Colorimetric Determination of Nitromethane in the Presence of Other Nitroparaffins LAWRENCE R. JONES and JOHN A. RlDDlCK Commercial Solvents Corp., Terre Haute, Ind.
Ln a specific method for determining nitromethane in the presence of other nitroparaffins sodium 1'2-naph~hoquinone-4-sulfonatereacts with nitromethane in an alkaline solution to yield a tiolet complex. This complex, when extracted with isoaniyl alcohol (3tiiethyl-1-butanol) can he quantitatikely measured at 585 mp. The complex follows Beer's law in the range of .ito 30 y of nitromethane. The method has an accuracj \rithin +2% and a precision within zkly'. 4 specific method for separating nitromethane from other nitroparaffins depends upon an azeotropic distillation with methanol. This separation step effectively extends the lower limits of the colorimetric procedure s o that as little as 1 part of nitromethane in 10,000 parts of other nitroparaffin ma? he quantitativelj determined.
T
HE quantitative determination of any one member of the nitroparaffin series in the presence of the others is difficult 1)rcaure of closely related chemical properties. However, nitromrthane, the simplest member, exhibits a few unique and charxteristic color reactions. Desvcrgencs (.3) mr:isured the yelloxv color formed with alkacl tfic. Grips-Ilosvay reactants to determine the off !)y alkaline degradation. m t l 11eyer and Imaiier (8 I formed nitrolic acids n i t h nitrous acid. These tests apply to nitromethane a+ ire11 as all other primary nitroparaffins. 1Iunzoff ( 4 1 dcscriheci a specific test for nitromethane wing vanillin :inti aniinoni;i as reagents, but AIachle, Scot?, xiid Treon ( , i found ) that t!ie test diil not follow Beer's luw antl had a limited u-e. 1Ie>.cr : i d .Xinhuhl ( ? ) coupled diazonium salts with the alkali mlts of t h r iiitropiidfins to obtain highl?. colored phen?-lhydrazones of nitro:iltlehj-des. Turba, Haul, itlid Uhlen ( 1 0 ) developed the diazonium reaction into a quantitative method ior ;dl primar). nitroparaffins. This reartion \vas more specific 101' nitromet!i:rne i \ - h ~ n a buffer was used. but the other primary nitropmitfins interfered appreeia1)ly \Then present in high conwntr:itions. Recently, T ~ i r hHaul. , antl Uhlrn ( I O ) presented :i qiialitativcx trnt for the tr:icr detection of nitromrthane. They fount1 that nitromethanc. contlerise- x i t h sodium 1.2-ii,iphtho(i"inoiie-~siili'onat(~i!: a n ;ilkaline solution to give a violrt colored complex. In the prescnt n-ork this complex \vas found to be suitable for cjunntitative measurement a t 585 mp when estracted from the re:wtion mixture \\-ith imamyl alcohol. The color develops
rapidly, is stable for 1 hour in the solvent, and f'ollom Beer's law in the range of 5 to 30 y of nitromethane. The reaction is 3pecific for nitromethane. BASIC PROCEDURE
Apparatus. Spectrophotometer, Beckman Model DC with 1-cm. Corex cells. Centrifuge, any laboratory type. Flowmeter, Fisher Lahoratory Model 11-163 or equivalent. Aeration apparatus ( 4 ) . Distillation column, Penn State-type column 16 mm. in inside diameter and 150 cm. long, packed with 3/10-inch single-turn glass helices. Wound with 182 turns of S o . 22 Xichrome \Tire. Reagents. Sitromethane standard, 99.99yo mole, by fieezing r..... i i r w f.9)
hlethanol, anhydrous. Phosphate Buffer, ILHP04, 20 zk 0.5% aqueous solution of dibasic aotaasium uhosDliatc adjusted to DH9.50 with a saturated solution of tripotkssium phospgate. Isoamyl alcohol, purified by fractional distillation and saturated with phosphate buffer, pH 9.50. Sodium 1,2-naphthoquinone-i-sulfonate, Eastman Kodak S o . 1372, O.lOyo aqueous solution. This solution is not stable, and is prepared fresh daily. Preparation of Calibration Curve. Preuare a solution of nitromethane standard in water to contain l.0b mg. per ml. Transfer 0.0-, 0.50-, 1.00-, 2.00-, and 3.00-nil. portions to separate 100-ml. volumetric flasks and dilute each to volume \Tith water. Transfer 1.00 nil. of each dilution into a test t,ube. Add 10.0 nil. of the buffer and 1.0 ml. of the color reagent and mix, -4llovi the solution to stand a t room temperature for 15 minutes. .4dd 5.0 ml. of t,he isoam?.l alcohol and mis. Separate the phases by centrifuging. Transfer a portion of the upper isoamyl layer to a 1-em. Corex cell and read the absorbance at 585 m p , using the solution containing 0.0 ml. of standard as the blank. Plot concentration against absorbance on linear graph paper. The above standards equal 0. 5, 10, 20, and 30 y of nitromethane, respec t i d y , Determination. The determination is the same as the calibration, except that the sample is prepared so that a 1.00-ml. aliquot contains no more than 30 y of nitromethane. A blank is prepared from 1.00 ml. of water treated with the same reagents as the sample. Interference. Several additional aliphatic nitroparaffins n ere tested to determine the specificity of the color reartion: nitroethane, 1-nitropropane, 2-nitropropane, 1-nitrobutane, 2-nitrobutane, l-nitro-2-methylpropane, 2-nitro-2-methylpropane, and 2,2-dinitrq~ropane. Only nitromethane gave a positive reaction. Applications. Although other nitrocompounds do not yield a colored complex, excessive amounts of primary nitroparaffin initroethnne, 1-nitropropane, 1-nitrobutane, and l-nitro-2-
Table I. Effect of Nitroparaffins on Determination of Nitromethane Nitroethane
1-Nitropropane
19.50 21.30 11.65 24,90 98 90
13.40 20.65 9.82 25 02
... ...
