Acid-Bleached Fuchsin in Determination of Sulfur Dioxide in the Atmosphere
''
PAUL F. CROIVE AND WILLI.4JI E. BOGGS' Division of Industrial Hygiene, O h i o Department of Health, Columbus, Ohio
GESERAL method for determining sulfur dioxide in air in the presence of sulfur trioxide, hydrogen sulfide, and other impurities was needed by this laboratory. It was desired that the procedure be simple and reliable, and that it allow the preservation of field samples for later analysis in the laboratory. Of the methods considered, Steigmann's colorimetric method ( 5 , 6 ) gave the most promise. The procedure is relatively simple and free from many of the interferences that coniplicate most of the previous methods (1, 2, 4-6). The color reagent consists of a mixture of basic fuchsin, sulfuric acid, and formaldehyde xhicli tlwrlops a red-violet color in the presence of sulfurous acid. Tlic inteniity of the color produced is proportional to the concentration of the sulfur dioxide and mag be measured accurately tiv nic:ms of a spectrophotometer or colorimeter. Steigmann (6) first developed the reagent for the determination of sulfurous acid, thiol compounds, and formaldehyde in coniniercial gelatins. For determining sulfurous acid he used mercuric cliloride to remove the interference of inorganic sulfides, mercaptails, and thio acids. Grant ( 2 )adapted the method for use in the dc?erniination of sulfur dioxide in biological materials, and ascertitined that treatment with mercuric chloride reduced the blank i'oi, Iiiological samples to a reasonably low value. Atkin ( 1 ) used a siinilar reagent in the determination of the relatively high con(witrations of sulfur dioxide in flue gases in the presence of sulfur t rioside. Steigmann ( 6 ) further improved the reagent as used by Grant :md stated that platinum and palladium chlorides as well as merc w i c chloride could be used to mask the interference of thiols and tliiosulfates. The mercury-precipitated thiols and thiosulfates must be removed by filtration or centrifugation before the sulfur dioxide test is carried out. Steigmann also stated that although the color effects of cysteine, tryptophan, enediols, and o-dih) tlmxybenzenes are slow in developing, they are not masked. Koelyaeva (4), using a modification of Steigmann's first reagent to determine sulfur dioxide in air, found that hydrogen sulfide and when disulfide did not interfere, whereas the nitrogen oxides did. His procedure c8onsisted of taking a sample of 1 to 2 liters of air in an evacuated flask. The sulfur dioxide was then absorbed in 10 ml. of distilled water, and after an absorption period of 1 hour, 6 ml. of the sampling solution were mixed with 4 nil. of the color reagent. As little as 0.2 p.p.m. of sulfur dioxide could be deIrrmincd colorimetrically by nieans of a photocolorimeter using a w l l with a 100-mm. light path and a capacity of 5 ml. Thr, method was not found entirely satisfactory for the needs of this laboratory. Difficulties were encountered with air sampling procedures, and tbe sensitivity of the method had to be increased not only to determine sulfur dioxide in concentrations as low as 0.01 p.p.m. but also to make possible the use of a 10-mm. cell in plavc of the 100-mm. cell used by Kozlyaeva. After consideralile c.xperimentation the procedure described below was developd. PREPARATION OF REAGENTS
Unless otherwise specified, all reagents are of anal\-tical reagent quality. Stock Solution I. To 228 i d . of distilled water 22 ml. of concentrated sulfuric acid (specific gravity 1.84) and 8 ml. of a 3 5 solution of basic fuchsin in ethvl alcohol are added. This solution is shaken vigorously for about 30 minutes until the deep bron-n color has faded. Then 142 ml. more of distilled water are added, and the solution is aged for 3 days and filtered to remove the precipitate which has formed. If the solution is kept for a long period, i t may be necessary to refilter. Stock Solution 11. Five milliliters of 4Oy0 formaldehyde arc diluted to 100 ml. with distilled water. Indicator Solution. One part of solution I1 is added to 10 part3 of solution I immediately before using. Sampling Solution, 5y0 glycerol in 0.1 N sodium hydroxide. Standard Bisulfite Solution. Sufficient sodium bisulfite is added to give a solution containing approximately 100 micro1 Present address, Research Laboratories, Carnegie Illinois Steel Corp., Pittsburgh, Pa.
