tion of formaldehyde. Porapak Q can be used to adsorb methylal. Previous work indicates that small polar gases elute very quickly on Porapak Q (IO). We find that formaldehyde and watt:r co-elute on a 6-ft Porapak Q chromatographic column. The dynamic capacity of a 500-mg Porapak Q adsorber was evaluated for possible breakthrough of methylal. The sampling conditions were those described earlier for the determination of formaldehyde: 5-liter sample collected at a flow rate of 1.0 liter/minute. A standard gas mixture of 50 ppm methylal was prepared in a Saran (Dow Chemical Co.) bag. The standard gas mixture was pulled through a sampling train into a chromotropic acid scrubber. The first sample did not have an adsorber in the sampling train. The second sample was identical to the first except a 500-mg adsorber was placed in front of the chromotropic acid scrubber. The absorbance of the second scrubber solution showed no increase over a blank indicating quantitative retention of methylal on the adsorber. The next major question is how quantitatively does formaldehyde break through the adsorber under our sampling conditions. The first experiment was designed to determine the residence time of formaldehyde on the 500-mg adsorber. The Porapak Q adsorber was placed in a gas chromatograph equipped with a thermal conductivity detector. A 37% aqueous solution of formaldehyde was injected into the adsorber gas chromatograph column. The elution time of formaldehyde was determined as a function of carrier gas flow rate at ambient room temperature. At carrier gas flow rates of 30, 400, and 480 ml/minute, elution times of 300, 30, and 17 seconds were obtained. This result implies that for our sampling rate of 1.O liter/minute, formaldehyde is retained for a few seconds. This is a very small fraction of the total collection time of 5.0 minutes. A second more direct quantitative experiment was performed to definitively establish this crucial point. A standard formaldehyde gas mixture was prepared in a large polyethylene bag (approximately 200 liters) by passing an air sample over solid paraformaldehyde. Gaseous formalde-
hyde tends to deposit on the walls of the sampling bag and in the pump and sampling train (11). Several runs of the sampling train were required to obtain a constant absorbance in the scrubber solution. Immediately after this was accomplished, adsorption tubes containing either 300 or 500 mg of Porapak Q were in turn installed just before the chromotropic acid scrubber. This was followed by a final run of the system with no adsorption tube present. The recovery of formaldehyde with the adsorption tubes in the sampling train was quantitative and independent of the amount of Porapak Q utilized. Formaldehyde is a commonly used industrial chemical and would be classified as potentially more hazardous to health that most other pollutants. For this reason, it would probably be one of the first pollutants studied to determine cgmpliance with the Occupational Safety and Health Act. In utilizing the chromotropic acid method for the determination of formaldehyde, it is recommended that one sample be taken with and one sample without an adsorber as a check for possible interferences. Previous work indicates that the chromotropic acid method for the determination of formaldehyde is affected by alcohols larger than ethanol, olefins, and aromatic hydrocarbons ( 2 ) . Most of these interfering species were evaluated at a five- to ten-fold excess over formaldehyde. In some plant environments, the relative values could often be considerably larger. Selective adsorption on Porapak Q could be utilized to remove virtually all of the interfering species with the possible exception of some of the volatile olefins. The approach developed here could be applied to selectively remove some interfering species when attempting to analyze other light polar gases. As a starting point, our results suggest that if the component of interest elutes before water on a Porapak Q gas chromatograph column, the approach is feasible. Clearly, the flow rate and sample time must be considered,
(10) H. M. McNair and E. J. Bonelli, “Basic Gas Chromatography,” Varian Aerograph, Walnut Creek, Calif., 1969.
(11) J. F. Walker, “Formaldehyde,” 3rd ed., Reinhold, New York, N.Y., 1964, p 37.
RECEIVED for review April 19,1972. Accepted July 27,1972.
