ANALYTICAL CHEMISTRY
562
in the concentration range anticipated for rock samples. Different amounts of National Bureau of Standards Standard Sample No. 97 (clay) were carried through the procedure. I n Table I the amounts of water absorbed are tabulated against the amounts of water contained in the several sample weights. The results obtained by two analysts have been used in the preparation of the table. In each case, the amount of water absorbed is a little less than that originally in the sample portions. Inspection of the data shows that a simple rule can be used to correct for the deficiencyin the amount of water absorbed. On samples for which the water absorbed is 20 mg. or less the value should be increased by IO%, and on samples for which the water absorbed is 20 to 160 mg. the value should be increased by 2 mg. Table I also shows the result of applying this rule to the set of data.
Table 11. Precision Sample Water, %
9a
loa
11 b 7.9 7.7 7.9 7.8 7.8 7.7
0.23 1.9 0.28 1.9 0.28 1.9 0.29 1.9 0.29 1.9 0.30 1.9 Average 0.28 1.9 7.8 Determined by Leonard Shapiro. 6 Determined by Harry F. Phillips, U. S. Geological Survey.
12 b 13.2 13.4 13.4 13.2 13.2 13.2 13.3
RESULTS
Precision of the method has been tested by running samples six times at each of four concentration levels. The results are shown in Table 11. Results obtained by the rapid method for total water in eight rock samples by two analysts are compared in Table I11 with results obtained by a third analyst who used the
Table 111. Comparison of Results by Rapid and Penfield Methods Water, yo Sample W-ld G-ld 1 2 3 4 5
4a 0.61 0.30 0.58 0.40 1 .o
Rapid
Bb 0.59 0.19 0.50 0.36 0.94 2.2 2.2 2.1 7.7 13.2
Penficeld
C
0.59 0.34 0.54 0.35 1.0 2.1 2.4 2.3 7.8 13.4
2.1 2.3 6 2.2 7 7.8 8 13.4 Determinations by Leonard Shapiro. b Determinations by Harry F. Phillips, U.S. Geological Survey. C Determinations by Paul W. Scott, U. S. Geological Survey. d Two carefully prepared rocks for collaborative study ( 8 ) .
conventional Penfield procedure. The values are in close agreement. A collaborative study of silicate rock analysis (2) reports the results of water determinations by the Penfield method and aome of its modifications. The reported values varied by 0.3 to 0.4% at the 0.5% level of water. The rapid method gives values well within the range indicated by this collaborative study. LITERATURE CITED (1) Dittrich, M., and Eitel, W., 2.anorg. Chem., 75,373 (1912). (2) Fairbairn, H. W., and coworkers, U. S. Geol. Survey, Bull. 980 (1951). (3) Federov, A. A., Zaoodskaya Lab., 1 1 , 3 5 4 (1945). (4) Penfield. S. L., Am. J . Sci., 3rd sei-., 48, 31 (1894). (5) Shapiro, L., and Brannock, W. W.. U. S. Geol. Survey, C i t c . 165 (1952).
RECEIVEDfor review October 23, 1954. Accepted December 16, 1954. Publication authorized by the director, U.S. Geological Survey.
Polarographic Determination of Sulfite DONALD 6. AULENBACH and JEAN L. BALMAT Department of Sanitation, Rutgerr University, N e w Brunrwick,
A method was needed for determining sulfite in the presence of other reduced compounds, in order to study the transformation of sulfur compounds in biological sewage treatment. The use of the polarograph is satisfactory for determining sulfite in concentrations of 1 to 250 p.p.m. as sulfur. Themethod involves the deaeration of the sample in neutral or alkaline conditions, acidification of the sample to convert all the sulfite to sulfur dioxide, determination of the height of the anodic wave produced by the sulfur dioxide, desorption of the sulfur dioxide, and determination of the height of the wave at the same applied potential in the absence of the sulfur dioxide. The difference between the two determinations is directly proportional to the concentration of sulfite originally present.
