D. A. Davenport Purdue University Lafayette, Indiana 47907
Baby-Bottles and Elementary Chemistry
W i t h o u t doubt the cheapest and most durable volumetric glassware on the market is a babybottle. Indeed as an act of propitiation they are often given away. For strictly chemical work the shape is not perhaps ideal but the graduations are commonly marked in cubic centimeters as well as in ounces and a pressure sensitive two-way valve is provided at no extra charge. We have found them to be useful in several elementary experiments and though it is unlikely that all such uses are novel they do not seem to be well documented. The following illustrate some of the possibilities. Equivalent Weights of Metals
A convenient volume of dilute acid (or base) is measured in the bottle. A weighed amount of metaI is placed inside the nipple and held there with the fingers (or a screw-clamp) while the collar is screwed in place. The bottle is inverted and the gas collected by downward displacement. When reactipn is complete the bottle is again placed upright and the volume of acid remaining read. The usual corrections and calculations are made. I n this form the experiment is perhaps best suited for high school students. I t is certainly very quick and it avoids the bodily injury which sometimes accompanies the tedious assembly and use of the standard water-displacement apparatus. We have used a variation as a salutary opening experiment for some of our general chemistry courses. The technique is demonstrated and the students are then required to study the Mg/HCl reaction from the point of view of stoichiometry, kinetics, and the effectsof acid concentration and temperature. Reports are due at the end of the laboratory period. They are a shrewd guide to past training and future potential. In principle any reaction between a solid and a liquid which yields a gas can be studied by this method. We have also looked at CaCdHCl and CaC03/HC1 and several others readily suggest themselves. One could even study the kinetics of a homogeneous gas-evolving reaction such as the dceomposition of a diazonium salt solution though here there is the added complication that reactant is expelled as the reaction proceeds and the mat.hematics becomes somewhat tricky. But a bright student might be usefully silenced for some time working the details out.
there some cosmic significance in the fact that the standard baby-bottle has a volume of about 273 ml ? ) The results are of course not overly precise but there is a certain charm in finding absolute zero on the kitchen stove and doing a statistical analysis with a sterilizer. Vapor Pressures of Volatile Liquids
A bottle is partly filled with water and clamped in an inverted position, and the volume of air is read. The nipple may be allowed to dip into water hut this is not necessary as the slight drip is self-limiting. A small sample of a volatile liquid (insoluble in, and lighter than, water) is injected through the hole in the nipple with a hypodermic syringe. When equilibrium has been established, the new vapor volume is read and the vapor pressure calculated using Dalton's Law. After a preliminary trial the student should he encouraged to choose that initial volume which affords maximum precision. The effect of temperature can be studied by obvious modification. Since the meniscus cannot be read to better than *1 cc some may prefer to find the volume of water displaced by weighing. Molecular Weights of Volatile Liquids
A familiar general chemistry experiment involves heating a volatile liquid in a flask closed by a piece of aluminum foil pierced by a small pin-hole. However, if heating is too brief not all thc liquid is volatilized; if too long air may diffuse in. Unfortunately there is no simple way to decide when to stop and the results are often disappointing. The use of a baby-bottle with its pressure sensitive valve avoids these difficulties and batch heating is conveniently carried out with a sterilizer. Care must, however, be taken to remove all water which tends to get trapped under the collar. Silicone nipples are available and they offer some advantages but they tend to absorb significant amounts of chlorinated hydrocarbons. Closed-Atmosphere Experiments
There are many interesting chemical species which are sensitive towards atmospheric oxidation, Clasused in order to exsically a Bunsen valve was clude air. A baby-bottle serves equally well and it
-
h--... a s the ~ - - -additional advantaees that aliauots mav be ~~~~~~
~
~
Charles' Law Expansion
removed with a hypodermic syringe' br the whole
A dry bott,le is heated at 10O0C, inverted into water and allowed to cool. The nipple is tweaked to ensure pressure 'qualization and the volume of water which has entered read. By invert'ing and again reading the meniscus the t,ot,al volume mav be calculated. (Is
' To avoid pressure eqnalisat~ionproblems it may be necessary to inject an equal volume of inert gas before withdrawing the sample. Such inert gases may be conveniently stored in, and sampled from, balloons sealed wit,h a serum stopple. See J. CHEM. EDUC., 39, 617 (1962).
878
/
Journcll of Chemical Education
sample titrated directly in the bottle by means of a buret inserted through the hole in the nipple. As an example we might cite the now fairly familiar titrimetric study of the oxidation states of vanadium. We have used the original Palmer procedureZtitrating the reduced samples directly in the bottles, a fine hypodermic need& piercing the nipple to ensure pressure as the titrant is added. The modified procedure recently described by Davis3 could undoubtedly be used. One can also utilize the pressure of hydrogen slowly generated by excess reducing agent
to force solution up into a pipet and in this way transfer an aliquot to a quenching solution of ferric ion. Grindley4has described a number of other reductor exercises which could readily be adapted to the baby-bottle technique. PALMER,W. G., "Experimental Inorganic Chemistry," Cambridge University Press, Cambridg-e, Mass., 1959. J. M,, J, CnEM, 45, 473 (1968), ' GRINDLEY,D. N., "An Advanced Course in Practical Inorganic Chemistry," Butterworths, London, 1964.
Volume 46, Number 12, December 1969
/
879
