Colorimetric Determination of Nitroparaffins LAWRENCE R. JONES AND JOHN A. RIDDICK Commercial Solvents Corp., Terre Haute, Ind. This work w a b begun because no sensitive or specific colorimetric method was available for the determination of small amounts of secondarq aliphatic nitroparaffins. The method as developed depends upon the decomposition of the nitroparaffin to nitrous acid by means of a treatment with concentrated sulfuric acid. The resulting nitrous acid is combined with resorcinol to form what is believed to he a p-nitrosophenolic compound. This compound gi\es a deep red-blue color, measurable at a wa\e length of 560 m p by means of a spectrophotometer. The color is suitable for the quantitative determination of most secondary and a few tertiary nitroparaffins in amounts as little as 3 to 5 micrograms with an expected accuracy of +5% and a precision of *1%. It is possible to determine 2-nitropropane in air, and in mixtures of primary nitroparaffins. The method has been adapted to the determination of halogenated nitroparaffin fumigants w-hen used on grains.
Primary nitroparaffins, when warmed with concentrated acid, decompose into fatty acids and salts of hydroxylamine, n hich do not interfere in the proposed test. EXPERIllERT4L
Experimental data in support of the above theory nere estalilished by adding the resorcinol to the reaction before and aftcr treatment with warm sulfuric acid. Khen the resorcinol n :IS added before the sulfuric acid, all of the nitroparaffins reactcd with some color formation. The primary nitroparaffins gave a transitory color which was not quantitative; and the secondary and tertiary gave a quantitative color, showing that a stable conipound was formed. The secondary nitroparaffins and a f ~ n tertiary nitroparaffins developed color \Then the resorcinol n :is added after treatment with sulfuric acid. This color was al\o quantitative. Apparently the primary nitroparaffins had berm converted to carboxylic acids and hydroxylamine salts. It n : t s determined that hydroxamic acids, carboxylic acids, and hydroxvl ammonium salts gave no reaction under similar conditions. The effects of several variables, such as temperature and concentration of water and acid during the reaction, have been investigated for their influence on the decomposition of the nitroparaffin and on the subsequent color formation, stability and intensity of the color, and conformity to Beer’s law.
T
HE vapor phase nitration of paraffinic hydrocarbons pro-
duces a mixture of primary, secondary, and tertiary nitroparaffins (10, 11, Sa), the composit,ion depending on the hydrocarbon used and nitration conditions. The det,erminaton of one nitroparaffin in the presence of another is difficult because of their closely related properties. Aliphatic nitrocompounds have some characteristic color reactions. These reactions have been investigated to determine the possibility of developing more rapid and specific means of analysis for specific types and compounds. Colorinietric methods of analysis for nitroparaffins and halogenated nit,roparaffin have heen described ( 3 , 6, 9, 15-15, 81-25, 31, 33). These methods are not sufficiently specific or sensitive for the secondary compounds, such as 2-nitropropane and 2-nitrobutane, or the tertiary compounds, such as 2-nitro-2-niethylpropane. 1Ieyer ( 2 5 ) !?-as the first to distinguish qualitatively between the three types of nitro compounds with the “red, vhite, and I h e ” reaction with nitrous acid as a reagent. The tertiary nitro compound does not react, leaving the solution colorless, or “white.” The secondary compounds form the blue pseudonitrole, while the primary compounds form nitrolic acids, the salts of 1vhic.h are red. More recently, Turba, Haul, and Uhlen ( S a ) developed the pseudonitrol reaction into a quantitative method. They point out that although the method is specific for the secondary nitro compound, it lacks the sensitivity needed for the determination of small amounts of secondary groups in the presence of the other types. During an investigation of the several types of nitroparaffin compounds, it was found that most secondary nitroparaffins and sonic tertiary nitroparaffins reacted with resorcinol in the presecnce of concentrated sulfuric acid to give an intense red-blue color suitable for quantitative measurement. All primary nitro compounds tested gave a negative reaction. This reaction ic similar to the extensively studied resorcinol test for nitrites, nitrates,aiidnitrosylsulfuric acid(l,d, 4 6 ,7,8, 12, 16-18, 20,26-30, 34-36). It is believed that the reaction involves the decomposition of the nitroparaffin with release of nitrous acid, which in the presence of concentrated sulfuric acid forms nitrosyl sulfuric acid. The nitrosyl sulfuric acid combines with the resorcinol to form a p-nitrosophenolic compound. This reaction would be similar to Lieberman’s (19) test for free nitrous acid.
