ANALYTICAL CHEMISTRY
1238
Table 11. Comparison of Chromous and Winkler iMethod Values with Theoretical Values after Nitrite Has Been Destroyed by Potassium Permanganate"
Sample Temp., NO:-, KO, C. p.p.m. A 2 6 9 3 B 28.0 3
c
28.0
45
Xitrite Destruction Per Liter Sample HzSOI, KXInOl, (COOK)?. ml. nil. ml. 1 0 1.0 0.5 1.0 1.0 1.5
1.0
4.5
1.5
Cr(I1) Titration Per 50.23-1Il. Sample C r ( I I ) , IO3-, 02, ml. I$. 0.p.m. 1 079 0.7705 7 . 9 7 1.080 0.7705 7 . 9 8 1 . 0 8 8 0.7705 8 . 0 4 1 . 0 8 3 0.7705 8 . 0 0 Av. 8 . 0 1 1 . 0 8 9 0.7705 8 . 0 5 1 . 0 9 1 0.7705 8 . 0 7 1 . 0 9 5 0.7705 8 . 1 0 -41..8 . 0 7
TheoWinkler Method retical Per P.P.M. 0 200-hI1. Sample (8. 7 ) SpOs--, 02, nil. p.p.m. ... 8.08 7'83 8.02 7.92
7.81 7 82
8.09 8 10
2
7.92
8.10
0.04M potassium permanganate: 3 6 s sulfuric acid: 0.1M potassium oxalate; O.O4957.\f chroinium(I1) ; 0.7705
id.
of iodate = 0.0690 ml. of chromium(I1).
-
sults is 0.12%, and the deviation between the chromium(I1) method results and the saturation data ( 2 , 7 ) is 1.1%, while the deviation between the Winkler method results and the oxygen saturation value is 1.3%. In Sample C, a 45-p.p.m. sample, the average deviation betwern runs is again only 0.25%: it differs from the results of t h e Winkler method by 0.37% and from saturation value by 1.9%, while the Winkler method deviates from the saturation value by 2.2%. SUMM4RY
-4method for determining the amount of molecular oxygen dissolved in nitrite-containing waters makes use of acid-chromous reagent to react with the oxygen after the nitrites have been
---_
-
destroyed with permanganate and the excess permanganate h a s been d e s t r o y e d w i t h oxalate. This determination may be carried out quickly, giving reproducible results of an accuracy comparable with other available methods. Where no interfering nitrite ions were present, the authors found the accuracy of this acid-chromous procedure to exceed that of the Winkler analysis. I t uses fewer reagents than the Winkler method and ones which can be stored for long periods of time without change.
LITERATURE CITED
(1) Am. Public Health Assoc., New York, "Standard Methods for the Examination of Water and Sewage," 8th ed., p p . 139-54, 1936. (2) Fox, C.J. J., Trans. Faraday SOC.,5 , 68 (1909). (3) Rideal, S., and Stewart, C. G., Analyst, 26, 141-8 (1901). (4) Stone, H.W., ANAL.CHEM.,20, 747 (1948). (5) Stone, H.W., and Eichelberger, R. L., Ibzd., 23,868 (1951). (6) Theriault, E. J., Public Health Bull., No. 151 (1925). (7) Whipple, G.C.,and Whipple, M. C., J.Am. Chem. SOC.,33, 362 (1911). (8) Winkler, L.W.,Ber., 21, 2843-54 (1888). (9) Ibid ,22,1773(1889). (10) Ibzd., 34,1410 (1901). RECEIVED for review October 14, 1953. Accepted February 25, 1954.
Quantitative Estimation of Aromatic Nitro Compounds ENNO WOLTHUIS, STEPHEN KOLKL, and LUKE SCHAAPZ Chemistry Department, Calvin College, Grand Rapids, M i c h .