Rapid Spectrophotometric Determination of Arsenic in Iron and Steel 0. P. Bhargava, J . F. Donovan, and W. G. Hines Chemical & Metallurgical Laboratories, The Steel Company of Canada, Limited, Wilcox Street, Hamilton, Ontario, Canada
INTHE PAST, reagents such as quercetin, morin, and rutin have been employed for the absorptiometric determination of arsenic. However, most recent methods employ either arsenomolybdenum blue ( I , 2 ) or the absorption of evolved arsine in a pyridine solution of silver diethyldithiocarbamate (3). A polarographic method ( 4 ) has been described for the ( I ) P. Paklans, A m l . Cliim. Acta, 47,225 (1969). (2) W. R. Nall, Aiialyst (Loridon),96, 398 (1971). (3) T. Kaneshige, M. Takizawa, and H. Nagai, Jap. Aual., 13, 780 ( I 964). (4) M. V. Susic and M. G. Pjescic, Analyst (Londo/i),91,258 (1966). 2402
determination of arsenic in steel but the lower limit is 0.01 arsenic. To our knowledge, arsine evolution and its consequent absorption in silver diethyldithiocarbamate to determine arsenic in steel has not been reported. While some methods note that the conditions must be followed exactly, none describe the precise conditions required for steel. The steel was dissolved under oxidizing conditions, the arsenic(V) was reduced to the trivalent state in the presence of potassium iodide and stannous chloride. Finally, the evolved arsine was absorbed in a pyridine solution of silver diethyldithiocarbamate.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 14, DECEMBER 1972
RUBBER STOPPER
Table I. Sample Weight
*3 As,
LEAD ACETATE COTTON
-
0.003-0.05 0.05-0.10 0.lc-0.15
CAP1LLARY 0.4rnm INNER JOINT
- 24/40
Add pure iron, g nil 0.050 0.070
Sample, g 0.100 0.050 0.030
73
FLASK -125ml- 24140
Table 11. Comparison with B.C.S. Steel Standards Standard Cert. av Cert. range Arsenic found,
Figure 1. .4pparatus for arsine evolution and absorption
However, the yield was erratic and a detailed investigation was called for. PROCEDURE
Table I shows the sample weight of pure iron, if any, required in the ranges 0.003 to 0.05, 0.05-0.10, and 0.10 to 0.15 % arsenic. As will be discussed, it is essential to keep the total weight of sample and pure iron constant. This is placed into a 150-ml beaker. Five milliliters of hydrochloric acid and 5 ml of nitric acid are added followed by 2 ml of 12N sulfuric acid. The contents are simmered until dissolution is complete and then taken to SO, fumes. The temperature must be kept below 200 “ C . After cooling, 5 ml of hydrochloric acid and 15 ml of water are added and warmed to dissolve the residue. To the cold solution, 2 ml of 15 w/v (freshly prepared) potassium iodide solution are added, mixed, and let stand for 5 minutes. This is followed by dropwise addition of 50% stannous chloride solution in hydrochloric acid, until the solution turns colorless. A 15-minute waiting period is allowed for completion of the reduction of arsenic(V) to the trivalent state. During this period, the arsine generation and absorption apparatus should be made ready by placing a small plug of lead acetate cotton in position and connecting the capillarydelivery tube. During the 15-minute waiting period, the solution should be transferred to the flask and diluted to 40 ml with water, and 5 ml of absorbing solution (0.5 solution of silver diethyldithiocarbamate in pyridine) pipetted into the absorption tube. Three grams of granulated zinc (arsenicfree) are quickly introduced into the flask and the delivery and arsine absorbing system are immediately installed. The apparatus is essentially the well known Gutzeit arsine generator and absorber. The capillary delivery tube with a tapered end, as shown in Figure 1 , produces small bubbles which facilitate efficient absorption of arsine. The reaction is continued for 1 hour, with periodic swirling of the flask. The absorbance of the solution is then read a t 540 nm in a 1-cm cuvette with the absorbing solution in the reference cuvette. The calibrations are carried out using 0.1 gram of pure iron and 0, 5 , 10, 20, and 50 p g of arsenic standard solution and treated exactly as the sample. Procedure for determining arsenic in copper is exactly the same except that iron is replaced by copper, both for the samples as well as for the calibration standards.
x
RESULTS AND DISCUSSION
The yield of arsine is influenced by such factors as the concentrations of iron, stannous chloride, and acid. It was also confirmed that the rate of evolution of arsine is important and should be as uniform as possible for standardization and for actual test samples.