Decomposition of 2-Sitropropane (50 y) at Room Temperature
Table I.
Time Hours 0 4 8 12 1G
18
”0 24
Optical Density 0 165 0 300 0.500 0.652 0.715 0.730 0.730 0.730
Decomposition of 2-Sitropropane (50 y) at 100” C.
Table 11.
Time, Minutes 0
1
1.5 2.0 3 0 4 0 .j 0
Optical Density 0 165 0.620 0.710 0.726 0.730 0.730 0.730
Decomposition of Nitro Compound. The decomposition of the nitro group to nitrous acid is accomplished by heating the nitroparaffin in concentrated sulfuric acid. I n order to find the most suitable conditions for this decomposition, several aliquot5 of the nitroparaffin containing 50 micrograms of 2-nitropropane, dissolved in concentrated sulfuric acid, were tested a t room temperature and a t 100’ C. The time needed for the decomposition of the nitroparaffin to nitrous acid after the addition of the sulfuric acid was followed by means of the color produced by the reaction with the resorcinol in the second step of the procedure. The results are shown in Tables I and 11. The results in Table I demonstrate that a t room temperature a t least 18 to 20 hours are necessary for total conversion to the nitroso compound, while the results in Table I1 demonstrate that a t 100” C. only 2 to 3 niinTable 111. Time, Minutes 0 2 4 6 8 10 12
1533
Color Development at 100’ C. Optical Density 0.350 0.400 0,460 0.635 0.730 0.730 0.730
ANALYTICAL CHEMISTRY
1534 Utes are needed. A 5-minute reaction time was chosen to ensure completion of this step. Formation of Color. After the decomposition of the nitro compound, the color is formed through a reaction with aqueous resorcinol solution and the hot sulfuric acid. A higher precision and reproducibility were obtained when the samples were heated on a steam bath a t 100" C. By varying the length of time, the most favorable conditions were found as shown in Table 111. A 10-minute heating period was used to ensure maximum color formation for 50 micrograms of 2-nitropropane. Absorption Spectrum. Spectral-transmittance curves of the red-blue complex resulting from the reaction of secondary nitroparaffin and resorcinol show a maximum absorption a t 560 mr.
Reagents. 2-Nitropropane standard and 1,l-dichloro-1-nitroethane standard, obtained from Research Department, Commercial Solvents Gorp. Sulfuric acid, concentrated, specific gravity 1.84, Mallinckrodt low-nitrogen, nitrite-free. Resorcinol, reagent grade, 1% aqueous solution. Preparation of Calibration Curve. Prepare a solution of 2nitropropane standard in concentrated sulfuric acid to contain 10 micrograms per ml. To six test tubes add 0, 1, 2, 3, 4, and 5 ml., respectively, of the 2-nitropropane standard. Dilute each to 10 ml. with sulfuric acid and mix. Stopper tubes and immerse in the boiling water bath for 5 minutes. Remove tubes from boiling water bath, remove stoppers, and immerse tubes in a cooling (20" t o 25' C.) water bath for 2 to 3 minutes. When contents of tubes are cool, cautiously stratify 5 ml. of the resorcinol solution upon the acid. Mix carefully, stopper tubes, and immerse tubes in the boiling water bath for 10 minutes. Remove tubes from boiling bath, remove stoppers, and immerse tubes once again in the cooling bath. Transfer the colored solution to a 1-cm. Corex cell and read the optical density a t 560 mp with the spectrophotometer, using the zero prepared standard as the blank for the zero optical density setting of the instrument. Plot concentration vs. optical density on linear graph paper, The above standards equal 10,20,30,40, and 50 micrograms of 2nitropropane, respectively. Analysis. Make the analysis as described above, but prepare the sample so that an aliquot of concentrated sulfuric acid of 10 ml. or less does not contain over 50 to 60 micrograms. APPLICATIONS
Figure 1. Apparatus for Extraction of Fumigant Aeration tube. Inside tube, 67 cm. long, 0.d. 32 mm., i.d. 28 m m . , wound with 43 turns of No. 22 Nichrome V resistance wire I/* inch between turns. Outside jacket, 57 cm. long, 0.d. 45 mm. B . Air scrubbing bottle, filled with sulfuric acid, sp. gr. 1.84 C. Standard taper 24/40 glass joints D. Leada t o Variac E . Tygon tubing F. Flowmeter, Fisher Laboratory Model No. 11-163 C . Glass wool plug T. Traps, 25 X 200 mm., borosilicate tubes, containing 10 m l . of sulfuric acid, sp. gr. 1.84 A.
