MICRO METHODS IN GENERAL CHEMISTRY* E. V. HJORT AND H. E. WOODWARD UNIVERSITY OF
PIITSBURGH. PITISB~OH,PENNSYLVANIA
Micro methods are becoming so important that it has seemed appropriate to begin training stdents to handle small amounts of material early in their careers. An experiment of this nature has sham that there i s an economy of time, space, and material. The @dagogical advantages are also significant. Teachersfaced with the necessity of cutting expenses without cutting the quality of work may well consider such methods.
. . . . . .
The modern tendency in the chemistry laboratory is in the direction of the application of micro methods. The term "micro methods'' is used to designate not only procedures involving the use of the microscope, but includes all methods in which very small quantities of material are used. Although the Curies worked with tons of ore, their work on the discovery of radium was on a micro scale, since means had to be applied to detect very minute quantities of the element. In the field of organic chemistry the application of quantitative analysis is well established. Methods for micro qualitative analysis have been worked out and are rapidly gaining favor. This paper is a preliminary report on the application of these methods to the teaching of a course in general chemistry. The advantages of micro methods consist in economy of time, material, and space. In addition to these advantages, theme of such methods in the general course serves to start preparing the student for micro work at the very outset of his career. From a pedagogical standpoint the time element is an important factor. If a student can make similar tests by micro methods in considerably less time than by macro methods, it follows that he can cover more material in the course of a given period of time. It may in some cases be found preferable to decrease the cost of the course by decreasing the number of hours of laboratory instruction. The authors have found that the material usually given by macro methods in two semesters can be covered more thoroughly in approximately one semester. This leaves the second semester available for additional work in such fields as inorganic preparations, elementary experiments of a physico-chemical nature, or qualitative analysis. The micro technic is easy to acquire and when proficiency is attained, individual tests may be performed very rapidly. A very common source of trouble in teaching a first course in general chemistry is the difficultyof persuading the student that he should avoid
* Paper No. 248 from the Department of Chemistry. University of Pittsburgh. 1815
1816
JOURNAL OF CHEMICAL EDUCATION
,
Ocrossn, 1932
the use of too much reagent, not only for reasons of economy but because secondary reactions are more likely to occur with such excess. Chemical reactions take place between ions, or atoms and molecules, and theoretically, a t least, only a few such groups are required for a reaction to occur. Practically, of course, sufficient material must be used so that the change may be observed. In most cases single drops of reactants are sufficient for this purpose. For example, if a drop of dilute ammonium hydroxide is added to one of copper sulfate on a microscope slide, the precipitated cupric hydroxide is easily visible. An extra drop or two of ammonium hydroxide shows the re-solution of the precipitate to form the copper-ammonia complex. It is often difficult to teach a student to keep his apparatus clean. Cleanliness is essential for good micro work. Thus, one speck of an impurity is a much larger percentage in a single drop of reagent on a slide than it is in a beaker of liquid. Also, traces of grease on a slide make difficult such operations as micro decantation and filtration. I t is no small advantage that with the use of minute amounts of material, odors are eliminated or greatly reduced, and the hazards of explosion, fire, and poisoning are negligible. For the most refined micro work, costly apparatus and a degree of manipulative skill are required that are out of place in working with beginning students. It was therefore decided that only the simplest apparatus was to be used. This apparatus consisted essentially of microscope slides, crucibles (2-cc. capacity), test tubes (8 mm. X 45 mm.), a small hand centrifuge, and sundry equi{&ent made by the students from glass tubing. Occasional use was found for larger test tubes, beakers, and flasks. It is obvious that the number of these larger pieces is greatly reduced, and expensive equipment, such as condensers, is entirely eliminated. The chemicals were essentially the same as those used in the regular course, but the quantity required was much less. Thus, in the use of solid reagents, a quantity the size of a pinhead was usually ample, and the standard size solution was a drop. The work was done in the regular freshman laboratory, although the space actually required was considerably less than for the macro course. The locker space per student in the micro course could easily have been cut in half. There was also considerablesaving in stockroom space. The cost of the course has not been computed, but the use of smaller quantities of chemicals and smaller equipment must necessarily have resulted in an appreciable saving. None of the methods used were new, but were adaptations of methods described in the literature. (See Bibliography.) A few illustrative examples follow :
