122 SUTCLIFFE AND PEAKE : THE SPECTROPHOTOMETRIC DETERMINATION [Vol. 83 The Spectrophotometric Determination of Nickel in Copper - Nickel Alloys BY G. R. SUTCLIFFE A.ND D. M. PEAKE (Research Department, Imperial Chemical Industries Ltd., Metals Division, Kynoch Wovks, Witton, Birmingham) From a study of the absorptiometric characteristics of solutions of copper and nickel, a method has been developed for the direct determination of nickel in copper-base alloys. The optical density of the sample solution in a nitric acid - phosphoric acid medium is measured at 3950 A, where absorption is due almost entirely to nickel. A similar measurement is made on the same solution at 4 9 0 0 ~ . where neither copper nor nickel absorb,, and this permits a background correction to be made.The method has been satisfactorily applied to typical copper-base alloys with nickel contents ranging from about 1.5 to 30 per cent. and a single determination can be completed in about 30 minutes, as opposed to 3 hours by a reliable gravimetric procedure. DETERMINATION of alloying amounts of nickel is usually based on the use of dimethy1glyoxime.l This gravimetric procedure is relatively straightforward and can be applied in the presence of small amounts of copper, but becomes increasingly difficult to apply when a large amount of this metal is present. In such instances, copper in solution can be reduced to the cuprous state,2 but the most satisfactory way of overcoming this interference is to remove the metal by electro-deposition. A further disadvantage ass,ociated with the dimethylglyoxime method, particularly in control laboratories, is the relatively small sample that must be used for the examination of alloys containing 15 per cent.or more of nickel. EXPERIMENTAL A study of the absorption characteristics of simple solutions of metals was carried out with the idea of developing rapid photometric procedures for the determination of constituents that are commonly present in copper-base alloys containing up to about 30 per cent. of nickel. Absorption characteristics of copper as sulphate, nitrate and perchlorate in correspondingMarch, 19581 OF NICKEL I N COPPER - NICKEL ALLOYS 123 acid solutions were determined and were shown to be substantially independent of acid concentration (see Fig. 1, curve A). 2.01, Wavelength, A Wavelength, A Fig.1. Absorption spectra of solu- Fig. 2. Absorption spectrum of nickel nitrate, sulphate and perchlorate solutions containing 0-5 g of nickel per 100 ml tions containing 0-2 g of copper per 100 ml : curve A, copper nitrate, sulphate and per- chiorate; curve B, copper chloride in 5 per cent. v/v hydrochloric acid; curve C, copper chloride in 30 per cent. v/v hydrochloric acid Absorption by solutions containing copper chloride and free hydrochloric acid is very largely influenced by acidity ; this is attributed to complex-ion formation? An intense absorption by copper chloride solutions occurs at 4000 A (see Fig. 1, curve C) and the optical density is related to acid concentration. Examination of the absorption characteristics of solutions of copper in hydrochloric acid show that it is undesirable to use this acid in photo- metric determinations involving absorption at about 4000 A or between 6000 and 10,000 A.Absorption characteristics of nickel as sulphate, nitrate and perchlorate in corresponding acid solutions (see Fig. 2), are substantially independent of acidity. By comparing curve A, Fig. 1, with Fig. 2, it is interesting to observe that nickel solutions absorb strongly at 3950 A, whereas copper solutions have no absorption at this wavelength. Absorption characteristics of iron solutions show that they absorb to some extent at 3950 A, although nitrate solutions of the metal absorb considerably less at this wavelength than either the corresponding sulphate or perchlorate solutions.It was shown that phosphoric acid represses the absorption due to iron without affecting the absorption characteristics of either copper or nickel. Hence, the optical density due to 10mg of iron, i.e., 1 per cent. of iron in a 1-g sample, was only 0.001, and it appeared likely that a single optical- density determination at 3950 A of a simple solution containing both nickel and copper could form the basis of a rapid method for the direct absorptiometric determination of nickel. Copper and nickel alloys are readily soluble in nitric acid, and curve A, Fig. 1, and Fig. 2 show that this acid is permissible as a solvent. Further, the presence of phosphoric acid prevents precipitation of tin, which is present in some commercial alloys, and a mixed solvent of nitric and phosphoric acids was therefore used. Solutions containing known amounts of copper and nickel were prepared and these showed good proportionality between optical density and nickel content, although the reproducibility of optical-density measurements was not entirely satisfactory.This was attributed to haze and cell-surface variations, but the difficulty was resolved when it was found that neither copper nor nickel absorbs significantly at 4900 A. At this wavelength the optical density therefore gives an approximate assessment of the haze and cell blank, and, by measuring the optical124 SUTCLIFFE AND PEAKE : THE SPECTROPHOTOMETRIC DETERMINATION [VOl. $3 density of the solution at 3950 A and deducting the value of the optical density measured at 4900 A, good reproducibility and proportionality were found.When, however, attempts were made to apply this principle to the analysis of alloys containing a small amount of manganese, an unexpected oxidation to permanganate occurred when the sample solution