Stability of Colored Complex. Pure 2-nitropropane was treated to develop the colored complex. The transmittancy of the sample was determined a t time intervals. The color was found to be stable for a t least 24 hours. Effect of Acid Concentrations on Colored Complex. The ratio of volume of acid to volume of water was varied to determine the optimum acid content. It was found that below 60% volume acid content, very little color developed. For convenience, 66% volume acid was chosen because it was easy to make this dilution with the two reagents. It was also found that when colors were too dark for measurement with the spectrophotometer, the solutions could be diluted with 66% volume acid to the proper range with only a minimum error. Transmittance and Concentration. Calibration curves were determined for purified 2-nitropropane by plotting optical density against concentration. The compound gave a straightline calibration originating a t zero concentration and optical density. The color obeys Beer's law for amounts of 5 to 50 micrograms of 2-nitropropane. Interference. The compounds that would be expected to be present in a mixture resulting from the vapor phase nitration of propane gas were tested. 2-Nitropropane and 2-nitrobutane gave a positive reaction. Xitromethane, nitroethane, l-nitropropane, 1-nitrobutane, and 2,2-dinitropropane gave a negative reaction. BASIC PROCEDURE
Apparatus. Aeration apparatus (Figure 1). Spectrophotometer, Beckman Model DU. Test tubes, 25 X 200 mm., borosilicate glass, equipped with 24/40 glass stopper. Steam bath, any type. Cooling bath, any type. Flowmeter, Fischer Laboratory Model No. 11-163.
Determination of 2-Nitropropane in Air. Pass the air containing 2-nitropropane vapor through a train of three test tubes containing concentrated sulfuric acid. The acid will quantitatively remove the 2-nitropropane. Combine the acid from the three tubes and assay an aliquot containing 50 micrograms or less of 2-nitropropane by the above procedure. Determination of 2-Nitropropane in a Mixture of Nitroparaffins. Weigh a sample of a nitroparaffin mixture into a volumetric flask containing Concentrated sulfuric acid. After dilution to volume with more acid, assay an aliquot containing 50 micrograms or less by the above procedure. Determination of 1,l-Dichloro-1-nitroethaneon Grain. The halogenated primary nitroparaffins, where the halogens are substituted on the same carbon atom that contains the nitro group, have become secondary or tertiary compounds, depending upon the number of hydrogen atoms that have been replaced with halogen. All the compounds of this type that have been tested by this method reacted in a quantitative manner. This study was undertaken to determine the residue of 1,l-dichloro-lnitroethane (Ethide) in the grain after fumigation with a carbon tetrachloride solution.
Table IV.
Decomposition of 1,l-Dichloro-1-nitroethane at Room Temperature and 100" C.
