Polarographic Analysis of Copper and Zinc in Brass Plate A Rapid Control Method WILLARD P. TYLER . ~ N DWALTER E. BROWS Technical Service Laboratories, The B. F. Goodrich Co., Akron, Ohio
of zinc and copper ions is measured instead of exact diffusion currents of the individual ions, exact temperature control is unnecessary, less care is required in measuring solutions, and the results are independent of the capillary used. The entire analysis requires an elapsed time of 20 minutes, but only about 10 minutes of the operator’s time. Thus, six analyses can be made per hour in routine work. The method may be depended upon to give results within * 1percent of the copper percentage in rapid control work and within 1 0 . 5 per cent when extra precautions are taken.
The existing methods of brass plate analysis for control purposes are either slow or inaccurate. A method is described whereby brass plate consisting of copper and zinc only may be analyzed by means of a polarograph if the base metal is iron or steel. The brass plate is dissolved in ammoniacal ammonium persulfate. This solution is added to a supporting electrolyte solution containing ammonium hydroxide, ammonium chloride, sodium sulfite, and gelatin and the polarogram is read after oxygen and persulfate have been reduced by the sulfite. Since only the ratio of diffusion currents
T
HE existing methods for the analysis of copper and zinc
the actual per cent of copper in special cases when the quantity of metal present other than copper and zinc is known.
in brass plate are not particularly suitable for rapid routine control. The most rapid methods, those involving separation with hydrogen sulfide, require 20 to 30 minutes of the analyst’s time and are not capable of very high accuracy or reproducibility under production conditions. Methods involving electrolytic separations are more accurate and require less of the analyst’s time. However, the total time required for the completion of the analysis is frequently so great that production lines may be held up if analysis is necessary for the release of plated articles. It was thought that the polarograph would offer a n excellent means for the determination of the composition of pure copper-zinc brass plate if suitable conditions could be devised. The method of Hohn (1) for brass analysis and the method of Kraus and Xovak, cited by Kolthoff and Lingane (S), for copper and zinc in zinc ores offered proof that a polarographic method would be practical for brass plate if i t could be made more adaptable to rapid routine control. With these facts in mind, a polarographic method was developed for the analysis of copper-zinc brass plated on iron or steel. The method makes use of the “pilot ion” system, in that the ratio of zinc to copper is determined rather than the actual amounts of each ion present. Thus, exact temperature control is not needed, less care is required in measuring solutions, and the results are independent of the capillary used. The entire analysis requires a total time of about 20 minutes and only 10 minutes of the operator’s time. It may be depended upon to give results within * 1.0 per cent of the true copper percentage in rapid control work, and within *0.5 per cent when extra precautions are taken. Solid brass samples are dissolved somewhat slowly by the reagent used in this investigation. However, the rate of solution may be increased to make the method applicable to solid brass by agitation, the use of a more concentrated solution of the reagent, and the use of small and finely divided samples. It should be possible to measure the zinc to copper ratio in the absence of interfering metals and to calculate
Apparatus The polarographic apparatus used in these studies was the Fisher Elecdropode, a manually operated instrtrment equipped with an adjustable galvanometer sensitivity control and a residual current compensating device. The measurements were made with the quiet mercury pool as the anode. In order to refer the potentials to the saturated calomel electrode (S. C. E.) it was necessary t o measure the pool to calomel electrode potential in a separate operation and correct the former readings with this value. The range of measurement of pool to calomel electrode potential is limited to h 0 . 2 volt. The conditions chosen for the analyses were such that the calomel-pool potentials were of the order +0.19 volt, and were somewhat variable. In those cases in which the potential was greater than 0.2 volt it was possible to amroximate the value with sufficient accuracy - by - extrapolatioh: The capillaries used in this work had initial drop times of approximately 3 seconds. The mercury used in the pool was purified by drawing air through it under 10 per cent nitric acid, washing with distilled water, drying, and “pinholing”, while that used in the capillary was further cleaned by vacuum distillation.