...
...
...
99:05 ,..
...
Composition of Mixtures, Wt. % I-Nitro%Nitro2-Iiitrobutane propane butane 14.55 16.10 15.20
... ...
9 s : 60
... ...
22.35 19.00 21.05 23.95
... ...
98: 94
...
20.15 16.60 18.63
... ... ...
...
99: 30
1493
Nitromethane 10.05 6.35 23.65 26.10
1,lO
0.95 1.40 1 06 0.70
Sitromethane Found, Wt. % 10.00 6.32 23.67 26.08 0.34 0.41 0.69 1.05
0.iO
Difference, Wt. % -0.05 -0.03 +0.02
-0.02 -0.76 -0.54 -0.71 -0.01 0.00
1494
ANALYTICAL CHEMISTRY Table 11.
The maximum a t 585 mp was the stronger and was chosen for use. TRASSMITTANCY AKD COSCENTRATIOS.Calibration curves were determined for nitromethane by plotting absorbance against concentration. A straight-line calibration curve, originating at zero concentration and absorbance, is obtained for the range of 5 to 30 y of nitromethane. TIME. ~ N DTEMPERATURE OF REKTIOX. The colored complex was prepared and spectrophotometer readings were made intermittently to determine the time required for complete color development. A time of 15 minutes a t room temperature was chosen as optimum. COLORSTABILITY.The absorbance of the colored complex m-as determined a t various time intervals (Table 111). The color was stable for only 6 minutes in the buffer solution after the 15-minute development period. However, it \>-as stable for 1 hour after extraction into isoamyl alcohol. PH OF REACTIOX.The color development was determined in several phosphate buffer solutions of varying pH. The buffers \yere prepared from a stock 20% dipotassium phosphate solution and adjusted to the desired p H with either 85% phosphoric acid or a saturated aqueous solution of tripotassium phosphate. Ten milliliters of buffer solution were used and the colors estracted with isoamyl alcohol. The optimum buffer solution, as indicated in Table IV, had a p H of 9.50 STABILITY A N D COYCESTR~TION OF REIGEYT. The stability and concentration of the sodium 1,2-naphthoquinone-lsulfonate are critical. Aqueous solutions of the reagent are not stable and must be prepared fresh daily to obtain reproducible results. The color development between nitromethane and the reagent was determined a t different levels of reagent concentration (Table V). Excess reagent inhibits the color development. A 1.0-ml. aliquot of a 0.10% aqueous solution of sodium 1,2-naphthoquinone-4-sulfonate was chosen as optimum.
Determination of Kitromethane in Air
Calcd., y 1000 500 100 50
Found, 995 500 99 51
y
Table 111. Stability of Colored Complex in Isoamyl Alcohol Time, Min. 0 15 30
10 Y 0.340 0,340 0.340 0.339 0.335
$? tJ
Table IV.
Absorbance of Nitromethane 20 7 0.680 0.680 0.679 0.680 0.676
30 Y 1.020 1,020 1.020 1.020 1.115
Effect of pH on Color Development Absorbance, 20 y i n < 0.350 0.615 0.680 0.670 0 640
PH 8.50 9.00 9.50 10.00 10.50
methylpropane) inhibit the color formation betlyeen nitro(Table I). methane and sodium 1,2-naphthoquinone-4-sulfonate \Then the nitromethane content is below 5% vieight, it must be separated before analysis. Determination of Nitromethane. IN PROCESS SAMPLES. I n the preparation of derivatives from nitromethane where relatively or better) nitromethane is the starting material, no pure interference is encountered due t o the presence of other primsrj. nitroparaffins.