A
Tu' ACCURATE method for the determination of sulfite
in the range of 0 to 250 p.p.m. was needed for the study of sulfur transformations occurring in waste treatment processes. The two methods more frequently used for determining sulfite in low concentrations, oxidation and subsequent determination as sulfate ( 4 ) and oxidation with iodine ( 5 ) , are subject to interferences from other inorganic sulfur compounds and other oxidizing or reducing compounds, which occur in sewage and industrial wastes. Therefore, a method had to be found which was sufficiently
N. 1.
sensitive and accurate to measure low concentrations of sulfite in highly heterogeneous mixtures, such as wastes. Kolthoff and Miller ( 8 ) showed that sulfite in 0 . 1 s nitric acid produced a polarographic wave in pure solutions of sodium sulfite having concentrations of approximately 5 X 10-4M (15 p.p.m. as sulfur). This indicated that the polarographic method was sufficiently sensitive; however, it had to be proved that this method would give accurate results when used to analyze waste samples of a highly complex nature. APPARATUS
The apparatus used was the Fisher Elecdropode. The scale of the galvanometer was calibrated and it was found that each division of the scale at the 1X sensitivity was equivalent to 0.0186 Ma. The temperature of the samples was maintained a t 25" C. by immersing the polarographic cell in a constant temperature water bath. \Tater-pumped nitrogen was used for deaeration of the samples. EXPERIMENTAL
Effect of pH. The nitric acid supporting electrolyte used by Kolthoff and Miller was tried. However, the deoxygenation of the sample prior to polarographic analysis resulted in the desorption of the sulfite, present as sulfur dioxide in such acid conditions.. Although the loss of sulfur dioxide from the sample initially appeared to be detrimental, it later served as the basis for the method of sulfite determination described in this paper.
V O L U M E 27, NO. 4, A P R I L 1 9 5 5
L
5 I
563
28
28
24
24
20
20
K 0
0
I I-
z
'6
16
12
12
K
2
z 0
v)
0
8
4
4
3 I
L
--02
n
~
-0.4 -0.6 -0.4 -0.6 APPLIED VOLTAGE vs. S.C.E.
Figure 1.
-0.4
-0.6
-0.8
"
Effect of pH on polarographic sulfite wave
It was found that neutral or alkaline sulfite solutions could be deoxygenated with nitrogen without the loss of sulfur dioxide. When this was followed by acidification with nitric acid, the sulfur dioxide formed could be quantitatively removed by further scrubbing with nitrogen. The difference between the diffusion current immediately after acidification and after removal of the sulfur dioxide was directly proportional to the sulfite concentration. That the sulfur dioxide was completely removed was shown by the return of the diffusion current to the same value as that of the base electrolyte alone. For the conversion of sulfite to sulfur dioxide and its subsequent quantitative desorption, a pH of 1 to 2 was needed. Figure I shows the effects of pH upon the determination. Above pH 2, the sulfite was not quantitatively converted to sulfur dioxide, as shown by the reduced diffusion currents accompanying the higher p H values. Also, the sulfur dioxide was not quantitatively removed by scrubbing with nitrogen, as shown by the fact that the diffusion current was greater than the residual current of the base electrolyte. Between pH 1 and 2 the diffusion current was directly proportional to the sulfite concentration. As the p H was decreased below 1, the diffusion current continually diminished for solutions of the same sulfite concentration. This may be due to the evolution of nitrogen dioxide gas, produced by the reaction between mercury and nitric acid, which partially desorbs the sulfur dioxide. The evolution of nitrogen dioxide was not observed a t pH 1 or above. I n addition to the polarographic wave produced by the reduction of sulfur dioxide another wave of different character appeared within the same voltage range, thereby complicating the sulfite determination below pH 1. This wave was very similar to the one produced by I N nitric acid alone. Within the pH range of 1 to 2, the current produced was maximum and directly proportional to the sulfite concentration. Therefore, this p H range was used for all the polarographic sulfite determinations. Prevention of Sulfite Oxidation. Pure solutions of sulfite a t p H 7 or greater are rapidly oxidized in the presence of air. However, Mangan (3) shows that the presence of 5% glycerol or sucrose greatly reduced the rate of oxidation. Xeither glycerol nor sucrose interfered with the sulfite determination, and they were responsible for more reproducible results. Nature of the Polarographic Curve. A typical polarogram for the reduction of sulfur dioxide in the p H range of 1 to 2 is shown in Figure 1. The diffusion current reached a maximum at -0.60 volt us. the saturated calomel electrode (S.C.E.). The potentials recorded in this figure have not been corrected for the IR drop of the solution. The resistance, R, of the solution was found to be 950 ohms. Each part per million of sulfite sulfur caused a galvanometer deflection of 18 divisions, which is equivalent to 0.335 Ha. The applied potential of -0.6 volt was chosen as standard for the determination. Interferences. Measurement of the sulfite concentration as the difference between the diffusion current after acidification
