THE ANALYSIS OF A SILVER-COPPER ALLOY OGDEN BAINE and JOHN BANEWICZ Southern Methodist University, Dallas, Texas
DURING the past two years an experiment dealing with the quantitative analysis of an alloy has been included in our general chemistry program with encouraging results. The experiment is an elaboration of the old and formerly popular procedure which involved the determination of the silver content of a tencent coin ( I ) , an experiment which has been generally abandoned due to the fact that destruction of coins is illegal (2). The materials used for analysis in this experiment are various silver brazing alloys, with a wide range of silver and copper content. Both metals can easily be determined in one laboratory period providing certain procedures are followed. Considering the type of equipment necessarily employed in the general chemistry laboratory for this purpose the results are very satisfactory. The experiment may be logically included in the laboratorv uDon the com~letionof the " nrozram . qualitative analysis studies. In general interest and instructional value, the experiment has been highly regarded by the students. Thirty-five silver-brazing alloys are listed in the literature of the manufacturers, and are composed largely of varying amounts of silver, copper and zinc.' The alloys are produced in the form of a wire about 2.5 mm. in diameter-from which it is convenient to cut sections that will result in optimum working weights and volumes for analytical purposes. These samples, incidentally, will cost about five cents each. The commercial limit for the variation of the silver and copper composition from the nominal values is 1.00'%. However, according to the manufacturer, the general agreement with listed values is much better than this (5). The copper analyses are carried out colorimetrically on a nitric acid solution of the alloy. A very simple phot,ometer, described in a following section of the report, Tvas assembled for this experiment. Silver determinations can be made gravimetricallv by ~recipitation of the chloride or volumetrically by &t&n with thioryanate. The latter procedure proved to be more accurate and much faster in the hands of our students. A diaeram of the ~hotometeris shown in the fieure. The unit is housed in a wooden chalk box ( A ) mounted upside d o m on a wooden base. A l-in. I. D. brass tube 41/2 in. long (B), which fits very snugly through a hole drilled in the box, serves as a test tube holder. The bottom of the brass. tube fits over a rubber stopper (C) fastened to the wooden base. The rubber stopper acts
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both to secure the brass tube and to cushion the sample tubes. A '/rinin. hole is drilled through the brass tube ll/? in. from its bottom, and a similar hole is drilled in the proper position in the side of the box. On the back of the box, in line with the holes in the brass tube and the box, a Weston model 594 Photrouic photocell (D) is secured. The two terminals of the photocell are connected to a Westinghouse RX35 0-60 microampere ammeter ( E ) mounted in the top of the wooden box. For pedagogical purposes the box is not fastened permanently to the baseboard but is fitted into a grooved recess cut in the base. Hence, the housing can be lifted and inverted to show the nature of the apparatus. The light source is a 100-watt bulb clamped to a ring stand situated close to the hole in the box. The intensity of light passing through the apparatus can be adjusted either by means of a variable transformer. and/or bv movine the rinestand. .
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EXPERIMENTAL PROCEDURES Dissolution of the Sample. Weigh two alloy samples (about 3 cm. in leneth and weizhine about 1.5 e.) an a small trinle-beam
space is not available, an inverted funnel may he clamped over the mouth of the flask and attached to an aspirator to dispose of the corrosive vapors. Another technique involves fitting a piece of bent glass tubing to the flask by means of s. rubber stopper, with the end of the tubing just above a few ml. of dilute alkali in s. small beaker. These two procedures involve the sacrifice of a short piece of rubber tubing or a stopper. After the samples dissolve it will be necessary t o boil'the solutions gently for about one minute to expel oxides of nitrogen which cause the solution to be green instead of blue. Determination of Copper. Transfer one of the solutions bo a 25-1111. graduated cylinder or graduated test tube; then rinse the flask with two successive 5-ml. portions of distilled water and add these to thegraduated container. Carefully add distilled water
E
B
The alloys were manufactured by Handy and Harman, 82 Fulton Street, Kew York 38, N. Y., and were purchased from Metal Goods Corporation, 6211 Cedar Springs Road, Dallas 9, Texas.