29 1 292 293 294 29.5 320 321 322 323 324 325 169
0.113 0.003 0.070 0.002 0.024 0.031 0.003 0.012 0.058 0.084 0.013 0.031
0.110-0.118 0.002-0.004 0.067-0.073 0.001-0.002 0.022-0.026 0.029-0.032 0.002-0.003 0.011-0.01? 0.054-0.060 0.079-0.086 0.011-0.014 0.027-0.039
0.117 0,001 0.076 0.002 0.026 0.033 0.0034 0.012 0.068 0.085 0.015 0.034
0.120 0.002 0.073 0.002 0.026 0.032 0.0035 0.013 0.063 0.086 0.014 0.035
Table 111. Comparison with NBS Steel Standards Cert. av Arsenic found, >; Standard 1161 1 I62
1163 1 I64 1165
1167 1168 1 174A 55E
0.028 0.046 0.100 0.018 0.010 0.14 0.008 0.022 0.007
0,027 0.043 0.099 0.019 0.010 0.143 0.009 0.022 0.008
0.028 0.049 0 . I13 0.019 0.011 0.142 0.009 0.021 0.008
Table IV. Comparison with J. M. Copper Standards Standard Cert. av Arsenic found, cc2 0 008 0 011 cc3 0 004 0 004 cc4 0 002 0 002
The effect of antimony (stibine is also evolved) was also investigated; Sb up to 0.1 % does not interfere, neither does copper in concentrations up to 1 %. Interference from larger amounts of copper can be attributed to the effect on the evolution rate of arsine. The iron content is critical and must be held constant. The concentration of sulfuric acid was critical. The capillary delivery tube with a tapered end as shown in the diagram. produces small bubbles which facilitate efficient absorption of arsine. Each of the above factors was investigated in detail. For reproducible and accurate results it is important to adhere strictly t o the recommended procedure. Table I1 shows the comparison of the average certificate values of arsenic in a number of B.C.S. (British Chemical Standards) Steel Standards and the value found. The precision of our values is much better than the spread listed in the certificate. Table I11 shows the comparison of arsenic values in the National Bureau of Standards Steels. In the entire series and throughout the range, the agreement is good and so is the precision of the values found. Table IV shows some results for arsenic in copper in Johnson, Matthey and Co. standards. These standards are in the
ANALYTICAL CHEMISTRY, VOL. 44, NO. 14, DECEMBER 1972
2403
pin form for spectrographic work. Again the agreement is quite satisfactory. CONCLUSIONS
A rapid spectrophotometric method is presented for the determination of arsenic in iron, steel, or copper. After dissolution of the sample under oxidizing conditions, nitric acid is eliminated. Arsenic(V) is then reduced to the trivalent state by the addition of potassium iodide and stannous chloride. Optimum conditions are established for the evolution of arsine and absorption of the latter in silver diethyldithiocarba-
mate in pyridine. The method is precise and accurate. Results from a wide spectrum of standard reference steels are presented. The method is suited for routine control, and shows promise for arsenic determination in iron ore, slags, or other matrices such as for pollution control. A batch of 12 samples can be analyzed in about 2 hours. RECEIVED for review May 22,1972. Accepted August 7,1972. Paper presented at the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, March 1972.
