Law
The simplest, hut not the best, way of verifying Charles' Law is to place a sample of gas in a syringe which is then immersed in liquid baths maintained a t various temperatures. No lubricant is used in this experiment because of viscosity and vapor pressure problems. Because it is run a t constant (atmospheric) pressure, leaks are not particularly troublesome. Data obtained by this method are plotted (Fig. 3) to give the value -275°C for absolute zero. Other experiments gave values between -260 and -290°C.
Table 1. Volume Increments when 273 ml of Air at 0°C Is Allowed to Warm to Room Temperature at Constant Pressure
Temverature ("C)
Volume increment ( m l )
warm to room temperature. As seen from Table 1, temperature and volume increase numerically in step. The 273-m1 flask is, of course, purely a mathematical convenience and Table 2 illustrates data obtained with a small reagent bottle of 76 ml capacity. The Charles' Law coefficient calculated from this data is 0.00372(°C)-' as compared with the ideal gas value (1/273) or 0.00366("C)-'. Table 2.
Charles' Law Data for Air
By piling hooks on the piston it is possible to verify Charles' Law for pressures greater than atmospheric. It is inconvenient to attempt Charles' Law experiments a t pressures below atmospheric by this method, though an arrangement involving the use of pulleys might he tried. Ideal Gas Law
Both pressure and temperature can be varied in the same experiment. When T / V is plotted against number of texts, we again get a straight line (Fig.4).
TEMPERATURE
Figure 3.
PC,
Chorler' Law plot for methane.
This method has two major drawbacks. First, the volume of gas is small and hence the changes in volume are also very small unless an inconveniently wide temperature range is used. Second, because of thermal conductivity through the piston, the temperature of the gas is probably somewhat lower (at 100°C) or higher (at say -55'C) than that of the surrounding liquid bath. This would tend to skew the graph somewhat. A much better way to perform the experiment is to use the syringe to measure the changes in volume of a larger volume of gas. A particularly graphic example of this utilizes a 273-m1 flask (a 250-ml wide-mouth Erlenmeyer flask may he adjusted with a little mercury) fitted with a rubber stopper carrying a sensitive (preferably pin-hcad) thermometer and hypodermic needle. The flask is cooled until the thermometer registers O°C. The syringe is then screwed into place and the apparatus allowed to
,
2
Number
Figure 4.
3
0,
4
T
