The ultramicroscope as a student project

The instrument is simele and instructive; it. (2) operation, (3) preMration of colloidal solutions, and is an immediate Possibility to one who has a m...
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The ULTRAMICROSCOPE as a STUDENT PROJECT* JAMES A. MILLER

AND

WALTER P. KETTERER

Dormont High School, Dormont, Pennsylvania

This article gives directions for the construction end operation of a simple ultramicroscope. The materials required are available in the average high school. The subject i s discussed under four heads: (1) construction, (2) operation, (3) preMration of colloidal solutions, and

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HE possibilities of the ultramicroscope as a student project are often overlooked. This is apparently due to the misconception that the instrument is very complex; a t least, such is the impression given by many texts of general chemistry. A simple instrument embodying the principles of the ultramicroscopic method, however, can be easily constructed. The materials required are to be found in almost any high-school laboratory. The best microscopes available are capable of resolving particles about one-tenth micron in diameter. Many of the particles contained in colloidal solutions are much smaller and in order to make these visible we must resort to the ultramicroscopic principle. This utilizes the well-known "Tyndall effect." In essence, this method consists of passing a powerful beam of light through a horizontal slit and, by means of a microscope objective, focusing it within a cell containing the colloidal solution. This beam is then observed by means of a microscope placed a t right angles to the path of the beam. When there is no object in the field of the microscope ouly darkness will be observed. If, however, a particle is present in the field and in the path of the l i ~ h beam, t diffraction occurs and the pariicles betray lheir presence by producing a group of concentric mlored r i n',~ s . Such a diffraction halo is -~ only an indication of the presence of a small particle and should not be considered as a true image. Thus colloidal solutions when viewed in this manner appear to be crowded with tiny moving particles. True solutions, however, appear optically void. The influence of matter in the colloidal state upon the economy of our everyday life and upon the reactions going on in living protoplasm can hardly be exaggerated. I t is the Purpose of this article to ,@e directions for the construction of a simple ultramlcroscope, and to describe experiments by which some of the characteristics of colloidal solutions may be demonstrated. ~

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(4) general obserwation. Apparatus for demonstrating the Brownian mmement in smoke as well as that for using high powers in observing the moument in liquids i s described. The instrument i s simele and instructive; i t i s a n immediate Possibility to one who has a microscope.

MATERIALS AND DETAILS OF CONSTRUCTION

The essential equipment consists of a microscope of moderate power having a t least two objectives, a powerful source of light, and a suitable cell for containing the colloidal solution. A piece of wood having the approximate dimensions of by 8" by 30" forms a satisfactory base. Ringstands will serve very well as supports in a temporary arrangement; however, with little extra effort, a permanent instrument may be constructed by the use of wooden or metal devices. Vibration from various sources located both within and without the building can be lessened by the mounting of the base of the iustmmeut upon a number of rubber sponges. An idea of the general set-up is conveyed by the accompanying diagram (Figure 1). A is the source of

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authors wish to express their gratitude to the members of the Science Department of Dormont High School for the help accorded them during the experiments made in conjunction with this article and to Miss Elizabeth Deacon for valuable criticism and suggestions.

1.-SIDE VIEW OP ULTRAMICROSCOPE FIGURE

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A-Light soume C-Focusing objective B-Horizontal slit D-Cell E-Observing microscope

light, suchas an electric arc or a slide-projector; the horizontal slit, B, serves to the beam of light to one plane; C, a microscope objective, focuses this flattened beam of light within the cell, D, which contains the colloidal solution; the beam traversing the colloidal solution is then observed by means of the microscope, E. ~h~ type of light source to be used depends upon the facilities available. Almost all high schools have among their equipment an electric arc or a slide-projector. The electric arc, while giving an intense light, has two important disadvantages, First, it requires constant attention unless it is automatic in operation.

