experimental situations. I n some cases it may still be necessary to perform a primary separation from the bulk material, but there should be no need for individual separations of all elements of interest. The analyses of minerals and rare earth element mixtures are particularly interesting in this regard, and experiments are now in progress to investigate these systems. ACKNOWLEDGMENT
The authors acknowledge the kind assistance of R. E. McCracken and the crew of the Livermore LPTR nuclear reactor for help in obtaining irradiations. LITERATURE CITED
(1) Allen, R . A,, Measurement of Source Strength, in “Alpha-, Beta-, and Gamma-Ray Spectroscopy,” K. Siegbahn, ed., North-Holland Publishing Co., Amsterdam, 1965.
(2) Breen, W. M., Fite, L. E., Gibbons, D., Wainerdi, R. E., Trans. Am. Nucl. SOC.4, 244 (1961). (3) Chinaglia, B., Malvano, R., Energia Nucleare 8, 571 (1961). (4) Ewan, G. T., Tavendale, A. J., Can. J.Phys. 42,2286 (1964). (5) Gaittet, J., Albert, Ph., Compt. Rendu. 247, 1861 (1958). (6) Goulding, F. S., Lawrence Radiation Lab. Rept. UCRL-11302, Feb. 1964. (7) Goulding, F. S., Hansen, W. L.,
Lawrence Radiation Lab. Rept. UCRL11261,Feb. 1964. (8) Goulding, F. S., Landis, D., “Proceedings of Conference on Instrumenta-
tion Techniques in Nuclear Pulse Analysis, Monterey, April 1963,” NASNRC publication 1184. (9) Hansen, W. L., Jarrett, B. V., Lawrence Radiation Lab. Rept. UCFU11589,Aug. 1964. (10) Heath, R. L., Scintillation Spectrometry Gamma-Ray Spectrum Catalogue, Vol. I (IDO-16880-1). (11) Jervis, R. E., Mackintosh, W. D., “Proceedings of the International Conference on the Peaceful Uses of Atomic
Energy, Geneva, 1955,” P/189,Vol. 28, p. 470, United Nations, New York, 1956. (12) Miner, C. E., Lawrence Radiation Lab. Rept. UCRL-11946 Feb. 1965. (13) Nuclear Data Sheets, compiled by K. Way et al., Printing and Publishing Office, National Academy of SciencesNational Research Council, Washington, D. C. (14) Robinson, R. L., Stelson, P. H., Bull. Am. Phys. SOC.10, 245 (1965). (15) Yakovlev. Y. V.. Kulak. A. 1.. ‘ Ryabukhin, ’ V . A., Rytchkov, R. S.; “Proceedings of the International Conference on the Peaceful Uses of Atomic Energy, Geneva, 1955,” P/2023, Vol. 28, p. 496, United Nations, New York, 1956. RECEIVEDfor review April 16, 1965. Accepted June 10, 1965. One of the authors (SGP) is indebted to the National Science Foundation for financial assistance in the form of a Postdoctoral Fellowship. This work was also supported by the U. S. Atomic Energy Commission, Contract No. W-7405-eng-48.
Application of Anodic Dissolution Technique to Automated Analysis of Metals Determination of Phosphorus in Copper SlLVlO BARABAS and SYDNEY G. LEA Noranda Research Centre, Pointe Claire, Quebec, Canada
b A novel, rapid, and automated method for phosphorus i s described which eliminates the need for weighing the samples and carrying out the usual acid dissolution. It consists of anodic dissolution of copper samples against an inert cathode by applying a fixed current for periods of time ranging from 45 to 70 seconds. As the current shuts off automatically, a solenoid valve located at the bottom of the electrolysis cell opens to release the liquid which descends by gravity into a sample cup placed on a rotating sampler plate. From there the solution moves by the action of a pump, and after being combined with the molybdovanadate reagent, passes through a flow cell inside the colorimeter. The resulting signal is either recorded on a strip-chart recorder or printed directly on a tape as per cent phosphorus. The total time of analysis from metallic sample to recorded phosphorus signal is 3 minutes. The new sample dissolution technique is applicable to the analysis of other metals for other elements.