VOLUME 34, NO. 6, JUNE, 1957
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dropwise until the volume is 25.0 ml.; then mix the solution and transfer i t to s. large, clean, dry test tube (25 X 150 mm.). This test tuhe should he free from scratches and stains. Placed by the instrument there will be a 25-mm. test tuhe partially filled with distilled wste-the reference tube. Place this tube in the photometer and adjust the transformer controlling the light source so that the meter reads 50.0 (microamperes). Next, insert the test tube containing the alloy solution into the photometer and note the meter reading, reoording i t a n the data sheet. Then place the distilled water tube back into the photometer. The meter reading now should be 50.0 again. If i t is not, the light intensity is fluctuating and the cycle of readings should be repeated. A graph is posted showing the relation of copper concentration t o the photometer reading. From this graph and the observed meter reading the weight of copper in the alloy sample is determined. Four standard solutions containing 0.10, 0.20, 0.30 and 0.40 g. of oopper dissolved in 25.0 ml. of nitric acid arc placed in a rack by the photometer to sllow the student to make a preliminary visual estimation of the copper content of his solutions. These solutions are also used in preparing the calibration curve for the photometer. The second alloy sample may be carried along following the same procedure, placed in a second large test tuhe.. and ohotameter readinas - made on both solutions in sequence. Pour the alloy solution into a 250-ml. Erlenmeyer flask; then rinse the test tube with about 10 ml. of distilled water and add this t o the same container. his solution is t o be used for the silver determination. Determination of Silver--Volumetric Methwl. T o each flask add 25 ml. of water and 2 ml. of saturated ferric alum indicator solution. Titrate each solution with standard KSCN solution (about 0.3 M). AgSCN is first precipitated, and then any KSCN added will cause the appearance of the ferric thiocymate complex. The original blue color of the solution will not seriously interfere with the detection of the limegreen color a t the end point. From the molarity of the standard solution of KSCN and the volume required, the weight of Ag in each alloy sample is calculated.
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RESULTS AND DISCUSSION
The results of several hundred alloys analyses by students are summarized in Tables 1 and 2. The determinations were made by laboratory classes containing science majors, engineers, and liberal arts majors. TABLE 1 Silver Determinations Stated
Check
-Student Au.
Ao
Ao
Ao
%
Name of dlov
%
%
ResullPSt. dev.
No. of detns.
Easy Fla 35 Easy Fla 45 Easy Flo Medium I T
The percentages listed in the check columns were determined by one of the staff members using the nrocedures nreviouslv described excent that doubledistilled water was used. Any student determination in which the per cent silver found varied by more than 15% from the average value was rejected and is not included in the above table. Copper results differing from the average by more than 12% were also excluded. L~~~~~~
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TABLE 2 Copper Determinations
The number of results rejected mas less than 5% of the total. In the photometric determination of copper, any cloudiness due to AgCl precipitation, caused by trace amounts of chloride in the distilled water used for rinsing, will result in a significant error. The distilled water in our laboratorv usuallv contains sufficient chloride to cause a very slight turbidity in the alloy 2, t,he b ~ results l ~~ , solution, ~ h ~as ~ ~in ~ f ~ ~ obtained for copper by the students are consistently high. It is interesting to note that problems similar to this have arisen elsewhere (4). Filtration t,hrough elimillate ~ ~ ~ kOOH ~ or~ similar l l filter , ~ paper due to this particles and any the factor. However, unless the Paper is very carefully after filtration and the washing added to the alloy solution after the determination of copper, the This filtrawill he appreciably Silver tion procedure is considered undesirable because of the increise in time required to perform the experiment, and the added possibility of error. If feasible, in ease a test with AgN08 solution produces cloudiness, the water should be redistilled or treated with deionizing materials before the experiment is begun. As showc by the check results in Table 2, this will improve the accuracy of the determination. I n our laboratory the chloride content of the distilled water was quite constant over the period of t,ime required to perform the experiment, and the copper results were reproducible to a satisfactory degree. Therefore no at,tempt was made to elimiuate this source of error. Small triple-beam balances were used by the students for all weighings. Since it is difficult to weigh more accurately than *0.01 g. with these balances, the minimum expected deviation is about one part in 150. For a few laboratory sections, torsion balances were provided, but no improvement in results was ohserved. LITERATURE CITED (1) FELDMAN, H. B., J. CHEM.EDUC.,26,225 (1949). (2) NAIUN, BARNET, J. CHEM.EDUC.,26, 571 (1949). (3) Personal communication, G. F. Donahue, Chief Chemist and Assayer, Handy and Harman, Bridgeport 1, Conn. (4) WITEKE,, R. A,, A N D E. H. SWIFT,Anal. Chem., 26, 1002 (1954).
JOURNAL OF CHEMICAL EDUCATION