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Second, and most important, the fluctuation of the arc causes minute displacements of the focused beam within the cell. These waverings of the beam are especially noticeable when the higher powers of the microscope are used. The beam is very often displaced entirely from the field and focus upon which the observing microscope is trained; obviously, these conditions do not permit an extended period of observation. These fluctuations, however, can be materially diminished if only cored carbons are employed. A slide-projector, on the other hand, furnishes a steady and strong beam of light. A five-hundred-watt projector easily serves the purpose, and while its light is not so intense as that of an arc, it provides a steady beam that permits the highest powers of the microscope to be used, and it will require little attention. If an electric arc is used it would be well to build a tin housing for it to prevent light leakage; with a slide-projector a simple cardboard shield serves the purpose. The horizontal slit may be made from two razor blades, a piece of tin, and a length of heavy wire. The details of its construction are shown in Figure 2. The

A-Sheet &Razor

EIGURE HORIZON^^, SLIT tin &Heavy wire soldered to A blade bolted to A D-Opening in A

slit can be held in position by the fastening of the wire in a Bunsen clamp holder. The length and width of the slit as well as its distance from the focusing objective will be discussed in the following paragraph. The focusing objective is nothing more than an ordinary microscope objective so placed that it will converge within the cell the light issuing from the slit. The lens used should be a 16 mm. or 10X objective or some power not too far removed from this. The working distance of this objective is about 7 mm. This is a convenient distance, for i t should be remembered that although the observing microscope objective, the cell, and the focusing objective must of necessity be close together, they must not interfere with each other. If a second microscope is available an adjustable focusing objective can be made (Figure 3). The mirror and stage are removed and the body-tube is taken from its track and replaced with the eyepiece end downward. The grip-arm is tilted a t right angles to the pillar; the body-tube should now be in a horizontal position, and the space about the projecting objective clear for the placement of the cell and the observing

microscope. This arrangement provides an adjustable objective, which is not absolutely essential; the objective itself can be used, secured in a horizontal position by means of a clamp and ringstand. When the adjustable arrangement described is employed the horizontal slit should be as long as the inside diameter of the body-tube and 3.-DIAGRAMOF ADJUSTABLE about 6 mm, wide FIGURE F o c u s ~ ~OBJECTIVE o f o r t h e normal A-Slit; B-Body-tube reversed in bodv-tube leuah of 160-mm. TG slit should be placed as close to the open end of the bodytube as possible. For the objective alone the slit should be as long as the inside diameter of the objective and about 1 to 2 mm. in width. As before, the slit should be as close to the objective as possible. The cell for the colloidal solution is an important part of the ultramicroscope. Its walls must not reflect light and it must be bounded on two adjacent sides by plane transparent surfaces a t right angles to each other. A simple cell satisfying these requirements may be constructed from glass, hard rubber, bakelite, or some similar substantial material which is waterproof and more or less workable. If glass is selected the piece should be about 6 cm. long, 2 cm. wide, and a t least 0.5 cm. thick. At a point on the upper surface about 3 mm. from one of the long edges and about 1 cm. from the end a tiny hole is scratched in the glass with the point of a file. The hole is then filled with a glass-grinding fluid1; using the hole as a center and a sharp 3/1a" metal drill, bore a hole 3 mm. deep. By an even and gentle procedure, and by generous use of the grinding fluid, a neat hole can be made. If a thin wall of glass still remains on one side take a file and cut it away until a rectangular opening is made. The walls of the niche should be filed as flat as possible, care being taken that all of the surfaces being filed are well lubricated with the grinding liquid. When the cell proper has been completed the inner surface should be painted black. The cell is now ready for the attachment of the vertical cover glass, which is cut from a thin microscope slide and should be 5 mm. in width and 1 cm. in length. The slide is cemented over the vertical opening of the cell with Canada balsam or a similar waterproof cement. The remaining opening is closed by an ordinary cover glass after the cell has been filled. A less fragile and more easily constructed cell can be fashioned from hard rubber or bakelite. These materials are easily shaped; as they do not reflect light very well the painting of the cell is avoided. The same general instructions that are given for the glass cell should be followed except that the block of material

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~HODGMAN AND LANCE, "Handbook of chemistry and physics," 13th ed. Cleveland Rubber Publishing Co., Cleveland, Ohio, 1928, Laboratory Arts and Recipes, p. 1044.

