Tested Overhead Projection Series Compiled by
HUBERT N. ALYEA Princeton University
Electrical Devices All electrical devices are inserted in the holder device E-1, Figure4.
Holder, E-1
cell an the stage, and project. Switch on the battery I?-2. Result: no electrolysis occurs with water alone. Eow add a drop (no more) of concentrated sulfuric acid. Result: the acid e m be seen swirling into the water, and immediately current flows, twice as many huhbles (of 2 H2) evolving from one pole as from the
City. We made a plastic case for the battery, attached a 10 in. length of lamp cord and phone plug (male) to fit into the jack of E-1, with s. toggle switch marked "charge" and "discharge" on top, and then wrapped the whole device in black electrical tape to make a compact waterproof unit.
Materials. Two pieces of clear Plexiglas 5 x 31/4X in. Cuts from a/, in. thick stock of black Plexiglas: two pieces 5 X '/P X one piece 2'/, X '/a X a/,, one piece 2 '/B X X a/, in. Two binding posts, one phone jack and plug two feet of lamp cord; two flat-headed machine screws, one strip of brass 3 X 1 x in., a. stainless steel base 3'/4 X 1 X in. Making. (a) Bare small holes in each end of the brass strip. (b) Bore four holes along the median of this strip '11, 11/,, l 3 / 4 , and 21/. in. from one end. (c) Thread them to receive the hinding posts and the flat-headed machine screws. Cut the plate in half, and bend each piece Lshaped as shown in Figure 5. (d) Bare a hole about '1, in. diameter down the long axis of each 5 X X in. plastic (with s. drill 3 in. long it will be necessary to bore it from both ends). Then cut into the shzppe ~hownin Figure 5. The top cut, on the outside of each piece, is to make the brass piece flush with the plastic sides. The bottom cut is to make a thin side for the lamp cord and to admit the wire. (e) Bore a I/. in. bole '/,in. deep in the top of each piece so that the hinding post, after screwing through the brass angle, can protrude into this hole. Bore a hole and countersink it in each piece in. from the top; the hole should allow free passage of the flaeheaded machine screw. (f) Bore a large hale for the lamp cord. (g) The pieces are now ready to assemble. Thread the lampcord wires through the holes, solder them to the hrass angles, and bolt the brass angle to each black piece. (h) Now place one clear piece of plastic on a flat surface and cement onto it one hlack side-piece (for cementing instructions see the J a n u a ~article). Then cement the middle cross-piece and bottom piece in b lace. Finally cement on the other sidepiece. (i) Knot the lamp card to anchor it in the device, cut a length of about eight inches, and attach the free ends to a female socket. ( j ) Cement on the second clear plate. (k) Attach the stainless steel base with epoxy-resin.
The Battery, E-2 Recently there have appeared on the market cadmium-nickel batteries with a 110-volt recharger for about $10. Use one delivering about 12 volts. We have found satisfactory the replacement battery ($20) and charger (510) sold far their cordless electric drill by the Black and Decker Company, 227 Varick Street, New York
Figure 4.
E-1 Bechical Cell
The Inserfs The inserts for cell E-l are shown in Figure 6. Their dimensions are not a t a11 critical, so that the reader is referred to this figure instead of being given construction details.
Making. Make two of the electrodes labelled GAS in Figure 6. The width of the brass strip is about 8/36 in. less than the innide of E-1, the strip notched a t the top, and the spiral of platinum wire soldered to the base of the strip. Then the whole is made waterproof by pressing two pieces of black pla~ticelectrical tape against both sides of the strip, the tape being about in. wider than the strip 80 that the overlapping edges of the tape can be pressed together to form a plastic pocket for the brassstrip. A simpler gas electrode can be made by taking 6 in. of plastic-coated annunciator wire (No. la), soldering a lug to one end and a platinum wire spiral to the other, and then bending the whole into the same shape as the brass strip device.
Using. Fi cell El four-fifths full of water. Plug in hattery E 2 , place the
other (of 0.). Switch ofl the battery. Result: the evolution of gas immediately ceases. Reverse the switch, and direction of the current. Result: the double volume of gas comes from the other electrode. Lower a gas bolder1 over one or hoth of the electrodes, and collect the off-gases. When full (this will take some minutes) test for hydrogen by bringing it up to a lighted match (POP). Test the other tube for oxygen by lowering it over a glowing taper (glows brightly). Finally collect a gas holder two-thirds full of hydrogen and one-third full of oxygen (by moving the holder from one spiral to the other a t the correct time, or by placing one holder so it straddles hoth spirals, or by reversing the current after 4/r tube of HS has been collected.) Bring the 2H1 O2 mixture up to the flame (POP).
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Making. Make one each of the devices labelled ELECTRODE and WIRE PLATER in Figure 6. The electrode can be cut of a strip of copper. Or it can be made by taking 6 in. of No. 12 bare copper wire, soldering one end into a. log, and (using a pencil as a form) bending the free end into a spiral 3 in. long. The
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wire plater can he made from plastic, with a copper or brass lug on top, and a length of No. 24 enameled copper wire held in place with set screws or binding posts. With this device new wires cexn he readily introduced. A simpler device is merely a 4 in. length of the enameled capper wire soldered to a lug, and bent L-shaped so that the wire will project down into cell E-1. Using. Burn off the middle 1 in. of the
Figure 5.
F!ectrical-Coude lnserfs Making. Prepare four half-cells as shown in Figure 6. These can he of plastic, with black sides and clear front and hack. The electrodes are of Cu, Mg (wire), Zn, and Ph, and are marked with pencil or waterproof ink in such a position that they will project, when inserted in E-1, as Cu/CutP, Mg/MgtZ, Zn/Zn+z, and P b / Pbta. On the side away from the lug, near the top of the cell, bore a I/, in. hole.
