Suppression of Interferences by Sodium Sulfate in Trace Chromium Analyses by Atomic Absorption Spectrometry J. A. Hurlbut and C . D. Chriswell Department of Chemistry, Metropolitan State College, Denver, Colo. 80204
CHROMIUM ANALYSES are frequently performed by atomic absorption spectrometry, and a large number of elements are known t o interfere with these analyses. The principal reported interferences are Ni, Ag, Co, Fe, Mn, Al, W, and Ti (1-4). In some cases these interferences can be overcome by duplicating the sample matrix in calibration samples ( 4 ) , by using the method of standard additions (4), by using an oxidizing, airacetylene flame ( 4 , 5 ) ,by using a nitrous oxide-acetylene flame (4, 5 ) , or by using an interference suppressor such as NHICl ( 4 , 6, 7), SrCh (3), LaCh (8), KzSzOa(21, or K&07 (9). The use of these chemical suppression agents appears to be the most common method of overcoming interferences; however, these suppression agents do not work on all interferences, and reports of their usage are limited and scattered throughout the literature. The presence of 1 to 2% NH4C1 is known to reduce the depressant effect of Fe on the absorption of Cr at 3579 A (6, 7), and more recently, both K2S207( 9 ) and 1 % K2S20e(2) were found to suppress the interferences caused by Al, Fe, and Ti. The addition of LaCL (8) was found to overcome interfering phosphate and Al, and SrClz (3, 6) has been used t o mask the interferences caused by low concentrations of various substances. Work in this laboratory has led to the use of a new wide range interference suppressor and absorption enhancer for Cr analyses. This new suppressing agent is Na2S04,and the purpose of this study is t o report and to demonstrate the effectiveness of Na2S04in trace Cr analyses. The effectiveness of 1 % NazSOIagainst relatively large amounts of many interfering substances will be compared to that of other suppressing agents. EXPERIMENTAL Apparatus and Instrumental Parameters. A Perkin-Elmer Model 303 atomic absorption spectrophotometer equipped with a Cr hollow cathode lamp and a single slot burner utilizing an air-acetylene flame was used to obtain the experimental data. The air-acetylene ratio was set at 7 . 5 : 10, and the fuel flow was then adjusted for maximum absorption using a standard containing only 10.0 ppm Cr for all runs except for those runs noted in the discussion. The air-fuel ratio was in terms of arbitrary flow units which were governed by the standard regula!or supplied with the instrument. A wavelength of 3579 A was used. No attempt was made to minimize interferences by instrument adjustment, and new calibra(1) M. A. Biancifiori, C. Bordonali, and G. Besazza, Chirn. Znd. (Milan),50, 423 (1968). (2) B. B. Elrod and J. B. Ezell, At. Absorption Newslett., 8, 40 ( 1969). (3) P. J. Belling, Efluenr Water Treat. J., 9, 314 (1969). (4) “Analytical Methods for Atomic Absorption Spectrophotometry,” Perkin-Elmer Corp., Norwalk, Conn., 1968, pp 1-24, Cr 1-Cr 6. ( 5 ) Walter Slavin, “Atomic Absorption Spectroscopy,” Interscience Publishers, New York, N. Y.,1968, p 97. (6) W. J. Price and P. A . Cooke, Spectrciuision, 18, 2 (1967). 38, 1083 (1966). (7) Lucien Barnes, Jr., ANAL.CHEM., (8) A. M. Tenny, Znstnrrn. News, 18, 14 (1967). (9) Y . Endo, T. Hata, and Y. Nakahara, Bunseki Kagaku, 18, 833 (1969).
Table I. Recovery. of 10.0 ppm of Cr from Mixtures Containing Acids and Bases, Both in the Presence of and in the Absence of Either 1 % NH4Cl or 1 Na2S04 Cr recovered, ppm With no With 1 With 1 Na2S04 Contaminant masking NH4Cl present agent addedb addedc addedd 0.10M “ 0 3 10.2 10.1 10.2 1.OM “01 10.2 9.9 10.0 0.10M HCl 10.3 10.1 10.1 1.OM HC1 10.1 10.1 9.4 0.10M HS04 8.3 10.0 10.2 1.OM HpSOa 8.4 8.6 9.2 0.10M H3P04 9.8 9.3 9.8 7.9 7.7 7.6 1.OM 0.10M HC104 10.3 10.1 9.9 1.OM HCIO4 10.2 8.7 9.5 0.10M NHiOH 9.6 10.1 10.0 1.OM NHaOH 9.0 10.3 10.2 0.10M NaOH 10.4 9.8 10.1 1,OM NaOH 9.9 9.4 8.3 a Any deviation from 10.0 ppm of Cr by more than 1t0.3 ppm was considered to be due to interferences, and all values are based on at least three determinations. * Values obtained from a calibration curve made by utilizing standard solutions containing only KzCrz07. c Values obtained from a calibration curve made by utilizing standard solutions containing K2Cr207and 1 NH4CI. d Values obtained from a calibration curve made by utilizing standard solutions containing K2CrZO7 and 1 Na2S04.
