Sister Denise Eby Saint Joseph College Emmitsburg, Maryland
Quantitative Analysis of Silver-Copper Alloys by Diffusion in Gelatin
Although diffusion in gelatin has been proposed as a technique for the quantitative analysis of ions ( I ) , this method has had limited applications because of its low resolving power (8). Quantitative estimates of Group I and Group I11 cations have been made by diffusing their nitrates individually in gelatin and measuring the radial distance travelled by each ion during a specified time interval. The procedure used in these determinations identified 0.1000, 0.5000, 1.0000, and 10.0000 N solutions of lead nitrate with reasonable accuracy (3). This paper describes a modification of the procedure employed by Antelman (4) and its application to the quantitative analysis of silver-copper alloys by differential radial diffusion of their component nitrates in concentrated gelatin. The resolution of the ions effected by this procedure permits a determination of their concentrations by direct measurements of the distances travelled by each during migration.
Alloy samples of about 1-2 g each were weighed on an analytic balance, placed in a beaker, and dissolved in about 5 ml 6 N HN03, with gentle warming until solution was effected. Nitrogen oxides were expelled from the solutions by heating them almost to dryness under an infrared lamp. (This may be accomplished by cautiously boiling the solutions on a hot plate or Bunsen burner.) Distilled water (75 ml) was added to each beaker; the pH was adjusted to 6 7 by adding 10% NaOH solution by drops (Hydrion indicator paper was used). Each solution was transferred quantitatively to a volumet,ric flask and diluted to 100 ml with distilled water. Determination of silver and copper: 0.050 ml (=t0.001 ml) of the alloy solution under test was delivered onto the center of a gelatin plate surface with a micropipet. The drops were allowed to diffuse for at least 8 hours. At the end of this time two concentric circular zones had formed in the gelatin. The larger zone was diffused silver and could be detected visually as a wellThe Experiment defined white area formed by the precipitation of Eight g of Knox Gelatine, Bloom 250 Lot B,' was silver chloride by the chloride contained in the gelatin. softened in 50 ml of cold distilled water and dissolved in This silver zone was measured by spanning the di150 ml of hot distilled water. 50-ml portions of the hot ameter with a bow divider and laying it against the liquid were poured into disposable plastic Petri dishes mm scale of a st,eel Vernier caliper. The silver zone of uniform diameter (10 cm) to insure uniform depth, can be accentuated by developing the area with sodium and gelled by refrigeration. chloride solution. Detection of the smaller conFive of the alloys used in this project were silver centric copper zone was made possible by developing brazing alloys in the form of wire 2.5 mm diameter and t,he area with oxine which produced a vivid yellomcomposed of varying amounts of silver and c ~ p p e r . ~ green precipitate (6). Copper may also be developed Two alloys were obtained in sheet form from a manuwith potassium ferrocyanide. This zone was measured facturer of j e ~ e l r y . ~Listed values of the silver and in the same manner as the silver. copper contents of the alloys were furnished by the A series of standard solutions of analyt,ical grade manufacturer (5). AgNO3, ranging from 0.0300 to 0.0800 N in intervals of 0.0100 N, was prepared and diffused on gelatin plates Gelatine Bloom 250 Lot B was provided by the Knox Geletine in the same manner as the alloy solutions. The zones Co., Inc., Csmden, N. J. were measured and a calibration curve constructed by Braeing alloys obtained from Handy and Harman, New York plotting diffusion zone diameters (d) vergus con28. --,N Y -. centrations (c). The silver concentrations .of the Sheet alloys obtained from Jenkins Sons, Baltimore 23, Md.
