Flame Spectrophotometric Determination of Microgram Quantities of Copper L. MANNA, D. H. STRUNK, and S. L. ADAMS Research Department, Joseph
E.
Seagram & Sons, /nc,, Louisville, Ky.
No detailed procedures have previously been published for the rapid, precise, and convenient determination of micro quantities of copper. Flame spectrophotometric studies at the copper line (324.7 mp) showed that the radiant power of copper was greatly enhanced by aspirating from an 80% methanol solvent. Interference studies indicated that nitric acid could be tolerated in appreciable quantities. RIost cations show considerable radiation interference; however, the increase in the radiant power of copper caused by the addition of nine cations to standard copper solutions was determinable at a wave length of 325.1 mp. Inasmuch as no preliminary separations are necessary, the method is adaptable to routine determinations. The results are as accurate as conventional microchemical procedures.
A
LTHOUGH there are many publications on the determination of alkali and alkaline earth metals by flame photometry, relatively few investigators have reported on the flame photometric determination of copper (1, 3, 9-11, 15-17). Of these, only Dean ( 3 ) and Jordan (11)have described methods in detail. Dean (3)worked with nonferrous alloys containing copper in the range from 0.3 to 2.5%, and Jordan (11) analyzed for small amounts of copper in a hydrochloric acid extract of gasoline. Work by Dean and Lady (5, I S ) indicates that copper can be determined in micro quantities by flame photometry after a preliminary extraction of the copper as the salicylaldoxime with either chloroform or amyl acetate.
Table I.
Radiant Power of 4 y per MI. of Copper with Various Alcohols as Solvent
Alcohol Methanol
%
Radiant Power, Scale Divisionso Cu+ROH ROH Net Cu
95 80 40 20 95 80 40 20 95 80 40 20
54.2 51.2 42.5 38.3 53.8 47.7 41.8 40.6 49.8 45.4 42.3 40.2
21.5 26.0 26.5 23.4 26.1 29.0 28.2 24.2 28.4 30.4 29.0 25.8
32.7 25.2 16.0 14.9 27.7 18.7 13.6 16.4 21.4 15.0 13.3 14.4
%Propanol
95 80 40 20
50.7 46.0 42.2 41.4
30.4 30.1 30.4 25.5
20.3 15.9 11.8 15.9
2-Methyl-2-propanol
95 80 40 20
45.9 43.7 42.4 42.3
34.0
11.9 9.7 12.7 16.5
Ethyl alcohol
1-Propano
34.0
29.7 25.8
a Instrument adjusted to read 100 on the transmittance scale with 12 ml. of copper in 80% MeOH.
y
termination of copper in complex solutions containing many inorganic components. APPARATUS AND CHEMICALS
Emission measurements were made with a Beckman Model DU spectrophotometer equipped with a Model 4300 photomultiplier accessory and Model 9200 flame photometry attachment. A Model -1020 atomizer-burner utilizing an osyhydrogen mixture was the source of excitation. Analytical grade chemicals we1 e used throughout the investigation. Stock solutions of copper, iron, aluminum, and magnesium ions were prepared by reacting each metal with a minimum of concentrated nitric acid and then diluting to volume with doubledistilled water. Solutions of sodium, potassium, manganese, nickel, and lead ions were prepared from their nitrates, and a solution of calcium ions was prepared by reacting calcium carbonate with concentrated nitric acid. The solutions were stored in polyethylene containers. A composite stock solution containing nine cations was used to study interference phenomena. This solution was prepared by pipetting into a 500-ml. volumetric flask 20 ml. of 0.57' aluminum, 150 ml. of 0.5% iron, 100 ml. of 0.5% magnesium, 50 ml. of 0.57c calcium, 5 ml. of 12.57' sodium, 10 ml. of 12.57' potassium, 1 ml. of 15.35% manganese, 100 ml. of 0.125% nickel, and 10 ml. of 0.57' lead. The flask was filled to the mark with double-distilled water and thoroughly agitated. This composite stock solution contained 1.5 y per ml. of copper aa determined by the technique described below. EXPERIMENTAL PROCEDURE
Preliminary esperiments with pure solutions of copper nitrate showed that the radiant power of copper was greatest a t a wave length of 324.7 mp with a slit width of 0.03 mm. This was also reported by Dean (3). The following Beckman D U spectrophotometer settings were used throughout the investigation: Wave length Copper line Flame background Selector Sensitivity on photomultiplier battery box Resistor Slit width
Hydrogen Oxygen
per
This study describes a rapid procedure for the determination of small concentrations of copper (1 t o 8 y per ml.) and presents a simple technique for reducing the error caused by the presence of varying amounts of interfering cations. Although the method was developed principally to determine micro quantities of copper in beverage alcohol, the procedure is applicable to the de-
324.7 mp 325.1 m p 0.1 Full, 60 volts per dynode 22 megohms 0.03 mm. 2 . 5 Ib. per sq. in. 10 Ib. per sq. in.
