Determination of Trace Levels of Chromium in Ilmenite by Atomic Absorption Spectrometry P. T. O'Shaughnessy Laporte Mining (Austra/ia) Pty. Limited. P.O. Box 245,Bunbury 6230, Wesfern Australia
The discovery of extensive deposits of relatively high chromium ilmenite in Western Australia has led to the need for a rapid method for the analysis of chromium. Very little published material exists on the analysis of ilmenite by atomic absorption methods. Kinson, Hodges, and Belcher ( 1 ) have shown that iron depresses the atomic absorption of chromium. On the analysis of steels, Barnes ( 2 ) has reported on the use of NH4Cl for reducing the depressing effect of Fe. Hurlbut and Chriswell ( 3 ) reported the use of NazS04 as a suppressing agent. As a result of this investigation, KHS04 is recommended as an interference suppressor and absorbance enhancer in the determination of Cr in ilmenite. EXPERIMENTAL Apparatus. A Techtron AA5 atomic absorption spectrophotometer was used with a small bore Cr hollow cathode lamp. The AA spectrophotometer was operated a t the following conditions: wavelength, 3578 A; slit width, 50 p ; lamp current, 5 mA; flame type, CzHZ/air and CzHz/N20 were run fuel rich to give maximum absorbance; and aspiration rate, 4.0 ml/min for the test solution. For interference studies, the absorbance of a 10 kg/ml of Cr standard was set to an arbitrary scale reading of 50 using scale expansion, and all determinations were duplicated. Reagents. All reagents used were analytical grade or B.D.H. Atomic Absorption Standard grade. Demineralized water was used throughout. Procedure. About 0.25 g of dried finely powdered ilmenite was fused with 3.0 g of KHSO4. The resulting melt was dissolved in 10 ml of 6N H2S04 and made up to 50 ml with demineralized water. Standards containing 2, 3, 5 , and 10 pg/ml of Cr were made up containing 3.0 g of fused KHSO4 and 10 ml of 6N HzS04 in 50 ml. The addition of Fe and Ti to the standards was studied but found unnecessary (Table I).
RESULTS AND DISCUSSION Preliminary interference studies indicated possible interference from Fe, Ti, Co, Mg, Si, AI, and Mn. In the air-acetylene flame, Fe and T i at ilmenite concentrations caused a large decrease in Cr absorption. In the nitrous oxide-acetylene flame T i caused a marked increase in absorption (Table 11). Several methods of overcoming these major interferences were tried. Nitrous oxide-acetylene was avoided because of the loss of sensitivity when using the short AB 50 burner (Table 111). NH4Cl ( I ) , KzS04, and KazSO4 ( 2 ) have been suggested as interference suppressors in the determination of trace levels of Cr (Figure 1). Of these NH4Cl proved unsatisfactory at Fe levels >5%. NazS04 provided a constant enhancement of the absorbance but gave a slightly unstable signal not suited to scale expansion. KzS04 gave a similar enhancement but also gave a quiet, stable flame. As KHSOI is an excellent flux for the attack of ilmenite, this was then tried and proved satisfactory, providing a conKinson. R. J. Hodges, and C. B. Belcher, Anal. Chim. Acta. 29. 134 (1963). (2) L. Barnes. Jr.. A n a / Chem , 38. 8 (1966) (3) J. A . H u r l b u t and C. D. Chriswell. Ana/. Chern.. 43. 3 (1971).
(1) K .
1946
Table I. Effect of KHS04 on the Absorbance of Cr in the Presence of Fe and Ti (Air/C2Hz Flame) Sample 5 pg/rnl Cr 5 pg/rnl Cr KHSO4 5 yg/rnl Cr KHS04 5 pglrnl Cr 4- KHS04 5 pg/rnl Cr KHSOl
+
+ +
50
68 67 68 68
+ Fe
+ Ti + Fe -t Tia
2 pg/rnl Cr 2 pg/rnl Cr KHS04 2 pg/rnl Cr -t KHSOl Fe 2 pg/rnl Cr KHS04 Ti 2 pg/rnl Cr 4- KHS04 Fe Tia Ti and Fe added equivalent to 0.25g of (FeTiOs)/SO ml.