M
OST of the methode employed for the analysis of aromatic
nitro compounds involve a reduction to the corresponding amine, followed by a determination of the amount of reducing agent consumed. One of the oldest and most common methods uses standard titanous chloride, the excess of which is determined by titration with standard ferric ammonium sulfate solution. This is the method of Knecht and Hibbert ( 3 ) ,which has been widely used in the dyeing industry, especially in the estimation of azo and triphenylmethane pigments and dyes. Some of its limitations when applied to the determination of nitro compounds have been reported by Callan et al. (1,2 ) . More recently its application to nitro compounds has been described by Siggia ( 5 ) . With proper precautions this method gives fairly good results, but suffers from the limitation that an inert atmosphere is essential throughout the procedure. More recently, Vanderzee and Edge11 (7) have suggested a reduction with tin in alcoholic acid solution followed by a gravimetric determination of the amount of tin used. For several years, one of the authors (Wolthuis) has used still another method with considerable success. This method, first suggested by Callan, Henderson, and Strafford (1 ), and more recently applied to the determination of parathion by O'Keefe and Averell ( d ) , depends upon the amount of primary amine formed, as determined by volumetric diazotization. It is fairly common knowledge, particularly in the intermediates industry, that many aromatic amines can be estimated best by quantitative diazotization with standard nitrite solution. Experience has 1 2
Present address, Wolverine Finishes Corp., Grand Rapids, Mich. Present address, Northwestern University, E r a n s t o n , Ill.
proved this one of the most reliable and most rapid methods for the determination of amine purity when the identity of the amine is known, or for the estimation of the amine equivalent weight when the amine must be identified. In the study reported here, this reduction-diazotization procedure for aromatic nitro compounds has been investigated to determine its range of applicability to various types of such substances, to obtain an estimate of its reliability, and to evolve a procedure generally applicable to most nitro compounds ordinarily encountered. The same general method can be used for a rapid qualitative detection of an aromatic nitro compound, and an improved procedure is described. QUAhTITATIVE AXALYSIS O F AROMATIC NITRO COMPOUNDS BY DETERMINATION OF EQUIVALENT WEIGHT
Reagents. Zinc dust, technical grade. Sodium bromide, U.S.P. grade. Sodium nitrite, standardized 0.1N. Dissolve about 7 grams of sodium nitrite, U.S.P. grade or better, in distilled water and dilute to 1-liter volume. For standardization use either of two primary standards, p-nitroaniline or sulfanilic acid, the latter being preferred because it can be obtained in purer form. As purchased, sulfanilic acid is in the form of its monohydrate. Most reliable results are obtained if this material is dried for 3 hours a t 120" C. to remove water of hydration. Dissolve 0.6927 gram (0.004 mole) of the anhydrous acid in 100 ml. of water containing about 0.2 gram of sodium hydroxide. Add 20 ml. of concentrated hydrochloric acid, cool to 15" C., and titrate with the nitrite solution, using starch-iodide paper as outside indicator. At the end point a drop of the solution touched to the paper gives an immediate, faint blue spot. Also run a blank to the eame end point color intensity. A standardized 0.1N nitrite
V O L U M E 26, NO. 7, J U L Y 1 9 5 4
1239
solution has been found to be absolutely stable for a t least 11 months. 40 Normality = ml. of nitrite - blank Procedure. About 3 meq., ordinarily about 0.5 gram, of the substance are weighed accurately and transferred to a 250-ml. Erlenmeyer flask, to which are then added 25 ml. of glacial acetic acid. The substance is dissolved with warming if necessary. Then 10 ml. of concentrated hydrochloric acid and 25 ml. of water are also added, and the mixture is heated to near boiling. Five grams of zinc dust are added in several ortions a t such a rate that no material is lost through excessive gothing. A small reflux condenser is attached to the flask, and the mixture refluxed gently between 20 and 30 minutes to complete the reduction of the nitro grbup to the corresponding amine. If the color formed during the reaction fails to disappear, it is advisable to add a little extra zinc and continue refluxing a few minutes longer. While still hot, the unreacted zinc is removed by filtration through a small Buchner funnel, the flask is rinsed several times with small portions of hot water, and these washes are poured through the funnel to remove all amine from the residue. (Lum s of zinc should be broken with a glass rod before washing.) T i e filtrate and washings are transferred quantitatively to a beaker, 1 to 2 grams of sodium bromide are added, and the solution is cooled to about 15' to 20 O C. The final volume is usually about 200 to 250 ml. It should be distinctly acidic to Congo Red indicator. The solution is then titrated with 0.1.V sodium nitrite, previously standardized, using starch-iodide paper as outside indicator. At the end point a small drop of the reaction mixture, touched to the paper, forme a faint blue spot immediately. A blank titration should be run also, using the same volume of water acidified to Congo blue with hydrochloric acid. This blank usually does not exceed 2 to 3 drops for the volume used. CALCCLATION.