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Hard Rubb., a Bahllte Cell

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Frcum 4.-DETAILS oa ULTRAMICROSC~PE CELLS A-Vertical cover glass B-Hale for bolting to rod C-Cell proper

D-Surface for clamping E-Colloidal solution F-Removable cover glass H-Wax for sealing

need be only 2 or 3 cm. long, 2 cm. wide, and 1 cm. thick. The niche is made in the manner described above exceptthat the grinding fluid is unnecessary; a co~inz-sawshould be used instead of a file for cuttingAawiythe wall. The glass block is made long in order to provide a surface for clamping. The hard rubber or bakelite block, however, is bolted to a rod which is then clamped in position. See Figure 4 for

tin formed into a cylinder and soldered around the free edges. The tube entering the microscope should have a diameter of about '/s"; the diameter of the eyepiece tube is approximately 1". The tubes are fastened to the inside of the box as shown in the diagram. A plane mirror is bolted a t an angle of 45' to the side of the box. This simple piece of apparatus permits the observer to sit down while watching the colloidal particles; it is almost a necessity if one desires to observe for any length of time. Difficulty may be experienced in holding the head steady and observing the particles a t the same time. TO correct this the observer may arrange some type of horizontal rod u ~ o nwhich he mav fold his arms in order to support head. The latter arrangement must be independent of the ultramicroscope itself as the vibrations set up will prevent observation if transmitted to the liquid in the cell. PREPARATION OP COLLOIDAL SOLUTIONS

Today it is recognized that the terms colloid and crystalloid distinguish between different states of mat-

FIGURE 5.-APPARATUS FOE OBSERVINGBROWNIAN MOVEMENT IN SMOKE A-Mouthpiece E-Bottle for catching tarry substances B--Mohr pinchcock F-Holder for cigaret C-Rubber stopper for clamping GCigaret D-Cell further information concerning the construction of ter and not between separate classes of matter. So many substances have been obtained in the colloidal these cells. In order to observe the Brownian movement in smoke form that this state is regarded as one in which almost a special cell must be made. 'With the exception of the every form of matter may exist. All of the different side tubes it is similar to the hard rubber cell described methods for the preparation of colloidal solutions may above (Figure 4). The holes are in diameter and be divided into two classes. are bored from each end of the cell block to the center These are: (a) the "couof each cell wall. The lead-in tubes are made by densation" methods in drawing apart a piece of glass tubing in the flame. which we pass from a true The capillary is broken off each tube until an end hav- solution to a colloidal sysing a bore of about is obtained. These tubes are tem, and (6) the "dispersecured in the holes with a cement of asbestos fibers sion" methods by which we and water-glass. The tubes are first coated with the pass from matter in mass cement and inserted in the holes; cement is then packed to colloidal particles. We about the tubes. The cell is kept in a warm place un- will deal here with only til the cement has hardened. The cover glasses should three of the six general be permanently attached to the cell block. The details of the remainder of the apparatus for the observation of smoke are shown in Figure 5. A-Hole far boltine mirror at The vertical microscope needs no detailed discussion angle of 45' GORTNER, "Outlines of hio- B-Tubeentering microscope here. In order to facilitate observation a right-angle John Wiley & Sons, C-Eyepiece tube extension tube may be built according to the directions chemistry," Inc., New York City, 1929, pp. D-Open top, covered with given in Figure 6. The tubes may be made from sheet 21-42. cardboard lid

HYDROLYSIS METHODS.Certain substances soluble in water can be hydrolyzed to produce colloidal dispersions of insoluble substances. An example is the hydrolysis of ferric chloride: 2FeClr