N
automated procedures based on color development and redox titrations have been reported in literature in recent years. The basic UMEROUS
1 132
ANALYTICAL CHEMISTRY
requirement of these automated techniques is that the elements to be determined be present in solution. Obviously, wherever the analysis of metals is involved, the automation can not start before the samples have been weighed, dissolved, excess acid eliminated, and the solution brought to a fixed volume. On occasions, the dissolution step alone may take longer than all the rest of the procedure. It was the purpose of this investigation to develop a technique allowing the handling of metallic samples from the beginning to the end of the analysis in a fully automated manner. The specific need for such development arose in connection with the analysis of phosphorus in copper. I n the copper industry, phosphorus is added to the molten copper for its deoxidizing properties. However, phosphorus is highly volatile and its losses are unpredictable. As a result, appreciable quantities of cast deoxidized copper have 10be scrapped because the phosphorus content is outside the specifications. These copper losses could be substantially reduced if the information relative to the phosphorus content of the melt could be obtained shortly before casting, in time to effect the necessary corrections. A rapid instrumental method for
phosphorus is available (3), based on the sharp drop in conductivity proportional to the phosphorus concentration. Unfortunately, this method is applicable only where pure copper is used. The generally accepted method of analysis of phosphorus in deoxidized copper and phosphorized bronzes is the ASTM Method E 62-60 (1). It is based on the formation of the yellowcolored molybdivanadophosphoric acid (MVP) when molybdate solution is added to an acidic solution containing orthophosphate and vanadate ions. This is a proven method, unaffected by most impurities. However, the analysis of a single sample takes about half an hour. We needed a method which would give a phosphorus analysis within less than 10 minutes after taking the samples of molten copper. Ferretti and Hoffman (8) reported an automated procedure for phosphorus in fertilizer materials after the sample dissolution in mixed nitric-hydrochloric acid. I n using a 9-minute time-delay coil and two double-length mixing coils, the total time of the automated portion of the procedure, apart from the sample dissolution, must have been very close to 15 minutes. A similar automated procedure for the determination of phosphorus in iron- and steel-making slags was described by Scholes and
PROPORTIONING PUMP
SOW. TO COLORIM.
SAMPLE TURNTABLE
I
REAGENT
Figure 1 . Schematic diagram of automated phosphorus analysis
Thulbourne (4). Apparently, the time factor was not considered critical in either paper since the time of sample digestion and dissolution was substantial. I n the present work, an entirely new technique of bringing solid, metallic samples into solution rapidly by anodic dissolution is described. I n accordance with this technique, weighing of samples is not required and the push-button dissolution takes no longer than 70 seconds. The anodic dissolution is fully integrated with the color development procedure which has been time-optimized to be completed in 1 minute and 50 seconds. Thus the total time of analysis from metallic sample to recorded phosphorus signal is 3 minutes. EXPERIMENTAL
Apparatus. Technicon AutoAnalyzer, laboratory model consisting of a Sampler I1 module, a two-speed proportioning pump, a heating b a t h with adjustable thermo regulator, a tubular flowcell colorimeter and a Bristol Dynamaster 560 recorder. Harrison Laboratories power supply unit, Model 6266, providing a maximum of 5 amperes, 36 volts. Labindustries Repipet, lO-ml., for fast dispensing of a n y desired volume from 5 to 10 ml. of electrolyte solution. Hoke solenoid valve, stainless steel, Series 90, 2-way normally closed. Electrolytic Cell. I n applying a current of 4 amperes to the electrodes immersed about 1 inch deep into the electrolyte solution, the heating of the solution is appreciable. T o avoid boiling, t h e volume of t h e electrolyte should be a t least 8 ml. I n this case a simple separator3 funnel of suitable dimensions will d o for electrolysis cell. K h e n it is desirable t o handle smaller volumes for greater sensitivity or even larger volumes for greater accuracy, t h e cell should be provided with a water-jacket. Flow Diagram. T h e automated system developed for t h e analysis of phosphorus in copper is shown schematically in Figure 1. After the com-