Time, Minutes 0 4 8 12 16 20 24 28 32
Optical Density Room temp. 1000 c. 0.455 0.455 0.525 0.805 0.595 0.805 0.650 0.805 0.685 0.805 0.725 0.805 0.765 0.805 0.805 0.790 0.805 0.805
Guillemard and Labatt (9) determined chloropicrin (trichloronitromethane) by using a reaction based upon the oxidation of resorcinol in an alkaline medium. Machle, Scott, Treon, Heyroth, and Kitzmiller (62) adapted this technique to the analysis of 1,l-dichloro-1-nitroethane. Their method lacks the required sensitivity necessary for residue analysis of stored grain, and was found to be too timeconsuming for routine
V O L U M E 24, NO. 10, O C T O B E R 1 9 5 2 Table V.
Analysis of Known Nitroparaffin Mixtures for 2-Nitropropane
7% by Weight N P ' s in Mixtures NM
KE
1-NP
0.0 26.02 99.49 0.0 0.0
0.0 23.74 0.0 99.39 0.0
50.36 29.31 0.0 0.0 95.25
Table VI.
2 - N P Recovered,
Weight %
2-KP 49.64 20.93
49.60 20.90 0.52 0.61 4.74
0.51 0.61 4.75
2 - N P Recovered, y
2000 4000 50 500 5000 100
1950
Table VII.
Air samples of known concentration were analyzed by the above method. The sample of 2-nitropropane was dissolved in ethyl alcohol and aliquots were added to a cotton plug in tube A . Air was passed through the chamber by means of vacuum and the 2-nitropropane was trapped in a train containing concentrated sulfuric acid. Typical data are presented in Table VI. A carbon tetrachloride solution of 1,l-dichloro-1-nitroethane of known concentration was added to several samples of corn that had not been treated with any fumigants, and the 1,ldichloro-1-nitroethane content was determined. Typical data are presented in Table VII.
Analysis of Known Air Samples
2-NP Added, y
Sample, G.
1535
4020 51 495 4950 99
Recovery of Ethide from Corn Ethide Added,
y
Ethide Recovered, y 98.0 97.5 95.0 48.0 47.0 93.5 99.0
analysis. 1,l-Dichloio-1-nitroethanein so far as the nitro group is concerned, is a tertiary type of nitro compound; howevrr, i t reacts readily by the proposed method and offers a test procedure that distinguishes i t from other common grain and soil fumigants. The conditions found necessary for the 2-nitropropane analysis were substantiated in the experimental conditions for 1,ldichloro-1-nitroethane. The rmults of the decomposition to nitrous acid after addition of sulfuric acid are shown in Table 11'. It is demonstrated that 1,l-dichloro-1-nitroethanereacts more readily a t room temperature than did 2-nitropropane. The color complex formed with 1,l-dichloro-1-nitroethane follow Beer's law from 5 to 100 micrograms. A s 1,l-dichloro-1-nitroethane has a low vapor pressure, it may be extracted from grain by a simple aeration procedure. Figure 1 is a diagram of the aeration apparatus used. An all-glass apparatus is necessary, because rubber, cork, or metal is attacked by the 1,l-dichloro-I-nitroethane fumes. Weigh a suitable amount of corn (50 to 500 grams, depending upon the expected concentration of 1,l-dichloro-1-nitroethane) and place in the aeration tube, A . Connect the receiving tubes, each containing 10.0 ml. of concentrated sulfuric acid. Adjust the air pressure to give 1 liter of air per minute, as measured by the flowmeter. Allow air to pass through the corn for 15 minutes. Turn off the air and remove the receiving tube, TI. Replace with another T I tube containing 10 ml. of sulfuric acid. Repeat this procedure, changing tubes every 15 minutes until all 1,l-dichloro-1-nitroethanehas been removed. To ensure complete removal of 1,l-dichloro-1-nitroethane from corn, the tube A may be heated to 50' to 55' C. I t is convenient to develop the color complex in each TI tube as it is removed. By this means it is possible to follow the extraction of the 1,l-dichloro-l-nitroethane to completion. At the end of the aeration period remove the three T tubes from the train, develop the color as previously described, and determine the transmittancy. By reference to the calibration curve, calculate the amount of Ill-dichloro-1-nitroethane present on the corn. Nearly all of the 1,l-dichloro-1-nitroethanewill remain in the TI tubes. However, after it has all been removed, tubes Ta and T 3are also developed for the color complex. 4 n y 1,l-dichloro-lnitroethane found present is added to the values obtained from the TI tubes. RESULTS
Solutions of known 2-nitropropane (2-NP) content in mixtures of nitromethane (NJI), nitroethane (NE), and l-nitropropane (1-XP) were analyzed. Typical data are presented in Table V.