Pressures of 1.5 and 12 pounds per square inch for hydrogen and oxygen, respectively, were also satisfactory. The wave length setting must be exact in order to obtain the maximum energy response and t o reproduce the standard curve. The following procedure was adopted t o minimize instrumental errors. -Make the instrument settings indicated above, light the flame, close the shutter, and zero the null meter by adjusting the dark current control. Set the transmittance dial to read 1007, transmittance, aspirate a copper solution containing 12 y per ml. and zero the null meter by means of the sensitivity control. Close the shutter and, if necessary, readjust the dark current control t o zero the meter. Aspirate the copper solution that is to be determined, open the shutter, and turn the transmittance dial to zero the null meter; close the shutter and record the radiant poLTer from the transmittance scale. Repeat this procedure at least twice. Each radiant power value reported is the average of three readings varying by not more than f 0 . 5 scale division taken directly from the transmittance scale. RESULTS ' Effect of Alcohols. Sumerous investigators have studied the effect of organic solvents on the flame photometric emission of certain elements Curtis, Knauer, and Hunter ( 2 ) showed that
1070
1071
V O L U M E 28, NO. 7, J U L Y 1956 Table 11.
Radiant Power of Copper in 80% Methanol Containing Various Acids
Radiant Power, Scale Divisionsa Cu Concn., Acid y per hll. Cu Acid Acid S e t Cu 30.3 25.2 0 0431 HiPOc 55.5 34.7 25.8 0 1M HsPOc 60.5 99.7b 2 O M Hapoi 9.0 108.7 26.2 0 05M "Or 50.5 76.7 26.8 50.6 77.4 1 0.w "08 26.2 55.7 81.9 2 O M HNOs 8 26.3 50.4 76.7 8 0 0 5 X HOAc 25.0 55.5 81.5 8 1 024 HOAc 23.9 62.2 86.1 8 2 0.M HOAc 26.3 75.9 49 6 0 05.11 HC1 8 27.5 43.2 70.7 1 O X HCI 8 27.7 40.5 1 6.11 HC1 68.2 8 25.8 47.9 74.7 8 0 05.11 HgSOa 31.1 41.6 72.7 8 1 oAv HzS04 34.5 72.2 8 37.7 2 O X H~SOI a Instrument set to read 100 on transmittance scale with 12 y per mi. of copper in 80% MeOH; 4 and 8 y per ml. of copper in 80% MeOH gave net radiant powers of 25.5 and 50.5 scale divisions, respectively. b Instrument set to read 60 on transmittance scale with 12 y per ml. of copper in 80% MeOH.
+
greater radiant power can be obtained by atomizing from hydrocarbons and ethers than by atomizing from aqueous solutions. They studied barium, calcium, and organosodium compounds. Dean and Thompson (6) studied the relative radiant power of boron from methanol-water solutions. Kingsley and Schaffert (IS)reported on the flame photometric determination of sodium, potassium, and calcium in various organic solvents including alcohols, and found that acetone produced the greatest enhancement of radiant power. Dean and Lady ( 4 ) used 2,4-pentanedione as the extracting reagent and combustible solvent for the determination of iron. Fink (8) investigated the use of alcoholic solutions for the flame photometric determination of calcium and magnesium and showed that pentyl alcohol produced a thieefold increase in sensitivity. A very recent general study (IS) of the solvent effects on flame emissions has been reported. Unfortunately, this report was called to the authors' attention a t the conclusion of this work. Preliminary experiments indicated that 9570 ethyl alcohol by volume increased the radiant power of copper. Consequently, different alcohols were tested a t varying concentrations to determine their effect on the radiant power of 4 y per ml. of copper. The data in Table I show that high concentrations of methanol produce the greatest net emissions. It was decided, therefore, to use 807, methanol as a solvent because this concentration contains sufficient water to dissolve large amounts of salts. The calibration curves for copper in 807, methanol and in water are linear. A comparison of the curves reveals that 0 to 12 y per ml. of copper in SOTomethanol gives a spread from 25 to 100 scale divisions, representing a total of 75 scale divisions with each division equivalent to 0.16 y per ml. of copper. When water is the solvent and the same sensitivity is used, the spread is from 18 to 49 scale divisions, or a net of 31 scale divisions with each division equivalent to 0.40 y per ml. of copper. Therefore, an error of 1 scale division represents less error, in terms of copper, when SOYo methanol ISthe solvent than when water is the solvent. A comparison of the curves obtained by aspirating 0 to 100 y per ml. of copper in water and in 807, methanol a t identical sensitivities was