ANALYTICAL CHEMISTRY
1392 fate, which form strong complexes with zirconium, interfere seriously. Phosphate forms a n insoluble zirconium salt and must be absent also. LITER4TURE CITED
(1) Bastian, R., ASAL. CHEM.,21, 972 (1949). (2) Zbid., 23, 580 (1951). (3) Bumsted, H. E., and Wells, J. C., Z b i d . . 24, 1595 (1952). (4) Fritz, J. S., and Fulda, 31. O., Zbid., 26, 1206 (1954). 15) Green. D. E.. Zbid.. 20. 370 11948). i6j Hahn,’ R.. Zbid.. 21, 1579 (1949);’ , (7) Hillebrand. H. F., Lundell, G. E. F., Bright, H. A, and Hoffman, J. I., “Applied Inorganic Analysis,” p. 569, Wiley, Kew York, 1953.
( 8 ) Zbid., p. 572. 21, 1440 (1949). (9) Hiskey, C. F., h 4 L . CHEM., (10) Kolthoff, I. XI., and Johnson, R. A , , J . Electrochem. Soc., 98, 131 (1951). (11) Kumin, C. d.,h.41.. CHEX.,19, 3 i 6 (1947). (12) Oesper, R. E., and Klingenberg, J. J., Zbid., 21, 1509 (1949). (13) Sanchis, J. AI., IND.ESG.CHEW,AS.AL.ED.,6, 134 (1934). (14) Thompson, T. G., and Taylor, H. J., Z b i d . , 5 , 8 7 (1933). RECEIVEDfor review March 2 5 , 1955. Accepted May 21, 1955. Presented a t the Southeastern Regional JIeeting of the ~ I E R I C A X CHEMICAL SOCIETY, Birmingham, 41a., October 21-3, 1954. Work carried out under Contract No. TI’-7405-eng-26 a t Oak Ridge Xational Laboratory, operated by Carbide and Carbon Chemical Co., a division of Union Carbide and Carbon Corp. for the Atomic Energy Coiiimission.
Spect rophotomet ric Dete rmination of Vanadium(V) with Benzohydroxamic Acid and I-Hexanol Application to Steels and Crude and Residual Oils WARREN
M. WISE
and WARREN W. BRANDT
Department o f Chemistry, Purdue University, W e s t Lefayette,
A spectrophotometric method for quantitatively determining minute amounts of vanadium is based on the extraction with 1-hexanol of the colored product, which is formed when vanadium(\’) reacts with benzohydroxamic acid, and the subsequent measurement of the absorbance of the extract. The effects of the following variables have been investigated: the amount of complex extracted with respect to the kind of alcohol used, amount of reagent, pH, volume of the aqueous phase, ionic strength, and diverse ions. The system obeys Beer’s law between the limits of 5.00 X and 1.00 X 10 -5 mole of vanadium(V). The method is accurate aitd precise. A satisfactory procedure for determining Vanadium in steels was developed. The interfering effect caused by large amounts of iron(II1) was eliminated by removing the iron. A n electrolysis using a mercury pool as a cathode was employed. The method was advantageously applied to crude and residual oils.
Tu)
HE results of the studies conducted by Bhaduri and Ray
show that in aqueous solutions minute amounts of vanadium(V) will react with salicplohydroxamic acid t o give a colored product. Das Gupta ( 4 ) used benzohydroxamic acid to develop a colorimetric method for vanadium. It was also observed that certain oxygenated organic solvents such as 1pentanol, amyl acetate, and isobutyl acetate have the ability to extract one or more of these colored products (1). Because versatile analytical procedures for determining minute amounts of vanadium are not abundant, the system was investigated in order to determine whether an extraction process could be employed to develop a more sensitive spectrophotometric method. APPAR &TU9
Beckman Model B spertrophotometer equipped with two matched 1.00-em. cells. General Electric recording spectrophotometer equipped with two matched 1.00-em. cells. Leeds & Korthrup p H meter (line-operated). REAGENTS
rlmmonium metavanadate, 1.00 X 10-3 *If Potassium hydroxide.
Ind.