(Moo
AZEOTROPIC DISTILLATION
ITeigh the sample into a volumetric flask containing water, neutralize, and dilute to volume with water. From this solution, prepare a diluted sample such that a 1.00-ml. aliquot will contain 30 y or less of nitromethane, and analyze. INAIR SA~IPLES. Pass the air containing nitromethane vapor through the aeration train with each tube containing 20 ml. of phosphate buffer. Use a flow rate of 0.2 liter per minute. .4t the end of the sampling period, combine the buffer solutions from the three tubes into a volumetric flask, dilute to volume with water, and analyze a 1.0-ml. aliquot containing 30 y or less of nitromethane.
The determination of small amounts of nitromethane in mistures of other primary nitroparaffins is subject to interference (Table I).
Table V.
Effect of Reagent Concentration
Concentration, G.1’100 M1.
Absorbance, 20 y X M
0.05 0.10 0.20 0.50 1.00
Sitromethane determinations in known concentrations of air samples are presented in Table 11. Effect of Variables. The influence of several variables on the color format,ion and quantitative applications of the reaction was investigated: ahsorption curve, Beer’s law, stability and intensity of color, time and t,emperature of reaction, concentration of reagente, pH of reaction, and azeotropic distillation. ABSORPTIOSSPECTRUM.The absorbance curve of the violet complex of nitromethane and sodium 1,2-naphthoquinone-4aiilfonate shove maximum absorbances a t 387 and 585 mp.
Table VI.
0 0 0 0 0
20.00 12.63 ZKGO ~~
...
1 00
1.60
I-Nitrobutane 6.46 10.18 32.89
... ... ...
2-Sitropropane 20.06 17.02
...
%Nitrobutane 10.02 7.80
50-MI. Distillate,
1.0 3.0 10.0 100.0
1 00; 1.01 3 00; 3.00 10 00; 10 03
...
I-Sitro2-methylpropane 10 08 10.92 2.69 ,.
2 12 4 40
..
515 385
Azeotropic Separation of Nitromethane
Composition of Mixtures, Weight % I-iVitropropane
672
Added t o Distillation Flask. Mg.
Found,, Mg.
99 0 0 ; 99.80
Table VII. Determination of Nitromethane Using Azeotropic Separation Sitroethane 33,67 41.42 37.81 99.50 96.40 93.00
680 680
Nitromethane 0.010 0.030 0.010 0.500 0.480 1.000
Nit romethane Found, Wt. Yo 0.010 0.030 0.010 0 490 0 480 0.990
Difference,
Wt.
70
0.000 0,000 0.000 -0.010
0.000 -0.010
V O L U M E 2 8 , NO. 9, S E P T E M B E R 1 9 5 6
1495
Desseigne and Belliot ( 2 ) found that nitromethane formed an ;ueotrope viith methanol containing 12.5 weight % nitromethane :md boiling a t 64.55' C. This provides a convenient method for separating nitromethane, as the higher nitroparaffins do not form azeotropes with methanol. The separation of nitromethane from other nitroparaffins, by nzeotropic distillation ivith methanol, ivas studied. Tests were made to establish the type and length of column needed, the most suitable rate of distillate removal, and the volume of distillate necessary to remove nitromethane quantitatively in the range of 1 to 100 mg. Removing 50 ml. of distillate with the described column a t a 30 to 1 reflux ratio gave good results. Typical data on the recovery of nitromethane in 50 ml. of distillate are presented in Table VI. Columns with shorter packed sections or increased take-off rates were not satisfactory. Determination of Nitromethane Using Azeotropic Distillation. Keigh 1 to 2 grams of the sample into a 500-ml. round-bottomed distillation flask, add 200 ml. of dry methanol, and attach to the fractionating column. Operate the column at total reflux for 1 hour. Remove 50 ml. of distillate a t a reflux ratio of 30 t o 1, transfer the distillate to a 100-ml. volumetric flask, and dilute to volume with water. analyze a 1.00-ml. aliquot for nitromethane.
Repeat the anal?-&, using a larger dilution of the distillate if the nitromethane content is high. Results of the determination of riitromethane in mixtures using azeotropic separation and the described color test are given in Table F'II. ACKNOWLEDG.MENT
The anthors \\-ish to thank Emory E. Toops, Jr., for thc preparation, purification, and puritj- characterization of the nitroparaffins iised in this work. LITERATURE CITED
(1) Bose, P. K., Andust 56, 504 (1931). (2) Desseigne, G., Belliot, Ch., J . chim. phus. 49, 46 (1952). (3) Desvergenes, L., Ann. chini. anal. chim. a p p l . 13, 321 (1931). (4) Jones, L. R., Riddick, J . A, Ax.4~.CHEM.26, 1035 (1954).