Solutions STAND~RD BRASSSOLUTIO^;^. Solutions of the desired composition were prepared by mixing measured volumes of standard copper sulfate and zinc sulfate solutions. The copper solution was standardized electrolytically. The zinc solution was standardized with potassium ferrocyanide solution which was standardized against a standard solution of zinc prepared from the pure metal. Diphenylbenzidine was used as an internal indicator in the zinc standardization. The solutions used in these experiments contained about 4.0 grams of copper and 1.7 grams of zinc per liter. The standard solutions for polarographic use were made from salts rather than from the pure metals because of the undesirable effect of nitrate on the polarogram. STANDARD AMMOSIUM PERSULFATE SOLUTION.This solution is 3.75 N in ammonium hydroxide and is made up to contain 17.5 grams of c. P. ammonium persulfate per 100 ml. of solution. Dilution of this solution according to the procedure for standardization results in a solution containing the amount of persulfate that is present when 15 mg. of 70-30 brass plate are stripped with 5 ml. of stripping solution. STRIPPING SOLUTION.The solution contains 80 grams of ammonium persulfate per liter of solution and is 1.5 N in ammonium hydroxide. c. P. grade salt was used in these experiments, but in
520
August 15, 1943
TABLE I. ANALYSIS OF SYNTHETICALLY PREPARED SAMPLES Sample
Temperature
Copper Polarographically
c. 1 2 3
521
ANALYTICAL EDITION
26.7 26.1 26.6 25.8 '! 26.5 Temperature of standardization:
Copper Known
Copper Error
70
70
%
54.0
54.30 60.25 69.45 65.80 73.47
-0.30 -0.15 -0.15
60.1 69.3 65.8 73.5 25.5' C.
i-0.00 $0.03
later work the several batches of technical grade material that were tested were found to he free of interfering substances. SUPPORTING ELECTROLYTE. This solution must be free from copper and zinc. I t has the following composition: 1 0 molar in c. P. ammonium chloride 1 . 5 molar in c. P. ammonium hydroxide 0 54 molar in c. P. sodium sulfate 0 67 gram of gelatin per liter
Procedure STASDARDIZA~~IOX PROCEDCRE. Pipet 3.0 ml. of standard brass solution into a small beaker. Add 2.0 ml. of standard ammonium persulfate solution and 15 ml. of supporting electrolyte solution. Mix and alloiv to stand covered for 10 minutes to effect complete reduction of persulfate and of oxygen. Cool to within 0.5' C. of the prevailing room temperature, place in the electrolysis cup containing the pool merzury, and measure the potential of the pool with reference to the saturated calomel electrode. Sote the temperature of the solution and record the currents (galvanometer deflections) at the following voltages (us. S. C. E.): A a t -0.36 volt; B a t -1.03 volts; D a t -1.605 volts. If the temperature has changed more than 0.2" C. during the run, repeat the readings. For very accurdte work a fourth current, C, a t -1.305 volts (us. S. C. E.) must be determined. -0.36 is the only point at which the voltage must be set accurately, since the other points oeeur in flat rqions of the polarogram. Variations of 25 millivolts usually have no effect on the current a t the other volhges. The above voltages were chosen at the flattest portions of the current-voltage curve. BRASSPLATE ASALYSIS. Strip the brass plate from a desired area of the sample by flowing a given quantity of stripping solution over the surface, using a pipet fitted viith a rubber bulb, until the brass has been entirely removed. Do riot allow solution to come in contact 7%-ithrubber. The arsa stripped should he such that 15 mg. of brass are present in a 5-ml. portion of the stripping solution, although in routine work this may vary from 5 t o about 25 mg. per 5 m l . portion. The Tvarnner the sample the more rapid the stripping, but it should not be hot enough to volatilize an appreciable amount of ammonia. hleasure 5 ml. of the brass solution into a beaker, add 15 ml. of supporting electrolyte solution, cover, and let stand for 10 minutes. Cool to within 0.5' C. of the room temperature and eontinue exactly as in the standardization. The proper sensitivity range of the galvanometer t o use may be determined from the magnitude of the current a t -0.36 volt (L's.S. C. E.).
Experimental Results A series of samples, made synthetically from measured portion< of the standard copper sulfate and zinc sulfate solutions described above, !>ere prepared t o contain the same concentrations of stripping solution and approximately the same amounts of ferric hydroxide as are found in samples obtained from plated articles. They were then analyzed by one of the authors without knowledge of their composition. The results, obtained by the accurate method of calculation described below, are compared in Table I with the known values. A second series of samples was taken from brass plate in production. A portion of each sample was analyzed with the Elecdropocle, and the results were calculated by the accurate method. The remainder of each sample was then analyzed for copper electrolytically. The solution after electrolysis F a s evaporated to sulfuric acid fumes, the small amount of iron was removed with ammonium hydroxide, and
the zinc was determined with standard potassium ferrocyanide, using diphenylbenzidine as an internal indicator. The data obtained by the two methods are compared in Table 11.