Sodium hydroxide, 2.11. Sulfuric acid, 6.W. All of the alcohols which were employed as extractants were distilled prior t o use. Solutions of diverse ions were prepared from analytical grade compounds and distilled Rater. The potassium benzohydroxamahe was prepared according to published techniques (2). The product, mas recrystallized from a solution containing 20% distilled tvater and SO% methanol. Benzohydroxamic acid is very difficult to dissolve in water, and since potassium benzohydroxamat,e decomposes rapidly upon standing in basic solutions, the reagent solution was prepared in the following manner: The required amount of purified p o t a e sium benzohydroxamate was dissolved in distilled water, the p H adjusted to 5.0, and the solution diluted to volume. A 0.200 JI reagent solution prepared in this manner is stable for a t least 1 month. DEVELOPWEST OF METHOD
Experimental. The required amount of reagent was added to the solution containing the vanadium in the +5 oxidation stat,e. (When the effects of diverse ions were investigated, they were added before the reagent.) The ionic strength, pH, and volume were adjusted. The extraction was performed with a single, pipetted 20.0-ml. portion of the alcohol, and the extract was cent,rifuged to remove the droplets of water. The absorbance of the extract was measured a t 450 mp. I n each case the alcohol which was employed for the extraction was used as the blank for the absorbance measurements. Results and Discussion. PRELIMISBRY STUDIES. Vanadium(V) in aqueous solution will react x i t h benzohydroxamic acid, yielding a colored product which can he extracted with some oxygenated organic solvents. I t was found that all of the vanadium must be in the +5 oxidation state in order to obtain immediate maximum color formation in the nonaqueous phase. Curve A in Figure 1 exhibit,s the spect,rophotometric properties of a system containing 5.00 X 10-6 mole of ammonium metavanadate and 2.00 X mole of benzohydroxamic acid in 20.0 ml. of water a t p H 2.0. Curve B shows the increase in sensitivity that can be obtained when the system is extracted with 20.0 ml. of 1-hexanol. Thus, the molar absorptivity of the complex a t 450 mp is increased from 1.0 x lo3 to 3.5 X l o 3 liter per moleem. by the nonaqueous solvent. EFFECTS O F USISG VARIOUS ALCOHOLS FOR EXTR.4CTION O F COMPLEX.An investigation showed that hydrocarbons and nonoxygenated solvents will not extract the colored complex,
V O L U M E 27, N O . 9, S E P T E M B E R 1 9 5 5 while aliphatic alcohols appear t o be the best and most inexpensive reagents for this purpose. A study was conducted a t pH 2.0 in order to find a satisfactory alcohol for the extraction of the complex. All the eytracts were found t o have absorbance masima at 450 f 5 mM. The data given in Table I show the efl’ecte of using different alcohols. Since with one extraction 1-hexanol removed 97% of the complex under the conditions used and was readily available, it was chosen for the purpose. The complex has a n absorbance maximum at 450 mp in this solvent, and this wave length TT as selected for all absorbance measurements. -
Table I.
Effects of Using Different Alcohols for Extraction of the Complexa
Mcohol A460 m p 2-l\Iethyl-4-butanol 0 885 Benzyl alcohol 0.885 1-Hevtanol 0 8R.i l-ocianol 0. 2-Ethyl- 1-hexanol 0,870 1-Hirxanol 0.865 1-Decanol 0.865 1-Pentanol 0.860 2-Hexanol 0,850 2-Ethylbutanol 0.230 Cycloliexanol 0.020 Vanudium(V) = 5 00 X 10-6 mole: reagent = 2 00 X 10-4 mole.
1393 obtained, and the standard deviation was calculated. If the permissible limits are established as this average &2u, constant results are obtained m-ithin the pH range 1.2 to 5.5. The system changes below pH 1.2, until a t pH 0.2 only 84% of the amount extracted a t pH 2.0 is removed. Above pH 5.5 a rapid change takes place, until a t p H 8.5 no color is present in the extractant. On the basis of these data, p H 2.0 was chosen for all subsequent work. It was found that the volume of the aqueous phase can vary between 30 and 90 ml. nithout effect. For simplicity the volume of the aqueous phase in all subsequent work was adjusted to approximately 50 ml. before extractions. When potassium chloride is used to adjust the ionic strength, the effect is negligible up t o 2.0. -4bove 2.0 the absorbance decreases slowly, until a t p = 4.0 it is 78% of what it is a t p = 0.04. This variable was neglected since none of the investigations were conducted a t higher ionic strengths.