DISCUSSION
I n general, most secondary aliphatic nitroparaffins and most halogenated aliphatic nitroparaffins, with the halogen and the nitro group attached to the same carbon, whether secondary or tertiary, give the same reaction with resorcinol. A halogenated nitro compound, where the halogen and the nitro group are on different carbon atoms, reacts according to the nature of the nitro group. Thus, 1-chloro-2-nitroethane reacts negatively, and 1-chloro-1-nitroethane reacts positively. Aromatic halogenated nitroparaffins or aromatic nitroparaffins do not react HOT\-ever, aromatic hydrocarbons that have an aliphatic nitroparaffin or aliphatic halogenated nitroparaffin side chain do react, if they are soluble in the water acid mixture. The aliphatic compounds tested and their reactions were: Positive. 2-Sitropropane, 2-nitro-2-methylpropane. 2-nitrobutane, 1-chloro-1-nitrobutane, trichloronitromethane, l-chloro1-nitroethane, 1,l-dichloro-1-nitroethane, l-chloro-l-nitropropane, 2-chloro-2-nitropropane, 1,l-dichloro-1-nitropropane, monobromonitromethane, 1-bromo-1-nitroethane, l-bromo-l-nitropropane, 1-bromo-1-nitrobutane, 2-bromo-2-nitropropane, 2bromo-2-nitrobutane, 1,l-dibromo-1-nitroethane,1,l-dibromo1-nitropropane, 1,l-dibromo-1-nitrobutane, 2-nitr0-2-chloro-1,lbis(p-chlorophenyl)propane, 2-nitro-1,l-bis(p-chloropheny1)-propane, and 2-nitro-l,l-bis(p-chlorophenyI)butane. Negative. Sitromethane, nitroethane, 1-nitropropane, 1nitrobutane, 2,2-dinitropropane, l-nitro-2-chloroethane, 2-nitro2-bromo-l-methoxy-l-phenylpropane, p-nitrostyrene, I-(pnitrophenyl)-2-nitropropene, l-(p-chlorophenyl)-2-nitro-l-propanol, l-(p-chlorophenyl)-2-nitro-l-butanol,trihydroxymethylnitromethane and 2-nitro-2-bromo-1-phenylethylene. Hydroxybenzene reagents other than resorcinol may be used. Phenol, orcinol, pyrogallol, and pyrocatechol were tested and developed a red-blue color. AT-(l-naphthyl)-ethylenedian~ine dihydrochloride forms a red-blue color complex. Oxidizing and reducing agents must be absent, as pointed out by Eichler (7) in his study of the detection of nitrite, nitrate, and nitrosyl sulfuric acid. The method will quantitatively determine all secondary nitroparaffins and some tertiary nitroparaffins in amounts as low as 3 to 5 micrograms and will detect amounts as little as 1 microgram. The method has an accuracy of =t5% with a precision of *1y0. ACKNOWLEDGMENT
The authors wish to thank R. J. Cotton and Leonard Redlinger of the U. S. Department of iigriculture for their help in obtaining the samples of stored corn treated with 1,l-dichloro-lnitroethane fumigant, and Joseph Creedon and Charles Henderson who ran many of the chemical tests for the residue assays for 1,l-dichloro-1-nitroethaneand air assays for 2-nitropropane. LITERATURE CITED (1) Adler, R . , and ildler, O., Z . physiol. Chem., 41, 206 (1940). (2) Alvarez, E. P., Chem. S e w s , 91, 155 (1905). (3) Bose, P. K . , Analyst, 56, 504 (1931). (4) Castiglioni, A , , Gam. chim. ztaZ., 62, 1065 (1932). (5) Denigirs, G . , J . Pharm., 2 ( 6 ) , 289, 400 (1895). (6) Desvergenes, L . , A n n . chim. anal. chim. appl., 13, 321 (1931). (7) Eichler, H . , Z . and. Chent., 96, 17 (1934). ( 8 ) Ekkert, L . , P h a r m . Zentmlhalle, 66, 733 (1926). (9) Guillemard, H . , and Labatt, A., BzclZ.soc. pharm., Boidenuz, 1919. (10) Hass, H . B., Hodge, E. B.,and Vanderbilt, B. AI., I n d . E n @ Chem., 28, 339 (1936). (11) Hass, H . B., and Riley, E. F . , Chem. Revs., 32, 373 (1943). (12) Heller, H . , Chem.-Ztg., 47, 701 (1923). (13) Jones, L. R . , and Rlddick, J. A., ASAL.CHEM.,2 3 , 3 4 9 (1951). Phys. Chem. Soc., 2 7 , 4 5 3 (1895). (14) Konovalov, J., J . RUSS.
