VOLUME 33, NO. 6, JUNE, 1956
8
APPARATUS AND EXPERIMENTS FOR SEMIMICRO EXPERIMENTATION IN GENERAL CHEMISTRY AT THE COLLEGE AND HIGH-SCHOOL LEVELS' NICHOLAS D. CHERONIS and HERMAN STEIN Brooklyn College, Brooklyn, New York
T H E attention which has been given to the use of smallscale experimentation in general chemistry is shown in the literature references of the table (next page). It is of interest t o note that it was considered and tried over 100 years ago, much earlier than is commonly conceded by even ardent supporters of these methods for laboratory instruction. It appears that D. R. Reid, in giving evidence before the Select Committee on Education in Ireland, in 1835, recommended the use of smallscale experimentation for science teaching in schools. His students obtained a blowpipe, a test tube, slips of paper, and broad and narrow strips of glass such as the glazier discards. The microscope was used when necessary. Tests were performed on paper and on glass slides; the latter were also employed for evaporation and heating. "Every school master might provide himself with apparatus sufficient t o show thousands of experiments on a small scale." With the development of systematic microchemical experimentation in Austria by Emich, there were sporadic attempts at smallscale experimentation in the teaching of general chemistry hoth a t the college and high-school levels, though for the most part instruction in general chemistry has followed the traditional pattern. On the other hand, the application of these meth-
' Presented before the Division of Chemical Education a t the 128th Meeting of the American Chemical Society, Minneapolis, September, 1955.
ods has had notable success in the teaching of inorganic qualitative analysis a t the college level. Their adoption for the teaching of hoth preparative and qualitative organic analysis is increasing. The advantages claimed for the use of micromethods for teaching have been widely discussed and may be summarized as follows: saving time and materials, developing superior habits of care, cleanliness, and manipulative skill in students, and the small initial investment for equipment. The last advantage is an important factor favoring the introduction of small-scale experimentation in many newly built high schools. Smaller breakage and upkeep costs and reduction in the seriousness of possible accidents are additional advantages. The present paper deals with our experience for the past several years in general chemistry a t Brooklyn College with experimental groups for whom the laboratory exercises consisted of small-scale work. The usual approach is t o adapt conventional experiments to the semimicro scale by a reduction in the size of apparatus and materials. The approach used in our work was slightly different. I n a number of cases, a new piece of apparatus was tried with the object of improving the experiments and increasing their instructional value. The economic question as to whether or not the equipment could be made available by manufacturers received secondary consideration.
JOURNAL OF CHEMICAL EDUCATION -
The gas generator shown in Figure 3 is a modified Pyrex 8-inch test tube with a side arm. It is suitable for the preparation of the common gases. The figure Year Reference also shows a convenient, but not eesential, bubbler 1835 Testimony before Select Com- which permits the students to see the rate a t which the mittee on Education in Ireland (Wilson. Mikrochic. Ada, 1, 58 gas is generated. The tip of the gas inlet is drawn to 0.1 (1953)). mm. diameter. 1911- Lehrbueh der Mikroehemie The combination pneumatic trough and water bath 30 (Bergman, Wienbnden) etc. 1924 Practical Chemistry by Micrw is shown diagramatically in Figure 1. It is a round methods (HeRor and Sons, metal vessel of galvanized iron and is 85-90 mm. in Cambridge, England, 1924) 1933 Laboratory Directions in Micro diameter and 75 mm. deep. A metal stand 40 mm. wide Inorganic Chemifitry (Edwarda placed 50 mm. from the bottom has in it three holes of Uros., Ann Arbor, Mich., 5-rnm. diameter through which the delivery tube of the 1933). -4 Laboratory Manual for H i ~ h gas generator can protrude. A removable top with three School Chemistry by Semi- holes of different diameters permits use as a water bath. micro Methods (Welch Mfg. Not illustrated are the student-made spatula and Co., Chicago, 1938). J. CHEM.EDUC..18. 543 (1941). deflagration spoon. These are made from a &in. length Chemi~tryL a b ~ k a t o ;andwork~ of iron wire on which an end is either flattened or made book by Semimioro Methods into a loop. (Globe Book Co., New York,
Selected References t o Small-scale Experimentation for Teaching General Chemistry Authm Reid, D. B.
Emieh. F. Grey, E. Hjort and Woodward
Schiller, W. J., and Sister M. Lawrence Schiller and Lawrence Schiller, et al.
Degering, E. F. Burrows, J. A,, P. Arthur, and 0. M. Smith
1945).
J . CHEM.EDUC.,16,276 (1939).
Weisbrush, F ,
Semimicro Laboratory Exercises in General Chemistry (Macmillan Co.. New York. 19521: J. CHEM.' Eouc., 2'6, 193 (1949). Semimicro Laboratory Exercises in Hieh Sohool (Heath and Co. ~ o s t c k , 1942): School Sei. and Math., 46, 768 (1946). J. CAEX EDUC.,26, 530 (1949).
Bray, W., W.Latimer, and R. Powell
Semimicro Experiments in General Chemistry (Prentieo Hall, Inc., New York, 1950). A course in General Chemistry, Semimicro Alternate Form (Mscmillan Co., New York, 1950).
DESCRIPTION OF APPARATUS
New pieces of apparatus were developed for use in the experiments described later. No originality is claimed for the ideas; they were borrowed from every possible source. Most came from the work of one of the authors in developing micro and semimicro experiments for both teaching and research in organic chemistry. The two-purpose clamp shown in Figures 2A and 2B is an ordinary Bunsen clamp with two arms welded to it. It is capable of holding glass tubing of any diameter (3-25 mm.)
P i g " .