sso
a
EWFECT OF AMOUNT OF REAGENT. The effect of the amount of reagent used was examined a t pH 2.0. The results listed in Table I1 exhibit the fact that a t least a 10 t o 1 ratio of reagent t o vanadium(V) is required for 1.00 X 10-5 mole of vanadium(V). This amount of vanadium(V) appears t o be the upper limit of the method when 20.0 ml. of 1-hexanol are used. I n all of the following work 2.00 X l o w 4mole of reagent was employed for each extraction.
Table 11.
Effect of Amount of Reagent
Molar Ratio of Reagent t o T’anadium (1’)
44kQ mp
0 400 0.910 1.480 1.720 1.760 1,780 1.750
1:l
2:l 4:1 8: 1 1O:l 2O:l 4O:l
Table 111.
Calibration Curve Data and Results of Precision Studies
hfoles of Vanadium(V) x 10‘
20.0 50.0 100 a
b
d(*
.ldSO m p a
0 085 0.357 0 878 1 754
5.00
Standard Deviation, u b 0,0038 0.0070 0.0080 0.0074
Average of 10 values. ~
=
-
?)*.
n - 1
w
0.E
0
z
a m a 0 v)
m
a 0.E
0.2
I
I
I
I
450
500
550
600
W A V E LENGTH
(mp)
Figure 1. Vanadium(V)-benzohJdroxamic acid complex 4. €3.
I n water I n hexanol
It has been noted that the 1-hexanol used in the extraction procedure must be redistilled at intervals of about 2 weeks. Apparently some decomposition product is formed upon storage which causes low results. EFFECTS OF pH, VOLUMEOF AQUEOCSPHASE,AXD IOSIC STRENGTH. The system %vasstudied with respect to the effect of the p H of the aqueous phase on the amount of complex extracted. The average of 10 absorbance values a t pH 2.0 was
BEER’SLAW. The data given in Table 111 show that the system obeys Beer’s law between the limits of 5.00 X 10-7 and 1.00 X 10-6 mole of vanadium(V). The standard deviation for each amount of vanadium(\’) given in Table I11 was calculated using 10 absorbance values. The results show that good precision can be obtained within the chosen concentrat’ion range. EFFECTS OF DIVERSEIors. The effects of diverse ions were investigated. The desired compound was added to the system before the reagent \vas introduced. The experimental procedure was not altered in those cases in which incomplete dissolution or precipitate formation resulted. An intrrference was considered mole of to be caused if the absorbance otitainpd for 5.00 X vanadium(\-) fell 1 2 u from the average value given in Table 111. The folloning compound? do not interfere n.hc11 the molar ratio of each to vanadiuni(T’) is 100 to I : animoriium sulfate, magnesium chloride, calcium nitrate, strontium nibat?, harium chloride, tantalum oside, chromic nitrate, copper sulfate, zinc chloride, cadmium chloride, potassium dihydrogen phosphate, sodium fluoride, pot,assium bromide, sodium carbonate, potassium thiocyanate, sodium citrate, sodium-potassium tartrate, potassium perchlorate, boric acid, disodium hydrogen arwnate, anisulfate, and wan!-1 acetate. in Table IT show t,he compounds which interfer? n-ith the method. The>- can he grouped into one of three main classifications. The powerful osidizing agents attack the reagent. Strong reducing agents convert the vanadium(V) to vanadium( IV). Removal of these groups should be accomplished without difficulty. Ions which form complexes with t,he reagent deprive the vanadium(V) of a sufficient amount of reagent to
ANALYTICAL CHEMISTRY
1394 Table IV. Substance Added
Interfering I o n s blolar Ratio to Vanadium(V1 100: 1 1O:l
KzTiO(C?04)?.2H?O ZrO(N03jz KsNbsOis. 16H20 0 0
0 0
KIOI NaCN Na?S 9Hz0 NaNO2 NazS03 0 - denotes low results. b denotes high results. c 3 denotes no interference.
