Application of Sodium Sulfate in Eliminating Interferences in Determination of Tungsten by Atomic Absorption Spectrometry
Richard M. Edgar Eutectic Corporation, Flushing, N.Y. 17358
Tungsten, besides being present in standard tool steels and cobalt base alloys, is also found, in the form of tungsten carbide, in various metal spray powders. Here it is added to increase the wear resistance of the sprayed deposit. Since our laboratory is responsible for the chemical analysis of routine and experimental tungsten-bearing powders, it is necessary that an accurate quantitative determination of tungsten be available. Tungsten is not a very sensitive element to determine by atomic absorption methods ( I ) . In some alloy systems, where it is present a t less than 1%,its poor sensitivity caused us to have sample concentrations as high as 2000 pg/ml. A literature search on possible interferences showed that little work has been done. Most of the published work on tungsten determination utilizes solvent extractions (2-4). This method eliminates matrix and chemical interferences and yields improved sensitivity. Other works deal with either additions of a strong base (5, 6), complex matrix matching techniques (7-9), or analysis by the standard additions method (10). With the exception of the standard additions technique, all of these methods determine only tungsten from the initial stock solution. Since we are required to determine numerous elements on the same stock solution, all of the above methods appear to be impracticable and too time-consuming. With the initial shock solution being as high as 2000 pg/ml, possible interferences seem likely. These had to be found and corrected. The following metals were studied for possible interferences on tungsten absorption; iron, nickel, cobalt, chromium, aluminum, magnesium, manganese, molybdenum, copper, titanium, vanadium, potassium, and sodium. Acids were also studied for their effects on tungsten determination. The acids studied were hydrochloric, nitric, sulfuric, and hydrofluoric. Nickel was first studied as the contaminant metal on tungsten absorption. Increasing additions of nickel, from 50 to 2000 pg/ml, were added to a tungsten test sample. This was compared against a sample that contained tungsten alone. The result was a progressive enhancement on absorption of the test sample. This enhancement was not believed to be produced purely because of a decrease in tungsten ionization since, when large amounts of sodium were added to both the test sample and tungsten blank solution to control ionization, enhancement was still seen in the test sample that contained the contaminate metal. The same response was found with most other contaminate metals studied. The method chosen with which to control interferences was that of the addition of a releasing agent. This compound would be added to both sample and standards. After trying many different releasing agents, including aminoriium chloride and lanthanum chloride, i t was found that alkali sulfates worked best. Ammonium sulfate was very successful but proved not as efficient as sodium or potassium sulfate. Since sodium is present most of the time in our unknown solutions due to a sodium peroxide fusion of the sample, it was decided to use sodium sulfate. It was found that sodium sulfate controlled all interferences encountered and also increased the sensitivity of tungsten in the acetylene-nitrous oxide flame. Therefore, the time-consuming methods previously discussed were not needed.
EXPERIMENTAL Instrumentation and Operating Parameters. The single beam Perkin-Elmer Model 290B atomic absorption spectrophometer was employed in the study. The fuel was acetylene and the oxident was nitrous oxide. A nitrous oxide burner head was used, and gas flows were adjusted with a Perkin-Elmer 303 gas regulator. The radiation source came from a Westinghouse Electric tungsten hollow cathode tube. Since flame adjustments had no effect on controlling interferences found, it was adjusted to produce maximum absorption. This resulted in a reducing flame with the inner red cone of the flame being 3.5 cm in height. The wavelength and slit width were 255.14 and 0.2 nm, respectively. Reagents. A 1000pg/ml stock solution of tungsten was prepared by dissolving 1.794 g of solium tungstate in 11. of distilled water. A dilution to 100 pglml was made in a 100-ml volumetric flask. Stock solutions, 10 000 pg/ml, of contaminant metals were also prepared. Iron, cobalt, aluminum, and magnesium were prepared from the pure metal, dissolving the pure metals in nitric acid. Sodium and potassium stock solutions came from their chloride salts. Chromium was prepared from sodium dichromate. The titanium stock solution was prepared by dissolving the pure metal in a mixture of hydrofluoric and nitric acid. Molybdenum stock solutions came from ammonium paramolybdate dissolved in water. Vanadium pentoxide was dissolved in a minimum amount of hydrochloricacid to prepare the vanadium stock solution. A 10%w/v solution of sodium sulfate was prepared from the purified salt.
RESULTS AND DISCUSSION Referring to Figure 1, enhancement of tungsten absorption, due to additions of contaminant metals, is easily seen. The decrease in formation of stable tungsten oxide may be the reason for this enhancement effect. It appears that most of the metals act as releasing agents for tungsten. It was found that sodium sulfate acts as the best releasing metals. Referring to Table I, it is seen that a 2% addition of sodium sulfate eliminated the metal enhancement effect found. W. J. Price (11) gives a possible reason for the action of sodium sulfate as a releasing agent. He states that it helps in preventing the for-
Table I. Recoverya of 100 pg/ml W from Mixtures Containing 2000 pg/ml Contaminant Tungsten Recovered, pg/ml Contaminant present at 2000 pg/ml
With no Na,SO added
Mo Mn Cr cu Mg A1 K
With 2% Na,SO added
Na co Ti
162 157 156 155 155 147 141 141 140 140 104 101
100 100 100 100
Fe
96
99
Ni V
100 100 99
100 100 102
100 100
aA deviation from 100 pg/ml W by less than 2% was considered to be due to interferences other than those caused by the metals studied. All values are based on three different determinations.
