Flame Spectrophotometric Determination of Copper, Nickel, and Manganese in Auminum-Base Alloys JOHN A. DEAN and CARL CAIN, Jr.' Department of Chemistry, University of Tennessee, Knoxville, Term.
b Copper, nickel, and manganese can be selectively extracted with chloroform as the metal diethyldithiocarbamates froni a buffered aqueous solution containing citrate. The elements can then b e determined successively by aspirating the chloroform extract into an oxyacetylene flame. This use o f an organic reagent in conjunction with extraction simultaneously separates several constituents of ihe sample from the matrix prior to the determination. The method circumvents many spectral and radiation interferences encountered when the elements are determined in an aqueous solution. In addition the chloroform enhances the emission o f each of the elements. Only cobalt interferes when this method i s applied to aluminum-base alloys.
T
sodium diethyldithiocarbamate for the separation of nickel, copper, and manganese from the other components in aluminum-base alloys has been suggested by the work of Bode (1-4) and hIalissa and Miller (9). I n the method presented here, these elements are precipitated as the respective metal diethyldithiocarbamates from an aqueous solution containing citrate to mask the iron, then extracted with chloroform. The chloroform extract is aspirated directly into an oxyacetylene flame. The emission intensity of each of the elements is increased severalfold when aspirated from a chloroform solution rather than from an aqueous solution. This enables accurate analyses to be made even though these elements may be present in the sample only as minor components. Serious interferences from large amounts of matrix elements or reagents added during the preparation of the sample are eliminated. By choosing a general extractant the copper, nickel, and manganese are simultaneously recovered, and, because their respective concentrations in the extract are sufficiently dilute to prevent mutual interference, they can be successively determined on the same extract. HE USE OF
1
Present address, Cramet Corp., Chat-
tanooga, Tenn.
530
ANALYTICAL CHEMISTRY
EXPERIMENTAL W O R K
A Elecknian Model D U spectrophotometer with Model 9220 flame attachment ttnd photomultiplier unit was used. A 10-mv. Bristol recorder with a 2/3-second pen response in conjunction with wave length motor drive operating a t 0.17 r.p.m. was used t o provide a 3ermanent record. Flowmeters were placed in the oxygen and acetylene lines to monitor accurately the quantity of gases consumed. Reagents. Staiidard solutions of copper, nickel, and manganese mere prepared from reagent grade salts. Each solution contained 1 mg. per ml. of the desired cation. Copper sulfate pentahydrate wati dissolved in demineralized water; nickel carbonate and manganese carbonate mere dissolved in hydrochloric acid, then diluted with deniinc ralized water. A solution of sodium diethyldithiocarbamate, 10% (n-./v.), was prepared by dissolving 13.2 g a m s of the reagent grade trihydrate SE It in mater, filtering the small amount of insoluble matter, and diluting to 100 ml. The solution was stored in a Dolvethvlene container away from light. Sodium citrate solution, 10% (w./v.), was prepared by d ssolving 57 grams of the dihydrate in mater, shaking with a 1% solution of dithizone in chloroform td -remove traces of metallic impurities, and diluting to 50(1 ml. with demineralized mater. Acetate buffer solution, pH 6.75, mas prepared by dissohing 247 grams of ammonium acetate, IO9 grams of sodium acetate, and 6 grams of acetic acid in mater and diluting t o 1 liter. Traces of metallic impurities were removed by shaking the solution with a 1% solution of dithizone in chloroform. Individual or cornbined standard solutions of the metal diethyldithiocarbamates (100 y of mea1 per ml.) were prepared from the aqueous standards. Ten milligrams of each metal was precipitated with the reagent a t pH 6.0 to 6.5, then extracted with three 20-inl. portions of chloroform. The combined extracts were diluted to 100 ml. with additional chloroform. Less concentrated standard solutions were prepared by appropriate dilution with additional chloroform. The nickel and copper carbamate solutioiis are stable on storage away from light, but the manganese solution decomposes after a few hours unless 0.1 ml. of acetylacetone is added per 100 ml. of solution. Apparatus.
-
I
1
Flame Spectrophotometer Settings.