+
effect maximum color formation in the alcohol layer. The latter appear to be the follon ing: titanium( IV), zirconium( IV), manganese(II), iron(III), thorium(1V j, aluminum( III), tin( IV), molybdenum(VI), and tungsten(V1). The addition of excess reagent should remove the interferences of those which give no color: zirconium( IV), manganese( 11), thorium(IVj, aluminum(111),and tungsten(V1). Suggested Procedure for Determining Vanadium(V) in Absence of Interfering Amounts of Diverse Ions. Add 10.0 ml. of 0.0200M benzohydroxamic acid to the solution containing not more than 1.00 x 10-6 mole of vanadium(V). Adjust the p H to 2.0 with 6M sulfuric acid, and dilute to between 30 to 90 ml. with distilled water. Extract the solution with a pipetted 20.0-ml. portion of 1-hexanol, and centrifuge the extract. Read the absorbance of the extract a t 450 mp against a 1-hexanol blank. Convert the absorbance to amount to vanadium(V) with the aid of a calibration curve. By using larger amounts of reagent and 1-hexanol, greater quantities of vanadium(V) can be determined.
with salicylic acid, thiocyanate, and benzohydroxamic acid and extracting the complexes with aliphatic alcohols, but it was observed that too many extractions are required, and some of the vanadium(V) is also extracted. An attempted distillation of the vanadium(V) from aqua regia was unsuccessful. The hydrous iron(II1) oxide was precipitated with ammonium hydroxide and by hydrolyzing urea, but some of the vanadium(V) was always coprecipitated. The previously mentioned methods which lyere tried in order to eliminate the iron(II1) interference failed chiefly because of the large ratio of iron(II1) to vanadium(V). A satisfactory procedure for quantitatively removing large amounts of iron( 111)from acidic solutions of vanadium(V) and iron(II1) was found. An electrolysis using a mercury pool for a cathode as described by Cain (3) is very satisfactory for this purpose. Results and Discussion. The method a i given can be used to determine quantitatively amounts of vanadium in steels ranging from 0.03 to 0.5%. The data listed in Table V show the results obtained using several different steels. The low values received for NBS No. 153 are probably due to the presence of some interfering constituent. However, the values obtained for the other samples agree with the accepted ones. It was found that as the concentration of the sulfuric acid is increased the time of electrolysis also is increased. The presence of nitrate and chloride ions in the electrolyte is undesirable. The former is reduced to nitrogen dioxide a t the cathode prolonging the electrolysis, and the latter is oxidized to chlorine which attacks the anode. The treatment of the electrolyzed solution with a n oxidant \vas found to be necessary. If the step is omitted, low results are obtained. Potassium dichromate solution is unsatisfactory; however, potassium peroxydisulfate in the presence of a trace of silver ion was found to be adequate. The treatment with peroxidisulfate and silver ion also converts the small amount of manganese(I1) left after the electrolysis t o permanganate. This serves to indicate that all of the vanadium is in the +5 state. The permanganate is subsequently destroyed when the solid potassium hydroside is added.
Table V.
Analyses of S o m e Steels Vanadium, %
Desciiption Cr-V steel Cr-V steel Cr-V-Co-hlo-W steel Cr-Mo-Si steel
Found 0 . 2 7 , 0 . 2 0 , 0.18, 0.18 0.2,5,0.2fi,0.22,0.26
Aver- Acage cepted value value 0.20 0.21 0 . 2 5 0.24
1 . 6 3 , 1 81, 1.86, 1 . 8 6 0.030. 0.033. 0 . 0 3 4 , 0 038
1.79 2.04 0.035 0.038
Cr-V-W steel
0.26, 0 . 2 4 , 0 . 2 2 , 0.23
0.24
4PPLICATION O F METHOD TO STEELS
As the method is capable of determining minute amounts of vanadium(V), an attempt was made to apply it to the determination of vanadium in steels and alloys. The chief difficulty that was encountered in this investigation was the elimination of the interfering effect caused by iron(II1) when its molar ratio to vanadium(V) was 100 t o 1 or greater. IROX Experimental. SUMMARY O F -ATTEMPTS TO ELIMINATE (111) INTERFERENCE. Qualitative tests showed that the extractable iron(II1)-benzohydrosamic acid complex is not readily formed at a low pH. Experiments were conducted a t pH 1.3 in the presence of iron(III), but the iron(II1) always interfered with the determination of the vanadium(V). Attempts ryere made to mask the iron(II1) nith acetylacetone, salicylic acid, fluoride, citrate, tartrate, Versene, and phosphate, but these agents did not prove t o be satisfactory. A cation exchange resin was employed t o separate vanadium(V) and iron(II1) without success. Complexing agents such as fluoride and thiocyanate were used to convert the iron(II1) present to negatively charged complexes EO that it could be removed with an anion exchange resin. However, some iron( 111) alm-ays passed through the column. Hydrochloric acid and isoprop? 1 ether were used to extract iron(II1). Hoxever, enough vanadium(1V) also m-as extracted to cause appreciably low results after the vanadium( IV) was oxidized to the +5 state and determined in the usual manner. Attempts were made to remove the iron(II1) by complexing it
Samplen NBS 30a NBS 30c NBS 153 NBS 160 Commercial sample a
0 25
h-BS. National Bureau of Standards.