ANALYTICAL CHEMISTRY, VOL. 48,
NO. 11,
SEPTEMBER 1976
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100
80
% Molybdenum
20
w
Manganese
-
Chromium
_ _ _ Nickel
10 L
I 100
I 250
I
I
I
I
I
I
750
1wo
I
500
1260
1500
1750
2w0
,ugml-' contaminate metal
Figure 1. Effect of increasing additions of various contaminant metals on a 100 Hg/ml tungsten sample
mation of metallic clotlets in the flame. The high vaporization temperature of tungsten, which is in excess of 5927 "C, could never be achieved in the acetylene-nitrous oxide flame. Sodium sulfate prevents tungsten clotlets from forming. The free atoms of tungsten are combined with the solid solution of sodium sulfate particles. They are then released as an atomic vapor when sodium sulfate is vaporized in the flame. The sodium sulfate, besides eliminating the enhancement caused by the contaminant metals, also increases the sensitivity of tungsten absorbance. With all conditions being the same except for the addition of sodium sulfate, a 100 pg/ml tungsten sample with no sodium sulfate gives an absorbance reading of 0.052, while a 100 yg/ml tungsten sample with 2% sodium sulfate gives an absorbance reading of 0.076. Acids, when added to a 100 Hg/ml sodium tungstate sample, produced no interferences. This was true even with a 10% addition. The only noticeable effect was the speed a t which the white precipitate of tungstic acid is formed. It was noticed that when sodium at concentrations greater than 100 pg/ml was present with the acids, an enhancement effect resulted. All acids studied, except hydrofluoric, showed this effect. Sodium sulfate, when added to the acid solutions that also contained sodium, eliminated the enhancement effect present.
All tungsten absorption interferences found in this study have been eliminated. Sodium sulfate, a t a concentration of 2%, increases the tungsten absorption sensitivity and eliminates interferences found from contaminant metals and acids.
ACKNOWLEDGMENT The author expresses his appreciation to B. Brachfeld and J. Wilkeyson for their helpful suggestions during the preparation of the final manuscript. LITERATURE CITED (1) (2) (3) (4) (5) (6) (7)
D. C. Manning, At. Absorp. News/., 5 127 (1966). P. D. Rao, At. Absorp. News/., 9 131 (1970). R. W. Morrow, RepiatomEnergy Commn., U.S.Y.-1812 (1972). I. G. Yudelevlch and N. P. Shabvrova, Chem. Anal., 19, 941 (1974). E. Keller and M. L. Parsons, At. Absorp. News/., 9 92 (1070). B. F. Quin and R. R. Brooks. Anal Chim Acta, 85 206-209 (1973). G. G. Welcher and Owen H. Kriege, At. Absorp. News/., 8 97 (1969). (8)J. Husler, At. Absorp. News/., 10 60 (1971). (9) D. M. Knight and M. K. Pyzyna, At. Absorpt. News/. 8 129 (1969). (10) R. C. Rooney and C. G. Pratt, Analyst, 97 400-404 (1972). (1 1) W. J. Price, "Analytical Atomic Absorption Spectrometry," Heyden and Son Ltd., New York, 1972, p 90.
RECEIVEDfor review March 9, 1976. Accepted June 14, 1976.
Temperature Compensation of a Hall Probe for Double Focusing Spark Source Mass Spectrometer E. F. Vozenilek Technical Staffs Division, Corning Glass Works, Corning, N. Y. 14830
The advantages of providing a magnetic field sensor for use with the Associated Electronics Industries (AEI) MS702R spark source mass spectrometer electrical detection system, or with similar instruments, have been discussed by Magee and Harrison ( I , 2). A table can be generated relating mass number to the magnetic field parameter, resulting in considerable time savings in peak switching analyses, rapid survey scans, or spectrum identification on an oscilloscope. Difficulty was encountered in this laboratory in obtaining long-term repeatability due to temperature excursions at the Hall probe on the order of 3 to 4 "C from week to week. While devices with better Hall voltage temperature coefficients are available, they are generally more expensive, and sacrifice sensitivity. A 1654
simple resistive compensating network that uses inexpensive, readily available components improves the effective temperature coefficient by about an order of magnitude. Equations are given to allow calculation of the component values suitable for a particular application. A circuit is presented which is easily assembled and consists of a constant current source for the Hall probe, and a stable amplifier.
EXPERIMENTAL Apparatus. T h e c i r c u i t diagram in F i g u r e 1 includes a 300-mA c u r r e n t supply, a n error amplifier (AI), a c u r r e n t booster (Ql), and a chopper-stabilized operational a m p l i f i e r (A2). S w i t c h 1 activates
ANALYTICAL CHEMISTRY, VOL. 48, NO. 11, SEPTEMBER 1976