406
/
lournol of Chemical Education
alloy solutions were found by locating the diffusion zone diameters on the calibration curve. From this concentration and the weight of the alloy sample the per cent of silver in the alloy was calculated. A similar calibration curve was constructed using copper(I1) nitrate standards ranging from 0.0100 to 0.0400 N solutions, and was used in like manner to determine the copper concentrations after having developed the zones with either oxiue or potassium ferrocyanide. Figure 1 shows the calibration curves obtained for the silver and copper standards. A preliminary diffusion of the alloy solution was made in order to estimate the per cent of silver and copper. A more precise determination was then made by diffusing the alloy solution and the appropriately selected standards simultaneously in gelatin plates. Simultaneous diffusion minimizes errors arising from variations in temperature, humidity, and age of g e l a h
of copper(I1) nitrate standards were made up by the class, under the direction of the instructor, and were used to plot the calibration curves for use by the students. Samples of the various alloys were issued as unknowns. The experimental results of 240 student alloy analyses are recorded in Tables 1 and 2. Table I. Silver Analyses
% Ag
Name of alloy
Easy-Flo Easy-Flo 35 Easy-Flo 45 Medium
IT
Ag-Cu
(listed by mfg.) 50 35 45 i0 80 93.5
% Ag -Experimental results-(deterNo. of mined Avg. determmsi yo Ag Avg. by titions tration) (diffusion) dev. 49.30 34.68 44.38 69.43 i9.35 93.35
50.43 33.26 44.38 69.67 76.86 93.14
1.22 2.02 2.32 1.81 3.01 0.88
30 40 30 40 40 40
(9'3.5/6.5)
Alloys Easy-Flo, Easy-Flo 35, and Easy-Flo 45 contained considerable amounts of cadmium which contaminated the silver and copper zones during migration, resulting in poor resolution; this made it difficult to measure these cations. Alloys containing predominately silver and/or copper are most readily and effectively analyzed by diffusion in gelatin. Table 2. C o m e r Anolvses
% Cu
Concentration I M I I ) Figure 1. Variation of cation diffusion zone diometerr with concentmtion for standard rolutionr of AgN03 and CulNOals
The silver nitrat,e standards and the per cent of silver in each alloy were checked volumetrically by titration with standard potassium thiocyanate using saturated ferric alum indicator. Copper nitrate standards and per cent of copper in alloys were checked by electrode-deposition. When d (diffusion zone diamet,er) is used to determine unknown concentrations of cations, the anion of the unknown must always be identical with the anion of the standard solutions--e.g., if silver nitrate is used to construct the calibration curve, the unknown must be determined as silver nitrate if consistent results are to be obt,ained. The details of the method described in this paper were worked out bv four student oarticioants in an Undergraduate ~ e s e a r c hparticipation program during the academic year 196&61. Diffusion analyses were repeated until reproducible results, consistent with those obtained by conventional analytic methods, were obtained. Any determinations which deviated widely from those found by titration or electrode-deposition were discarded after having ascertained the source of error. This technique was used by students enrolled in the fall 1961 quarter of the Quantitative Analysis course at Saint Joseph College. One set of silver nitrate and one
Name of allov
(listed by mfe.) -.
Easy-Flo Easy-Flo 35 Easy-Flo 45 Medium
15.5 26 15 20
IT
16
% Cu -Experimental
resultsNo. of de.terAvg. mmsi dev. tions
(electrodedeposition)
(diffusion)
6.92
6.81
0.10
40
94.56
93.82
018
40
Avg.
% Cu
Ag-Cu (93.5/6.5)
6.5
Cu-Zn (95%)
95
The amount of chloride in the gelatiu is significant in the analysis of silver. If silver chloride is precipitated in appreciable amounts during migration the diffusion of the silver ion will be arrested after several hours and the measurement of small zone diameters will result in error. Of all the types of gelatin tested, only Knox gelatin, Bloom 250 Lot B, contains sufficiently low chloride to permit overnight diffusion of silver. Although the procedure described in the paper lacks the degree of precision obtainable by many of the conventional analytical procedures, i t does permit students to analyze silver and copper simultaneously by a single simple procedure, and to appreciate the potential of differential radial diffusion as a tool for the analyst. Acknowledgment
This project was developed in an Undergraduate Research Participation Program supported by an NSF grant. Student participants included Maureen Monks, Philomena Liscio, Mary Colombrita, and Jean Lentz, all of Saint Joseph College, Emmitsburg, Maryland. Volume 39, Number 8, August 1962
/
407
Literature Cited (1) ANTELMAN, M., EBY, D., AND KAUFMAN, G. B., Anal. Chm., 31,829-33(1959). (2) SPAIN, J. B., Anal. Chm., 32, 1622-24 (1960). M., ET AI., op. tit., p. 831 (3) ANTELMAN,
408
/
Journal of Chemical Education
(4) ANTELMAN, M., Anal. Chem., 26, 121&19 (1954). (5). "Braaine Technical Bulletin No. T-1." Hendv and Harman. New Fork, 1960. (6) LANGE,N. A,, "Handbook of Chemistry,"HandbookPublishem Inc., Smdusky, Ohio, 1946, p. 1125.
.