Equivalent weight
=
1000 X grams of sample ml. of nitrite X normality
Discussion. If the nitro compound is not soluble in the solvent mixture prescribed, it is permissible to use less water and more acetic acid to facilitate its reduction, although this is usually not necessary. If the amine crystallizes out of the solution during cooling prior to diazotization, it is recommended that more acetic acid be added to redissolve it. In both the reduction and diazotization one may exercise considerable liberty in varying the composition of the solvent to suit the properties of the substance being analyzed. A good diazotization requires that sufficient acid be present throughout to ensure a p H of 3 or less-that is, acidic to Congo Red indicator. Ordinarily diazotizations are carried out a t fairly low temperatures to avoid decomposition of the diazonium salt, However, for analytical purposes, where there is no interest in isolating the product of diazotization, a temperature of 15' to 20' C. suffices. The decomposition of the product has no appreciable effect upon the amount of nitrite consumed, as is evident from the following data on the titration of aniline a t various temperatures. Temperature. 3 10 17 22
O
C.
Aniline, Gram 0,3689 0.3630 0.3609 0.3482
Nitrite, (0.12065), MI. 31.4 31.9 31.85 30.5
Equivalent Weight 94.8 94.4 94.0 94.7
The presence of an alkali bromide during diazotization is desirable because it greatly accelerates the rate of diazotization as reported by Ueno and Sekiguchi (6). Generally speaking, a t least 0.2 gram of nitro compound should be used for analysis. Smaller samples can be used if necessary, together with a weaker nitrite standard such as O.OLV, but considerable experience is required to determine the end point with such a weak nitrite solution. Results. A variety of nitro compounds has been analyzed by this method from time to time, and the accumulated data are assembled in Table I. Wherever possible, the purest available compounds were used for analysis. In some cases only the technical grade was available.
Table I. Equivalent Weight Determination of Aromatic Nitro Compounds by Reduction-Diazotization Nitrite Sample, 0.110N, Grams MI. 0 3136 23.03 0.4284 28.25 0.4885 32.18 o-Ni trotoluene 0.5109 33.70 o-Kitrochlorobenzene" 0 , 4 4 7 7 25.25 0.5489 31.21 m - S i t r ~ c h l o r o b e n z e n e ~ 0.5874 33.43 0.4609 25.54 2-Chloro-6-nitrotoluene 0.6212 27.51 p-Xi trophenol 0.4158 26.59 0.5376 34.35 p-N itroanisole 0 5497 32.22 0.4864 28.43 p-Nitrobenzoyl chloride 0.5135 25.68 0.6073 30.10 3-Sitro-4-chlorobenzene 0.8921 28.40 sulfonic acid, K salt 0.9270 29,48 2-Chloro-&nitrobenzene 1.3619 46.99 sulfonicacid, N a s a l t 0.8747 30.10 2-Nitrotoluene-4-sulfonic 0,9573 33.16 acid, K salt 0.7252 25.48 Z-Amino-4'-nitrobiphenyl 0,3018 25.50 4,4'-Dinitrocarbanilide 0,3022 18.50 Technical grade. Compound Nitrobenzenea p-Nitrotoluene
Equivalent Weight Calcd. Theory 1 2 2 . 6 123 I 1 136.6 137.13 136.8 136.6 137.13 159.7 157.56 158.4 158.2 I 5 7 , 5 6 159.0 170.7 1 7 1 , 7 4 1 4 0 . 9 . 139.11 141.0 153.7 153.13 154.1 1 8 0 . 1 185.57 181.8 283.0 275,7 283.3 261.1 259.5 261.9 260.0 255 256.4 1 0 6 . 6 107 148 5 151 . I
Average Error, 0.41 0.31 0.39 0.96 0 . ($6
0.61 1.32 0.50 2.49 2.70 0.77 1.25 0.37 1.72
Mononitro compounds generally give good results, certainly comparable to those obtained by other methods for the determination of equivalent or molecular weights, and usually better. The only exception encountered is o-iodonitrobenzene. In its reduction by the method prescribed, the iodine was reduced to iodide which, in the subsequent nitrite titration, was oxidized to free iodine and so immediately reacted with the starch of the indicator paper, obscuring the end point. Presumably p-iodonitrobenzene would react similarly because of the activation of the halogen by the nitro group. Dinitro compounds, in which both nitro groups