Anion Analysis by Gas Chromatography Kevin L. McDonald MacMillan BIoedel Research Limited, 3350 East Broadway, Vancouver, B.C., Canada
WE HAVE BEEN CONCERNED for some time with the analysis of aqueous solutions containing carbonate, sulfide, or sulfite, sometimes in mixtures and frequently contaminated with large amounts of organic material. The usual wet chemistry methods involving potentiometric titrations (1) were sometimes unsatisfactory owing to difficulties in end-point detection and the possibility of air oxidation of the sample during sample handling. The method described here avoids these drawbacks and, to our knowledge, has not been previously reported. EXPERIMENTAL
Apparatus. The gas chromatograph used was a Pye Series 104 with thermal conductivity detector. The design of the injection system on this apparatus lends itself readily to modification as the integral carrier gas and injector head connection is made via washers directly to the column and not to the injection port. The connection may easily be interrupted by the insertion of additional accessories external to the instrument. An acid-washed 60-80 mesh deactivated silica gel (2) (Applied Science Laboratory) packing was used to fill a 15-inch X 1/8-inch Teflon (Du Pont) column, leaving no void at the inlet end. This was operated under a helium flow rate of 25 ml/min. The oven and injection heater were maintained at the same temperature, either 65 or 130 "C according to requirements, and the detector was operated at 200 "C with a katharometer current of 150 mA. A Disc Integrator was used with a 1mV recorder to record the area of eluted peaks. The acidifying device (Figure 1) was connected at one end to the column inlet, which protrudes slightly from the injection heater, using a Swagelok union and O-ring seals and at the other end to the existing Pye injection head. The borosilicate glass tubing was6mmo.d.,4mmi.d., or 6mmo.d.,l-mmcapillary and the bulb capacity was about 1.5 ml. The overall height was about 11 cm. None of the dimensions are rigorous so long as the internal void volume is not large, the abovementioned connections can be easily made, and the syringe needle will extend just beyond the initial length of capillary during injection. The straight limb of the device was packed with 14 to 22 mesh Anhydrone (British Drug Houses) and 0.1 to 0.2 ml of approximately 60% phosphoric acid was injected into the other limb with the carrier gas flowing prior to a series of analyses. The bulb prevents any small amount of the acid foaming over to the drying tube by allowing room for bubbles to break. A Hamilton 10-pl microsyringe was used for injections, and to ensure close reproducibility this was fitted with a Shandon Repro-Jector (Chemical Research Services). Procedure. For samples containing carbonate and sulfide, the column oven temperature was set at 65 "C while carbonate-sulfite solutions were run at 130 "C. Five-microliter injections, in duplicate, were made. Calibration was by injection of a standard carbonate solution when the relative response of H2S or SOz to CO, was known. This was determined by injection of identical volumes of standard carbonate and standard sulfite or sulfide solution and calculation from the resulting peak area ratios. The concentration of anion in (1) Technical Association of the Pulp and Paper Industry, Test
Method 625, Tentative Standard (corrected) (1964). (2) W. L. Thornsberry, Jr., ANAL.CHEM., 43, 452 (1971). 1298
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Figure 1. Acidifying device 1. 2. 3. 4. 5. 6.
Helium Injection head Glasswool Anhydrone Column coupling Injection heater
the sample was then determined from the peak area ratios either directly or after application of the appropriate response factor where required. RESULTS AND DISCUSSION
Normal chromatograms of the evolved gases were obtained with symmetrical peaks and base-line separation. The presence of the acidifier caused a slight increase in retention times and very little peak broadening; the bubbling of carrier gas through the acid had no noticeable effect on baseline stability even at highest sensitivity. The evolved gases were eluted from the column in about four minutes and no interference from water vapor or organic material was experienced. Calibration plots using Na2C03, Na2S03, and NazS solutions were linear up to 10% in salt concentration and passed through the origin. Estimates of precision at about 2 % concentration gave relative mean deviations of 0.37 % for sulfide and sulfite, 0.25% for carbonate at 65 "C, and 0.55% for carbonate at 130 "C(n = 8). Tests for sensitivity showed that carbonate could be detected down to the 0.01 Z level and the other salts to the 0.02 % level under the given conditions. Restrictions to the method apply where mixtures of anions give either the same volatile acidification products, e.g., carbonate and bicarbonate, sulfide and hydrosulfide, sulfite and bisulfite, or where the products are mutually reactive, e.g., sulfide and sulfite. In these cases a prior chemical treatment
may be needed, such as removal of sulfide from sulfite solutions by precipitation with zinc acetate. Other anions which may be analyzed include chlorite and nitrite (the latter will also interfere with sulfide analysis). The method will be useful for quality control purposes, particularly where automatic print-out of results can be obtained, and for cases where only small amounts of sample are available (for instance, determination of blood bicarbonate). The amount of sample injected was taken arbitrarily as 5 pl; this may be varied within wide limits according to the concentration of the analyzed solution. For very dilute solutions, such as may occur in pollution testing work, it would probably be advisable to use a polyphenyl ether column to minimize adsorption of sulfur gases on the packing and also a more sensitive detector (3). A Porapak Q-S (Waters Associates) column used initially in our studies was abandoned be-
cause of gross adsorption problems, although excellent separation and peak symmetry were obtained. The use of a sodium carbonate solution as a standard avoided the problems associated with storage of standard solutions of sulfide and sulfite. For greater accuracy or convenience a noninterfering anion could be added as an internal standard, but we did not find it necessary for our purposes. The initial charge of phosphoric acid is sufficient for many determinations ; over fifty subsequent 5-pl injections of solutions ranging up to 10% in Na2C03concentration have been made with no loss in potency and no significant increase in acid volume. Presumably the passage of dry carrier gas through the acid prevents undue dilution. The temperature of the acidifying device is above ambient, owing to its proximity to the column oven, and control is unnecessary.