AIDS FOR ANALYTICAL CHEMISTS
Modified Attapulgite-A
New Support Material for Gas Chromatography
Rijhwani Moolchandral and Kanai L. Mallik2 Indian Institute of Petroleum, Dehra Dun (U.P.), India
THE RAW MATERIAL for most gas chromatography solid supports has been primarily diatomaceous earth (1, 2). In the recent past, however, several nondiatomite support materials, such as polymeric solid supports (3),vermiculite (4,sephadex (the cross-linked dextran polymer) (3, nonporous Teflon (Du Pont) (6), Kel-F type solid supports (7), and porous glass bead supports (8-10) have been reported in the literature. In an effort to substitute some other suitable siliceous material which is abundant in the country for the scarce diatomaceous earth material, we explored the possibility of using attapulgite ( I I ) and its modified form ( I 2 ) as a potential solid support for gas chromatography. Present address, Institute fur Fettchemie, Adlersholf, Berlin. To whom correspondence should be addressed. (1) J. F. Palframan and E. A. Walker, Analyst (LOI?dO/I), 92, 71 (1967). (2) D. M. Ottenstein, “Advances in Chromatography,” J. C.
Giddings and R. A. Keller, Ed., Marcel Dekker, New York, N.Y., 1966; p 137. (3) 0. L. Hollis, ANAL.CHEM.,38, 309 (1966). (4) R. W. McKinney, J. Gas Chromutogr., 3, 388 (1965). ( 5 ) N. Cockle and G. R. Fitch, Chem. Did. (London), 1966, 1970. (6) R. I. Sidorov and A. A. Khvostikova, Zh. Prikl. Khim., 39, 942 (1966). (7) H. Shinohara, N. Asakura, and S. Tsujimura, J. Nucl. Sci. Technol. 3, 373 (1966). (8) C. L. Guilleman, M. Le Page, A. J. de Vries, and R. Bean, Sixth
International Symposium on Gas Chromatography, Rome, Sept. 1966; also ANAL.CHEM.,39,940 (1967). (9) A. M. Filbert and M. L. Hair, J . Chromatogr. Sci., 6 , 150, 218 (1968); 7, 72 (1969). (10) C. L. Guilleman, M. Le Page, and A. J. De Vries, ibid., 9, 470 (1971). (11) K. L. Mallik, Discussion in Gas Chromatography Seminar (1968) Indian Institute of Petroleum, Dehradun(1ndia); Lab. Prucf., 18, 1077 (1970). (12) R. Moolchandra and K. L. Mallik, Indian Patent 122,025 (Filed June, 1969; Accepted Dec. 1970).
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According to Bradley (13), attapulgite has a structure of lath-like crystallites with long chains of tetrahedrons of silicon and oxygen (Siloll)running parallel to the long axis, the upper and lower parts of each double chain being joined together by Mg and A1 atoms in 6-fold coordination and to other chains by shared oxygen atoms along each edge (14). Attapulgite is highly sorptive in its natural form and has an unusually large surface area which has made it particularly useful as an industrial sorbent (IS). The mineral in its natural form, has limitations for use as a support material for gas chromatography (GC) since it contains more than 2 0 2 moisture, thus precluding its use above room temperature. Interestingly enough, the untreated mineral at room temperature was able to resolve a good many number of lighter hydrocarbons ( I I ) , especially some of the C4 isomers. Its performance is perhaps somewhat comparable to that of pure silica microbeads (IO). The mineral attapulgite, as received from the commercial (local Indian) source, contained 56 2 silica besides moisture and metallic oxides like Fe208,A1203,TiO?, MgO, and CaO. From theoretical considerations, this mineral might appear to be difficult to be tackled as a GC support, especially when one finds that the intercrystalline channels which run parallel to the crystal length normally contain water molecules half of which are closely coordinated to the outermost Mg cations sandwiched into the strips (16). This is besides its other inherent major handicaps-e.g., large surface area, low silica content, and presence of higher percentage of metallic oxides, We have attempted to overcome some of these difficulties by (13) W. F. Bradley, Amer. Minerul., 25, 405 (1940). (14) S. Callere and S. Henin, “The X-Ray Identification and Crystal
Structure of Clay Minerals,” G. Brown, Ed., Minerological Society, London, -1961, (15) W. L. Haden. Jr.. and I. A. Schwint, Ifid. En,?. Chem., 59 (9), ‘ 59 (1967). (16) R. M. Barrer and N. Mackenzie, J . Phys. Chem., 58, 560 (1954).
ANALYTICAL CHEMISTRY, VOL. 44, NO. 14, DECEMBER 1972