are in the same aromatic ring, are not readily determined by this method, since their reduction products, the phenylenediamines, react incompletely with the nitrite or are subject to side reactions. +Diamines react partly to form triazoles. m-Diamines undergo the familiar Bismarck Brown reaction, obscuring the end point. Some p-diamines can be diazotized nearly quantitatively but only in the presence of a large excess of nitrous acid, making a titration as described impossible. Any nitroamines containing the primary amino group would be titrated directly without prior reduction. In these cases it should be remembered that negatively substituted amines, such as the nitroamines, very readily form insoluble diazoamino compounds during diazotization, and it is necessary to increase the finera1 acid concentration and to titrate with nitrite as rapidly as possible a t a lower temperature than usual. QUALITATIVE T E S T FOR AROMATIC NITRO COMPOUNDS
In the characterization of organic compounds, the ferrous hydroxide test and/or the zinc and ammonium chloride reduction follom-ed by Tollens reagent, are usually employed to test for the nitro group. Each is valuable, but has its limitations, and neither is specific for the aromatic nitro group. For many years the test described has been used with excellent results. It is specific for the aromatic, as distinguished from the aliphatic, nitro group and is simple to perform. Procedure. A drop or two of the liquid, or a comparable amount of solid, is dissolved in 3 ml. or more of glacial acetic acid, with warming if necessary. One milliliter of concentrated hydrochloric acid and 3 ml. of water are then added. About 1 gram of zinc dust is added carefully, and the mixture boiled for about 5 minutes, usually until it is colorless or nearly so. It is
-
1240
ANALYTICAL CHEMISTRY
filtered to remove unreacted zinc, the filtrate is cooled to 10” to 20” C., and 3 to 5 d r o p of 0.1N sodium nitrite are added with shaking. If desired, a test with starch-iodide paper may be made while the nitrite is added to see if the solution absorbs nitrite. After shaking well, the solution is spotted on a piece of filter paper alongside a spot of R-salt solution. A color, usually reddish, forming where the spots meet, is an indication of coupling, and therefore that the original substance is very likely an aromatic nitro compound, or, much leas frequently, a reduction derivative of such a nitro compound, such as a nitroso, azo, azoxy, hydrazo, or similar substance.
Discussion. All mononitro compounds encountered have given a good positive test by this procedure. Polynitro compounds are reduced and consume nitrite, but often fail to give the typical coupling test because of side reactions during diazotization. The spot test on filter paper is more sensitive than the usual test wherein the diazonium solution is mixed with a solution of a coupling agent. The white background of the paper makes it possible to detect as little as a few milligrams of amine. The use of R-salt is preferred to a solution of %naphthol, which Is sometimes used because such a solution is stable for a long time and generally gives azo colors of greater intensity. R-salt is
the sodium salt of 2-naphthol-3,6-disulfonic acid, and couples, like %naphthol, in the 1- position. A good R-salt test solution contains about 2% R-salt in a solution which is 5% with respect to sodium carbonate. The same diazotization-coupling test can be applied to identify any primary aromatic amine with certainty. LITERATURE CITED
(1) Callan, T.,and Henderson, J. A. R., J. SOC.Chem. Ind. (London). 41T, 157-61 (1922). (2) Callan, T., Henderson, J. A. R., and Strafford, N., Ibid., 39T, 86-8 (1920). (3) Knecht, E.,and Hibbert, E., “New Reduction Methods in Volumetric Analysis,” 2nd ed., London, Longmans, Green and Co., 1928. (4) O’Keefe,K., and Averell, P. R., ANAL.CHEM., 23, 1167-9 (1951).