Calculations In calculating the diffusion currents from a polarogram it is usually necessary to take into consideration two factors, the residual current due to the supporting electrolyte and the change in the current with potential above and below the electrocapillary zero potential. Correction for the former was found to be unnecessary, a t least for quantities of brass greater than 5 mg., because of the very low and almost linear residual current-voltage curve of the supporting electrolyte. I n order to correct for the change in current with increase in potential it is necessary to measure the values of m2'3t1 6 a t the potentials used and to correct the diffusion currents as described b y Kolthoff ( 2 ) . The mass of the mercury dropping per second, m, and the drop time in seconds, t , were measured a t the four potentials employed in the procedure, using a typical brass plate analysis solution for the electrolyte. The electrocapillary zero potential appears to be betxeen -0.40 and -0.45 volt (os. S. C. E.). The values of m2'3t16 a t -0.36, -1.03, -1.305, and -1.605 volt (os.S. C.E.) Tvere2.035, 2.009, 1.982, and1.939mg.2 3sec.-1 2, respectively. The correct diffusion currents of copper and zinc are calculated as follows: id id
(CU)
(Zn)
=
B - (2.009/2.035) /l = B - 0.987 A D - (1.939 '1.982) C = D - 0.978 C
where A , B , C, and D are the currents at the above-mentioned potentials as previously defined. The above method of calculating the diffusion currents is referred to in this paper as the accurate method of calculation.
TABLE
11. COMPARIBOS O F POLAROGRAPHIC A S D CHE>IICAL ASALYSIS OF BRASSPLATE
Sample
Temperature
c. 1
Copper Polarographically
70
Copper Chemically
Difference
70
69.3 69.10 2 76,s 76,50 3 69.3 69,45 68.80 4 68.9 72.50 5 72.8 6 71.1 71.05 Temperature of standardization: 1 and 2 = 24.8' C., 3 t o 6 = 25.1 25.0 24.3 23.8 23.8 24.1
% f0.40 +0.00
-0.15
i o . 10
-0.30 +O 0 5 24.1' C.
I n order to decrease the time for analysis and calculation in routine work it n-as found permissible to measure only diffusion currents A , B , and D and to make a n empirical correction of the final copper percentage. For this method, the diffusion currents are defined as follows:
I n either case the calculation of per cent copper is 5s follo\vs: The ratio of the diffusion currents of the analyzed sample E , = id (Zn)/id (Cu)
1%
In order to obtain the weight ratio, R,, of zinc to copper in the analyzed sample, divide R, by the corresponding diffusion current ratio for the standard brass solution and multiply by the weight ratio of zinc t o copper in the standard brass solution. Then, % c u = 100/(1 R,) If the empirical correction method is to be used when the standard solution is 70 per cent copper, add 0.1 per cent to the
+
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INDUSTRIAL AND ENGINEERING CHEMISTRY
calculated value for each 1 per cent belox TO per cent and subtract 0.1 per cent for each 1 per cent above TO per cent. The limits within which this method has been applied are 60 to 80 per cent copper. No data are available using other standard solutions.