noteworthy. Dean (3) obtained almost the complete spread of the transmittance scale %-hen burning 0 to 100 y per ml. of copper in water. With 807, methanol as the solvent the 8ame spread was obtained with 0 to 28 y per ml. Effect of Anions. The effect of various anions was investigated because acids are generally used in the analysis of metals. Table I1 illustrates the data obtained when acids are present in SOYo methanol containing 4 and 8 y per ml. of copper. -4 peculiar effect is encountered when phosphoric acid is present. The addition of either 0.04M or 0.1M phosphoric acid increases the background emission without affecting the net emission by copper, Phosphoric acid, 2 M , exerts a pronounced inhibiting action on
the radiant power of copper. This phenomenon was observed by Parks, Johnson, and Lykken ( I 4 ) , who found that phosphate had an inhibiting effect on the radiant power of sodium and potassium. Dippel, Bricker, and Furman ( 7 ) demonstrated that phosphoric acid shows a continuous emission between 320 and 700 mp. Nitric acid was tolerated in appreciable quantities. Concentrations of 0.05X and 1M had no effect on the radiant polyer of copper, whereas a 231 concentration showed an increase in the radiant power of copper corresponding to 5 scale divisions. The radiant power of copper is definitely enhanced by 1M and-2M acetic acid in 80% methanol. Hydrochloric and sulfuric acids caused the radiant power of copper to decrease as the concentration of these acids mas increased. Effect of Cations. Various metal ions in concentrations of 50 and 200 y per ml. were added individually to 4 y per ml. of copper in 80% methanol to ascertain the extent to which these ions contribute radiation interferences. The data presented in Table I11 show that both concentrations of aluminum, iron, magnesium, calcium, lead, potassium, and nickel cations repress the radiant power of copper while the manganese ion appears to exert a slight enhancement. The addition of 50 y per ml. of lithium ion had no effect, while 200 -!per ml. of lithium ion and 50 and 200 y per ml. of sodium ions increased the background emission slightly without affecting the net copper emission. Dean (6).in his work with aqueous solutions, found that large quantities of aluminum, lead, magnesium, manganese, iron, nickel, and potassium ions did not contribute radiation interferences.
Table 111.
Effect of Cations on Radiant Power of 4 y per M1. of Copper in 8070 &lethanola Radiant Power, Scale Divisions
Cation Added AI+++ AI+++ Ca::
g:Fe++ + ++ +
+
K
KLi Li
+ +
Mg++ hlg ++
MU++ &In++ Na Na + h-i + + Ni++ Pb++ Pb++ +
a
Y
per
MI.
50 200 50 200 50 200 50 200 50 200 50 200 50 200 50 200 50 200 50 200
cu
+
cation 38.8 39.9 39.0 39.9 39.0 40.2 39.5 38.8 51.5 54.6 38.5 39.9 52.3 52.6 54.2 55.7 38.7 38.6 38.1 38.3
Cation 25.1 25.0 25.5 27.2 25.1 26.9 25.0 25.2 25.8 28.0 25.2 25.5 24.5 25.3 28.2 29.7 25.2 26.1 24.8 26.0
Net Cu 13.7 14.9 13.5 12.7 13.9 13.3 14.5 13.6 25.7 26.6 13.3 14.4 27.8 27.3 26.0 26.0
13.5 12.5 13.3 12.3
4 y per ml. of copper gave a net radiant power of 25.5 scale divisiona.
Experiments were than performed to determine the extent of radiation interference caused by the addition of a combination of nine cations to standard copper solutions, and the degree to which this interference could be corrected. Aqueous and 80% methanol solutions n'ere prepared containing known amounts of copper and nine cations in varying concentrations. The instrument was adjusted t o read 1007, transmittance when aspirating 12 per ml. of copper and the standard curve was checked with solutions containing 1,4,and 8 y per ml. of copper. Actual emisEion readings were then taken on each solution containing the copper with added cations. 9 ,
In order to correct for the radiation caused by the interfering cations, the following technique was employed. Adjust the instrument to read l O O ~ ,transmittance while aspirating 12 y per ml. of copper and then move the transmittance dial to the radiant power (T,)obtained by aspirating the solvent alone. (These values were 25 and 39 scale divisions for 80% methanol and water, respectively.) Rotate the wave length dial very slowly
ANALYTICAL CHEMISTRY
1072 Table IV.
Effect of Cation Mixture on Radiant Power of Copper
Composite Stock S o h . Added, % (v./v.) 5"
Copper, Present
7 per
8 0 6 0 4 0
1 0
so
2 0 15*
40 2 0
6.0 3.9 1.9
1 0
1 1
10 b
8 8 4 9 1
0 0 0 0 0
7.9 6.0 3.9 1.9 0.9
16 b
so
8.0 6.1 4.1 1.8 0.9
6 0
ti0
4 0 9 0 1 0
40 b
8 0 6 0 4 0 ' 0 1 0
a
b
Found __ 7.9 6.0
so
8 0 6 0 4 0
10"
M1.
