ANALYTICAL CHEMISTRY
1236 curves, considerable accuracy has been achieved in calculating the biphenyl content of solutiom containing known amounts of biphenyl dissolved in a variety of orange oils. The measurement of the absorption peak is arbitrary but convenient. It is done using a millimeter rule and measuring the vertical h t a n c e between the parallel lines shown in Figure 2. A new standard curve must be prepared for each group of samples because Of variations in the machine’s response with time* Using a 0.4mm. spacer, analyses are accurate within the range of about
0.08 to 0.8% biphenyl. Samples more concentrated than 0.8% may be diluted with d-limonene. LITERATURE CITED
(1) hsoc. offic.Agr. Chemists, Washington, D. C., “Methods of Analysis,” 7th ed., p. 479,1950.
(2) Knodel, L. R., and Elvin, E. J., ANAL.CHEM.,24, 1824 (1952). (3) Steyn, A. P., and Rosselet, F., Analyst, 74,89-95 (1949). (4) Tomkins, R. G., and Isherwood, F. A., Ibid., 70,330-5 (1945). R s c s ~ vfor s ~review December 2, 1953, Accepted March 5, 1954, Agricultural Experiment Station Journal Series NO.230.
Florida
Quantitative Determination of Dissolved Oxygen in Nitrite-Containing Water Using Acid-Chromous Reagent HOSMER W. STONE and PAUL SIGAL University o f California, Los Angeles, Calif.
I
N A paper by Stone and Eichelberger (6) on the determination of dissolved oxygen in aqueous solution a method was developed for quantitatively determining dissolved oxygen by titration with a standard solution of acid-chromous reagent. However, this method did not give accurate results when nitrites were present in the water sample. The purpose of this research waa to investigate a suggested modification of the original method, involving first the determination of the total number of equive lenta of chromous solution necassary to titrate the nitrite and oxygen in an aliquot portion of the sample, and then the determination of the equivalents necessary to titrate the nitrite in another aliquot portion from which the oxygen had been removed by boiling. From these data the dissolved molecular oxygen could be determined by dgerence. If this suggested modification proved unworkable, a method was to be investigated of determining the dissolved oxygen directly after first destroying the nitrites in the water sample and then determining the oxygen. This nitrite destruction method is essentially that suggested by Rideal and Stewart ( 3 ) . It involvea the use of permanganate to destroy the nitrite and then oxalic acid to destroy the e x c w of permanganate. This is followed by the use of the manganous sulfate, alkaline-iodide Winkler method ( 6 , 8, 9, IO) of determining dissolved oxygen in the water sample.
chromium(I1) reagent was increased to 0.05M in chromium and in 0.1M hydrochloric acid. Use of the iodate storage buret ( 4 ) was eliminated, and instead the iodate was dispensed from a plunger-type pipet with a known volume of less than 1 ml. The 0.0035N iodate solution was kept in an ordinary glass-stoppered bottle. This meant that the iodate solution was nearly saturated with dissolved oxygen. However, when the iodate was standardized against the chromium(I1) reagent and used in the titration of a 50-ml. aliquot of water sample, the error introduced by fluctuations in the oxygen content of the iodate solution did not exceed O.l%, which is less than the random error.
M
EXPERIMENTAL
In the acid-chromous method employed by Stone and Eichelberger (6) carbon dioxide, free from oxygen, is run through the titration flask at a rate of approximately 400 ml. per minute for 3 minutes before the titration is started and a t the same rate during the titration. An estimated excess of 0.02M chromium(11)reagent is run into the titration flask from the storage buret (4). The volume necessary to react with the oxygen of the sample and yield an excess is determined by B reliiinary titration. A 5ml. aliquot of the water sample is t i e n added to the excess of chromium(I1) reagent. The excess chromium(I1) reagent reacts with an excess of 0.0035N potassium iodate dispensed from a storage buret. A crystal of potassium iodide is added and the iodine equivalent to the excess iodate titrated with the chromium(I1) reagent using starch as the indicator. The total equivalents of chromium(I1) less the equivalents of iodate equal the number of equivalents of dissolved oxygen in the aliquot. During the preliminary work, it was found that an end-point correction was necessary. This correction increased with an increaae in the concentration of the starch in the reaction mixture. This concentration was necessarily high because of the small total volume of reactants. This factor and the many mechanical difficulties caused by the small size of the aliquot led to the introduction of a larger scale in the method. The volume of the sample waa increased to 50 ml. and the concentration of the
‘
I
1
S l i r r i n a bo,
Magnetic s t i r r e r
Figure 1. Diagram of Titration Flask Trap on titration flask prevents diffusion of air into titration vessel. Tube next to capillary tip is used t o introduce indicator reagent; it is stoppered at all other times
The grease used on the stopcocks of the chromium(I1) reagent storage buret created a problem by “creeping” into the Zml. microburet bore. This difficulty was overcome by substituting Teflon-coated stopcock plugs with pressure adaptors. REVISED PROCEDURE
Oxygen-free nitrogen is passed through the titration flask (Figure 1) a t the rate of approximately 400 ml. per minute for 3 minutes before the titration is started, and a t the same rate during the titration. After the 3-minute deaeration period, an estimated excess, determined by a preliminary titration, of 0.05M chromium(I1) reagent is run into the flask. A 50-ml. aliquot
V O L U M E 26, NO. 7, J U L Y 1 9 5 4
1237
