ANALYTICAL CHEMISTRY
732 sponsorship of the Bureau of Ships, Navy Department, which initiated this investigation. Thanks are expressed to James McCambridge and Leonard Zoole, under whose supervision this work was conducted, for their continued interest. The views expressed by the authors are their own and are not to be construed as representing the official views of the Navy Department.
LITERATURE CITED
(1) Bureau of Ships, Navy Department, ~ p e c i f i c a t i o n51-S-47(IXTT) (Oct. 1, 1947). (2) Hanawalt, J. D., Rinn, H. W., and Frevel, L. K., ISD. ENG. CHEY.,ANAL.ED.,10, 467-513 (1938).
RECEIVED September 15, 1948.
Spectrophotometric Determination of Hydrogen Sulfide Methylene Blue Method JAJlES K . FOGO' AND MILTON POPOWSKY Southern California Gas Company, Los Angeles, Calif. Hydrogen sulfide is absorbed from gases and precipitated as zinc sulfide. The precipitate is then redissolved and allowed to react with p-aminodimethylaniline in the presence of ferric chloride. The optical density of the resulting methylene blue solution is measured at 670-millimicron wave length and the corresponding quantity of sulfide is read from a previously prepared calibration curve. The method is sensitive to about 3 micrograms and the range up to about 500 micrograms. The procedure is convenient for occasional as well as frequent use.
T
HE determination of hydrogen sulfide in gases has usually
been accomplished by iodometric methods (2,6). These give accurate results on appropriate samples but are often too insensitive for samples containing very little hydrogen sulfide. A much more sensitive method is that of Field and Oldach ( S ) , in which the sulfide is converted to bismuth sulfide which is determined photometrically while in suspension. This method, although very sensitive (1.4 micrograms), is not well suited for the occasional user, because very rigid control of technique is said to be necessary and all solutions must be protected against oxygen. The method described herein is a refinement of the methylene blue method (1, 5, 7 ) . The technique has been improved by use of optimum conditions for the principal reaction and by applying modern spectrophotometry to the measurement of concentration. The manipulation is simple and the results are not affected by minor variations. The method has been in successful use in the form given for several pears. The hydrogen sulfide is absorbed from a stream of gas in a suspension formed by adding sodium hydroxide to a solution of zinc acetate. The stripped gas is then suitably metered. The suspension then containing the absorbed sulfide as zinc sulfide is treated with an acid solution of p-aminodimethylaniline, followed by the addition of a small amount of ferric chloride solution. Bfter time has been allowed for the formation of the methylene blue, the solution is diluted in a volumetric flask and an aliquot is transferred to the spectrophotometer for measurement. The correspondin quantity of sulfide is then determined from a previously preparef calibration curve, plotted from similar measurements on methylene blue solutions prepared in the same manncr with known amounts of sodium sulfide or hydrogen sulfide. The method is sensitive to about 3.5 micrograms of sulfide when used as given. Greater sensitivity could be obtained fairly easily by appropriate reductions in the volumes of solutions used. The upper limit of the method as given is about 500 micrograms. The 1 Present addreas, Chemistry Department, University of Southern California, Los Angeles, Calif.
precision at such high concentration is somewhat poorer than a t about 100 to 200 micrograms, where it is *3%. APPARATUS
The list of apparatus includes the items necessary for taking two samples simultaneously and thereafter treating them consecutively. Two 250-ml. coarse sintered-glass type gas washing bottles. (Those made by Corning Glass Works are suggested.) Two test meters, either wet or dry type. One pipet, 25-ml. Two pipets, 5-ml. One graduated cylinder, 250-ml. One glass tubing cross, 8-mm. Three tubing clamps, screw type. Ten meters of 7-mm. Tygon tubing. Three volumetric flasks, 250-ml. One spectrophotometer or filter photometer. REAGENTS
KO especial care need be taken in the preparation of the reagents. Deviations up to 5y0 in the concentrations given are allowable. If the diamine used produces a dark colored solution, a fresh supply should be obtained. Zinc acetate, c.P.,1% solution in distilled Kater. Sodium hydroxide, c.P., 12y0solution in distilled water. Ferric chloride, c.P.,0.023 molar solution in 1.2 molar hydrochloric acid. p-Aminodimethylaniline sulfate, Eastman white label, 0.5 gram in 500 ml. of 5.5 molar hydrochloric acid. SAMPLING
A dual sampling procedure in which two samples are obtained simultaneously is recommended. The absorption should be done if possible directly a t the source. Gas samples brought into the laboratory in metal or rubber vessels usually give low results due to the reaction of hydrogen sulfide with the metal or its oxide or to its solubility in rubber. The pressure a t the source must be a t
V O L U M E 2 1 , N O . 6, J U N E 1 9 4 9
733