z
z
tion curves were prepared for every separate run in order to help minimize experimental deviations. Reagents. A stock solution containing 100 ppm of Cr was made from a standard stock solution containing 1.414 g of dry primary standard K2Cr207in 500 ml of water. Dilute standard solutions were freshly made before use and were used for preparing the calibration curves and for preparing the test solutions. These test solutions always contained either no suppressing agent or 1% (w/v) of a suppressing agent. The suppressing agents, Na2S04, Na2S03, K2S04, K2S20s, and NH4C1, were reagent grade chemicals. The compounds used for preparing the stock, interference solutions are given in the footnotes t o Table 11,and stock solutions containing 10000 ppm of each interference were prepared.
RESULTS AND DISCUSSION The presence of small amounts of Na2S04in a Cr sample was found to enhance the absorption at 3579 A. For example, when an air-acetylene ratio of about 7.5:lO was used, the presence of 100 to 20000 ppm of Na2S04increased the absorption signal at least 10%. When an air-acetylene ratio of about 7.5 :14 was used in the presence of Na2S04,then the signal was enhanced at least loo%, but the noise level was also increased; so, this rich mixture was not used. Ammonium chloride, K2S208,KzS04, and Na2S03 also enhanced the absorption. Because of this interference it was necessary to prepare several calibration curves as given in the footnotes to the tables. These absorption-concentration curves passed through the origin and were linear up to 13 ppm of Cr. CaliANALYTICAL CHEMISTRY, VOL. 43, NO. 3, MARCH 1971
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Table 11. Recovery. of 10.0 ppm of Cr from Mixtures Containing 1000 ppm Contaminant Both in the Presence of and in the Absence of Either 1 Z NH4CI or 1 Z NanS04 Cr recovered, ppm Contaminant With no With 1% With 1% present at masking NHiCl Na&04 lo00 ppmb agent addedc addedd added6 Na 10.0 10.0 10.0 K 10.0 10.0 10.1 Mg 8.9 8.3 6.5 Ca 9.9 9.8 9.3f Ba 7.9, 7.6f 10.0f*s Ti 6.3h 7. lh 9.8h Mo 8.9 10.0 10.0 W 10.6 10.0 10.1 Mn 7.2 10.0 9.9 Fe 2.8; 10.2i 10.1; co 3 9.1 10.0 Ni 3.7 9.0 9.7 cu 8.9 10.1 10.0 Ag 5.9f 10.01 9.9f Zn 9.8 10.0 10.0 Cd 11.1 10.5 10.1 Hg 9.5; 9.4i 10. 1; AI 9.1i 9.1i 9.9< Si 10.2 8.6f 10.2 Pb 10. If 9.8f 9.8f.k Bi 10.7f 10.0, 10 Of Ce 10.8' 10.4; 1o.oi Any deviation from 10.0 ppm Cr by more than f 0 . 3 ppm was considered to be due to interferences, and all values are based on at least three determinations. All added contaminants were present as the nitrate salts except for the following. Titanium tetrachloride was the source of Ti; (NH4)~Mo7024.4Hz0 was the source for Mo; NazW04.2Hz0 was the source of W; and NazSi03.5H20 was the source for Si. Mercury, Mn, Fe, and Ce were in the $2, +2, f 3 , and +3 oxidation states, respectively. Values obtained from a calibration curve made by utilizing standard solutions containing only K2Cr2O7. Values obtained from a calibration curve made by utilizing standard solutions containing KzCr2O7 and 1% NHaCl. Values obtained from a calibration curve made by utilizing standard solutions containing KzCrz07and 1% Na2SO4. f The solution was 1.OM in "0,. 0 Barium sulfate precipitated. * The solution was 0.10M in HCI. The solution was 0.10M in FINOa. i The solution was 1.OM in NH40H. k Lead sulfate precipitated. I
