Elimination of Nitrogen Dioxide Interference in the Determination of Sulfur Dioxide PHILIP W. WEST and FE ORDOVEZA Coates Chemical laboratories, Louisiana State Universify, Baton Rouge, l a .
b In the spectrophotometric estimation of sulfur dioxide with hydrochloric acidbleached pararosaniline hydrochloride and formaldehyde, interference due to oxides of nitrogen has been eliminated without complication of the original procedure of West and Gaeke (5). Addition of 0.06% sulfamic acid to 0.1 M sodium tetrachloromercurate(l1) used as absorbing solution for sulfur dioxide from the atmosphere immediately destroys any nitrogen dioxide present.
T
HE COLLECTION and fixation of sulfur dioxide by scrubbing into tetrachloromercurate(I1) with subsequent spectrophotometric estimation by means of acid-bleached pararosaniline hydrochloride and formaldehyde are now widely used for both continuous and spot determinations of SO2 in air pollution studies. The method is very sensitive and accurate. Other forms of sulfur do not interfere, and the only significant interference is that of nitrogen dioxide ( 5 ) . Concentrations of NOzin amounts exceeding 2 p.p.m. induce a fading of test colors and, therefore, cannot be toIerated. A slight modification of procedure is now proposed which serves to eliminate the oxides of nitrogen interference. Sulfamic acid’s ability to decompose nitrite has been utilized in the manufacture of dyestuffs to remove interfering nitrites. Ruchhoft and Placak ( 4 ) have inhibited the interference of nitrites in the determination of dissolved oxygen in activated sludge and sewage mixtures by using sulfamic acid, and Marshall and Litchfield (5) have used it to destroy excess nitrous acid in the determination of sulfanil-
amide in blood and urine. It has also been used by Baumgarten and Marggraff (2) in the quantitative estimation of nitrite in the presence of nitrate. Altshuller, Schwab, and Bare (1) have studied the effect of adding sulfamic acid along with phosphoric acid for acidification in the evaluation of oxidizing systems using potassium iodide reagent. Decomposition of nitrite by sulfamic acid takes place quantitatively and rapidly, releasing nitrogen gas. Based on the ability of sulfamic acid to destroy nitrites, elimination of nitrogen dioxide interference in the spectrophotometric determination of sulfur dioxide (5) is effected. EXPERIMENTAL
Apparatus. Beckman model B or D U spectrophotometer. Reagents. Sodium tetrachloromerDissolve 27.2 curate(II), 0.1M. grams of reagent grade mercury(I1) chloride and 11.7 grams of reagent grade sodium chloride in distilled water and dilute to 1 liter. Modified sodium tetrachloromercurate(II), in which is dissolved 0.G gram of reagent grade sulfamic acid. Dilute to 1 liter with 0.1M sodium tetrachloromercurate(I1). Hydrochloric acid-bleached pararosaniline hydrochloride, 0.04%. Four milliliters of 1% aqueous solution of pararosaniline hydrochloride (aged 24 hours) is mixed with 6 ml. of concentrated hydrochloric acid. The mixture is shaken until the brown color disappears and is then diluted to 100 ml. Highest purity reagent available should be used, and the aging step is important in keeping the blank a t a minimum (by means of private communication it has been learned that Fisher Scientific Co. will now market a special grade of pararosaniline hydrochloride having purities of 98 to 99%).
Table 1.
Run
4 a
Results of Chemical Analysis of Treatment Conc. SO*, Conc. NO,, Absorbancies~ at 560 mp pg.lm1. rg./n-J. 0.00 0.1 0 0.025 0.06 0.1 0 0.022 0.06 0.1 10 0,023 0.00 0.1 10 0.000
Concn. NH*SOsH, %
Averages of 12 determinations in each case.
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ANALYTICAL CHEMISTRY
Table 11.