least 50 mm. of mercury above atmospheric or a pump must be used to draw the sample through the absorption bottle. One arm of a glass cross is connected to the source with Tygon tubing. A 5-cm. length of tubing is attached to another arm and a screw clamp is placed on the tubing. Each absorber is then charged with 130 ml. of 1% zinc acetate solution and 5 ml. of 12% sodium hydroxide solution, and the solutions are mixed by swirling. The parts of the bottles are assembled with petrolatum and fastened with rubber bands. The inlets of the bottles are connected to the remaining arms of the cross and screw clamps are placed on the connecting tubing. A test meter is then connected to the outlet of each absorber. M'ith all screw clamps open, the gas is turned on a t the source a t a rate considerably in excess of the sampling rate. Then the clamp on the bleeder arm is slowly closed until gas passes through the gas washing bottles a t a rate of about 170 liters per hour (6 cubic feet per hour). The rates through the two bottles may be equalized by adjusting the screw clamps on the connecting tubing. The amount of sample to be taken should be that which will contain betiveen 35 and 350 micrograms. Where it is necessary to use samples smaller than 50 liters, the sampling rate should be reduced correspondingly. If the method is being used to determine the amount of hydrogen sulfide resulting from the conversion of other sulfur compounds to hydrogen sulfide for analysis (4), only one gas washing bottle is used and the test meter can be replaced by a simple flowmeter. PROCEDURE
After the sample has been passed through the gas-washing bottle, the inlet and outlet of the bottle are closed by slipping the ends of a 25-em. length of tubing over them. Just before beginning the methylene blue reaction the temperature of the bottle and contents is adjusted to 24" * 3 " C.; the temperature of the diamine reagent should be similarly adjusted. Then the top of the gas-washing bottle is rai;ed and 25 ml. of diamine reagent are pipetted into the bottle. The bottle is closed quickly and the contents are snirled until all the precipitate is dissolved. Then by alternately applying slight pressure and suction on the inlet, a small amount of the solution is forced back and forth through the sinter in order to dissolve any zinc sulfide that may have concentrated there. When all the sulfide is dissolved, the top is again raised and 5 ml. of ferric chloride reagent are pipetted into the bottle, followed by mixing as before. The use of pipets designed for short delivery time rather than great accuracy is recommended.
Table I.
Specimen Calibration Data
(Coleman Universal spectrophotometer, wave length 650 millimicrons) Sulfide Inserted, Micrograms/2SO M I . Optical Density 64.8 0.21 64.5 0.22 129.6 0.41 129.6 0.43 2 59 0.80 239 0.82 1.12 389 359 1.16
After the closed bottle is allowed to stand for 10 minutes the blue solution is transferred to a 250-ml. volumetric flask and diluted to the mark with distilled water. Before the optical density is measured, the solution should be allowed to stand a t least 20 minutes but not more than 20 hours in a place out of direct sunlight. A blank solution is made by mixing the same amounts of the four solutions used above in a 250-ml. volumetric flask and diluting to 250 ml. with distilled water. This solution should be allowed to age for about 30 minutes before use in the spectrophotometer; the solution may be stored for several days in a dark or dimly lighted place. The optical density or transmittance of the test solution is determined by making the initial adjustment of the instrument while the cell is filled with the blank solution. Sormally, and for highest sensitivity, the measurements are made with light of 670millimicron nave length. Light of 750-millimicron wave length may be used if the solution is unusually opaque. CA LIBR 4TION
If the measurement of the optical density of the test solution is to be useful, a calibration curve must be prepared by making up several standards in the manner described above but using carefully measured quantities of sodium sulfide solution or hydrogen sulfide in place of the sample. A solution of sodium sulfide containing about 20 micrograms of sulfur per milliliter is satisfactory. The lumps of sodium sulfide should be thoroughly washed immediately before making the solution, in order to remove any sodium sulfite. Oxvnen-free distilled water should be used in making the solution. The solution is standardized iodometrically. Care must be taken throughout the preparation of the standards t o protect the sodium sulfide solution from more than a minimum amount of contact with oxygen. The calibration is completed bv measuring the optical densities of the standard methylene blue solutions and plotting the values obtained against the corresponding mass of sulfide used in preparing the 250-ml. solution. The resulting curve should be nearly linear in the lower half of the useful range of concentrations. Specimen calibration data are given in Table I. Once made the calibration may be used indefinitely. Data should be obtained a t 670 millimicrons and also if possible a t 710 and 750 millimicrons. The apparent peak absorption wave-length may vary somewhat from 670 millimicrons when instruments of low spectral purity are used. For example, with the Coleman Universal spectrophotometer the apparent peak is a t 650 millimicrons; this is apparently due to this instrument's band width of about 35 millimicrons. The absorption spectrum for a methylene blue solution compared to a blank solution with a Beckman Model DU spectrophotometer using a 2 to 3 millimicron band width is shown in Figure 1. I
-
WAVE LENGTH-MILLIMICRONS
Figure 1. Transmittancy of Methylene Blue Solution 157 microgram. of sulfur i n 250 m l . Spectral band width, 2 to 3 millimicrons. Cell thickneae, 1.00 cm.