+ +
Q
Absorbance
+ + + +
20 28 28 29 28
Table II. Effect of Ti and Fe on Cr Atomic Absorption Absorbance Sample 10 pg/rnl Cr 10 pg/rnl Cr 10 pg/rnl Cr
Air/C2H2
NzO/CzHz
50 20 15
50 60 49
+ Tia + Fea
n T i and Fe added equivalent to 0.25 g Of
(FeTiO3)/5Oml.
Table Ill. Sensitivity of Cr (Concentration to Give 1% Absorbance) Nitrous oxide/acetylene: Air/acetylene:
~
~~~
6-cm Burner, 0.12 yg/rnl 10-cm Burner, 0.07 pg/rnl
~~~
~~~~
Table I V . Comparison Check for Accuracy Cr. %
Sample No. 1112 1116 1168 909 EP 139
Source Capel W.A. Capel W.A. Capel W.A. Upgraded ilmenite CSI RO
Conventional analysis 0.04 0.04 0.05 0.045 0.107
AAS
0.038 0.034 0.046 0.046 0.105
stant enhancement when used alone or in the presence of Fe and Ti (Table I). R e c o v e r y . Interference from Mg, Si, Mn, and A1 was minimal at the levels present in ilmenite, and recovery experiments indicated 96% recovery of 20 ppm Cr added to a solution of ilmenite treated as proposed.
ANALYTICAL CHEMISTRY, VOL. 45, NO. 11, SEPTEMBER 1973
Figure 1.
Calibration curves for Cr standards
These standards contain Ti, Fe, and the following interference suppressors: ( 0 )K H S 0 4 , ( A ) Na2S04, ( X ) aqueous, ( B ) NHICI
Effect of Anions. Some anions interfere with Cr absorption ( 3 ) , but as standards and sample are closely matched and recovery was satisfactory the effect of anions can be practically ignored. Precision and Accuracy. Lack of a suitable analyzed standard ilmenite was a problem. The accuracy of the method was checked against various ilmenites analyzed by conventional techniques (Table IV). Precision was established by replicate analysis of E P 139 over several months. A standard deviation of 0.0036%
was obtained. The relative standard deviation was 3.4%. The method is simple, rapid, and is applicable to a wide range of ilmenite types and alteration products.
ACKNOWLEDGMENT My thanks are due to E. S. Pilkington of CSIRO, Melbourne, for kindly providing the analyzed ilmenite E P 139 and to A. Pepper for critically reading the manuscript. Received for review November 30, 1972. Accepted March 28, 1973.
Determination of Low Concentrations of Cobalt in Plant Material by Atomic Absorption Spectrophotometry W. J. Simmons Department of Soil Science and Plant Nutrition, lnstifute of Agriculture, University of Western Australia, Nedlands, W. A. 6009
Co deficiency develops in animals when the Co concentrations of their pastures and fodders fall below 0.07 to 0.10 pg/g (dry weight) for some time ( I ) . In many areas levels as low as 0.01 pg/g (dry weight) are encountered. There is, therefore, considerable interest in simple, rapid, and accurate procedures for measuring these extremely low concentrations. The most common procedures in current use for determining Co in biological materials are based on the nitroso R salt colorimetric method (2-4). This technique is not ( 1 ) E. J. Underwood, "Trace Elements in Human and Animal Nutrition," 3rd ed. Academic Press, New York, N . Y . , 1971, p 150.
ideal for routine analyses because it is long and great care is required during the color development of the cobalt nitroso R salt complex. Also a t low Co concentrations, a t least 20 g of dry plant material is required for sufficient sensitivity. A more rapid technique using small samples was required for Co assays of a large number of samples collected in a survey of the trace element content of a range of (2) H. R. Marston and D . W. Dewey, Aust. J. Exp. Biol. Med. Sci., 18, 343 (1940). (3) D. W . Dewey and ti. R. Marston, Anal. Chirn. Acta., 57, 45 (1971). (4) B. E. Salzman, Anal. Chern., 27, 284 (1955).
ANALYTlCAL CHEMISTRY, VOL. 45, NO. 11, SEPTEMBER 1973
1047