was digested with the phosphoric acid - nitric acid mixture. This reaction is not quantitative and is probably due to oxidation by a perphosphoric acid. Addition of hydrogen peroxide, after dilution, is sufficient to reduce the permanganate and eliminate this interference, and excess of peroxide can be readily decomposed by boiling. A Unicam SP600 spectrophotometer was used for all optical-density measurements, and as the density scale is logarithmic, the lower end of the scale up to 0-25 was used, which permits accurate readings to be made to within &O.OOl.When 4-cm cells are used, this length of scale is equivalent to about 60 mg of nickel, ie., 6 per cent. of nickel in a l-g sample. For larger amounts of nickel, say up to 30 per ce:nt., the determination was made by difference, use being made of a reference liquid containing a solution of known nickel content These general principles were embodied in a provisional method that was applied to a series of commercial alloys. Nickel in these alloys was also determined by a gravimetric dimethylglyoxime procedure, after preliminary electro-deposition of copper. Two sets of results are shown in Table I. Hence, by dissolution of the sample in a nitric acid - phosphoric acid mixture, dilution to a standard volume and subsequent measurement of the optical densities at 3950 and 4 9 0 0 ~ against a solution containing either copper, or copper and a known concentration of nickel, a rapid and accurate determination of nickel can be made in about 30 minutes once the calibration graphs have been prepared, as opposed to 3 hours by a reliable gravimetric procedure.TABL:E I COMPARISON OF RESULTS :BY DIFFERENT METHODS Typical manganese content, % 0.25 1.2 0.1 0.2 0.1 0.15 1.4 Typical iron content, % 0.05 3.7 < 0.05 0.05 - (0.05 2.2 Nickel found by Nickel found by gravimetric method proposed method - \ . A . . 1 Analyst A, Analyst B, Analyst A, Y O % YO 1.69 4.29 { 4.62 5-02 7.27 9.39 18.51 24-98 30-34 30.26 30.37 30.05 1-75 1.70 1-84 1-95 2.18 2.17 4-37 4.43 - 4.66 - 5.01 - 7.32 - 6-92 - 6 7 3 - 6.64 9.46 9.38 9-74 9.7 1 10-03 9-99 - 18.40 - 25.03 - 30.22 - 30.25 - 30.32 - 30.03 Analyst B, 1.72 1.98 2.13 4.43 4.66 7-36 6-89 6.70 6.6 1 9.33 9.68 9-96 18.37 25-05 30.20 30.20 30.42 30- 13 Y O - METHOD APPARATUS- A Unicam SP600 spectrophotometev was used.Grade A caZibrated$asks must be used throughout, and, for work of the highest accuracy, The cells used for the preparation of the calibration graph must also be used in deter- the certified volumes must be used in the calculation of the nickel contents. mining nickel in the test solution. REAGENTS- Distilled water should be used for the preparation of all solutions. Nickel-Hilger H.P. quality was used.March, 19581 OF NICKEL I N COPPER-NICKEL ALLOYS 125 Cofifier-The metal nominally free from nickel was used, Nitric acid, diluted (1 + 1)-Dilute 1 volume of nitric acid, sp.gr.1.42, with 1 volume Phosphoric acid, sp.gr. 1-75. Hydrogen peroxide, 2-volume-Dilute 10 ml of 20-volume hydrogen peroxide with 90 ml of water. Standard nickel y e ference solutions-Reference solutions contain a combined weight of 1 g of nickel plus copper per 100 ml. The ratio of nickel to copper used for a particular solution is governed by the range of nickel contents to be determined, e g . , a reference solution for nickel in the range 5 to 11 per cent. should contain 0.0500 g of nickel plus 0.9500 g of copper per 100 ml. Transfer the calculated weight of nickel and copper to make 1 litre of solution to a large beaker. Dissolve it in 100 ml of diluted nitric acid (1 + 1) and add 300 ml of phosphoric acid.Evaporate to ensure removal of all nitrous fumes; it is not necessary to remove all the nitric acid by evaporation. Dilute to about 800 ml, add 20 ml of 2-volume hydrogen peroxide and boil for 5 minutes. Cool, transfer the solution to a l-litre calibrated flask, dilute almost to the mark and place in a thermostatically controlled water bath at 20" C for 1 hour. Dilute to the calibration mark and mix well. PROCEDURE FOR PREPARING A CALIBRATION GRAPH FOR BETWEEN 5 AND 11 PER CENT. OF Weigh accurately about 0.06, 0-07, 0.08, 0.09, 0.10 and 0.11-g portions of nickel and to each add copper to make a total sample weight of 1 g. Dissolve the samples in 10 ml of diluted nitric acid (1 + l ) , add 30 ml of phosphoric acid and evaporate to small volume to ensure removal of nitrous fumes.Cool, dilute to a volume of about 85 ml, add 2 ml of 2-volume hydrogen peroxide and boil gently for 5 minutes. Cool, transfer to a 100-ml calibrated flask and dilute to the mark at 20" C. Measure the optical density of each solution in a 4-cm cell at 3950 and at 4900 A with respect to the 5 per cent. reference solution, and record the difference in optical density. (For other ranges of nickel contents, the procedure is similar and measurements are made by using a reference solution appropriate to the range.) PROCEDURE FOR DETERMINING NICKEL- Weigh 1 g of sample and continue exactly as described for preparation of the calibration graph. Measure the optical density with respect to the appropriate reference solution and calculate the nickel content from the calibration graph. Acknowledgment is made to Mr. W. T. Elwell, Division Chief Analyst, Imperial Chemical Industries Limited, Metals Division, for his interest in this investigation and for assistance in the preparation of this paper. REFERENCES of water. NICKEL- 1. 2. 3. American Society for Testing Materials, "Methods of Analysis of Metals," 1956, p. 270. Bayley, W. J., unpublished work. Orgel, L. E., Quart. Rev. Chem. SOC., 1954, 8, 422. Received September 4th, 1967