The instrument settings used were as follows: Sensitivity control, turns from clockwise limit
5 . 0 (Cu) 8.0 (Ni) 8 . 0 (Mn) 0.1
Selector switch, position Phototube resistor, megohms 22 Phototube, volts per dynode 60 (Cu, Ni) 50 ( A h ) Acetylene, cubic feet per hour 2.15 Oxygen, cubic feet per hour 4.55 Slit, mm. 0.030 The intensities of the principle lines of manganese, copper, and nickel are in the ratios of 8 to 2.5 to 1, respectively. Calibration Curves. Individual calibration curves for copper, nickel, and manganese were obtained by measuring the emission intensity observed from the standard solutions in chloroform. The calibration curves for the nickel 352.5-mp line and the manganese 403.2-mp line are linear for concentrations less than 20 y per ml. The curve for the copper 324.7-mp line is linear for concentrations less than 15 y per ml. but begins to bend toward the concentration axis with larger amounts of copper because of self-absorption. PROCEDURE
Dissolve the aluminum alloy sample by conventional methods and evaporate to dryness. Dissolve the residue with dilute hydrochloric acid, filter any insoluble matter if necessary, and dilute with demineralized mater to a suitable volume. Transfer an aliquot of the solution containing 25 to 500 y of the least abundant component to a 100-ml. beaker. Add sufficient 10% sodium citrate solution to complex the metals present (approximately 1ml. per 10 mg. of sample). Adjust the pH to between 6.0 and 6.5 with acetate buffer solution, then transfer the solution t o a separatory funnel. Add a quantity of 10% sodium diethyldithiocarbamate calculated to be in excess of the amount necessary to react with the copper, nickel, manganese, iron, lead, and zinc present. (One milliliter reacts with approximately 13 mg.
of divalent metals and 8 mg. of trivalent metals.) Add 10 ml. of chloroform to the separatory funnel and shake for 30 seconds. TF7hen the emulsion has separated, draw off the chloroform layer into a 25-ml. volumetric flask. A small plug of glass wool inserted into the stem of the funnel is helpful in retaining droplets of the aqueous phase, which invariably find their may into the stem. Repeat the extraction with two or three additional 5-ml. portions of chloroform. A colorless chloroform layer indicates complete extraction. Combine the extracts and dilute to the mark with chloroform. Aspirate the chloroform extract and measure the line emissions and flame backgrounds a t the following wave lengths : Element CU Mn Ni
ditional chloroform before determining copper. Samples whose copper content is greater than 20 times that of the nickel or manganese are subjected to the following modified procedure prior to the diethyldithiocarbamate extraction step. Following the initial evaporation to dryness, dissolve the sample in 1N hydrochloric acid. Add sufficient 5% thioacetamide solution to precipitate about 90% of the copper present. Warm the solution to 80" C. for several minutes, then filter. Boil to expel excess hydrogen sulfide. Cool, then treat the filtrate as described to extract the nickel and manganese. Determine copper on a smaller aliquot or by direct measurement on the aqueous solution (6).
TdineEmission, Background, M p
324.7 403.2 352.5
325.3 401 353.5
RESULTS AND DISCUSSION
Table I summarizes the results obtained for the analysis of NBS samples of aluminum-base alloys. The values obtained include the mean of replicate samples and the standard deviation. Flame Spectra of Copper, NickeI, and Manganese. T h e major flame emission lines of copper have been studied extensively (6); the lines occur in the ultraviolet a t 324.7 and 327.4 mp. The emission intensity of the 324.7-mp line is increased sufficiently in chloroform solutions to enable less concentrated solutions of copper to be used. From these solutions self-absorption is negligible. Background readings were taken a t the head of the weak hydroxyl band a t 325.3 mp. This
13racket the unknown samples with standard samples of copper, nickel, and manganese in chloroform. The standards may be either single solutions or a mixture of the three. Subtract the background readings from the unknown and standard emission readings to obtain net relative emissions and read the amount of metal present from the appropriate calibration curve. For recording instruments, scan the regions from 323.5 to 325.6 mp for copper, 351.5 to 353.5 mp for nickel, and 401 to 404 mp for manganese. Samples containing amounts of copper considerably in excess of the nickel and manganese may be diluted with ad-
~~
~
Table I.
