Improved Sulfur-Reacting Microcoulometric Cell for Gas Chromatography Donald F. Adams, George A. Jensen, J. Paul Steadman, Robert K. Koppe, and Thomas J. Robertson, College of Engineering Research Division, Washington State University, Pullman, Wash.
IJLFUR-CONTAINING compounds
are common constituents in the gaseous emissions from combustion and many chemical processes. In some instances, a wide variety of compound types may be present. For example, inorganic gases including hydrogen sulfide, sulfur dioxide, and organic compounds such as mercaptans, alkyl sulfides, and disulfides may be simultaneously present in kraft pulp mill effluents. There is a need for a sulfur-specific gas chromatographic detector which will respond to all of these types of compounds in the partsper-million to parts-per-billion concentration range. The microcoulometric iodine (7) and silver-silver sulfide (4) titration cells, the flame ionization detector (IO, 11), the electron capture detector ( I ) , and a modified Titrilog (6, 8, 9) either did not give a complete sulfur analysis or were unsuitable for gas chromatography. A Dohrmann iodine microcoulometric cell (Model C-100 coulometer and T-200 cell) provided the best overall sensitivity for all the various types of sulfur compounds found in typical pulp mill emissions ( I ) , but this system was not considered suitable for routine process analysis. The present studies were begun with a transistorized microcoulometer (Dohrmann Instrument Co., Palo Alto, Calif., Model C-200) and an iodine, microtitration cell (Dohrmann Model T-300). Used according to the manufacturer's recommended operating practice, several objectionable characteristics were observed, including tailing of the titration peak upon direct injection of the sample into the cell (Figure I), pressure response of the cell upon injection of air, temperature sensitivity not related to the microcoulometer electronics, short electrolyte life (3090 minutes) requiring frequent flushing and recharging with fresh electrolyte, and slow return of the freshly-flushed or overloaded cell to operating equilibrium (15-45 minutes). The number of samples which could be analyzed in an 8-hour day was significantly reduced because of excessive downtime required for flushing and awaiting return to equilibrium. None of the undesirable characteristics had been observed with the Titrilog macro-bromine cell when used as a gas chromatographic detector. The Titrilog responded rapidly even though the overall sensitivity was limited by the need to extend each volume of chromatograph column effluent with 1094
ANALYTICAL CHEMISTRY
1 wsu-I
T-300 '2
Br2
Figure 1 . Comparison of H2S peaks from T-300 and WSU-1 microtitration cells
at least six volumes of dilution air to provide the minimum gas flow requirement for proper operation. Therefore the Dohrmann T-300 iodine microtitration cell was converted to a bromine cell to determine whether the differences in response were related to differences in oxidation potentials, cell design parameters, or coulometer electronics. EXPERIMENTAL
Titration Cell Modification. The elemental iodine reference electrode of the Dohrmann T-300 cell was first replaced with elemental bromine contained in a small vial but open to the electrolyte. Elemental bromine slowly diffused into the titration cavity, changing the operating characteristics of the cell. A reference electrode was then prepared from mercury-mercurous bromide paste. The polarity of the reference battery in the C-200 microcoulometer had to be reversed to obtain a positive output from the bromine-type titration cells. No other coulometer changes were required. No improvement in response of the bromine-modified T-300 cells was observed. The slow response therefore, was attributed to the design of the reference electrode arm of the T-300 cell which the patentees claim to be essential to proper cell operation (3). The reference electrode in the T-300 cell is separated from the titration cavity by two fritted-glass disks and a captive electrolyte path of approximately 4.25 cm. between the two frits. It appeared that the slow response was due to unnecessary delay in ion diffusion within the sidearm. To test this hypothesis, a new micro-
titration cell (WSU-1) was designed and constructed to bring the reference electrode into close proximity with the electrolyte of the titration cavity. The new reference electrode follows conventional calomel-type electrode design and is readily removable, being equipped with a ground-glass joint. The details of the WSU-1 bromine cell are shown in Figures 2 and 3. The electrolyte contained 0.72 gram of potassium bromide and 6.0 ml. of sulfuric acid per liter The mercury-mercurous bromide paste is prepared by dissolving 20 grams of mercurous nitrate in 10 ml. of concentrated nitric acid. Eight grams of potassium bromide dissolved in distilled water are slowly poured into the mercurous nitrate solution. The precipitate is filtered with suction to a pasty consistency and washed thoroughly with 200 ml. of water and finally with 100 ml. of electrolyte. The precipitate is transferred to a flask, a few drops of mercury are added, and the flask is stoppered with a cork. The flask is shaken thoroughly until the mercury and paste are intimately mixed. This procedure is continued until 75 grams of mercury have been added. The paste is stored under electrolyte in a cork-stomered flask. The shelf life is indefinitk: The WSU-1 cell body was designed with a 34/28 ST outerioint at the tor, to accept the Dohrmkn T-300 pc'. containing the generating and sensing electrodes. Following extended use of the Dohrmann electrode cap, the titration system failed because an independent potential developed between the generating and sensing electrodes. The Dohrmann T-300 generating and sensing electrode cap is constructed of borosilicate glass. The two platinum electrodes and lead wires are sealed into two borosilicate-glass tubes extending downward from the cap into the electrolyte in the cell titration cavity. Because borosilicate glass does not adhere to platinum, electrolyte penetrated the capillary spaces between the two surfaces during continuous use. This produced an independent concentration cell between these two electrodes. The potential generated between these electrodes prevented the proper operation of the coulometer and titration cell. This difficulty was overcome by sealing the electrodes and lead wires into soft glass tubing. The electrode tubes were then positioned through the electrode cap and attached with epoxy resin. Graded seals could also be used. However, the cost would be much greater. Preparation of Standard Gas Samples. Standard gas concentrations of hydrogen sulfide, sulfur dioxide, methyl mercaptan, dimethyl sulfide, and dimethyl disulfide were prepared
Figure 2.