pletion of t h e electrolysis, t h e contents of the cell are emptied automatically through t h e solenoid valve into the sample cup. The sample solution and the vanadomolybdic acid are then drawn each through polyvinyl tubes 0.08-inch i.d. a t an approximate rate of 2.5 ml. per minute. Air is drawn through a polyvinyl tube of 0.045-inch i.d. The two solutions and the air stream are combined into one by means of a three-pronged glass connector (Gd) and m5xed i n a singlelength mixing coil of approximately 3 feet in total path. Finally, the solution from which the air bubbles have been removed in a n F-shaped debubbler enters a 15-mm. flow cell for absorption measurement. Reagents. Molybdovanadate Solu0.01 0.02 0.01 0.04 0.05 0.06 X PWOSPRORUI tion. Dissolve in a 600-ml. beaker 20 grams of ammonium heptamolybdate tetrahydrate, ( N H & h 1 0 7 0 ~ ~ . 4 H ~ O , Figure 2. Calibration curves for conventional and anodic dissolution in 300 ml. of hot water, and cool. I n another 1000-ml. beaker dissolve 1 gram of ammonium metavanadate, solution using actual samples are NH4V03, in 200 ml. of hot water. shown in Figure 2. Cool and add 120 ml. of 70% perchloric Sequencing of Samples. T o allow acid. After cooling, transfer the molybt h e detection of very low phosphorus date solution to the vanadate solution signals coming immediately after very while stirring and adjust to the mark high signals, t h e filling of intermediate in a 2000-ml. volumetric flask. sample cups with wash water between Standard Phosphorus Solution, 25 successive samples is recommended. p.p.m. of phosphorus. Dissolve 0.1146 I n setting t h e sampling rate a t 60 gram of disodium phosphate, Na2HP04, samples per hour, 30 samples per in about 200 ml. of water. Add 20 ml. hour will actually be analyzed. If of nitric acid and dilute to 1 liter in a 70 seconds are allowed for anodic disvolumetric flask. solution, the operator will have 50 Electrolyte Solution. Fifty grams seconds remaining in which to pull of ammonium persulfate, (h”,)zSzOs, out the used sample electrode from the in 8 volume % nitric acid made u p to pressure clip and insert a new one. a liter with the same acid. The total capacity of the system could Calibration. Conventional Dissolueasily be twice as much by eliminating tion: Dissolve six 1.25-gram portions wash water cups and by either slightly of pure copper containing less than 2 reducing the electrolysis time or by p.p.m. phosphorus i n 7.5 ml. nitric operating with two electrolytic cells. acid. Boil to expel brown nitrogen The indicated power supply unit is oxide fumes, then cool and transfer more than adequate to provide current each solution to a 50-ml. volumetric t o two cells simultaneously. flask. Carry one solution through as a blank and to the others add 2, 5, 10, DISCUSSION 20, and 30 ml. of the standard phosphorus solution corresponding to 0.004, The experimental work covered in 0.01, 0.02, 0.04, and 0.06% phosphorus. this investigation centered around the Adjust to the mark with distilled water. optimization of the MVP color developRun portions of these solutions on the ment, and the anodic dissolution. The AutoAnalyzer using 420 mp filters. discussion concerning the factors afFor greater accuracy use carefully analyzed samples for the preparation of fecting the two items will be presented the calibration graph. to the same logical order. Anodic Dissolution: Select five acColor Development. EFFECTOF curately analyzed samples in the form PERMANGANATE A N D HYDROGEN PERof 0.25-inch rods covering the same OXIDE. I n accordance with the A S T M concentration range from 0.004 to Method E 60-62, potassium per0.06% phosphorus. For the blank use manganate is added to the hot sample a rod of pure copper. Effect anodic solution to oxidize a n y suboxides of dissolution of each rod by placing it phosphorus t o orthophosphate which into the electrolytic cell containing 5 ml. of electrolytic solution against a is the only form in which phosphorus graphite cathode. Apply 4 amperes reacts with molybdovanadate to form for a period of anywhere from 45 to the yellow M V P complex. The excess 70 seconds and run the resulting solupermanganate is then destroyed with tions on the AutoAnalyzer using 420 hydrogen peroxide which in its turn is mp filters. subsequently decomposed by boiling. Procedure. Carry out t h e analysis Since the MVP method, as used in the in accordance with the previous secanalysis of phosphorus in copper, is tion on Calibration. Calibration based on the method originally applied graphs for analysis by conventional to iron and steels containing varying dissolution using synthetic standards and actual samples, and anodic disamounts of carbon, it was felt that the VOL. 37,
NO. 9, A U G U S T 1965
1133
Table 1.