. (5) hlachle, W. F., Scott, E. IT.,Treon, J., J . Ind. HUB.T o ~ i c o l22, 315 (1940). (6) JIansoff, D. D., 2. .Yahr.-Genuss?n. 27, 469 (1914). (7) JIeyer, V., Ambuhl, G., B e r . 8, 751, 1073 (1875). (5) Aleyer, V., Locher, J., Ibid., 8, 219 (1895). (9) Toops, E. E., J . Phw. Chem. 60, 304 (1956). (10) Turba, F., Haul, R., Uhlen, G., Angew. Chem. 61, 74 (1949). RECEIVED f o r review January 23, 1966. Accepted May 12, 1956.
Determination of Chromium and Vanadium in Silica-Alumina Cracking Catalyst T. A. HIETT and PAUL KOBETZ' Houston Manu~acturing-ResearchLaboratory, Shell
Oil Co.,
A rapid and accurate wet chemical method for the determination of small amounts of chromium and vanadium in silica-alumina cracking catalyst and similar materials involves amperometric titration with ferrous iron to determine chromium plus vanadium. Vanadium is titrated directly after reoxidation with potassium permanganate, followed by selective reduction of chromium with sodium azide. Chromium is calculated by difference. The precision varies from within = I = O . O O O S ~ for less than 0.005 weight 7' to =kl% of the mean for greater than 0.10 weight Yo.
C
HROMIUbl and vanadium are two contaminant metals t h a t are known to produce adverse effects on the perform-
nnce of silica-alumina catalysts in fluid cracking operations. llIany methods are reported in the literature for estimation of these metal?. Chromium has been determined colorimetrically in many materials with s-diphenylcarbazide (1); however, large amounts of vanadium interfere. T'anadium is frequently determined colorimetrically with phosphotungstate ( 2 ) . Here again, interfering ions, such as chromium and iron, must be removed. TVillard and I-oung ( 7 ) have reported a volumetric method for determination of chromium and vanadium in steel. They used ferrous iron arid potassium permanganate \Tith o-phenanthroline as indicator. Diicret (4)employed ferrous iron as a titrant, with diphenylamine as indicator, to determine total chromium plus vanadium after permanganate oxidation. He then selectively oxidized vanadium with permanganate, destroyed the oxidant, with sodium azide, and titrated the vanadium directly. Parks and Agazzi ( 6 ) employed Ducret's technique, hut substituted amperometric titration for the visual indicator 1
La.
Present address, Kaiser Aluminum and Chemical Corp., Baton Rouge,
Houston 7, Tex.
titration. They digested the saniple briefly with sulfuric. : t i i d perchloric acids before addition of permanganate. Johi~son, Keaver, and Lykken (6) used electrodeposition to remove cahromium and similar ions from solutions to prevent their interferc~nre with the determination of other ions. The methods th:tt appeared most promising for the authors' purpose [6-7] \\-('re evaluated. I n the method presented here, a modification of the Parks and Agazzi procedure ( 6 ) , samples are digested with hydroHiioric and sulfuric acids to remove silica and with perchloric ti(*id to oxidize chromium and vanadium. Total chromium pliis vanadium content is determined by amperometric titration \\ ith ferrous soliltion. Then, after reoxidation, chromium is sclwtively reduced with sodium azide, and the vanadium is titrated directly. Chromium is calculated by difference. EQUIPMENT
The equipment consists of a Sargent Model X X I polarograph equipped with a saturated calomel reference electrode, salt bridge, constant-speed stirring motor, and rotating platinum indicator electrode. REAGEYTS
. I 0.1.V stock solution of ferrous ammonium sulfate in 1 to 9 sulfuric acid is made 0 0 1 S and O.OOIAr by dilution. Potassium permanganate is approximately 0.1N. Sodium azide, 10 weight 70aqueous, is prepared fresh daillRECOMMENDED PROCEDURE
Weigh 1 to 2 grams of silica-alumina catalyst into a 100-ml. platinum dish. Heat slowly to 650' C. in a furnace and ignite about 30 minutes a t this temperature. (This ignition step may be omitted if carbonaceous matter is known to be absent.) Wet the catalyst thoroughly n i t h water and add 5 ml. of C . P . concentrated sulfuric acid. Add 15 ml. of 50y0 hydrofluoric acid in