Results of Statistical Analysis of Treatments
Run
~ ~ Deviation
1
0.005 0.002
2 3
0.001
Variance ~ f Ratio 0.7763
Formaldehyde, 0.2% in distilled water. Procedure. Collect a sample of appropriate size (6) by bubbling it through tetrachloromercurate(I1) solution containing 0.06% sulfamic acid. Within 24 hours, develop color with acid-bleached pararosaniline hydrochloride and formaldehyde and measure spectrophotometrically. For purposes of study, 1 pg. of freshly prepared sulfur dioxide as NaHSOa and 100 pg. of freshly prepared nitrogen dioxide as NaN02 were added to about 5 ml. of 0.1M sodium tetrachloromercurate(I1) containing 0.0670 sulfamic acid. The sample was shaken gently until the evolution of nitrogen gas w119 complete (approximately one minute). One milliliter of 0.04% hydrochloric acid-bleached pararosaniline was added, followed by the addition of 1 ml. of 0.270 formaldehyde. The solutions were made up to 10 ml. with 0.1M sodium tetrachloromercurate(I1) containing 0.0G70 sulfamic acid. After 30 minutes of color development, the absorbancy of the solution was read a t 560 mp on a Beckman model B or DU spectrophotometer. The solution contained 0.1 pg. per ml. of sulfur dioxide and 10 pg. per ml. of nitrogen dioxide. RESULTS AND DISCUSSION
T o study the efficiency of sulfamic acid in eliminating NO2 interference, four systems were analyzed using the established colorimetric method, modified only by the addition of sulfamic acid in the absorbing solution. The value of the modified procedure can be seen from the data shown in Table I. It is clearly indicated that sulfamic acid is efficient in removing NO2 interference. The absorbancies listed were an average of 12 determinations, as compared with the reagent blank, Reagent blanks without sulfamic acid
~
~
and with sulfamic acid had the same average absorbancies (0.040). Table I1 shows the statistical analysis of the results of the chemical studies. An investigation of variance showed that the differences between average absorbancies are not significant. To determine the optimum concentration of sulfamic acid required to remove NOz interference without introducing complication to the original method, solutions containing 0.04 to 0.08% sulfamic acid in 0.1M sodium tetrachloromercurate (II) were prepared and used to determine sulfur dioxide in solutions containing 0.1 pg. SO2 and 10 fig. NOz per nil. Comparison was made with' the original method using Norfree SO1 solutions as the control. It was found that 0.06% sulfamic acid in 0.1M sodium tetrachloromercurate(I1) gave an absorbancy (0.025) nearest to that of the control (0.023). Using sulfamic acid in concentrations lower and higher than 0.06T0, absorbancies much lower than that of the control were obtained. Good results were obtained over normal concentration ranges of SO2 and a NO2 level of 10 p g . per ml. To ascertain the effect of sulfamic
acid on the stability of 0.1M sodium tetrachloromercurate(II), analyses of solutions containing O i l pg. sulfur dioxide per ml. were made using a solution of 0.1M sodium tetrachloromercurate(I1) which contained 0.06% sulfamic acid. No evidence of instability of the scrubbing solution was noted over a period of 21 days. A series of absorbing solutions (0.1M sodium tetrachloromercurate(I1) with 0.06y0 sulfamic acid) containing 0.01, 0.1, and 1.0 pg. per ml. of sulfur dioxide was prepared. To each solution 10 pg. per ml. of nitrogen dioxide, in the form of sodium nitrite, was added. Sulfur dioxide was then determined. At the hundredth microgram level of sulfur dioxide with 10 pg. of nitrogen dioxide, sulfamic acid still proved to be effective in preventing nitrogen dioxide from interfering in the colorimetric determination of sulfur dioxide, and the red-violet color of the test developed rapidly. Spectrophotometric measurements could be done within 30 minutes. At 560 mp the absorbancies of the samples followed the BeerLambert law. Sulfur dioxide can be quantitatively determined within 48 hours after col-
lection, with no loss of SO2 from the sample. Longer storage of sulfur dioxide in sodium tetrachloromercurate (11) with 0.06% sulfamic acid is not recommended because erratic results may occur. In this regard, the modified procedure suffers in comparison with the original method. The effective elimination of interference from the oxides of nitrogen makes possible the adaptation of this general method to the determination of sulfur in the micro- and ultramicroanalysis of organic substances and will be the subject of a later communication. LITERATURE CITED
(1) Altahuller, A. P., Schwab, S. M., 31,1987 (1959). Bare, M., ANAL.CHEM. (2) Baumgarten, P., Marggrd, I., Ber. 63, 1019 (1930). (3) Marshall, E. K., Litchfield, J. T.,
Science 88, 85 (1938). (4) Ruchhoft, C. C., Placak, 0. R., Sewage Works J. 14, 638 (1942). ( 5 ) West, P. W., Gaeke, G. C., ANAL. CHEM.28, 1816 (1956).