Effect of Time on MPV Color
Time color Total time develop.,b cycle," seconds seconds Absorbance 418 375
378 335
0.261 0.260
a Total time cycle is the time from aspirating the sample solution to obtaining the recorded signal. Time of color development is the time elapsed after the sample solution combined with the reagent leaves the three-pronged glass connector and before it enters the flow cell.
addition of permanganate and hydrogen peroxide, mandatory in the analysis of the latter materials, might be dispensed with in the analysis of copper. T o corroborate this hypothesis, three portions of a sample containing 0.029% phosphorus were run by the ASTM method-Le., adding KlhfnOa and Hz02; by omitting K M n 0 4 but adding HzOz; and by omitting the additions of both K h l n 0 4 and HzOz. The absorbances read were 0.32, 0.31, and 0.30, respectively, indicating some but not significant drop in sensitivity. As a result, the additions of K M n 0 4 and HzOzwere omitted in all subsequent tests. EFFECT OF TEMPERATURE. T o establish the effect of temperature on the color development of the MVP complex six portions of a sample solution assaying 0.040y0 phosphorus were run on the AutoAnalyzer with the thermostat set a t 26', 40°, 61') 76", go', and 100' C., respectively. The absorbances recorded at 420 mp did not vary from the mean by more than 0.01 corresponding to O.OOlyo phosphorus, which is insignificant. As a result all subsequent tests were carried out a t room temperature. Other experimental conditions were as follows : the MVP reagents were prepared as per Reference (2); the sampling rate was 40 samples per hour; the sample solution, after being combined with the color reagent was passed first through two single-length mixing coils, then through a 40-ft coil in the thermostat. The total time for each sample solution from aspiration from the sample cup to recording the transmission peak was 7 minutes. EFFECT OF TIME. The effect of time on the color development of the yellow M V P complex was established by comparing the absorbances of portions of the same sample solution passing through coils of various lengths before entering the colorimeter. The pertinent data are shown in Table I. The data presented show clearly that nothing is gained by extending the time 1134
ANALYTICAL CHEMISTRY
of color development beyond 45 seconds. This is the time required for the solution to pass through a single-length mixing coil. EFFECTOF REAGENTCONCENTRATION. A reagent solution containing 2 grams of ammonium vanadate, 40 grams of ammonium molybdate, and 450 ml. of perchloric acid per liter was prepared. Stepaise dilutions from this solution were made down to 1/32ndof the original concentration. KO loss in signal was experienced in the analysis of deoxidized copper samples in using the reagent solutions that were as low as one-sixteenth of the original reagent concentration. Signal, however, was reduced by about 17% for the solution that mas diluted by a factor of 32. Subsequently, solutions were prepared to contain 10 grams of ammonium molybdate and 0.5 gram of ammonium vanadate per liter. The perchloric acid concentration of these solutions was adjusted to be 1.2, 2.5, 5.0, 7.5, 11.25, 15.0, 17.5, and 20.0 volume yo. In trying each of these reagents in a sequential manner with the same sample solution, it was observed (Figure 3) that for the two reagent solutions that were the lowest in acid content, the recordings were of a peculiar shape not amenable to easy measurement. This behavior is due to the surprising fact that the absorption of the copper blank solutions comprising the reagents turned out to be far less that that of the reagents alone. The best performance was realized with the reagents containing 5.0 and 7.5 volume yo of perchloric acid. For solutions of higher acid concentration, the signals obtained decreased as the acid concentration was increased. Finally, reagent solutions were prepared to contain 6.0 volume % of perchloric acid and increasing amounts of the two chromogenic reagents. The highest sensitivity resulted