EXPERIMENTAL
The amount of methylene blue finally formed in the reactions involved is a function of the tem-
ANALYTICAL CHEMISTRY
934
Then solutions were prepared with greater amounts of sulfide and these solutions were diluted with blank solution sufficiently so that the diluted solution should have corresponded to the one in which complete conversion was assumed. Invariably the optical densities of the diluted solutions were found to be less than that of the reference solution, indicating a loss. The results of these experiments are given in Table \-. Because corresponding losses occur in preparing the calibration curve, this effect is not considered to be important for methylene blue solutions representing less than 470 micrograms of sulfide in 250 ml. of solution. This effect accounts for the deviation of the calibration curve from Beer's law.
Table 11. Effect of Temperature on Yield of Methylene . Blue Temperature, C. Relative yield, %
5 76
20 99
25 100
30 98
40 79
55 64
75 43
Table 111. Effect of Acid Concentration of Diamine Reagent on Optical Density of Methylene Blue Solutions" Molarity (HC1) of diamine reagent' Optical density a t 660 millimicrons a -411 solutions
2
4
5
5.5
6
7
8
0.33
0.66
0.69
0.71
0.70
0.66
0.63
10 0.57
contained 222 micrograms of sulfide per 250 ml.
Table IV.
perature and other variables. A t higher temperatures the reaction is rapid but greater amounts of hydrogen sulfide escape from the acid solution into the vapor space of the gas-washing bottle before reacting; a t low temperatures little hydrogen sulfide escapes but the methylene blue reaction becomes so slow that side reactions occur to a greater extent. The over-all effect of temperature on the relative yields of methylene blue fromidentical reaction mixtures is shown in Table I1 Fortunately, the maximum yield occurs a t about 24" C. and a reasonable tolerance may be allowed. The effect of final acid concentration on the optical density of a methylene blue solution formed from a given amount of hydrogen sulfide was investigated by preparing the solutions as described above but with diamine reagents of various acid concentrations. All the solutions contained 222 micrograms of sulfide per 280 ml.; the results are given in Table 111. The effect is believed to be due largely to the influence of acidity on the absorption spectrum of methylene blue rather than to influence on the yield of the reaction. When the diamine reagent is added to the suspension containing zinc sulfide, hydrogen sulfide is formed. Some of it escapes into the vapor space of the bottle and is lost. The amount which escapes is a function of the solubility and the total amount present. When only small amounts of sulfide were present no hydrogen sulfide was detectable over the solution and this was arbitrarily assumed to indicate complete conversion to methylene blue.
Recovery of Hydrogen Sulfide as Rlethylene Blue
Sulfide inserted, micrograms Recovery, %
35 100
122 99
243 98
366 97
487 96
610 94
730 88
855 80
The reaction time of 30 minutes allowed in the procedure includes a considerable safety factor. Periodic determinations of the optical density of a solution during the reaction period indicated that the reaction was just completed after 10 minutes-that is, no further increase in the optical density was detected after 10 minutes. After about 20 hours a decrease due to fading may begin to be measurable. LITERATURE C I T E D (1) Almy, J . Am. Chem. Soc., 47, 1381 (1925). (2) Calif. Natural Gasoline Assoc.. Los Angeles, Calif., "Determination of Hydrogen Sulfide in Natural Gas," Bull. TS 413, 1943. (3) Field and Oldach, IND. EKG.CHEX.,A N ~ LED., . 18, 665 (1946). (4) Fog0 and Popowsky, ATAL.CHEM.,21,734 (1949). ( 5 ) Mecklenburger and Rosenkranzer, 2 . anorg. Chem., 86, 143 (1914). (6) Shaw, ISD.ENG. CHEM..ANAL.ED.,12, 668 (1940). (7) Sheppard and Hudson, Ibid., 2, 73 (1930). RECEIVED August 30, 1948.
Conversion of Sulfur Compounds to Hydrogen Sulfide In Air, Fuel Gas, or Mixtures JAMES K. FOG01
AND
MILTON POPOWSKY
Southern California Gas Company, Los Angeles, Calif.
T
HE sulfur content of fuel gases is usually determined either by oxidation or by hydrogenation. Oxidation methods (5, 8, I O , 1 1 ) are capable of accurate results on gases containing as little as 114 micrograms of sulfur per cubic meter (0.005 grain per 100 cubic feet), but the technique is cumbersome and the apparatus is likely t o be capricious. Hydrogenation methods 1 Present address, Chemistry ~ fornia, Los Angeles, Calif.
~
~university ~ of~ southern t tali~
~
( 2 , 4,6, 9 ) have some advantages but are limited in scope by the interference of oxygen, which is plentiful in certain types of gases. Even the oxidation methods cannot be applied to explosive mixtures or nonflammable gases. A method that can be used on any mixture of air and fuel gas became necessary for this laboratory in order to determine whether the natural gas present in Soil~gasest was , that normally present in the soil of certain areas or leakage from gas distribution lines which carry natural gas