VOL. 9. No. 10
MICRO METHODS IN CHEMISTRY
1817
1. Precipitation tests are made on a microscope slide or a spot plate. 2. Filtration may be carried out by a micro method, hut it is usually an advantage to make the separations in a centrifuge. The residue may he washed by stirring with the washing liquid and again centrifuging. 3. A distillation can he performed in a crucihle, receiving the vapors in a drop of liquid hanging from the under side of a microscope slide which is placed over the crucible. Thus, in the test for ammonia, the ammonium salt is placed in the crucihle together with one drop of sodium hydroxide. Gentle heat is applied and the evolved ammonia distils into a ,drop of water on the slide containing an indicator. Similarly, the action of sulfur dioxide of permanganate can he tested by receiving the gas in a drop of dilute Dermanganate solution. 4. ?3ublim~tioucan he carried out in a glass tube sealed at one end or in a crucible. In the latter case the sublimate is received on a glass slide placed over the crucihle, or on the under side of a watch glass in which is olaced a ~ i e c of e ice. 5. Reactions requiring the passing of a gas over a solid (for example, the reduction of copper oxide by hydrogen or the bleaching of colored cloth by chlorine) may be carried out by placing the solid in a short length of 4-mm. glass tubing and connecting this directly to the gas generator. In place of the usual Kipp or other type of gas generator, the student may make his own from a six-inch test tube with a delivery tube attached. A U-tube, with acid in one arm, separated by a porous plug from ferrous sulfide in the other arm, serves as a convenient generator for hydrogen sulfide. The acid is brought in contact with the solid by tilting the tub@and the evolved gas escapes from a delivery tube in the arm containing the ferrous snlfide.
The work has been in progress a t this University for three years. The first two years were devoted to small groups, doing the same experiments in the same laboratory with larger groups. During the past year a section of forty-six students devoted one and one-half semesters to micro experimentation, the experiments being especially written so as to he adapted to this work. The question is frequently asked: "Does the micro work unfit the student for handling larger equipment?" To answer this, half of the second semester was devoted to experiments as usually performed. The students showed no inferiority to students who had been working with the regular macro equipment during the whole year. On the whole, the authors feel that the micro course has been an advantage to the student in training him in habits of closer observation. The interest in the work has been excellent and many valuable suggestions have come from the students themselves. During the coming year i t is planned to give each student an individual set of reagents in small vials set in a wooden block. It is expected that a
1818
JOURNAL OF CHEMICAL EDUCATION
OCTOBER. 1932
considerable saving of time will be effected thereby, in that the student will not waste his time in running to the side shelf for materials. It is also planned to make a more quantitative study of time consumed and actual cost as compared with the regular course. The authors are convinced that by the use of the micro method, chemistry may be made available to schools which now feel they do not have the space or cannot afford the macro equipment. After the first cost of equipment is paid the upkeep cost will be low, and any room equipped with water, gas, and electricity will serve as a laboratory.
Bibliography 1. Enarcn AND SCHNEWER, "Microchemical Laboratory Manual," John Wiley and Sons, Inc., New York City. 1932,180 pp. 2. GREY.E. C., "Practical Chemistry by Miuo Methods,'' W. Heffer and Sons. Ltd., Cambridge, England, 1925, 124 pp. AND MASON,"Handbook of Chemical Microscopy." Volumes I and 11. 3. CHAMOT John Wiley and Sons, Inc., New York City, 1930, Vol. I, 474 pp., and Vol. 11, 411 pp. 4. PREGL,F., "Quantitative Organic Microanalyses." 2nd edition, J. & H. Churchill, London, 1924, 190 pp. 5 . TIEDE, E., AND RICRTER,F. (Editor$). "Handbuch der Arbeitsrnethoden in der Iuargaueschen Chemie," Zweiten Band, Walter de Gruyter and Co., Berlin and Leipzig, 1925, pp. 655 to 882.