and after the desorption of the sulfur dioxide eliminated most interferences. In addition to sulfite, there may be other substances present in waste samples which are reduced a t a potential of -0.60 volt us. S.C.E. or less. Those which are not desorbed with the sulfite will produce the same increase in the diffusion current both before and after the removal of the sulfur dioxide from the acidified sample and will produce no error. Those which are desorbed with the sulfur dioxide by nitrogen will produce a positive error. Sulfide in any concentration interfered with the sulfite determination (Table I ) due to an anodic current produced by the sulfide in the region of -0.6 volt ( I ) . The effect of sulfide may be eliminated by precipitating it with zinc acetate or by the method of Mangan (3). Thiosulfate also interfered with the sulfite determination. While not producing a wave of its own, the thiosulfate in the presence of sulfite produced a rapidly increasing diffusion current with any small increase in the applied potential, so that a limiting diffusion current was never reached. Thiosulfate may be removed by the differential solubility of the lead salts of sulfite and thiosulfate as described by Mangan (3). METHOD
Standard Curve. To prepare a standard curve of sulfite Concentration us. diffusion current, a solution of known sulfite concentration is used; this is standardized by adding a known volume of iodine and back-titrating the excess iodine with standard sodium thiosulfate, using starch as an indicator ( 5 ) . Sufficient glycerol is added to produce a 5% concentration in the standard sulfite solution. Using this standardized sulfite solution, the height of the polarographic wave is determined as described below. Knowing the galvanometer deflection for a known concentration of sulfite, the deflection for each part per million is determined. This relationship is linear for the concentration of 1 to 250 p.p m. of sulfite sulfur. Preparation of Sample. To the sample is added sufficient glycerol to produce a 57? concentration. Particulate matter is removed by centrifugation. Any sulfide present is precipitated by the addition of zinc acetate and removed by centrifugation. Thiosulfate present is separated from the sulfite by the method of Mangan ( 3 ) . The pH of the sample is adjusted to 7 or higher. Analysis. A 15-ml. portion of the prepared sample containing 1 to 200 p.p.m. of sulfite sulfur is placed in the polarographic cell. The oxygen is removed by scrubbing with nitrogen for 5 minutes. The nitrogen bubbler is then removed, and the sample is acidified with a few drops of concentrated nitric acid to lower the pH to 1 to 2. Five drops of acid are usually sufficient to adjust the pH of a 15-ml. sample, but for exceedingly alkaline or well-buffered samples, the amount of acid to be added should be determined on a se arate portion of the prepared sample. The mixture is stirred a few seconds with a small stirring rod. The bubbler is placed so as to maintain a nitrogen atmosphere over the sample. With an applied potential of -0.6 volt us. S.C.E., the diffusion current is noted immediately. Nitrogen is bubbled through the solution for 5 minutes to remove the sulfur dioxide. Removing the bubbler and again maintaining the nitrogen atmosphere over the sample, the diffusion current is Table I.
Effect of Sulfide and Thiosulfate on Polarographic Determination of Sulfite (P.p.m. as sulfur) Detd.
o
Calod. Sulfide BO: Concn. Concn. 22 56 0.8 62 ca. 200 84 0 ca. 200 82 4 82 4a ca. 200 ca. 200 82 46 247 25 5 247 25 5 15 346 37 1 12.5 371 12 5 371 12 5 450 5 352 25 23 391 78 33 Excess zinc acetate added.
803 Concn. 61 58 66.8 74.4 83.1 84.6 24.0 25.5 13 18 14.5 13 14 11 30 35
Error. %
-17.2 -8 0 +0.7 +2.2
5 0 -2 +5.5
-1
+2 0
i , E" l
- 14 4-7 +2
564
ANALYTICAL CHEMISTRY
Table 11. Polarographic Determination of Sulfite in Various Solutions (P.p.m. as sulfur) Calcd. Detd. 908 Concn. SO8 Concn. 15.2 16.1 3.8 3.9 7.6 6.9 11.4 11.7 15.2 15.3 31.0 30.0 225 222 31.0 31.7 29.0 29.4 29.0 26.7 22.5 21.7 45.0 47.8
Solution Dist. water
Sewage Sludge liquor
Average
Error, Error +0.9 +O.l -0.7 f0.3 +O.l -1.0 -3.0 +0.7 +0.4 -2.3
-0.8 +2.8 11.2
%
5.9 2.6 9.2 2.6 0.7 3.2 1.3 2.3 1.4 7.9 3.6 6.2 3.8
noted at the same applied potential. The difference between the two galvanometer readings is directly proportional to the sulfite concentration in the sample. This proportionality has been determined previously above. DISCUSSION