(5) Siggia, S., “Quantitative Organic Analysis via Functional Groups,” pp. 824,New York, John Wiley & Sons, 1949. (6) Ueno, S.,and Sekiguchi, H., J. Soc. Chem. Ind. Japan,37B, 23.56 (1934). (7) Vanderzee, C.F., and Edgell, W. F., ANAL.CHEM.,22,572(1950). RECJEIVED for review August 20, 1953. Accepted April 23, 1954
Ultraviolet Absorption Spectra of Some Inorganic Ions In Aqueous Solutions R.
P. BUCK, SAMANG SINGHADEJA’,
and L.
B. ROGERS
Department o i Chemistry and Laboratory for Nuclear Science, Massachusetts lnstitute o i Technology, Cambridge 39, Mass.
A
LTHOUGH ultraviolet measurements in the range 210 to 400 m r are universally accepted for characterization and analysis of organic compounds, few investigations (6) have been directed toward inorganic compounds until very recently. Little information of analytical value is available about simple or complex ion spectra in general, though some studies may be found in the reviews of Rosenbaum (9). For that reason, a systematic survey has been made of a number of common anions which might absorb in the ultraviolet either by themselves or in the form of complexes with metal cations: sulfate, thiosulfate, peroxydisulfate, sulfite, nitrate, nitrite, chlorate, perchlorate, bromate, iodate, metaperiodate, vanadate, tungstate, molybdate, cyanid?, cyanate, thiocyanate, borate, phosphate, phosphite, hypophosphite, and pyrophosphate. In addition, the organic ions, acetate, citrate, tartrate, and oxalate, were studied because they are frequently encountered in analytical procedures. After most of the anions had been found to have low general absorption without characteristic absorption maxima, spectra of 15 metal and metal-containing ions, a t a level of approximately 10 p.p.m., were determined in tartrate, citrate, pyrophosphate, phosphoric. acid, and cyanide media wherever possible: zinc( 11), cobalt(II), cadmium(II), mercury(II), copper(II), nickel(II), manganese(II), lead(II), thallium(I), silver(I), indium(III), gallium(III), iron(III), thorium(IV), and uranyl. Arsenite, molybdate, tungstate, and vanadate were investigated in phosphoric acid and pyrophosphate media. Sulfuric acid medium was not examined because it had already been studied by Bastian ( 2 ) ,while hydrochlorir acid has been examined in this laboratory ( 4 , 8).
in preparing solutions of the anions with the exceptions of p h o s phoric and boric acids and ammonium metavanadate. Approximately 0.1Jf stock solutions of the cations as perchlorates were usually prepared by twice evaporating the nitrate with excesa perchloric acid before diluting to volume. Stock solutions of thallium(1) and uranium(V1) were made from the acetates by gentle evaporation with perchloric acid on a hot plate. In these two cases. the acetate may not have been completely
I SODIUM NITRITE
U SODIUM NITRATE
IU
SODIUM THIOSULFATE SODIUM PERSULFATE Y POTASSIUM BROMATE p1 POTASSIUM IODATE YU SODIUM SULFITE WII POTASSIUM CHLORATE IX SODIUM SULFATE Lp
6-
REAGENTS AND SOLUTIONS
Reagent grade chemicals and distilled water were used throughout this investigation. Sodium or potassium salts were used 1 Present addresa, Department of Science, Vatana College, Bangkok, Thailand,
220
240
260
280
300
WAVE LENGTH, M r
Figure 1.
Ultraviolet Absorption Spectra of
Aqueous Solutions of Simple Inorganic Salts