ANALYTICAL CHEMISTRY
1536 (15) Konovalov, M., Ber., 28, 1860 (1895). (16) Korenman, I. M.,2. anal. Chem., 93, 438 (1933). (17) Leffman, H., Am. J . Pharm., 95, 110 (1923). (18) Ibid., 96, 295 (1934). (19) Lieberman, Ber., 15, 1529 (1882). (20) Lindo, D., Chem. A-ews, 58, 1, 15, 28, 176 (1888). (21) Machle, IT, F., Scott, E. K., and Treon, J., J . Irid. Hug. Toricol., 22, 315 (1940). (22) Machle, V.F., Scott, E. W.,Treon, J., Heyroth, F., and Kitzmiller, K., Ibid., 27,95 (1945). (23) Mansoff, D. D., 2. Xahr.-Genussm., 27,469 (19141. (24) hleyer, V., andAmbuh1, G., Ber., 8, 751, 1073 (1875). ( 2 5 ) hIeyer, V., and Locher, J., Ibid., 8, 219 (1895). (26) Xovelli, A., AnnZes asoc. q u i m Argentina, 13, 13 (1925).
Xovelli, -4., Chirn. Ind.,744 (1926). Rodillon, G., J . Pharm. Chim.,26, 376 (1922). Ibid., 27, 64 (1922). Rodillon, G., Schweir. Apoth.-Ztg., 1923 160. Scott, E. W.,and Treon. J., ISD. EXG.C m x . .\SAL. ED.,12, 109 (1940). Siegle, L. W., and Ham, H. B., I n d . Eng. Chem.. 31, 648 (1939). Turba, F., Haul, R., and L-hlen, G., A n g e r . Chem., 61, 74 (1949). Vagi, S.,Z. anal. Chem., 66, 101 (1925). Vare, A. H., Chemist and Druggist, 123, 232 (19351. Wilson, J. H., Pharm. J., / 3 / , 20, 541.
Photometric Determination of Sulfide and Reducible Sulfur in Alkalies MAX S. BUDD AND HOW.4KD i. BEWICK The Solvay Process Division, Allied Chemical & Dye Corp., Syracitse, S.Y.
4 method for the determination of small quantities of sulfide and reducible sulfur compounds in alkalies was desired. N o methods applicable to carbonate samples or for the determination of sulfide alone were found in the literature. Methods applicable to all alkali products were developed for the determination of sulfide and reducible sulfur compounds. The hydrogen sulfide evolted on acidification, or by stannite and nascent h:drogen reduction, is passed into zinc acetate and converted to methylene blue bj reaction with p-aminodimethylaniline. The range of determination is 0.2 to 100 p.p.m. of sulfttr using a 3-gram sample. 4 relatively simple apparatus has been designed for the evolution of hydrogen sulfide, precipitation as zinc sulfide, and coniersion to methylene blue for photometric measurement. Losses due to oxidation of sulfide in alkali carbonate samples are minimized by pelleting such samples before acidification.