Ports for Cell E-1
wire and polish i t with steel wool. Fill cell El four-fifths full of cupric chloride solution. Clamp the electrode and the wire plater in the cell so that the two surfaces run parallel to each other and about I/. in. apart. Plug in battery E-2, place on the stage and project. Throw the switch 80 that the oopper wire is the uegs, tive pole. Result: in a. few seconds the wire swells noticeably. The fact that the wire is thin, a,nd that the lower portion of i t is insulated by the enamel, enhances the plating effect: only the h a e portion of the nire is electroplated, and swells Like s sausage. Now reverne the switch. Result: the wire grows rapidly thin again, and flakes of copper fall from the wire. Be sure t o point out to the class that thisis improper electroplating, i.e., the current density is too great, so that the plating scuffs off; but that when the plating is done slowly with the proper current the copper adhercs. Silver will also plate strikingly well, u i n g the same electrodes and copper wire immersed in aqueous AgNOa.
Moisten around the hole with Plexiglas cement, wait 30 seconds for the plmtio to soften, then press a 1 X in. piece of filter paper against the hole. Touch tiny drops of cement t o the four corners of the papcr to seal it to the cell, hut take care that the psper immediately over the hole has no cement on i t for it is to act as a saltbridge to the other half-cell.
lon-Migrotion lnserf
Using. Fill cell E-1 one-quarter full of dilute salt (NaCI) solution. Fill the copper half-cell with 1 M CuC12,and clamp i t in cell E-1. Unscrew and remove the other binding post. Fill the zinc half-cell with 1 M ZnCL, lower i t alongside of the copper h d f - d l . If the salt solution in E-l does not rise shove the center spot in the paper on the half-cells, add mare salt solution to E-l. Press the zinc lug against the brass top. Result: a current of 1.1 volts is generated by the couple Cu/ Cut'// Zntz/Zn, and registers an a meter plugged into E-1.2 The couples Cu/Zn, of Cu/Mg, and of Cu/Pb, each eell being filled with its corresponding 1 M salt solution, give approximately 1, 1'/. and I/* volts respecbively. Have the class confirm this from the E.M.F. values in the electromotive force series. Standard hydrogen The standard far measuring E.M.F. is the H2/H+ half-cell, arbitrarily nssigned the value of zero. Thus if H2/H+//Znt+/Zn gives an E.M.F. of +0.76 voiti;, and
Making. Make the migration cell so i t fits loosely into cell E-1. Prepare two clcctrndes by soldering 2 in. of a platinum wire spiral to brass or copper lugs. Fasten the lugs b the migration cell hy boring and threading holes on the top of the migration cell, also holes in each lug, and fastening the electrodes to the cell with roundheaded machine screws. Using. Fill the migration eell with dilute KI-aq., containing a, few drops of phenolphthalein indicator. Insert and clamp i t in cell E-1. Insert battery plug E-2 and place the device on the stsge and project. Switoh on the current. Result: almost immediately the solution around the negat,ivepole turns red (KOH withphenolphtbalein), the solution around the positive pole turns brown (1%). Reverse the battery switch. Result: the colors a t the poles reverse. Later experiments will describe migrstion of ions through gelatin in the cell.
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Journal of Chemical Education
Gas
Figure 6.
Hn/Hf//Cu++/Cu gives an E.M.F. of -0.34 volts, we should expect the couple Zn/Znt+//Cu++/Cu to give an E.M.F. 0.34 = 1.10 volts. Thi8 can he of 0.76 confmned by the use of haE-cells. The standard hydrogen electrode is made the same as the other half-cells, but with a small platinum-black foil or wire immersed in 1 Molar HCI. A few moments before the voltage is measured, drop in small pieces of mossy zinc, or coarse granulated zinc, so that hydrogen is generated and bubbles up to surround the platinumblack electrode and saturatet it with hydrogen a t one atmosphere pressure. This arrangement is, of course, not accurate since the HCI is being consumed; for correct voltages generate Ha in device G-4 and hubble it artround the platinum-black electrode. Calaml electrode. Another standard is the cdomel half-cell KCl,,t. or ur,a.t., Hg,CI*/Hg. I t is a half-cell in which a Pt-tipped copper wire dips into 1 ml of in. of a Hg on top of which is placed paste (made by grinding calomel with mersat. KCI) and the cell filled with cury sat. KCI. With sat. KC1 the voltage a t 25°C. is 0.244; with I N KC1 it is 0.2802. Coneatration cells. I t is also of interest to use two half-cells containing the same M/MC hut of different ionic concentrs, tions. These are known as concentration cells. The p H meter, used for measuring hydrogen ion concentration, is an applic* tion ai a concentration cell.
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1 Electrode
Half Cell
Electrical Inserts
Glass Cell Inserf Occasionally electrolysis experiments will he carried out in organic solvents (e.g. glacial acetic acid) which will attack Plexiglas; for these experiments i t is convenient to insert in cell E-1 a glass liner made by cutting a rectangular bottle to the proper size.
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Various gas holders will he described in next month's article. The one used here consists of a Pyrex brand culture tube, No. 9200, cut to 8. length of 3 in., with a 2-in. length of 6 mm tubing sealed to its bottom. Stopper with a cork, ajler i t has been immersed in the liquid in cell E-1. *Adapt an ordinary 1-3 volts D.C. voltmeter to projection and plug i t into E-1. Details for making such a device (Me-volts) will appear in a later article. a For further discussion on the hydrogen and calomel electrodes see Daniels, Matbews, ~rilliams,Bender and Alherty, Experimental Physical Chemistry, pp. 174-76, and 393-94. McGraw-Hill, 1956.
