V O L U M E 2 5 , NO. 4, A P R I L 1 9 5 3 amount of the same absorber was placed in the reference beam so that the pen was recording in the 0.01 to 0.10 absorbance range. Thus the spectrum is traced on the portion of the chart paper which permits accurate measurement of small absorbance differences. A practical drawback lies in the servo response limitations of the instrument which decrease the response time of the pen during the most important portion of the spectrum-while the pen is traversing the peak of the band. To overcome this dificulty, the gain is increased at a reasonable point along the slope of the absorption band. This permits more rapid pen response
669 near the absorption peak. Once past the absorption peak the gain is reduced in order to maintain stable operation of the servo system. B wide slit width is used to permit the passage of sufficient energy for optimum amplifier gain and loop response conditions throughout the scan. LITERATURE CITED
(1) Hiskey, C . I'..; i s i ~CHEM., . 21, 1440 (1949)
(2) Robinson, D. Z.,Zbid.. 23, 273 (1951). (3) Zbid., 24, 619, (1952). RECEIVED for review September 6. 1962.
Accepted Derernber 2. 1952.
The Determination of Zinc in Brass by Controlled Potential Electrolysis DUNCAN G. FOSTER Swarthmore College, Swarthmore, Pa. purpose of this investigation x a s to find an electrolytic would be more reliable than existing methods and a t least as rapid. Existing methods are mostly constant amperage methods, and are slow and often disappointing. illthough in the analysis of brasses, zinc is the last element to be determined, and there is consequently no problem of separation involved, the writer felt that the application of cathode potential control might make the duplication of conditions easier and result in a saving of time. This turned out to be the case, and a method was developed which is accurate and reproducible and which requires an electrolysis time of less than 1.5 hours. Specifically, the problem involved the deposition of the zinc from solutions from which copper had been deposited by the method of Torrance (4),and lead and tin by the method of Lingane (3). These solutions contained, in addition to the zinc, 14.3 grams of disodium tartrate, 1 gram of succinic acid, 2 grams of urea, 21.1 grams of sodium chloride (from the neutralization of 30 ml. of concentrated hydrochloric acid with sodium hydroxide), and 3 grams of hydrazine dihydrochloride, all in a volume of about 500 ml. In order to diminish the hydrogen ion concentration and lessen the possibility of hydrogen evolution, ammonia solutions R ere used according to one method of standard practice, but for better control the solutions were buffered with ammonium chloride. Both were made 1 molar for simplicity. The zinc in such solutions would be present chiefly as ammonia complex ion, which has the additional advantage of lowering its reduction potential. The electrolyte from the previous procedures might also be expected to affect the redurtion potential. This was determined by a polarographic examination of solutions made up to duplicate the composition of this electrolyte, both with and without zinc present, a t the rotating platinum electrode ( 3 X 0.6 mm., 600 r.p.m.). The reduction potential was estimated by extending the nearly vertical portion of the polarographic wave to its intersertion with the extension of the nearly horizontal portion (residual current). Solutions were made u p as described above. Enough solid ammonium chloride and 0.1 % gelatine solution were added to make the final concentrations 1 J E in ammonium ion and 0.01% in gelatine. They were then adjusted to pH 9 on a pH meter M ith concentrated ammonia, and the polarogram was recorded. Without zinc present a hydrogen wave appeared a t -0.91 v. 2s the saturated calomel electrode (S.C.E.). Upon the addition of enough zinc chloride to make the solution 0.02 molar the wave appeared a t -1.20 v., but there was no distinct plateau. Increasing the zinc concentration to about 0.05 molar produced a definite maximum on the lower portion of the wave, indicating that the zinc was being reduced first, the hydrogen overvoltage being higher on zinc than on platinum. r THE
1method for the analysis of zinc in brass which
Calculation of the reduction potential of zinc in 0.02 molar solution, using the value -1.03 v. for the molar potential in ammonia solution (t?)gave - 1.32 v. us. the S.C.E., which is somewhat more negative than the observed value, indicating that the residual electrolyte did affect the potential. Calculation of the potential of a 10-6 molar zinc solution (the concentration corresponding to satisfactory analytical removal), using the observed value of - 1.20 v., and assuming that it is simple zinc ion which is reduced, gave the value - 1.34 v. us. the S.C.E. This, within the limits of experimental accuracy, should be the reduction potential of zinc from these solutions a t 10-6 molar concentrat ion. Electrolyses a t - 1.4 v. were successful from the start, except that it was found necessary to increase the potential for the final 20 minutes or so to -1.5 v., to ensure complete removal of the zinc. A series of runs %-as made with solutions containing zinc in amounts varying from 100 to 350 mg. Runs were made also in the presence of iron, aluminum, and manganese separately, and since iron was found to interfere, of aluminum and manganese together. The results of these experiments are shown in Table I, Six analyses of a brass of known composition were then carried out using a single sample of 20 grams dissolved and made up to 1 liter. A 50-ml. pipetful was used for each run. These results are set forth in Table 11. Table I. Taken, M g . 100 0 150 6 200 0 250 0 300 2 350 2 100 4 200 4 200 2 200.2 200 1
Table 11.