+ %Ha0+FerOa.nH10 + 6HCI

A ferric hydroxide sol may be prepared by ebullition of 100 cc. of distilled water in a beaker with slow addition of 1 cc. of a 30% solution of ferric chloride to the boiling water. Hydrolysis of the salt occurs and a brown colloidal solution of ferric hydroxide is formed. If allowed to remain the hydrochloric acid formed during the reaction will cause a gradual flocculation of the colloid. The sol must be purified of most of the acid by dialysis. Make a diilyzer cup by folding an 8" square of'wet parchment paper over a 150-cc. beaker; the paper is held in place by a piece of string tied about the beaker. When dry the cup is removed and fitted with a loop of string to form a support. Suspend the cup in a large beaker of distilled water after filling it with the colloidal solution. After three hours change the water, and leave overnight. This solution will be fairly stable. DOUBLEDECOMPOSXTXON METHODS.Colloidal arsenious sulMe. Prepare a 1% solution of arsenic trioxide by boiling 1 g. in 100 cc. of water and filtering the solution. Bubble hydrogen sulfide through this solution until i t assumes a light yellow color. Filter if necessary. Colloidal Prussian blue. Mix 10 cc. of N / 5 0 ferric chloride solution with an equal volume of N / 5 0 potassium ferrocyanide. Filter and dilute to 40 cc. with distilled water. REDUCTION METHODS. Reduction may be carried out in a number of ways; the preparations described below use tannic acid as the reducing agent.8 Collo'dal gold. Dilute 2 cc. of a 1% gold chloride solution with 98 cc. of water. Prepare a solution of 0.5 g. tannic acid in 100 cc. of water. Heat both solutions and, using equal parts, slowly add one to the other. A colloidal suspension of gold results; the color of this may vary from a clear ruby to blue depending upon the size of the particles formed. Colloidal silver. Add very dilute (1:30) ammonium hydroxide, drop by drop, to 5 cc. of a 1% solution of silver nitrate until the precipitate which forms just disappears. After diluting with 100 cc. of water mix equal volumes of the resultant solution and a tannic acid solution as prepared above. In this case, tannic acid reduces silver oxide to colloidal silver. As with gold, the color varies with the size of the particles; it may be red, brown, or greenish brown. The colloidal solutions whose preparations have been described above represent only a few of the number which may be examined with this instrument. For more information concerning the methods and the preparation of colloidal solutions as well as colloids in general the reader should consult a text on the subject. 'HOLMES, "General chemistry," The Macmillan Co., New York City. 1922, p. 351.

OPERATION

The apparatus is assembled as shown in the diagram, the light source, slit, and horizontal objective being placed as close together as possible. When these three parts are aligned the cell should be filled and the cover glass sealed k i t h wax. It is then clamped in position and the beam of light is focused within the cell. The microscope is placed over the cell and the beam of light is centered. In order to do this the observer should remove the eyepiece and bring the beam into view by looking down the tube and raising or lowering the objective; once the beam is brought into view i t can be centered easily by shifting the base of the microscope. The eyepiece is then replaced and the objective is lowered almost to the point of contact with the cover glass. The observer, while viewing the unfocused field, should then slowly raise the objective until a clear image of the beam is formed. All centering should be done with a low-powered objective. In the use of the higher powers of the microscope, it will be troublesome to bring the objective of the observing microscope and the beam of light close enough to permit the particles to be resolved. This difficulty

A-Thin cover glass B-Slanting hole C-Cell well for objective D-Hole for boltine to rod

may he overcome by one of the following methods. The cell may be placed a t an angle to the beam of light and the observing microscope tilted to the same angle; this arrangement permits the beam to pass immediately below the objective. The same end may be gained by the use of the cell described in Figure ?A. This cell is made from bakelite or hard rubber and the same precautions should be observed as in the construction of the ordinary cell. The upper opening should be permanently closed with a very thin cover glass. The vertical opening is sealed with an ordinary cover glass after the cell has been filled. Still another method dispenses with the cell entirely. A small block of some non-reflecting material is clamped in such a position that one surface is parallel to and just below the beam of light. A drop of the colloidal solution is placed near the edge of the block directly in the path of the beam. The microscope is placed over the drop of solution and the high-power objective lowered into the solution and focused upon the beam. The end of the hard rubber or bakelite cell can be used in place of the block for supporting the drop of colloidal