The accuracy of the polarographic method is shown (Table 11) by the recovery of sulfite added to distilled water, sewage, and sludge liquor from which the sulfite, thiosulfate, and sulfide were removed (3). The method is sensitive to at least 1 p.p.m. of sulfite sulfur. The diffusion current produced by this concentration corresponds to 18 divisions (0.335 pa.) on the scale of the galvanometer used. The method is not recommended for
use with samples containing more than about 200 p.p.m. of sulfite sulfur because of the longer time required for the desorption of the sulfur dioxide a t higher concentrations. Of all the substances present in sewage and sludge liquor, only sulfide and thiosulfate were found to interfere. When these were removed, the average error of the method was reduced to about 4%. The removal of sulfide and thiosulfate from the sample by the method of Mangan (3) required about 2 hours. Centrifugation consumed the major portion of this time. Several samples may be treated simultaneously, thereby reducing the time per sample. In most wastes, thiosulfate and sulfide are seldom found in the presence of sulfite, The conditions under which these compounds occurred together were artificially created in order to study the transformations of these sulfur compounds during sewage treatment. After the preparation of the sample, to remove these interfering compounds, the polarographic determination required only 15 minutes. LITERATURE CITED
S.,J. Am. Chem. Soc., 62, 2171 (1940). ( 2 ) Ibid., 63,2818 (1941). ( 3 ) Mangan, J. L., New Zealand J. Sei. Technol., 30B, 323 ( M a y 1949). (4) Scott, W. W., “Standard Rlethods of Chemical ilnalysis,” 5th ed., p. 925, Van Nostrand, New York, 1948. (5) Ibid., p. 926. (1) Kolthoff, I. M., and Miller, C.
RECEIVEDfor review June 1, 1954. Accepted November 13, 1954. Paper of the Journal Series, New Jersey Agricultural Experiment Station, Rutgera University, the State University of New Jersey Department of Sanitation, New Brunswick.
Spectrophotometric Determination of Aliphatic Sulfides S. H. HASTINGS and B. H. JOHNSON Humble
Oil and Refining Co., Baytown, Tex.
A procedure for spectrophotometric determination of aliphatic sulfides has been modified to improve its accuracy and reproducibility.
I
S A previous paper ( 1 ) a procedure was given for the determination of aliphatic sulfides, utilizing the intense ultraviolet absorption spectrum of the complex formed between these sulfides and molecular iodine. Certain modifications have been made to improve the accuracy and reproducibility of the method and an error in the original paper has been noted. The decrease in absorbance of the iodine-sulfide complex with time while in the cell compartment of the Beckman DU quartz spectrophotometer was thought to be due to the fact that the sample was in the dark; however, it was caused simply by a shift in the equilibrium brought about by the higher temperature of the cell compartment. The thermal effect noted on the Beckman spectrophotometer also occurs with the Cary ultraviolet spectrophotometer (Model 11). The equilibrium involved is
R’SR
+
I1
C---r-
R’SR.12
(1)
where R’SR is an aliphatic sulfide, Ip is molecular iodine, and R’SR.12 is the complex. This reaction has been found to be temperature-sensitive, lower temperatures favoring the formation of the complex. As a result, it is necessary to perform the analysis under controlled temperature conditions or apply a temperature correction. The temperature coefficient of the “apparent” absorptivity of the complex is approximately 13 absorptivity units per degree Fahrenheit a t 79’ F., and absorptivity is 372 liters per gram cm. (at thermal equilibrium in the cell compartment). The term
-
“apparent” is employed because the absorption due to the complex has been correlated with the initial sulfide sulfur concentration and this relationship is significantly affected by the temperature. The iodine concentration specified in the original work was later found to be far from optimum. Study of the iodine-sulfide complex has shown that the complex is a 1 to 1 combination of iodine with the sulfide, as indicated above. The equilibrium constant for this reaction is
where i indicates the initial concentrations. Calculations based on the iodine and sulfide concentrations specified in the analytical procedure (1) show that the two species must be present in very close to a 1 to 1 mole ratio, and as a consequence, the complex concentration is very sensitive to slight changes in the concentration of either of the partners making up the complex. This is undesirable, as it is known that aromatic and olefinic hydrocarbons tie up some iodine and consequently the iodine concentration is not constant, as was previously assumed. However, if the iodine concentration is made large with respect to the sulfide concentration, the factor ( [I214 - [R’SR.Iz]) in Equation 2 is essentially constant and the absorbance due to the complex is significantly affected only by changes in [R’SR],. Another important advantage results when [I21,>>[R’SR];. The equilibrium constant can then be written, to a good approximation, as [R’SR.I,] K = [I214[R’SRIi - [R’SR.Iz])
(3)