for the regent solutions containing 0.5 and 1 gram of ammonium vanadate and 10 and 20 grams of ammonium molybdate per liter. Anodic Dissolution. GENERAL CONSIDERATIONS. Anodic dissolution accompanied by cathodic deposition is a n old and well-established practice in the electrolytic purification of numerous metals. T h e novelty of this application consists in effecting the anodic dissolution of the sample presented in the form of a rod or tube without the undesirable deposition at the cathode. T o this end, a spectrographic graphite rod was selected for inert cathode. The hydrogen evolution at this electrode replaced the undesirable metal deposition. However, for the purposes of a more general application of the anodic dissolution technique to the analysis of metals for a number of elements, the absence of cathodic deposition of the
% TRAUSMITTAUCC
11.21%
20 %
Figure 3. Effect of perchloric acid on absorbance
main constituent is not a prerequisite. For as long as the element to be determined does not deposit on the cathode or deposits only partially in a reproducible manner, one could safely use as cathode a rod of the sample material. Based on the Faraday's Law, the amount of copper dissolved at the anode can be calculated as follows: Cu (dissolved)
Ite
= -
96,500
where I is the current in amperes, t time in seconds, and e the copper equivalent of 31.79 grams. I n setting the maximum operating current a t 4 amperes, it can be easily calculated that the dissolution of from approximately 60 to approximately 90 mg. of copper needed for the phosphorus analysis of deoxidized copper will take from 45 to 70 seconds of electrolysis. If the electrolysis is carried out for longer periods of time, trace amounts of dissolved copper will tend to deposit on the graphite electrode. These trace quantities of copper dissolve promptly in the nitric acid electrolyte solution by stirring it with the graphite rod after completion of the electrolysis. By slightly greasing the graphite electrode with silicone stopcock grease, the deposition of copper a t the cathode can be either prevented altogether or a t least delayed. EFFECTOF AMMONIUM PERSULFATE. I n the beginning of this work, anodic dissolution was carried out in an 8 volume yonitric acid solution containing no ammonium persulfate. The phosphorus signals obtained were some 20% less than when the dissolution was carried out as per the ASTM method. Moreover, the reproducibility was not
too satisfactory. I n the area surrounding the anode, one could observe in the course of the electrolysis some yellowish substance forming and promptly disappearing in the solution. This might have been phosphorus hydride of the type (P2H), (6). I n one instance electrolysis was performed in a solution containing the molybdovanadate solution as the electrolyte. The solution became a deep blue, a clear indication that strongly reducing conditions prevailed in the course of the electrolysis. Obviously, in the course of the ancdic dissolution in nitric acid electrolyte solution a t least some lower valent phosphorus compound formed which did not react with molybdovanadate to form the M V P complex. Attempts made to remedy this situation by adding some potassium permanganate to the electrolyte solution were not successful. However, when ammonium persulfate was added no yellow substance could be seen during the dissolution. The phosphorus signals obtained on the recorder were larger than obtainable by the conventional dissolution of phosphorized copper and equal to signals obtained when pure standard phosphorus solution was allowed to react with molybdovanadate (Figure 2). This indicates a complete recovery of phosphorus present in copper as orthophosphate. The slightly weaker signals obtained following the conventional ASTM dissolution technique are caused by the presence of some residual nitrogen oxide which has the effect of depressing the M V P color. Anodic us. Conventional Dissolution. Precision studies were carried out by t h e automated phosphorus procedure on a sample of deoxidized
Table II.