solution. The main disadvantage of the latter method lies in the fact that the changing curvature of the drop due to focusing and evaporation displaces the horizontal beam of light. This may be avoided by use of the cell described in Figure 7B. The details of its construction are given in the diagram. I t will take some time to become proficient in placing-the cell and bringing the beam into view; but if one is deliberate and unhurried in his actions he will soon learn to operate his instrument quickly and eftidently. The room in which the apparatus is used should be as dark as possible. Both eyes should be kept open a t all times while the instrument is being used. A piece of black cardboard should be placed before the eye not employed in viewing the beam; this arrangement is easy to become accustomed to and prevents the observer from unconsciously injuring his eyes. The cells described above are easily emptied and cleaned. If the cell has been sealed the wax is broken off and the liquid is absorbed by a thin roll of filter paper. The cell is then flushedwith distilled water, and dried with another roll of filter paper. A micro-pipet, made by drawing a piece of glass tubing in a flame, is used in filling the cell. When the cover glass is lowered over the filled cell it should be so manipulated that no air bubbles are formed beneath it. The cell is then sealed about the edges with drippings from a wax candle. GENERAL OBSERVATION

The most obvious characteristic of colloidal solutions when viewed under the ultramicroscope is that the apparently homogeneous solution now appears to be heterogeneous. Colloidal solutions, unless specially prepared, generally contain particles of varying size. The diameters of the particles vary from above 1 X mm., which is the limit of microscopic visibility, to 1 X lo-' mm., which borders upon molecular dimension~.~The following table shows the nomenclature used for particles of varying degrees of dispersity : Term Micron.

Dioad6r 1 X 10-r mm. to 1 X 10-4 mm. (visible vnda microscope) Submicrons 1 X 10-I mm. to 5 X 10" mm. (visible under ultremierormpe) Amieronn about 1 X 10- mm.

The ultramicroscope described above is capable of resolving the larger submicrons; however, many sols when viewed with this instrument will show only a faint licht - cone. This cone is due to the presence of

' GETMAN,"Outlines of theoretical chemistry?

& Sons, Inc.. New York City. 1918, p. 239.

John Wiley

the smaller submicrons and amicrons. Even the best ultramicroscopes cannot resolve an amicroscopic light cone, as it is termed. The Brownian movement, which is the effect produced by the molecules of the medium upon the colloidal particles, is easily discernible with this apparatus. As a result of this motion the particles describe complicated zig-zag paths. Other things being equal, the magnitudes of these paths depend upon the sizes of the particles and the density of the medium. In visible smoke, for instance, the motion is evident even when low powers are used. However, when the medium is a liquid the average free path is not as great, and high powers must be used to show the movement clearly. Certain precautions must be observed in.the viewing of the Brownian movement in tobacco smoke. The entire system should, of course, be air-tight. The smoke is imprisoned, after being drawn through the apparatus, by release of the two pinchcocks in the order of their positions from the mouthpiece. The number of particles and the magnitude of their movements will probably astonish the observer when he first sees the smoke under the ultramicroscope. The smoke will gradually disappear. I t is worth while to wait until only a few particles are visible; their motions may then be easily followed. There is a marked diierence between the ultramicroscopic character of suspensoid and emulsoid colloids. The suspensoids are in general well defined, the metallic sols especially so; but while the emulsoids show the Tyndall phenomenon, they are very poorly defined. This lack of distinctness is probably due t o the small difference between the refractive indices of the particles and the medium. Conversely, if the difference between the refractive indices is considerable the definition will be good. The ultramicroscopic character of visible smoke is generally rather well defined. In addition to the above there are many other experiments relating to colloids that could be carried out with the aid of this apparatus. For instance, the ultramicroscopic nature of many common substances could be investigated. Or the instrument described could be refined by having the moving parts made more easily adjustable and by being changed in any other respect that would make it more efficient. An adjustable slit permitting thin slices of the colloidal solution to be taken is but one example of this. More difficult experiments such as the photographing of colloidal particles, the determination of particle size, or the dernonstration of catapboresis might be attempted. There is no reason why the average high-school student of chemistry cannot make a similar instrument and go much beyond the scope of this article if he is willing to undertake the necessary study and experimentation.