Construction details; WSU-1 microtitration cell
in bags made from a Mylar-Dacron laminate (Schjeldahl Co., Northfield, Minn., Type X-308) heat-sealed with a thermosensitive Mylar film (Schjeldahl Type GT-400). The value of these containers for production of known gas concentrations has been reported by Altshuller and coworkers (8). Sulfur-containing compounds were injected into the bags using Hamilton microsyringes. Measured volumes of nitrogen were used for dilution, The initial dilution of each compound was 50&2000 p p m . by volume. Further dilutions were then prepared from each initial dilution to yield concentrations down to 20 p.p.b. Standardization and Analysis of Gas Samples. Replicated bags of standard gas mixtures were prepared and colorimetrically analyzed. The relative standard deviation ranged from 5.3 to 12.6% as shown in Table I. The hydrogen sulfide concentration of the bags was determined by the methylene blue method (5) and methyl mercaptan with Bindschindler’s green (18). Tho relative standard deviation associated with the colorimetric analyses was determined on 14 aqueous hydrogen sulfide standards and was 1.5%. A gas flow of approximately 170 ml./ minute is required for maximum detector response. Since the optimum gas chromatographic separation on a 3/lsinch X %foot column was obtained with a carrier gas flow of 50 ml./minute, an auxiliary gas flow of 120 ml./minute was introduced into the system between the gas chromatograph and the detector. Sample volumes between 1 and 20 ml. were either directly injected into the microtitration cells through a septum-equipped tee in the inlet to the titration cell or into the gas chromatograph.
RESULTS
The WSU-1 cell attained equilibrium within 10 minutes following preparation of a fresh reference electrode paste, flushing and refilling with fresh eleetrolyte, or overloading with high concentration of sulfur compounds. The useful life of one filling of electrolyte is in the order of 4 to 9 hours, depending upon the usage. The sharpness of the titration peak was also improved (Figure 1). The sensitivity of the cell configuration having the reference electrode in direct contact with the titration cavity is approximately 30 times greater than that obtained with the original T-300 iodine cell and eliminates its major difficulties. Direct syringe injection of the sample into both the iodine and bromine microcells produces a pressure peak on the recorder. However, the peak with the bromine cell is not as great as with the iodine microcell. Sulfur Sensitivity. The minimum detectable sensitivity for each compound was that concentration which provided a peak height twice the base line noise. A microcoulometer attenuation setting of 400 gave a base line fluctuation within the limits of +5% full scale. Table I1 gives the minimum detectable concentrations of sulfur compounds analyzed. DISCUSSION
Because of the differences in electron requirements for oxidation of H2S, SO2, mercaptans, and organic sulfides and disulfides, i t is necessary to standardize the titration cell against each
Figure 3. tion cell
WSU-1 bromine microtitra-
Table 1. Dilute Gas Preparation HIS, BV. canen., Rel. p.p.m. N std. dev., yo 0.17 5 10.8 0.78 9 12.6 1.58
4.48 10.31
3 17 8
5.9 10.3
5.3
Table II. Minimum Sulfur Sensitivity
p.p.b.
type of compound or calculate the electron equivalents for each oxidation reaction for quantitative gas chromatographic analysis. To incresse the sulfur specificity and to eliminate the need for individual compound calibration, the column effluents may be either oxidized to SO1 or reduced to Hi3 in a suitable furnace prior to introduction into the microcoulometric titration cell to obtain an equivalent response for an equivalent number of sulfur atoms. The furnzce oxidation prior to titration should also destroy any olefinic compounds which VOL 38, NO. 8, JULY 1966
?c?5
might he present in complex gas mixtures, be titrated by bromine, and be emumusly reported as sulfur. Reduction to H2S rather than oxidation to SO2 has the added advantage of providing a four-fold increase in sensitivity because of the greater electron change required for the bromine oxidatiou of H2S. Although the need for individual calihration for each compound favors the use of a furnace between the chromtograph column and the detector, it complicates the analytical system and thus may not be suitable for process control analysis under mill conditions.