It is frequently desirable to know the approximate average thickness of a brass plate. If the brass is 70 * 5 per cent copper and the temperature of the analysis is within 1O C. of that of the standardization, this can be done by comparing the sum of the diffusion currents of the sample and the standard. It is thus possible to calculate the quantity of brass in the analyzed sample if the quantity in the standard solution is known, and to calculate the thickness of the brass plate to within 5 per cent of the true average value without resorting to carefully controlled conditions. Discussion The problem of selecting the best supporting electrolyte solution was simplified by a limited choice of methods of stripping the plate from the base metal. The three common reagents used for this purpose are nitric acid, ammoniacal ammonium trichloroacetate, and ammoniacal ammonium persulfate. The use of nitric acid is not practicable because of the large amounts of iron that may be dissolved. Trichloroacetate solution produces cathodic waves in ammoniacal solution and therefore cannot be used. Persulfate also produces a cathodic wave, but this ion can be reduced successfully with sodium sulfite which simultaneously removes the oxygen. Experiment showed that the final solution should be a t least 1.5 molar in ammonium hydroxide and 0.75 molar in ammonium salts, and should contain about 0.5 gram of gelatin per liter; 0.4 molar sodium sulfite provides a safe excess of reducing agent. Higher concentrations of ammonium hydroxide and ammonium chloride can be used and appear to make the readings less sensitive to slight changes in supporting electrolyte concentrations, but the above concentrations proved to be satisfactory. Lo\ver gelatin concentrations have a tendency to give erratic results because of failure to remove maxima completely and because of more pronounced variations in the diffusion current ratio Eith slight differences in the gelatin content. Lower sulfite concentrations increase the time of standing required before the polarogram can be obtained and increase the error due to variations in final sulfite concentrations. The effect of temperature on the ratio of the diffusion current of zinc to that of copper is small compared to its effect upon the individual diffusion currents of zinc and copper. This ratio increases only about 0.25 per cent per degree change in temperature, indicating a slightly greater temperature coefficient of diffusion current of the zinc ammonia ion than of the copper ammonia ion. Thus a temperature difference of 2’ C. between an analysis and the standardization will affect the ratio by 0.5 per cent which is within the limits of accuracy of the measurements. The temperature must, however, be maintained constant to 0.2” C. during the current-voltage readings of any given analysis or standardization because of variation with temperature of one diffusion current without an equivalent change in the other. This is comparatively simple, since the time required for a given set of readings is only about 2 minutes. The current-voltage curves are normal in form, showing a portion of one wave and a full second wave for copper and a single wave for zinc. They also show the decrease in current due to decrease in drop time with increase in potential above the electrocapillary zero potential. The half-wave potentials are very close to those given by Kolthoff and Lingane (d), -0.48 (us. S. C. E.) volt for cuprous ion to copper and -1.43 volts (us. S. C. E.) for zinc ion to zinc, in
Vol. 15, No. 8
supporting electrolytes high in ammonium hydroxide and ammonium chloride. Fresh standard ammonium persulfate solution should be made each week before standardization. The supporting electrolyte solution will keep indefinitely out of contact with air. The stripping solution should be kept in a stoppered bottle and may deteriorate after 1 or 2 weeks. None of the solutions should be allowed to come in contact with rubber before or during use. Koroseal tubing has been found satisfactory for dispensing the supporting electrolyte solution, but no tests have been made with any tubing for dispensing the stripping solution. If the sample is to be rinsed, only stripping solution should be used for this operation. Rinsing is not essential in routine work. All dilutions of the stripped “brass” solution must be made with stripping solution. Experience will enable the operator to decide whether such dilution is necessary to keep the solution within the specified concentration limits. If the solution must be more concentrated, more brass can be stripped with the solution which contains the previously stripped brass. Because the action of persulfate on mercury produces ions which affect the residual current, the reduction of the persulfate with the sulfite must be done before the solution comes in contact with mercury. The action of ammonium persulfate on the iron from which the brass has been stripped mill produce “rust” spots, particularly when the sample is hot. A normal amount of this reaction is permissible, but an excessive amount may decompose too much persulfate and thus change the concentration of sulfite ion in the final solution. A number of elements such as nickel, cobalt, manganese, and cadmium, which are reduced under the conditions of this determination, interfere with the analysis. Small amounts of lead do not affect the polarograms, while the insolubility of the hydroxide prevents the interference of iron. The presence of the precipitate of iron hydroxide in the electrolysis solution does not measurably affect the polarogram. Since the success of the method depends on the maintenance of a constant supporting electrolyte concentration, a large excess of sodium sulfite is used. This leaves a relatively constant amount of sulfite ion in the final electrolysis solution, regardless of slight variations in the quantity of persulfate used up in the stripping process. Thus it is permissible to use only one standardization for a range of samples containing from 5 to 25 mg. of brass per 5 ml. of solution without introducing appreciable error into routine analysis. The difference between the ratio of the diffusion currents which would be obtained when the minimum ( 5 mg.) and the maximum (25 mg.) of brass are present in 5 ml. of solution has been found to be about 1.5 per cent. When this value is applied to the calculation of per cent copper in a 70-30 brass, it represents an error of about 0.5 per cent copper. The mean value between these limits is found when the solution contains approximately 15 mg. of brass in 5 ml. Therefore, if the standardization conditions simulate those existing when 15 mg. of brass are stripped by 5 ml. of stripping solution, the maximum error which can be attributed to this source will be only *0.25 per cent copper. I n more accurate work, empirical corrections or calibration of the instrument with persulfate concentrations corresponding to various brass concentrations will reduce this error. The temperature effect on the ratio of diffusion currents has been previously noted as causing an error of about 0.25 per cent per degree. This causes an error of about 0.1 per cent per degree in the copper percentage in 70-30 brass. This icrror may be eliminated in more accurate work by the
August 15, 1943
523
ANALYTICAL EDITION
use of more exact temperature control or of temperature coefficient corrections. The reproducibility of the method was determined by repeated standardizations with varying “brass” concentrations using different galvanometer sensitivities, with and without compensation for the residual current of the copper wave. The diffusion current ratio was found t o be reproducible to within +0.5 per cent average deviation from the mean if the galvanometer has a linear response and if the temperature is constant within 1’. The maximum probable error is about 1 per cent of the ratio. This represents a n error of about +0.3 per cent copper in 70-30 brass, and can be safely said to represent the maximum error of the method using carefully controlled conditions and the accurate method of calculation. The error introduced by using the rapid empirical method of correcting the copper percentages is very small. In 28 determinations the maximum difference between values obtained by this method and the accurate method of calculation was 0.3 per cent copper and the average difference was 0.1 per cent copper. The maximum probable error of the routine method of analysis in which the temperature is controlled only to *2’ C.,
one standardization is used for all quantities of brass from 5 t o 25 mg. per 5 ml. of solution, and the rapid empirical method of calculation is used, can be found from the above discussion t o be about *I per cent copper. The average error will be considerably less than this value.