RECEIVED for review March 12, 1962. Accepted July 19, 1962. Work BU ported by the National Institutes of Healti under Public Health Service research grant RG-7992.
Studies in Paper Chromatography of a Few Cations with Solvents Containing Chloroform R. P. BHATNAGAR' and N. S. POONIA Deparfment o f Chemistry, Holkar College, Indore, India
b Chloroform and chloroform mixtures were studied as possible solvents for the paper chromatographic separation of inorganic ions. R, values for C U + ~ ,Ni+z, Cof2, and Fe+3 are given for the various solvent compositions tested. Although chloroform itself is not an effective solvent for inorganic paper chromatography, chloroform mixtures containing alcohols, ketones, esters, or phenols give qualitative separations on paper disks and strips for the four inorganic ions studied.
P
solvents having donor properties have been used extensively in the paper chromatography of inorganics. However, little work has been reported with polar solvents which are not electron donors. Such a solvent is chloroform, which has been virtually neglected as a solvent for this type of chromatographic work. Lacourt, SomOLAR
Present address, Government Science College, Gwalior, India. 1
mereyns, DeGeyndt, arid Jacquet made an unsuccessful attempt to separate nickel, cobalt, and copper ions with chloroform (8). However, a 10% chloroform solution in acetone separated nickel from copper quantitatively (3). Laskowski and McCrone (4) successfully separated several ions using chloroform on paper impregnated with 8-quinolinol. But none of these studies can be regarded as a complete, systematic investigation of the usefulness of chloroform. The present work explores the possibility of using chloroform and chloroform mixtures for inorganic paper chromatography. Ion migration studies and R, measurements are included for some 35 mixed solvents. Separations are indicated in those cases for which the differences in R, values were reasonably large. EXPERIMENTAL
Apparatus. A glass chamber (20 X 20 x 50 cm.) and a cylindrical battery jar (12 cm. in o.d., and 35 cm. high)
with the usual accessories were used for descending and ascending paper chromatography, respectively. Disk chromatograms were prepared in air-tight glass chambers. The disks were supported over the rims of Petri dishes which held the solvent. Watch glasses were placed over the disks to keep them in position. Papers. Whatman No. 1 paper was used for all preliminary studies, while 3-mm. Whatman was used for the final strips (30 x 3 cm.); the disks were 11 cm. in diameter. Disks with wicks of about 3 cm. cut radially were used instead of disks with capillaries. The wicks were 2 to 3 cm. wide a t the connecting end and about 1 mm. wide a t the free end. Reagents. Cobalt, nickel, copper (11), and iron(II1) solutions were prepared from their nitrates (E. Merck, pro-analysis quality). Solutions of 5 to 10 pg. of cation per 100 ml. were used for R, measurements; solutions of 20 to 50 pg. of cation per 100 ml. were used for the separations. All organic solvents were doubly distilled. Organic mixtures were prepared on a volume to volume basis, VOL 34, NO. 10, SEPTEMBER 1962
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