Air P o U u i h Conirol Aasw. 8, 338
UTEFATURE CITED
(1) Adam, D.
F., Koppe, R. K., Tuttle,
W. N., J. Air Pollution Control Assoc.
15, 31 (1965). (2) Altshuller, A. P.,
BeUar, T. A,, Clemons, C. A,, Vander Zanden, E., Intern. J. Air Water PoUuticn 8, 29 I1 Qfi4)~ \_"_.,_
(3) Coulson, D. M., Cavanagh, L. A,, U. S. Patent 3,032,493 (May 1, 1962). (4) Fredericks, E. M., Harlow, G. A,,
ANAL.CHEM.36, 263 (1964). (5) Jacobs, M. B., Braveman, M. M., Hochheiser, S., Ibid., 29,1349 (1957). (6) Klaas, P. J., Zbid., 33, 1851 (1961). (7) Martin, R. L., Grant, J. A,, Ibid., 37, 644 (1965). (8) McKee, H. C., Rolliwitz,
W. L., J.
(1959). (9) Nader, J. S., Dolphin, J. L., Ibid., 8 , 336 (1959). (10) Ruus, L., Waste Water Laboratory,
Stockholm, private
communication,
1964. (11) Thomas, E. W., Toppi 47, 587 (1964). (12) Wright, R. H., Schoening, M. A,, Hayward, A. M., Zbid., 34,289 (1951).
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~
~
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pGovement. Air ~Poll&ion Symposium, 150th Meeting, ACS, Atlantic City, September 1965.
Evaporation Manifold for Septum-Sealed Vials Gerald J. Litt' and Norman Adler,' Merck Chemical Manufacturing Division, Mcrck & Co., Inc., Rahway, N. J.
c
hy evaporation is quite often a requisite step in the analysis of trace components. Typically, micro amounts of the component must be recovered quantitatively from tens to hundreds of milliliters of solution. The evaporation of such large volumes is often done in stages, starting with a container of large surface area to hasten the evaporation. The residual contents are then transferred after each stage to a container of progressively smaller volume until the final micro container is reaxhed. It is readily apparent that a one-step evaporationi.e., directly into the final containerwould not only be more convenient but would avoid the risk of loss of sample associated with multiple transfer steps. The suitability of septum-sealed pharmaceutical vials for the convenient yet quantitative preparation and sampling of micro solutions for analysis by gas-liquid chromatography, infrared spectrometry, and other techniques has recently been described ( 1 ) . The viab would thus be of considerahle value as the final container for the evaporation process. It is the object here to describe an evaporation manifold designed expressly to permit controlled, one-step evaporation of solutions directly into this type of container. The apparatus is shown in Figure 1. The receiver, 8 3-ml. vial (Wheaton Glass Co., New York, N.Y.) designed to accept a septum closure and reshaped to a conical form, is plugged into the body of the evaporation manifold through a tightly fitting rubber grommet ('7/,,inch 0.d. X 0.5-inch i.d., No. HHS-2186, Federated PurONCENTRATION
' Prment address, New England Nuclear Gorp., Boston, Mass. 'Present address, Artbur D. Little, Inc., Cambridge, Mass. 1096
ANALYTICAL CHEMISTRY
chaser, Springfield, N.J.). The solution to be evaporated is fed from the separatory funnel to the vial via a 0.047inch i.d. tube made of Teflon which is fitted onto a ring seal extension of the inlet from the standard taper joint. This tube terminates about 1 em. from the bottom of the vial. The rate of
Figure 1.
Evaporation manifold
flow, regulated by the Tenon SWPCOCK, is preferably adjusted to prevent the liquid level from reaching this tube, yet to maintain a definite liquid phase during the evaporation. In practice each unit requires an occasional inspection after the initial adjustments to ensure optimum conditions. The low wettability of Teflon minimizes the spread of solution up the out.side of this tube. A stream of air or an inert gas, iutroduced via the other Teflon tube, sweeps the vapors from the vial. The opening of this tube is directed against the side of the vial, rather than straight down, to avoid excessive sweeping action. A third port in the manifold serves as the exit line and permits simultaneous application of vacuum. When air is used as the sweeping gns, the flow rate may be conveniently and reproducibly regulated by using hypodermic needles of various sizes as vents. In this case, the needle is first connected to a drying and filtering tube to prevent entrance of water or dirt. I n general, the optimum combination of the degree of vacuum and the flow rate of the sweeping gas is dependent upon the solvent being removed and the vapor pressure and physical state of the residue. Wide variations in operating parameters may be tolerated, however, and only a few trials are necessary to select settings that provide a reasonably rapid evaporation rate while minimizing the risk of blowing the dried sample out of the vial. The system may also be heated to accelerate evaporation. For solvents boiling helow 75' C., the bottom half of the receiver vial is immersed in a water bath of appropriate temperature. If necessary, the upper chamber of the manifold may be heated simultaneously with a hot air blower or an infrared lamp. For solvents with