Summary
A polarographic method is described for the determination of copper and zinc in brass plate on iron or steel, which is capable of determining the copper percentage to within *1 per cent in routine analysis and within 0.5 per cent under carefully controlled conditions. The total time for a single analysis is approximately 20 minutes, and since the attention of the operator is required for less than 10 minutes, about six analyses can be made per hour. Literature Cited (1) Hohn, H., 2. Elektrochern., 43, 127 (1937). (2) Kolthoff, I. M., IND.ENQ.CHEM.,ANAL.ED.,14, 195 (1942). (3) Kolthoff, I. M.,and Lingane, J. J., “Polarography”, p. 339, New York, Interscience Publishers, 1941. (4) Ibid., pp. 482, 487.
Water Thermoregulator WM. E. BOYD, Inspection Board of the United Kingdom and Canada, Nobel, Ontario, Canada
A
N IXEXPENSIVE, easily portable thermoregulator, capable of maintaining a variation of *0.5’ C. in bath temperature, is quickly constructed from a branched-tube regulator using a mercury valve. No electricity is used, power being obtained directly from the feed water.
B
I
C
FIGURE1
As shown in Figure 1, the bath was used a t 20” C., with warm water entering A to keep it up to temperature. Above 20’ C. an external heater is to be preferred, with cold water entering a t A controlling the excess quantity of heat supplied. The regulator consists of two parts: the pendulum group and the valve group. The pendulum group consists of a tube, C, of 7-mm. bore, about 45 cm. (18 inches) long, suspended by a piece of rubber tubing, B firmly clamped a t its upper end. The amount of ming allowed the pendulum is regulated by the slotted guide plate, E. A tube, F , holding a t least 10 ml. of water, is held on an arm a t right angles to the vertical tube, C , but free to move vertically up and down it. A weight, G, counterbalances the weight of F . The valve group consists of a toluene-mercury regulator, P, with a branched tube serving as a valve. -4vessel holding at least 75 ml. of toluene is recommended. The adjusting device consists of a large-diameter knurled setscrew entering a 7-mm. glass side arm. The vertical tube and side arm are made of 3mm. bore tubing. N is a constant-level device, while R is a bleeder emitting water to the drain a t approximately 30 ml. per minute. The action is as follows: A small amount of water enters the valve group a t M , flowing into the constant-level tube, N . Any exceys is led to waste. Water flows down tube 0 into the vertical arm of the regulator and up the inclined arm through tube Q to fill vessel F . The weight of F is now great enough t o force tube C over and deliver water into the bath. As the temperature rises the mercury assumes the shape as shown in the diagram and seals off tube &, preventing water from reaching F. Bleeder R, though operating rontinuously, is now able to empty F in a few seconds and allows tube C to swing back over the baffle. When the temperature drops the mercury falls, allowing the water to reenter &, and the cycle is repeated. No cutting off of the mercury column takes place when the water leaves by way of the inclined tube. A small head, h, of water is to be preferred. Good results were obtained when a head of 15 to 20 cm. (6 t o 8 inches) was used. A temperature variation of *0.5” C. was easily maintained in 6-day runs.