1.
Combination Pn."motic
Trough m d water Bath
EXPERIMENTS USING SEMIMICRO TECHNIQUES
The course of experiments which has been developed includes small-scale experiments to demonstrate topics normally included in a year's course in general chemistry as well as several inorganic preparations. The two described here make use of t,he smal1,scale apparatus described in the preceding paragraphs. Constant Composition of Compounds. This type of experiment appears in almost every laboratory manual that has been published since 1900. All 32 laboratory manuals examined contain one or more experiments dealing essentially with the composition of chemical compounds. These are designed to illustrate various principles such as the law of definite proportions, the law of multiple proportions, and the concept of chemical formulas and valence. For example, Alexander Smith's "Experimental Inorganic Chemistry" which might be considered the most widely-used laboratory text in general chemistry during the first two decades of this century includes experiments on the composition of the oxide and sulfide of a metal and the composition of carbon dioxide. The composition of the sulfide of a metal is still widely used to illustrate the principle of the constant composition of compounds. The experiment which was developed in the present investigation, besides illustrating these principles, is aimed to give instruction as far as possible in what is considered to be the fundamental rule for small-scale work. This rule, stated in its simplest form, is: "Utilize small quantities and small apparatus, and avoid or minimize transfers of material from one vessel to another." The experiment consists of the following operations: (1) Weighing an amount (about 250 mg.) of pure silver oxide in a 3-inch test tube. A weighing ring (shown in Figures 2A and 2B) made from a piece of copper wire is used to hold the tube on the balance. (2) Heating the tube for about 30 seconds in a microflame and testing for oxygen with a glowing splint (Figure 2A).
VOLUME 33, NO. 6, JUNE, 1956
(3) Reweighing the tube t o determine the loss of weight and then calculating the weight of silver and oxygen and the composition of silver oxide. The formula and valence relationship can be derived from this. (4) Adding asmallamount of concentrated nitric acid (0.3-0.4 ml. or about 10-12 F i w r e 2A. Decomposition drops) to dissolve the silver of Silver O x i d d p l i n t i n with evolution of brown position t o Detect Oxygen fumes of nitrogen dioxide (Figure 2B). (5) Precipitation of hydrated silver oxide by addition of sodium hydroxide. (6) Washing the silver oxide free from sodium and nitrate ions2 (Figure 2C). (7) Drying the oxide and weighing it to determine the loss of mass in the cycle of reactions. The experiment can be terminated after steps (I), (2), and (3) have been completed. The average student requires about two laboratory periods (four hours) to complete the experiment in duplicate. I n the opinion of the authors, the completion of the cycle serves t o illustrate the principles of constant composition of compounds and conservation of mass in chemical reactions and in addition provides an excellent method of training in the techniques of small-scale experimentation. The difficulties encountered by beginners are as follows: (1) Overheating the silver oxide. This causes the ~ilverto adhere t o the glass so that it does not all react with thenitric acid. (2) Use of a large excess of nitric acid. This requires a large volume of sodium hydroxide in the neutralization and precipitation steps. The volume of the tube may he exceeded. (3) Washing the hydrated silver oxide. Generally the average student obtains good results in the first step, that is, the composition of the silver oxide. However, losses of from two t o five per cent or more on the first trial are common whereas on the second trial, the losses are usually less than two per cent. Generation and Collection of Gmes by Water Displaeement. The small-scale generation and collection of gases is illustrated by a brief description of the generation and collection of oxygen with the apparatus shown in Figure 3. Hydrogen peroxide is used since its decomposition in the presence of a catalyst requires no heat. The timehonored experiment in which oxygen is generated by heating potassium chlorate was eliminated for two reasons-first, the accident factor which is too obvious 2 The hydrated silver oxide is washed twice with water. The process involves centrifugation and then removal of the centrif"gate by a long capillary pipet. A final washing is made with methanol and then the silver oxide is dried in an oven.
rigvre aB. Addition of concentratd Nitric Acid to D%sol.. the Silvar
Pipure 2C. 1nse*ing Cap& 1-7 Pip.* with Long Tip into th. Tub. Containing the Precipitated Hydzat-d Silver Oxide i n 01de. to Wash th. Solid. Noto that th. Tip of the pipet Should 8. I n s v t s d About 10 mm. &or. th. S d i d
to require any elaboration, and second, the fact that from the authors' point of view it is easier t o explain t o a beginner the preparation of oxygen from hydrogen peroxide t'lan from potassium chlorate. The problem is approached by considering the breaking or cleavage of bonds and by bringing in a t the same time the accelerak ing effect of a catalyst, in this case manganese dioxide. Five 3-inch tubes are filled with water and inverted in the trough as usual. The generating flask is charged with 5-6 ml. of 3 per cent hydrogen peroxide and stoppered with a solid rubber stopper. About 0.5 g. of manganese dioxide is added with the stopper of the generator flask raised momentarily. The first gas, which is mostly air, is allowed t o bubble without collection. This takes about a minute during which 7&80 ml. of gas escapes. There is no suction back when the gas bubbling stops. When the generation of gas ceases, the delivery tube is brought into position. The stopper is raised and 5-6 ml. of hydrogen peroxide is added. This will generate sufficient oxygen t o fill five 3-inch or 4-inch tubes. A third charge may be necessary when 5inch tubes are used. The samples of gas collected are subjected to the traditional tests. The deflagration spoon prepared by the student by making a loop a t the end of iron or copper wire serves admirably for the burning of sulfur and phosphorus.
F i r 3. G.n-+ion Assembly for oxygen a"d Other G.nd Collection i n Small Tub- by Meof tha Combinntion Rough-Bath