grams of sulfur dioxide per ml. This solution is accurately standardized with 0.01 N iodine solution. EQUIPMENT
Spectrophotometer or Colorimeter. Throughout this study a Beckman Model DU spectrophotometer with a 10-mm. cell was used. I t was set a t 580 mp with a slit width of 0.04 mm. 31SA midget fritted-glass bubblers. =lir pump. hir-volume measuring device. PROCEDURE
Ten milliliters of the sam ling solution are placed in a midget fritted-glass bubbler and 10 Titers of air are passed through it a t a rate of 20 liters per hour. Depending upon the concentration of tlic sulfur dioxide in the sampling solution, 1 to 3 ml. of this solution (readjusted to 10 m].) are mixed thoroughly with 4 to 2 ml. of the indicator solution, respectively. This mixture is thermostated a t 25" C. for a 30-minute color development period. The extinction, E, of the color solution is measured on a spect'rophotometer against water a t a wave length of 580 mp. A blank is made using the same proportions of sampling and indicator solutions, and the difference in extinction between the blank and the sainple ( E - Eo) is compared to a standard curve. If interferences are known or suspected, the sampling solution 11141~he mixed with an equal volume of saturated mercuric chloride tolution and cent,rifuged before mixing with the color reagent ( 2 , /)), blanks and standards being adjusted accordingly.
Table I.
R a t e of Loss of Strength of Sulfite Solutions
Solution
Time, Days
YO! Found, ,/All.
r. 1.o-3
100.5
80.6 46.i (J 1
.V S a O H
.Yc
glycerol
100 3 46 1 10 4
0 1
1
97.0 63,3 93.6
36
91.8
r>
3 r ; glycerol 0 1S S a O H
0 3 12 Determined colorimetrically
1 8 4
0 4 0 9
STANDARD1 ZhTlON
h portion of the standard sodium bisulfite solution is diluted in such a manner that the resulting solution contains 10 micrograms of sulfur dioxide per milliliter and 5% glycerol, and is 0.1 S in sodium hydroxide, Four milliliters of the indicator solution are placed in each of five 5-ml. volumetric flasks and 0, 0.2, 0.5, 0.8, or 1.0 ml. of the 10 microgram per milliliter sulfur dioxide solution is added to each flask. Fresh sampling solution is added to each to bring its volume to 5 ml. The color is developed and read in the usual manner, and the standard curve is cowtructed from the i.esults (Figure 2). EXPERIMENTAL
During preliminary investigations, solutions of sodium bisulfite in distilled water were used to check the conformity of the color reaction to Beer's law. However, successive standardizations using 0.01 iV iodine solution showed that the sulfite solutions were unstable even when made with doubledistilled water (Table I). The presence of base increased this instability, and when air was bubbled through the sulfite solutions, in simulation of actual air sampling, there was a still greater loss in strength. According to Haller (3) the oxidation of the sulfite proceeds only in the presence of certain catalysts, chiefly traces of salts of
1517
A N A L Y T I C A L CHEMISTRY
1518
copper and iron. It was suggested that the use of a “negative catalyst” to form a nonionizable compound with these salts would prevent the oxidation of the sulfite. Btkin ( I ) used a solution of 5y0glycerol and 10% sodium hydroxide in his scrubbers and successfully reduced the oxidation. To study further the stability of sulfite solutions, small amounts of sodium bisulfite were added to pure water, 0.1 N sodium hydroxide, and 5% aqueous glycerol solutions. The concentration of sulfite W M determined by titration with 0.01 N iodine immediately and again after different periods of time. Table I shows the results of these determinations. The oxidation of sulfite was substantially reduced in the 5% glycerol-0.1 N sodium hydroxide solution.
Thermal Effect on Color Development. Both Atkin ( I ) and Kozlyaeva ( 4 ) suggested allowing 30 minutes for color development. The color development, which is rapid a t first, levels off within 30 minutes. K i t h an increase in temperature, a corresponding increase occurred in both the extinction of the blank, EO,and the extinction of the actual sample, E. Although the extinction of the solution minus the extinction of the blank, E - EO, remained nearly constant for the temperature interval between 23” and 26’ C., it seemed best t o thermostat the solutions a t 25” C. during the 30-minute color-development period. Table I1 shows the effect of temperature on the extinction of the blank, EO, and the amount of color, E - EO,developed per microgram of sulfur dioxide.
Table 11. Effect of Temperature on Color Developed Y so2 E - Ea c. E1 E E - Eo Delivered per y SO?
Temp.,
31 30 28 27 26 25 24 23
0.250 0,242 0.232 0.221 0.216 0,203 0.195 0.182
0.612 0.610 0.996
0.362 0.368 0.764
6.64 6.73 11.36
0.055 0,056 0.067
O:Qi4 0.703 0,794 0.684
0:728 0.500 0.599 0.502
9.70 6.60 7.68
0:075 0.076 0.078 0.076
...