Sample Aluminum alloy 85a
Aluminum alloy 86c
Aluminum-silicon alloy 87
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Analysis of NBS Samples
Cn
Certified Value, % 2.48 f 0.01
Xi
0.41 2IZ 0.01
1111
0.66 rt 0.01
cu Ni
7.92 2IZ 0.03 0.030 f 0.004
1.111
0 . 0 4 1 f 0.001:
c 11
0.30 f 0.01
Xi
0.59 2IZ 0.01
A h
0.30 f 0.00
Value Found, yo 2.40,2.43, 2.45, 2.52, 2.45,2.51, 2.49,2.50 Av. 2.48 Std. dev. 2IZ0.03 0.43,0.38,0.42, 0.47, 0.43.0.42.0.46, 0.41 Av. 0.43 ' Std. dev. f 0 . 0 3
0.68,0.65,0.67,0.66, 0.69,0.63, 0.65, 0.64,O.68 Av. 0.66 Std. dev. fO, 02 7.96, 8 05, 8 . 1 6 0.027, 0.034, 0,036, 0.025 Av. 0.031 Std. dev. i ~ 0 . 0 0 5 0.039, 0.037, 0.042, 0.042
Av. 0.040 Std. dev. &0.002 0.29, 0.30, 0.30, 0.32, 0.30, 0.29, 0.30 Av. 0.30 Std. dev. 1 0 . 0 1 0.61, 0.60, 0.61, 0.61, 0.61, 0.59, 0.59 Av. 0.60 Std. dev. f O . 0 1 0.29, 0.26, 0.29, 0.28 Av. 0.28 Std. dev. & O . 01
point is preferable to the minimum between the peak of the copper line and the hydroxyl band head because the background a t the minimum is slightly affecited by the amount of copper present. Nickel exhibits a series of closely spaced lines between 340 and 360 mp (IO, 11). The most sensitive line occurs a t 352.5 mp. The flame background is taken a t 353.5 mp in a region devoid of any nickel lines for a span of about 2 mp. The major flame emission line of manganese at 403.2 mp has been studied by Dippel and Bricker (7). These authors reported serious interference from aluminum and iron, both of which possess a series of band spectra due to the oxides of the element in this region of the spectrum. The selective extraction in the presence of citrate eliminates the interference of both elements. Nature of Extraction. Sodium diethyldithiocnrbamate is stable in alkaline solution, b u t decomposes as the acidity is increased ( 1 ) . The upper p H limit is set b,y the extraction characteristics of manganese, which is incompletely extraoted when the p H exceeds 9 (4). However, the manganese complex and extract are more stable when the p H of the aqueous phase is maintained between 6 and 7. As the copper and nickel inay be extracted over a wide pH range, the optimum conditions for the manganese extraction must be adhered to for the simultaneous extraction. The nickel and copper carbaniates precipitate rapidly and are easily extracted by chloroform. Manganese precipitates inibially as the divalent salt and is converted t o the trivalent complex after a few secoiids by air oxidation
(4)*
Interference Studies. Lead, zinc, and iron (partially) react with diethyldithiocarbamate and are extracted under the operating conditions selected. The influence of various conceiitrations of these metals on the emission of copper, nickel, and manganese, and alrio the mutual interference of the lattey elements on each other, was studied. No interference was found when each element was present in amounts less than 400 y per ml. Larger amounts of copper could be tolerated, but when the copper concentration exceeds the amount of manganese or nickel by a, factor of approximately 20, it is deciirable to remove the majority of the copper in order to avoid difficulties attendant to the extraction of large amounts of material and to avoid weak spectral interference upon the flame emissions of nickel and manganese. A partial sulfide precipitation, utilizing thioacetamide, separates the bulk of the copper from the nickel and VOL. 29, NO. 4, APRIL 1957
531
manganese without danger of coprecipitation of these elements. Nickel emits a weak line a t 324.3 mp and iron emits a weak line a t 352.1 inp. No spectral interference from overlap of these weak lines with the copper 324.7-mp line or the nickel 352.5-mp line was noted when a slit width of 0.030 mm. was employed. Iron also emits a series of weak oxide band spectra in the 1-icinity of the manganese 403.2-mp line. Correction for these band systems and general flame background is achieved by subtraction of the background reading from the total line emission. Cobalt, although not encountered in any of the standard aluminum alloys, would interfere seriously in the nickel determination if it were present because i t xould also be extracted. Cobalt has rather strong emission lines a t 352.2, 352.7, 352.9, and 353.0 mp. Cobalt could be removed and determined colorinietrically by the alumina-ion exchange method (6), and the eluate could be used for the carbamate extraction. Fuel and Oxygen Flow Rates. The optimum acetylene flow rate was obtained by adjusting the oxygen pressure to the rated pressure of the burner, 12 pounds per square inch, which corresponded to an oxygen flow rate of 5.2 cubic feet per hour, and then observing the emission of the metals individually a t various acetylene flow rates. For each of the metals a linear increase in emission intensity was observed for increased acetylene consumption. The rate of increase on the background radiation was less than the rate of increase of metallic light for flow rates less than 2.15 cubic feet per hour. A t higher flow rates the background radiation increased a t a greater rate than did the metallic emission. The optimum oxygen flow rate was determined by setting the flow of acety-