I
S THE analysis of alkali carbonates and hydio\ides a practi-
cal and rapid method was required for determining small amounts of sulfide sulfur and “reducible sulfur coiiipounds.” The latter term refers to compounds that are reducible to sulfide under the conditions of the test. A number of methods are to be found in the literature, but none \\-a3 directly applicable to the authors’ requirements. Johnson ( 7 j compares the iodometric and antimony tartrate colorimetric methods for sulfide in sewage The Association of Official Agricultural Chemists ( 3 ) gives an iodometric method for sulfide in mineral wateis. Scherer and Sweet (11j describe a method for trace amounts of reducible sulfur wherein the sample ie acidified and treated with aluminum strips, the evolved hydrogen sulfide forming a stain on lead acetate paper. The stain 1s matched against standard stains, Heinemann and Rahn (6) found incomplete recovery with aluminum reduction alone. They developed a method for total reducible sulfur in sodium hydroxide, reducing the sulfur to sulfide in t n o steps: alkaline reduction with sodium stannite and completed reduction with aluminum in acid medium. a f t e r reduction the evolved hydrogen sulfide was absorbed in ammoniacal cadmium chloride solution. The sulfide was titrated with iodate-iodide and sodium thiosulfate. Field and Oldach ( 5 ) absorbed hydrogen sulfide in gases in caustic soda solution and subsequently determined the sulfur turbidimetrically as bismuth sulfide. Ethrington et nl. ( 4 ) absorbed sulfide in gases in a n arsenite solution and determined the sulfide colorimetrically as arsenious sulfide in the presence of a protective colloid.
For trace determination of sulfide and ”reducible sulfur,” the methylene blue method appeared superior. Almy ( 1 ) applied the method to sulfide in foods, sweeping the hydrogen sulfide with carbon dioxide into 0.6% zinc acetate solution. K i t h the reagents Blmy used, the color development time was 2 hours. Pomeroy (9) studied the application of the method to sen age analysis, and developed the modification that is now the standard colorimetric procedure of the American Public Health Association (a). Enough ferric chloride is added to get complete color development in 1 minute. Subsequently ammonium phosphate is added to eliminate the ferric chloride color. Kosior (8 absorbed the sulfide present in natural gas in a solution of zinc acetate and sodium hydroxide, subsequently forming methylene blue. H e allowed 2 to 10 hours for full color development. Sands et a/. ( 1 0 ) determined low concentrations of hydrogen sulfide in gas by the methylene blue method, absorbing the sulfide in 2 5 zinc acetate solution. These authors conducted an intensive investigation and present a wealth of information on the method. J
The method developed involves the Heinemann and Rahn reduction of sulfur to sulfide, follo~?edby the formation of meth) 1ene blue in the zinc acetate absorbing solution. The principal findings of this work \yere:
It is absolutely necessary to sweep all osj-gen from the apparatus with an inert gas before releasing trace quantitieE of hydrogen sulfide-this has not been stressed by other workers. Use of a photoelectric instrument permits determination of smaller quantities of sulfide than by the visual method, R hich 13 limited probably by the interference of ferric chloride. Higher concentration of reagents accelerates formation of methylene blue color, resulting in complete development in 15 minutes. The photometric methylene blue method permits the deteimination of a low limit of 0.2 p.p.m. of sulfur in sodium hydroxide on a sample considerably smaller than that used in the volumetric method of Heinemann and Rahn. Proper conditions are established for determining reducible sulfur and sulfides in alkali carbonates. Heinemann and Rahn are confirmed in the use of stannous chloride as well as aluminum metal for the quantitative reduction of thiosulfate to sulfide. APPARATUS
All-glass reaction and absorption assembly, Figure 1. Photoelectric colorimeter or spectrophotometer, with 10-mm. absorption cells. Filter &I-660, obtainable from Photovolt Corp., Sew- I-ork, K.Y., or eauivalent filter, for use with filter colorimeter. Graduated 50-ml mixing cylinders. Press and die for making pellets of carbonate samples. The diameter of the die should be 0.5 inch, and the pressure exerted in