6.60
Original Standardization. In order to determine accurately the extinction versus concentration curve, a gas measuring and mixing system was devised (Figure 1) which would give an approximation of actual mmpling conditions. With this apparatus sampling media were checked, a satisfactory sampling rate was determined, and a curve of concentration versus extinction was constructed. With the system adjusted as shown in Figure 1, the mixing tube and capillary buret were flushed thoroughly with sulfur dioxide from the tank. The capillary was filled by lowering the leveling bulb and then closing stopcock 2. Stopcocks 1 and 4 were reversed, and the mixing tube was flushed thorouehlv with air. The eas in the buret was broughi & atmospheric pressure by adjustment of the leveling bulb, the length of the gas column was recorded, and the barometric pressure was noted. Stopcock 3 was reversed and the rate of bubbling was adjusted by means of needle valve a. The leveling bulb was raised 10 to 15 em. to put the as in the buret under a slight pressure, and a voyume of sulfur dioxide was bled into the mixing tube through stopcock 2. Stopcock 2 is unique in that a fine line was cut about ‘/a cf the way around it from the ends of the bore in a clockwise direction. This allows the gas in the buret to be bled slowly into the mixing tube under pressure and prevents the air in the mixing tube from entering the buret. The final length of the gas column was recorded after adjustment to atmospheric pressure with the leveling bulb, and the amount of sulfur dioxide introduced into the air stream was calculated. Aliquots of the solutions in the bubblers were mixed with the proper amount of color reagent, and the color was developed. E - EO was
A
Table 111. Efficiency of Various Sampling Solutions SO, rldded, Aging E - Eo Normality % y/Llter E - Eo Time After of NaOH 1.5
Glycerol 5
Air
2.53 4.67 9.39
Initial 0.00
0.08 0.25
0.5
5
2.96 6.02 9.45
0.28 0.38 0.67
0.1
3
2.71 5.20 9.25
0.21 0.44 0.81
2.34 5.26 9.36
0.18 0.44 0.73
0.1
1
Days, N
7 7 7
3 3 , .
3 3 3
N Days 0.00 0.00 0.00
0.19 0.40
..
0.17 0.41
0.74
measured and plotted against the calculated concentration of sulfur dioxide to make the standard curve (Figure 2). Sampling. Since, aa is shown in Table I, a solution of 5% glycerol in water has excellent preservative qualities, it was first tried for sampling purposes. Considerable sulfur dioxide was carried over into the second and third bubblers of the train even when scrubbed a t a relatively low rate. A solution of sodium hydroxide and glycerol was found to be almost 100% efficient, and an investigation was conducted to determine the optimum proportion of sodium hydroxide and glycerol in the sampling solution. Table I11 compares the efficiency and preservative qualities of different concentrations of sodium hydroxide and glycerol used as sampling solutions. The sulfur dioxide was added slowlj- to approximately 10 liters of air as the air was passed through the standardization apparatus a t a rate of 20 liters per hour. It was decided that a sampling solution of 0.1 iV sodium hydroxide and 5% glycerol Ras the most satisfactory. With this solution, excellent recovery was obtained in concentrations as high as 139 micrograms (47 p.p.m.) of sulfur dioxide per liter of air. Bubbling rates up to 20 liters per hour had little effect on the efficiency of sampling, while rates exceeding 30 liters per hour reduced the scrubbing efficiency and increased the oxidation rate. Sensitivity. The sensitivity of the method wm increased by using 3 ml. of sampling solution to 2 ml. of the color reagent and developing the color as outlined above. Inasmuch as the usual proportions were 1 ml. of sampling solution to 4 ml. of color reagent, this actually amounted to tripling the concentration of sulfur dioxide in the solution being analyzed. Figure 2 shows that
Figure 1. Standardization Apparatus
V O L U M E 23, NO. 10, O C T O B E R 1951
0
06-
1519 concentrations. At lower concentrations, when 2 or 3 ml. of sampling solution are used, the accuracy is increased accordingly. If the concentration of sulfur dioside is greater than 11 micrograms per liter of air (3.7 p.p.m.) a smaller aliquot must be used. If the concentration of sulfur dioxide falls below 0.1 microgram per liter (0.03 p.p.m.) a greater volume of air must be ssmpled. The minimum volume of sampling solution which could be used in the midget fritted-glass bubblers was 10 ml. If 10 liters of air are sampled, the amount of sulfur dioxide in 1 mi. of the sampling solution is the same as the amount in 1 liter of air. Field Test. The method was tested in the field by comparing results with those obtained Tvith a potentiometric titration method which used a neutral 0.45y0 hydrogen peroxide sampling solution. Samples taken simultaneously gave comparable results (Table IF').