(3) R. K. Stevens, J. D. Mulik, A. E. OKeeffe, and K. J. Krost, ANAL.CHEM., 43, 827 (1971).
RECEIVED for review December 27,1971. Accepted February 8, 1972. The author thanks the management of MacMillan Bloedel Limited for permission to publish the above article.
Mercuric Iodate as an Analytical Reagent Measurement of Chloride Ion by Ultraviolet Absorption of Mercury Complexes Ray E. Humphrey, Rufus R. Clark, Lauretta Houston, and Donald J. Kippenberger Department of Chemistry, Sam Houston State University, Huntsville, Texas 77340
CHLORIDE ION is commonly determined spectrophotometrically by reaction with a mercury(I1) compound to form soluble, slightly dissociated mercuric chloride and either release an absorbing anion or an anion which can then undergo another reaction to form an absorbing species. Relatively insoluble mercuric chloranilate reacts with chloride ion to release the chloranilate ion which absorbs both in the visible and ultraviolet regions ( I , 2) while soluble, slightly dissociated mercuric thiocyanate reacts to release the thiocyanate ion which is then combined with ferric ion to form the red ferric thiocyanate complex (3-5). These two reactions are probably the most common procedures for the spectrophotometric determination of chloride. The reactions involved are shown in Equations 1-3 below.
Mercuric iodate, a very slightly soluble compound (6),has apparently been used very little as an analytical reagent. A
chemical procedure for chloride was reported which involved the reduction of the iodate released to iodine by adding acid and iodide ion and titrating with thiosulfate solution (7). This method was termed an “amplification” since six iodine atoms resulted for every chloride ion in the sample. In another study, mercuric iodate containing some of the radioactive isotope *03Hgwas prepared and used to react with solutions containing chloride (8) or cyanide ions (9). The activity of the solutions containing soluble HgClz or Hg(CN)2 incorporating the z03Hgwas then measured. These appear to be the only reports in the literature in which the mercury compound in solution was measured and the amount related to the anion being determined. It seemed likely that some spectrophotometric procedures for measuring mercury(I1) in solution could also be used for the determination of chloride ion. The amount of mercuric chloride in solution is directly related to the amount of chloride in the sample as is shown in Equation 1. One of the simplest procedures to determine mercury(I1) in solution is to add a large excess of bromide, chloride, or iodide (IO) or thiocyanate ions ( I I ) to form the complex ion HgX4*-, as shown in Equation 4 below.
(1) J. E. Barney I1 and R. J. Bertolacini, ANAL.CHEM.,29, 1187 (1957). (2) R. J. Bertolacini and J. E. Barney 11, ibid., 30,202 (1958). (3) I. Iwasaki, S.Utsumi, and T. Ozawa, Bull. Chem. Soc. Jap., 25, 226 (1952). (4) R. P. Marquardt, ANAL.CHEM., 43,277 (1971). (5) T. M. Florence and Y. J. Farrar, Anal. Chim. Acta, 54, 373 (1971). (6) R. Castagnou and M. Devasle, Bull. Trau. SOC.Pharm. Bordeaux,84,70(1946).
(7) R. Belcher and R. Goulden, Mikrochim. Acta, 1953,290. (8) F. Szabadvary,E. Banyai, and L. Erdey, Chim. Anal. (Paris), 45, 289 (1963). (9) E. Banyai, F. Szabadvary,and L. Erdey, Talanta, 10,499 (1963). (10) C. Merritt, Jr., H. M. Hershenson, and L. B. Rogers, ANAL. CHEM., 25,572 (1953). (1 1) G . E. Markle and D. F. Boltz, ibid., 26,447 (1954).
-
+ 2‘21- HgClz + Ch*Hg(SCN)z + 2C1- e HgC12 + 2SCNFe3+ + SCN- e FeSCN2+ ___ HgCh
(1)
(3)
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