bration curves made from Cr(II1) appeared to be identical to those made from Cr(V1). Since many Cr analyses are performed in the presence of acids or in some cases in the presence of bases, it was desirable to first determine the effects of acids and bases upon the Cr absorption. Table I summarizes this data. Of the seven compounds tested at a concentration of 1 .OM, three of them, H 3 P 0 4 ,HzS04,and N H 4 0 H , were found to interfere with the absorption by Cr. The presence of 1 % NazS04corrected this interference in the case of N H 4 0 H , but the solutions which contained 1 Na2S04and which were 1 .OM in either HCl, HC104,H3P04, H2S04, or NaOH gave low absorption values. Solutions which contained 1% NH4Cland which were 1.OM in either HC1O4, HaP04, or NaOH also suffered from interferences ; however, solutions which contained either 1 % NH4Cl or 1 % Na2S04and which were 0.10M in either "03, HC1, HzS04, HC1O4, N H 4 0 H or NaOH gave satisfactory absorption values. Table I1 summarizes the effects of 1000 ppm of various contaminants upon the absorption by 10.0 ppm of Cr. Changes in the instrument settings, such as a change in the air466
ANALYTICAL CHEMISTRY, VOL. 43, NO. 3, MARCH 1971
Table 111. Recovery. of 10.0 ppm of Cr from Mixtures Containing 1000 ppm Each of Mo, Mn, Fe, Co, Ni, Cu, AI, and Cd Both in the Presence of and in the Absence of Masking Agents Masking agent presentb Cr recovered, ppmc,d None 6.3 1% NaS04 10.0 1Z NazSOa 9.3 1% 8.2 1% KzSzOa 7.7 7.1 1% NHiCl a Any deviation from 10.0 ppm Cr by more than A0.3 ppm was considered to be due to interferences, and all values are based on at least three determinations. b The solutions were 0.10M in "01. c Values obtained from calibration curves made by utilizing standard solutions containing K2Cr207and 1% of the indicated masking agent. d These solutions had to be analyzed shortly after preparation because of the slow formation of a precipitate. acetylene ratio, caused changes in the degree of interference in some cases. For example, A1 was an absorption depressant when an air-acetylene ratio of about 7.5:lO was used, but it was an absorption enhancer when an air-acetylene ratio of about 7.5:14 was used. Also, slightly different results were sometimes obtained when the chloride form of the contaminant was used in place of the nitrate form; so, only the nitrate salts were used. Of the twenty-two contaminants added at a concentration of 1000 ppm each, sixteen of them, Mg, Ba, Ti, Mo, W, Mn, Fe, Co, Ni, Cu, Ag, Cd, Hg, Al, Bi, and Ce, interfered with the absorption by Cr. When the same contaminants were in the presence of 1 % NHdCl, nine of them, Mg, Ba, Ti, Co, Ni, Cd, Hg, Al, and Ce, still interfered; and, in addition, Si now interfered. One per cent NH4Cl effectively suppressed the interferences caused by 1000 ppm of Mo, W, Mn, Fe, Cu, Ag, and Bi. When the contaminants were in the presence of 1% Na2S04, only Mg still interfered, and a 7% interference in the sample containing Ca appeared; however, this interference in the sample that contained 1000 ppm of Ca and 1 Na2SO4disappeared when the Ca concentration was reduced to 500 ppm. One hundred ppm of Mg still caused at least a 20% depression in the absorption, both in the presence of and in the absence of the masking agents. One per cent Na2S04effectively suppressed the interferences caused by 1000 ppm each of Ba, Ti, Mo, W, Mn, Fe, Co, Ni, Cu, Ag, Cd, Hg, Al, Bi, and Ce. Solutions which contained 10.0 ppm of C r along with 1000 ppm each of eight species present in the same solution, which interfered with the Cr analyses, were analyzed both in the absence of and in the presence of various masking agents. Table I11 summarizes this data. This combination of contaminants produced a 37 % depression in the absorption in the absence of a suppressing agent, a 29% depression in the presence of 1 % NH4Cl, a 23y0 depression in &hepresence of 1% KzS208, and no depression in the presence of 1% Na2S04. These results demonstrate that the addition of 1% Na2S04to samples to be analyzed for Cr will eliminate the effects caused by large amounts of known contaminants. Ammonium chloride and KzSzOsare probably two of the more effective suppressing agents for Cr analyses, but they are not as effective as Na2S04. The data in Table I11 also shows that Na2S03and K2S04are interference suppressors, but they too are not as effective as Na2S04. RECEIVEDfor review September 22, 1970. Accepted NOvember 23,1970.