3.2 PROPORTION OF SO, SOLUTION TO COLOR REAGENT
0
y
0.4-
w
02
-
-
N A z S t 4 SOLUTIONS
STD CURVE DETERMINED w i T n GAS MICROBURET
Table IV. 1
0
2
I
1
I
4 6 MICROGRAMS OF Sop
Figure 2.
I
I
8
Standard Curves
Sample 1 2 3
0
;:
4
the E - Eo values, obtained from known amounts of sulfur dioxide using a 2 t o 3 or a 3 to 2 proportion of sampling solution to color reagent, agree well with the results obtained with the usual 1 to 4 proportions. The curve of extinction versus concentration was found to follow Beer's law between the concentration limits of 0.1 and 11.0 micrograms of sulfur dioxide. The average deviation was 0.11 microgram of sulfur dioxide or 0.037 p.p.m. This represents accuracy within 1.1% a t 10 micrograms or 11% a t 1-microgram
Results of Field Test
Sulfur Dioxide Determined by Each Method Colorimetric fuchsin Potentiometric titration 1.1 1.1 0.4 0.0 2.3
LITERATURE CITED
(1) Xtkin, Sidney, ANAL.CHEM.,22, 947 (1950). (2) Grant, W. M.,Ibid., 19, 345 (1947). (3) Haller, Percy, J. SOC.Chem. Ind., 38, 52T (1919). (4) Korlyaeva, T. N., Zhur. Anal. Khim., 4, 75 (1949) (5) Steigmann, Albert, ANAL.CHESI.,22, 492 (1950). (6) Steigmann, Albert, J . SOC.Chem. Ind., 61, 18 (1942).
RECEIVED January
24, 1951.
Dicyanatodipyridine Copper( 11) Complex for Colorimetric Determination of Cyanate ERNEST L. MARTIN AND JEAN MCCLELLAND University of New Mexico, Albuquerque, N. M.
HE existence of the complex [CU(PJ-I)(CXO)~] was estabTlished by Ripan (3). The blue color of the chloroform estract of this complex formed with copper sulfate, pyridine, and cyanate solutions was utilized by Bailey and Bailey ( 1 ) for the colorimetric determination of cyanates. The chloroform e s tracts were compared directly with similar solutions containing known amounts of cyanate. The absence of absorption spectra datn for this system prompted a reinvestigation of the mcthod. APPARkTUS 4 N D MATERIALS
All absorption spectra measurements were made with a Beckman Rlodel DU quartz spectrophotometer using 1-cm. Corex cells with a nominal entrance band Ridth from 1 to 3 mp. A Beckman Model G pH meter, the glass electrode of which was calibrated frequently against potassium dihydrogen phosphate buffers, was used for all pH measurements. Potassium Cyanate Solutions. 1 ml. = 0.01 gram of OCN- or 0.0193 gram of IIOCX in 0.04 N NaOH solution The sock solution was diluted with water to give the desired cyanate ion concentration. Copper Sitrate-Pyridine Solution. 1 ml. = 1.38 X 10-4 mole per ml. of Cu(SO,)2 and 2.1 X mole per mi. of pyridine in water The potassium cyanate, copper nitrate, and sodium hydroxide were of analytical grade. The pyridine was Eastman 214. The chloroform was technical grade.
DEVELOPMENT O F PROCEDURE
Choice of Copper Salts for Complex. Previous investigators ( I , 3) used copper sulfate as the source of copper ions for the dicyanatodipyridine copper(I1) comples. In order to determinr. thz effect of another copper salt,, determinations were carried out in which solutions of copper sulfate and copper nitrate were usetl for complex formation. Ten milliliters of the reagent containing 1.38 X 10-4 mole per ml. of copper sulfate or copper nitrate and 2.1 x 10-3 mole pet' ml. of pyridine were mixed with 5 ml. of cyanate solution, were allovied to stand for 10 minutes a t 25" C., then extracted wit,h four 5-ml. portions of chloroform. The combined extracts were diluted to 25 ml. with chloroform anti the transmittancy was determined at 680 mp. When copper sulfate solutions were used, the transmittancy of the chloroform extracts of the complex formed was higher and the slope of the concentration curves less steep than when the comples was formed wit,h solutions of copper nitrate (Figure 1). In view of these results, copper nitmte solutions were chosen as the source of copper(I1) ions for the complex. Effect of Concentration of Reagents. Investigations of similar copper complexes formed with amines have shown that the comples dissociates in chloroform extracts unless the amine is present in great excess ( 2 ) . The authors' investigations showed that the trnnsmittancy varied as the nniount of pyridine was increased,