lene a t its optimum value of 2.15 cubic feet per hour and varying the oxygen flow rate. Vari,ition in the oxygen flow rates has only a slight effect on the metallic light anc. the background radiation. The maximum emission of the metals occurred a t 4.55 cubic feet per hour. A flow rate less than 4.4 cubic feet per hour was insufficient for complete combustion of the acetylene and solvent, and resul ;cd in luminous, erratic flames. This minimum flow rate was also necessary for steady aspiration of the sample with the particular burner employed. The optimum ratio of acetylene to oxygen for any burner is approximately 1 to 2. The oxjgen flow rate is more or less fixed by the orifice characteristics of the particular burner used. Effect of Chloroform on Flame Emissions. The emission of metallic light from chloi*oform solutions exhibits a considerable enhancement over the light emitted from aqueous solutions. Sevei a1 other solvents investigated by La3y (8) exhibit greater enhancement th,m does chloroform; however, the ease of extraction with this high density liquid weighed heavily in its favor for this study. Adequate ventilation should be provided above the liurner housing to dispose of the proiucts of combustion of chloroform, pvhich include large amounts of hydrochloric acid and some phosgene. The flame is of ;en more erratic when organic solvents are aspirated. Fluctuations increase with increasing concentrations of mttal present, which is why the calibralion curves are confined to 20 y per i d . or less of metal. CONCLUSIONS
The extractior of copper, nickel, and manganese as their diethyldithiocarbaniate cainplexes into chloroform offers sewral advantages for
flame spectrophotometric analyses. By contrast with water, the organic solvent is a fuel and adds rather than detracts from the energy of the flame. The emission intensity of the three elements is increased four- to sixfold. A selective extraction avoids the introduction of high concentrations of diverse ions into the flame, and in particular, avoids the serious interference of aluminum and large amounts of iron. Considerable time is saved by isolating all three elements simultaneously and then scanning their emission lines and adjacent flame background successively on the same extract. ACKNOWLEDGMENT
Carl Cain, Jr., is indebted to the Tennessee Eastmaii Corp. for a fellomship under which this work was carried
out. LITERATURE CITED
(1) Bode, H., 2. anal. Chenz. 142, 414 (1954).
( 2 ) Ibid., 143, 182 (1954). (3) Ibid., 144, 90 (1955). (4) Ibid., 145, 165 (1955). (5) Dean, J. A,, ANAL.CHEJI. 23, 1096 (1951). Ibid., 27, 1224L (1955). Dippel, W.A ., Bricker, C. E., Ibid., 27, 14S4 ( I (255). .--,. Lady, J. H., Ph.D. dissertation, University of Tennessee, August
1955. MalisFa, H., Miller, F. F., Mikro-
cheinie ver Mikrochem. Acta 40,
63 (1952/53). Mavrodineanu, R., Boiteux, H., “L’Analyse Spectrale Quantitative par la Flamme,” Masson, Paris, 1954. ~. ~.
Whisman, If., Eccleston, B. H., ANAL.CIIE~I. 27, 1861 (1955). RECEIVEDfor review July 5, 1956. Accepted December 26, 1956. Taken in part from a dissertation submitted by Carl Cain, Jr., to the Graduate School of the University of Tennessee in partial fulfillment of the requirements for the degree of doctor of philosophy.
Determination of Titanium in Plutonium-Titanium AIIoys KARL S. BERGSTRESSER University o f California, 10s Alamos Scientific laboratory, 10s Alamos, N. M.
b Titanium in plutonium-titanium alloys can b e determined by adding hydrogen peroxide to a perchloric acid solution of the sample. A colored solution with titanium and a precipitate of plutonium are formed simultaneously. The insoluble plutonium peroxide is removed by centrifuging 532
ANALYTICAL CHEMISTRY
before the titanium peroxy complex is measured c:)lorimetrically. The method is applical3le from 0.1 to 1 .O% titanium. A statitlard deviation of 2 y is observed for samples containing from 50 to 500 * I of titanium and 50 mg. of plutonium. Thorium does not interfere, but uranium and niobium do.
C
OLORIMETRIC
DETERXIKATIOKS
Of
titanium have been reported extensively in the literature. Frequently, the photometric measurement is made of the yellow color formed with hydrogen peroxide in a titanium(1V) solution. In many cases the solution contains 5 to 10% sulfuric acid, both for trace amounts
