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 3 ~ 1 %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
1496
ANALYTICAL CHEMISTRY Table I.
Recovery of Known Amounts of Chromium and Vanadium
Amperometric a i t h Electrodeposition (6) Chromium, mg. Added Found Vanadium, nig. -4dded Found
Fe and
0.508 0.510
0.040 0.040
0,020 0 023
0 020 0 019
0,012 0.008
0.020 0.025
0 020 0.024
0 020 0.034
0,000 0.010
0.060 0.020
0.500 0.460
0.200 0.200
0.200 0.200
0.200
0,200 0,201
0.200 0,200
0 200 0.208
0.200 0.198
0.240 0.245
0.300 0.254
0.214
Amperometric with Selective Oxidation (6) Chromiuni. mg. Added Found Vanadium, mg. Added Found a
Willard and Young with 0.01.V 0.005.V KMnOa ( 7 )
-
Amperometric with Selective Reduction
0.040
0.020
0.000
0 040
0.026 0.032
0.020 0.019
0.14a 0.125
0.200 0.20B
0.200
0,200 0.200
0 200 0 206
0 500 0 505
0 400 0 402
0 14 & 0 01a 0 151
0.200
Calculated from NBS r-aiues for SBS sample 98.
small increments (caution) and evaporate t,o sulfur trioxide fumes. Repeat the hydrofluoric acid addition and evaporation. Wash down the walls of t,he dish with about 15 ml. of water and evaporate to sulfur trioxide fumes. Add 13 ml. of 727, perchloric acid and transfer the solution t o a 150-ml. beaker containing a few boiling stones. Cover, heat to fumes, and allow to simmer 10 minutes after development of a full yellow or orange color. Cool rapidly by dipping and sxirling in ice water for G to 8 seconds. Add 15 ml. of water, wash down the cover and sides, and boil the solution for 3 minutes. Cool, and titrate amperometrically with ferrous iron solution. Make the solution up to approximately 75 ml., boil, and add potassium permanganate dropwise until a pink color remains 1 minute. Add sodium azide solution dropwise to the boiling solution to destroy excess oxidant and 1 ml. ia excess (add a correspondingly larger excess when more than 0.05 meq. of total tit,rant is expected). After 30 seconds, remove the solution from the heat and cool in ice water. Titrate the vanadium as above and obtain chromium by subtracting the millieqiiivalents of vanadium (serond titrat,ion) from the total niimtm of milliequivalents (first titration). APPLICkTIOI T O KYOWY .MIXTURES
Solutions of known chromium and vanadium contents TS ere analyzed by the method presented and bv three other methodg for comparison (Table I). Amperometrica titratlor1 n ith electrodeposition permlts good recovery of chromium and vanadium; howeve1 this pi ocedure requires extra equipment (electrodeposition unit) and about 1 hour more time per analysis than the proposed method. The method of Killard and Young showed the poorest accuracy. End point determination was difficult 111th the visual indicator in the prescnce of large amounts of huffering sidts, especially for thr 1015 er toncenti ations.
Table 11.
Wt. ro
.ANALYSIS OF CATALYST SilMPLES
Two silica-alumina cracking catalysts were analyzed by the four procedures (Table 11). The Willard and Young method is untenable for the catalyst samples stiidied because of difficulties with buffering in the presence of large :mounts of alumina, and indicator difficulties with the low concentrations involved. Electrodeposition removes the chromium satisfactorily; hoLvever, it requires too mucth time. I*alues obtained -1, the selective oxidation and reduction methods are of the same magnitude; hoivever, those by the selective reduction method show better repeAtability. The deviation, at 95% confidence level ( 3 ) ,for duplicate determinations has been calculated for the new procedure using approximately 100 analyses for various concentration ranges. 0.0003 for less thitn 0.005 weight yo, =t The deviation was
*
Determination of Chromium and Vanadium in Catalysts
Selective reduction of chroiniuni
T',
The method of Parks and Agazzi, which utilizes selective oxidation of vanadium, gives good recovery of vanadium. However, oxidation and reduction in the cold are slow. While n-orking Tvith the Parks and Agazzi method, it, was found possible to oxidize the boiling solution with permanganate and, then, selectively reduce the chromium with sodium azide. This method of selective reduction of chromium is neiv. The data show no difference in accuracy betiyeen the selective oxidation and reduction methods; hoxever, the latter requires about 10 minutes less time per analj-sis. One Kational Bureau of Standards sample (Yo, 98) similar in composition to the fluid cracking catalyst was :analyzed by the new procedure (Table I).
0 0 0 0
041 041 041 041
0.0030 0,0029 0.0028 0.0028
Amperometric Titration Selective oxidation of vanadium
Electrodeposition
Willard and Young Procedure
0.0058 0.0049 0 0036 0 0038
0.0052 0.0052
0.0056 0,0043
0 044 0.041 0 040 0 042
0.043
0.048 0.050
0.0012 0.0011 0.0011 0.0015
0.0011 0,0007
0.0011
0,0029 0.0029 0.0026 0.0020
0.0030
0.0029
0,043
1497
V O L U M E 2 8 , N O , 9, S E P T E M B E R 1 9 5 6 0.0005 f o r 0.005 to 0.01 15-eight %, & 0.001 for 0.01 t o 0.10 and i 1 % of the mean for greater than 0.10 weight 7& weight 70. CONCLIJSION
Amperonietric titration TT-ith ferrous iron combined with either selective reduction of chromium or oxidation of vanadium affords a satisfactory method for determination of chromium and vanadium in silica-alumina cracking catalyst,. Selective reduction gives better repeatability and can be handled in about 10 minutes less per 'analysis than selective oxidation. It is therefore recommended.
LITER-ATURE CITED
(1) C'hunmann, H. J., Bisen. Ruth. ;Is.AL. C'HFX. 24, 1341 (1952). (2) Cooper, 11. D., TTinter, P. I