Precision of Automated Anatysis Using Conventional and Anodic Dissolution
Each sample contained o.o36Y0 phosphorus Conventional, 25 mg. Cu/ml. Anodic, 15.4 mg. Cu/ml. Deviation Deviation Absorbance P, % from mean Absorbance P, yo from mean 0.275 0.036 0.000 0,205 0.036 0.000 0.290 0.038 0.002 0,200 0,035 0.001 0.280 0.037 0,001 0.200 0.035 0.001 0.255 0,034 0.002 0.195 0 034 0.002 0.250 0,033 0.003 0.203 0.036 0,000 0,275 0.036 0.000 0.200 0.035 0,001 0.270 0,036 0.000 0.210 0.037 0.001 0.210 0.037 0,001 Mean 0.036 0.0011 0.036 0,0009 Std. dev. 0.0017 0.0011 copper containing 0.036% phosphorus starting with conventional and anodic dissolution. I n the case of conventional dissolution 2.5-gram portions were dissolved in nitric acid and made u p t o 100-ml. in a volumetric flask. I n the case of anodic dissolution, a copper rod of the same sample was repeatedly electrolyzed at 4 amperes for 70 seconds in 6 ml. of electrolyte solution. T h e data obtained are shown in Table 11. The superiority of the method involving the anodic dissolution does not lie so much in improved precision as in the elimination of the skill from the analysis. Any inexperienced technician following the automated anodic dissolution technique will get from the very first run results as good as those obtained by the most experienced technician following the ASTM method, Anodic dissolution is a highly reproducible push-button technique. Conventional dissolution requires full attention of the operator in the dissolution of the
K TRANSMITTANCE 0
m
4
m
0 (L
0
U
1179 x
a
Table 111. Comparative Results by ASTM and Automated Methods Using Anodic Dissolution
Phosphorus, % ASTM Automated Difference 0 036 0 038 0 002 0 020 0 018 0 002 0 036 0 036 0 000 0 019 0 019 0 000 0 038 0 038 0 000 0 037 0 03.5 0 902 0 037 0 034 0 003 0 023 0 024 0 001 0 037 0 036 0 001 0.042 0 039 0.003 0,020 0,018 9 002 0 036 0 034 0 00z 0 022 0 024 0 002 0 037 0 037 0 000 0 019 0.018 0 001 Mean 0 0014 Std. dev. 0 0018
samples and elimination of excess acid, A comparison of the results from the two methods is given in Table 111. The difference between the results by the two methods is no larger than between the duplicate analyses by the same method. A typical series of recordings displaying the sequence of high (0.07%) and low (0.005%) phosphorus values is shown in Figure 4.
.006 %
ACKNOWLEDGMENT -+
P33X
,018
,022
x
x
dl424
,01496
,01651 .OIOU
.036%
Figure 4. Typical strip chart recordings of phosphorus in copper
The authors thank W. J. Wright of Noranda Copper Mills for providing phosphorus results by the ASTM method and for encouraging this work in automation. LITERATURE CITED
(1) Am. SOC. Testing Materials, Philadelphia, Pa., “ASTM Methods for Chemical Analysis of Metals,” 1960. ( 2 ) Ferretti, R. J., Hoffman, W. PI., J. Assoc. Oqic. Agr. Chemists 45, 993 (1962). (3) Garatoni, P., Iron Age 193, 92 (1964). (4) Scholes, P. H., Thulbourne, C., Analyst 88, 702 (1963). (5) Sidgwick, N . y., “The Chemical Elements and Their Compounds,” Vol, I, p. 730, Orford at the Clarendon Press, London, 1962. RECEIVEDfor review March 22, 1965. Accepted May 28, 1965. 48th Chemical Conference and Exhibition, Chemical Institute of Canada, Montreal, June 1965. VOL 37, NO. 9, AUGUST 1965
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