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portion of the sample (Figure 2) i~ then run into the excess Table I. Determination of Dissolved Oxygen by Difference Method" of chromium(I1) reagent. A Before Boiling 0.75-ml. portion of standard Sample (NOS- + On), M1. 1 0 1 - , M1. Cr (II), M1.b DiB., M1. Cr(II)c potassium iodate solution is 50.23 0.7705 1.231 1.162 dispensed, from the plunger50.23 0.7705 1.229 1.160 t e pipet, into the titrating 50.23 0.7705 1.223 1.154 This amount of potasAv. 1.159 eium iodate is sufficient to 0.7705 ml. 1 0 1 0.0690 ml. Cr(I1) react with all of the unused After Boiling c h r o m i u m ( 1 1 ) reagent and Apparent leave some iodate in excess. Amt. of Solution A crystal of potassium iodide sample (NO*-), MI. Boiled Off 1 0 1 - , MI. Cr(II), M1. Diff. 1.159-DS. Is introduced into the flask 1.541 0.375 0.237 0.922 7.28 50.23 1/5 0.210 0.072 1.541 1.087 8.66 along with 0.3 ml. of 0.5% 50.23 1 /3 0.198 0.060 1.099 8.68 1.541 starch indicator solution. The 50.23 1/2 titration is then resumed with Theoretical p.p.m. 0% 7.92 (3,7) the chromium(I1) solution to 0 Water contained 3 p.p.m. of nitrite and was saturated with oxygen at 28.0' C . the first disappearance of the b Total milliliters of ohromium(I1) re uired by oxygen, potassium iodate, and sodium nitrite. blue color, and the total volume C Milliliters of chromium(I1) requiredgy sodium nitrite and oxygen. of chromous reagent used is recorded. The total number of equivalents of chromous less the equivalents of iodate dispensed is equal t o the number of equivalents of oxygen in that it was due to the fact that the nitrites existed as nitrous the aliquot. acid in the boiling solution and that oxides of nitrogen were lost during the boiling process. The extent of the decomposition DIFFERENCE METHOD appeared t o depend upon the temperature and length of time of heating. For the difference method ( 5 ) , samples were prepared by The data obtained in this laboratory seem to substantiate this serating distilled water containing 3 p.p.rn. of nitrite a t constant theory and so prove the method impractical. In Table I it is temperature for 4 hours and then allowing the sample to equiliseen that when one fifth of the solution had boiled away, the apbrate for 1hour. This equilibration period was found necessary parent oxygen content was 7.28 p.p.m. When one third of because the oxygen content would fall off for about a half hour the solution had boiled away, the apparent oxygen content and then reach a steady value ( 5 ) . increased to 8.56 p.p.m., and when one half had been boiled away, the apparent oxygen content increased still further to 8.68 p.p.m. The oxygen content of a saturated solution a t this temperature is 7.92 p.p.m. ( 2 , 7). When one fifth had been boiled away, it appeared that the oxygen was not completely driven off, and when one third had been boiled away, the high apparent oxygen content may be explained by assuming that some of the nitrite had also decomposed. This decomposition would result in less chromous reagent being necessary for titration. The milliliters of chromium(I1) reagent necessary to titrate both the nitrite and the oxygen before boiling, 1.159 ml., less the amount of chromium(I1) reagent necesaary to titrate after boiling would then be too large and would lead to a value of the oxygen content which is too high.
Gk.
-
26%;
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PERMAYGAYATE METHOD OF DESTROYING NITRITES
Figure 2. Diagram of Sample Pipet Nitrite destruction procedure i s carried out in sample pipet. Any selected volume may be dispensed by using a pipet with a delivery bulb of the desired volume
In this method ( 1 , 3 , 6 ) ,the samples were saturated with oxygen as described previously. The nitrite in aerated samples containing 3 and 45 p.p.m. nitrite, respectively, was destroyed by adding to 1 liter of sample 1 ml. of concentrated sulfuric acid and then 0.04M potassium permanganate in 0.5-ml. portions until the permanganate color persisted. It was found that 5 minutes waa a sufFicient period of contract to destroy the nitrite in 3 to 45 p.p.m. nitrite samples. The excess of permanganate was then destroyed by adding 0.1M oxalic acid in 0.5-ml. portions until the color disappeared. Fifteen minutes was the longest period of time required to decolorize the solution providing a sufficient amount of oxalate was present. Excesses up to 1.5 ml. of 0.1M oxalate apparently had no effect upon the subsequent titration (8).
Results consistent to 0.3% were obtained upon titrating aliquot portions of an aerated sample containing 3 p.p.m. of nitrite, indicating a quantitative reaction between the nitrite and chromium(I1) reagent. However, it was found upon boiling other aliquots of the same sample and titrating that the amount of the chromium(I1) reagent required varied inversely with the volume of solution that had been boiled off. This would cause the calculated concentration of dissolved oxygen to be high. Stone and Eichelberger ( 5 ) also found this error and concluded
Determinations by the Winkler method were run on aliquots of the same samples for the purpose of checking the chromium(I1) method against a method which is commonly used, with the results shown in Table 11. These data were also checked against physical data for saturation of water with dissolved oxygen a t the specified temperature ( 2 , 7). In Table 11, Sample B, the average deviation of the three runs is 0.25%, which is less than experimental error. The deviation between the chromium(I1) method and the Kinkler method re-
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 7.97 1.080 0.7705 7 . 9 8 1.088 0.7705 8 . 0 4 1.083 0.7705 8 . 0 0 Av. 8 . 0 1 1.089 0.7705 8 . 0 5 1.091 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).
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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
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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, G r a n d 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, Eranston, 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