Electrolytic Deposition of Zinc in Buffered Tartrate Solutions Found, Llg. 100.2 160.4 200.0 250.4 300.2 360.1 134.7 200.0 200.6 200.6 201.0
Difference M g . 0.2 - 0.2 0.0 f 0.4 0.0 - 0.1 f34.3
+ -
0.4
f 0.4 ++ 0.4 0.9
Other metals present, JIg Sone Sone Sone None None None Fe, 40 Al,100 4 h l n , 10 h l n , 100 d l , 100; h l n , 100
~-
.4nalysis of a Typical Brass, Thorn Smith's Sample No. 320 7~Cu
% P b f Sn 78.24 9.7aa 78.12 9,95a 78.I 1 9.995 78,23 10.03b 78.23 10.04b 78.23 10.05b Mean 78.19 9.97 AI-. dev. 0.06 0.07 Pts./1000 0.8 1.4 Llanufacturer's valueC 78.01 9.93 a P b and Sn determined separately. b P b a n d Sn determined simultaneously. C Analyzed entirely b y gravimetric methods, without precipitation.
% Zn
Total
11.83 11.88 11.82 11 86 11.85 11.86 11.86 0.01 0.9 11.98
99.87 99.95 99.92 100.12 100.12 100,14 100.01 99.92
corrections for co-
ANALYTICAL CHEMISTRY
670 Table 111. Duplicate Analyses of Typical Brasses % Pb
+ Sn
% Zn
Total
7.76 7.76
99.93 99 86
Sampleo
70 Cu
Diff, Manufacturer’s ralue
82.25 82.21 0.04 82.03
9.92 9.89 0.03 9.90
Diff. Manufacturer’s value
76.66 76.60 0.06 76.39
9.83 9.85 0.02 10.03
13.35
99.77
Diff.
72.21 72 2 1 0.00
1.50 1.48 0.02
25.19 25.17 0.02
99.90 99.86
Diff.
60.49 60.37 0.12
2.81 2.90 0.09
36.67 36.68 0.01
99.97 99.95
1.
2.
3. 4.
0.00
7.83
99.86
13.43 13.39
99.92 99 84
0.04
0 Samples 1 a n d 2 were Thorn Smith’s samples KO.318 a n d 321 respectively. Samples 3 a n d 4 were “free cutting” brasses from our own shops, for which prerious analyses for copper a n d for lead and tin combined were available.
Finally four different brass samples varying in zinc content from about 7y0 to about 36% were analyzed in duplicate. Table I11 gives the figures for these determinations. To save space and, also, because in some caBes tin and lead were determined simultaneously, the sum of the percentages of tin, and lead are reported in Tables I1 and 111. Detailed directions for the analysis of brass for copper, lead, tin, and zinc are given below. EXPERIMENTAL
Apparatus. The polarograph was a recording instrument built in this laboratory, using a General Electric photoelectric recorder. It was of conventional design and had a maximum sensitivity of about 0.02 microamperes per mm. The potentiostat was also built in this laboratory and is described elsewhere (1). For these experiments an external laboratory potentiometer was used for control. pH measurements and adjustments were made with a Fisher Titrimeter, using glass and calomel electrodes. Materials. The zinc used was Baker’s analyzed grade, 30 mesh granular, low in arsenic, lead, and iron. ill1 other chemicals were reagent grade. Techniaue. The solutions used as sumorting solutions for the polarograhs were made as described above. - A standard zinc solution was made by dissolving pure zinc in hydrochloric acid and making up to 1liter. For the zinc electrolyses a weighed sample of pure zinc was dissolved in 10 ml. of concentrated hydrochloric acid and 5 ml. of water, 10 ml. of 12 M sodium hydroxide were added, then, after cooling, 14 grams of sodium chloride were added. The solution was diluted to about 500 ml., and the constituents mentioned above, except the sodium chloride, were added. Final adjustment of pH was then made. When interfering metals were present, they were introduced as pure metal with the zinc, except in the case of manganese which was introduced as pure manganous chloride. DIRECTIONS FOR THE ;COMPLETE ANALYSIS OF BRASS
Copper. Lingane’s method (3) for electrolyzing copper from a buffered tartrate solution was found to be too slow for the high copper percentages found in brasses. In any event the tartrate is unnecessary when bismuth is not present. The quicker method of Torrance ( 4 ) was used. Weigh a 1-gram sample of the brass into a 250-ml. electrolytic beaker. Dissolve in 15 ml. of 2 to 1 hydrochloric acid with the aid of 5 ml. of 307, hydrogen peroxide, following the method of Willard ( 5 ) . After boiling out the excess hydrogen peroxide, dilute to about 50 ml., add 2 grams of hydrazine dihydrochloride, and heat to just below the boiling point until the color, which becomes quite dark a t first, is considerably lighter, indicating appreciable reduction to the cuprous state. This requires about 15 minutes. Cool, add 2 grams of urea, set up on the potentiostat, dilute just enough to cover the electrodes, and electrolyze a t -0.40 v. until the current has fallen to less than 10 milliamperes and remained constant for 10 minutes. Total electrolysis time is from 20 minutes to 1.5 hours, depending on the quantity of copper present.