4.0 1.9 0.9 8.0 6.0 4.1 2.0 0.9
2 0 1 0
Read the copper equivalent of T, from the standard curve. Subtract the amount of copper occurring as a contaminant in the composite stock solution to obtain the amount of copper recovrred. Table IV illustrates the data obtained by utilizing this technique. It is apparent that radiation interferences produced by the addition of large concentrations of foreign ions can be corrected by determining the excess emission at a wave length of 325.1 m9. The results obtained with 80% methanol as solvent xere comparable to those obtained with water. LITERATURE CITED
Cholak, J., Hubbard, D. LI., ISD. EX. CHEM.,ANAL.ED. 16, 728 (1944).
Curtis, G. SV., Knauer, H. E., Hunter, L. A , Am. SOC.Testing Materials, Tech. Pztbl. 116, 67 (1952). Dean, J. A , , AIBAL.CREY.27, 1224 (1955). Dean, J. A., Lady, J. H., Zbid., 27, 1533 (1955). Dean, J. A., Lady, J. H., Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, February 1956. Dean, J. A., Thompson, C., ANAL.CHEX 27, 42 (1955). Dippel, W.A , , Bicker, C. E., Furinan, H. W., Zbid., 26, 553
8.0 6.0 4.0
(1954).
Fink, A. Mikrochim. Acta 1955, 314. Gerber, C. R.. Ishler. iY.H.. Borker, E., AA-AL.CHEM.23, 684
2.0 1.0
(1951).
SO% methanol solvent.
Griggs, 11. A , , Johnstin, R.. Elledge, B. E., IND. EKG.CREM., A N A L . ED. 13, 99 (1941). Jordan, J. H., Jr., Petroleum Refiner 33, 158 (1954). Kingsley, G. R., Schaffert, R. R., J . Biol. Chem. 206,807 (1954). Lady, J. H., Ph.D. dissertation, University of Tennessee,
Water solvent.
until the null point is obtained on the meter. With this instrument the wave length was found to be 325.1 mp. Measure the interfering radiation of each copper solution containing the intcrfering cations. Calculate the radiant power due to copper by the following equation:
.lugust 1955.
Parks, T. D., Johnson, H. O., Lykkeii. L., IND. ENO.CHEW, ANAL.ED.20, 822 (1948). Robinson, A. R., Newman. K. J., Schoeb. E. J., AIBAL. CHEM.22 1026 (1950).
Tc
=
T ~ . -T (T3tc.1 - Ts)
Raring, - C. L.. Zbid., 21. 425 (1949). (17)
where
T, T.
Weichselbaum, T. E., T-arney. P. L.. 3Iargraf. H. W.. Ibid., 23, 684 (1951).
= radiant power due only to copper = background of pure solvent
RECEIVED for review October 19. 1955. Accepted April 16, 1956.
Dif f e rential Spectrophotomet ric Method for Determination of Uranium C. D. SUSANO, OSCAR MENIS, end C. K. TALBOTT Analytical Chemistry Division, O a k Ridge National Laboratory, O a k Ridge, Tenn.
A rapid spectrophotometric method is described for the determination of uranium in high concentrations with a precision equal to that of conventional volumetric or gravimetric methods. The relative absorbance of uranyl ion is measured at a wave length of 418 mp against a highly absorbing reference standard. From the difference in absorbance, the concentration of uranyl ion in excess of that in the reference standard solution is determined with a high degree of precision. A method for the detertnination of the optimum concentration of the reference standard is presented. The effects of trace impurities are evaluated. In the optimum range of 20 to 60 m g . of uranium per ml., the precision is within 0.3yG.
A
LARGE number of methods have been described (9) for the gravimetric and volumetric determination of macro (luautities of uranium. Several apectrophotometric methods,
dependent on colors developed by the addition of chromogenic reagents, have been applied to the determination of uranium, particularly in low concentrations. However, few procedures for uranium determination, based on the color of the uranyl ion, have been published. A colorimetric method was used by Scott and Dixon (11) for the determination of uranium in leach liquor. Rodden (IO) noted that a differential spectrophotometric technique was used by Brackenbury for the estimation of uranium in alkali peroxide solutions. Recently, a method in which uranium is determined spectrophotometrically in perchloric acid n a s reported (8). While the present paper was being reviewed, Bacon and Milner of the British Atomic Energy Research Establishment reported the results of a similar study ( 1 , 2 ) . They make use of differential spectrophotometry for the determination of uranium in the metal, binary and tertiary uraniumbase alloys, and uranium oxide. Ordinary spectrophotometric methods dependent on the absorbance of uranyl ions fail t o yield satisfactory precision for macro amounts of uranium, because the absorbance scale must