When large quantities of copper are present (more than about 750 mg.) two precautions may be necessary. Instead of setting the controls for complete automatic operation a t first, set the down control only. Then set the current a t about 2 amperes and leave until the down control begins to operate. The up control may now be cut in and the electrolysis finished in the normal manner. If this is not done, the very high currents attained in the beginning may cause the deposit to be so loose as to flake off on washing. When more than about 800 mg. of copper are present the 2 grams of hydrazine dihydrochloride are insufficient to reduce it the necessary amount. In such cases 4 grams should be used. Even so, reduction may still be insufficient to allow immediate deposition of copper, but this merely has the effect of lengthening the electrolysis. The author has observed some cases in which i t was more than an hour before reduction had proceeded to the point where any appreciable amount of copper began to deposit. At the end of the electrolysis, wash the ca’thode with the current still on, by lowering the beaker slowly away from the electrodes. Dip the cathode in alcohol or acetone, dry 3 to 5 minutes in the oven, cool, and weigh. This procedure is the same for all determinations. Lead. Transfer the solution from the copper deposition to a 400 ml. beaker which has been roughly calibrated a t 200,250,300 and 350 ml., and marked with a wax pencil. Wash the original beaker with 3 small portions of distilled water, keeping the volume as small as possible. Add 10 ml. of 12 M sodium hydroxide, and note the volume. Allowing for the addition of tartrate, adjustment of pH, and washing the Titrimeter electrodes, estimate the final volume of the solution. ( A little practice makes this quite easy.) Now add enough solid disodium tartrate to make the solution 0.25 31, and 1 gram of succinic acid. Set the solution up on the Titrimeter and adjust the p H to 3.9 to 5.1 with 12 JI sodium hydroxide. The electrolysis of lead may be done on the copper-plated electrode or on a fresh one. This is also true of the tin and zinc determinations. A potential of -0.6 v. is employed and electrolysis is continued until the current has fallen below 10 milliamperes and has remained steady for 10 minutes. Tin. .4dd 20 ml. of concentrated hydrochloric acid and 1 gram of hydrazine dihydrochloride to the solution from the lead determination, and electrolyze a t -0.6 v. Time and final current are the same as for the lead determination. The time for each is about 40 minutes to 1.25 hours. If desired, tin and lead may be determined simultaneously by simply omitting the lead electrolysis. The procedure is otherwise identical. Zinc. Transfer the ’solution from the tin determination to a calibrated 600-ml. beaker, add 20 ml. of 12 JI sodium hydroxide, and cool. Estimate, as in the case of lead, the final volume, and add enough solid ammonium chloride to make the solution 1 M, then adjust to pH 9 on the Titrimeter, using 15 J1 ammonia. Electrolyze a t a potential of - 1.40 v. until the current has become essentially constant. This will take about 40 minutes; the current is usually between 60 and 100 milliamperes. Now raise the potential to -1.50 v. and electrolyze an additional 20 minutes. This last stage is usually accompanied by hydrogen evolution, and the fall of the current is no criterion of the progress of electrolys1s.
It was a practice, in all experiments, to run an additional 15 minute electrolysis a t -1 50 v., although zinc was obtained in less than half the cases. This procedure is recommended, and it should be done on a fresh electrode, as there is some danger of redissolving zinc. The zinc deposit is dense and adherent and light gray in color. In no case was a loose deposit obtained, as is so often true with existing methods. LITERATURE CITED
Foster, D. G., J . Chem. Edztc., 28, 626 (1951). Latimer, W. M., “The Oxidation Stages of the Elements and Their Potentials in Aqueous Solutions,” p. 157, New York, Prentice-Hall, Inc., 1938. (3) Lingane, J. J., and Jones, s. L., ANAL.CHEM.,23, 1798 (1951). (4) Torrance, S., Analyst, 6 2 , 7 1 9 (1937). (5) Willard, H. H., quoted by H. Diehl, “Electrochemical Analysis with Graded Cathode Potential Control,” p. 39, Columbus, Ohio, G . Frederick Smith Chemical Co., 1948. RECEIVED for review October 21, 1952.
Accepted December 4, 195’2.