In the following procedure an excess of reagent is required in order to give the correct result for the amount of vanadium(V) present. At least 4.00 X 10-4 mole must be used. This is probably due t o a reaction between the excess peroxydisulfate and the reagent. Suggested Procedure for Determining Vanadium in Steels and Alloys. Dissolve the sample, 0.02 t o 0.1 gram, Kith gentle heating in an acid or suitable mixture of acids. After the sample has dissolved, add 1.5 ml. of sulfuric acid if none was present initially. Evaporate to fumes of sulfur trioxide, cool, and dilute to 40 ml. with distilled water. Heat the solution to boiling, and stir to dissolve any solids. Add 500 grams of mercury, and make the cathode contact with a platinum wire lead sealed in a piece of glass tubing filled with mercury. For an anode immerse a strip of platinum 0.5 inch wide to a depth of about 1 inch in the electrolyte. Stir and electrolyze with 6 or 7 volts and 4 or 5 amperes until the solution gives no pink color when a drop is placed on a few crystals of 1,lO-phenanthroline. From 20 to 40 minuter are required for complete removal of the iron.
V O L U M E 2 7 , NO. 9, S E P T E M B E R 1 9 5 5 Isolate the electrolyte with a separatory funnel, and transfer it to a 400-ml. beaker. Add 1 gram of potassium peroxydisulfate, 2 drops of 0.3M silver nitrate, a few glass beads, and boil until the volume is about 50 ml. Cool to room temperature, and slowly add solid potassium hydroxide with stirring until the p H is about 1.4. Cool the solution, and add 20.0 ml. of 0.0200M reagent. Adjust the p H t o 2.0 n-ith 251 sodium hydroxide, and extract with 20.0 ml. of 1-hexanol. (The volume of the aqueous layer should be between 70 and 90 ml. a t this point.) Wash the beaker and the glass beads n-ith a portion of the aqueous phase, and return the rTashings to the separatory funnel. Shake the mixture again. Centrifuge the alcoholic extract for a few minutes, and read the absorbance at 450 mp against a 1-hexanol blank. Convert the absorbance t o per cent vanadium with the aid of a calibration curve. APPLIC4TION OF METHOD T O CRUDE AND RESIDUAL OILS
Current methods ( 5 , 6, 8) for determining vanadium in crude and residual oils are frequently time-consuming or yield unsatisfactory results. An attempt mas made t o utilize the sensitivity of the method in its application to samples of crude and residual oils.
Table \-I.
Sample Crude oil Residual oil Residual oil Residual oil Residual oil Asphalt
Results of Analyses of Some Crude and Residual Oils % V (Found) (Wet-Ashing)
A B C D
0 021 0 028 0.0015
0,0044 0.013 0.031
0.022 0.030 0.028 0.0014 0,022
0.0038
0.015 0.030
Average Value 0,022
0.029 0.0014 0.0041 0.014 0.030
%V (Reported) (Dry-Ashing) 0.020 0 034 0 0010 0 0032 0 013 0 051
Experimental. The sample of oil was wet-ashed, using SUIfuric, nitric, and perchloric acids in that order. I n order to maintain a high reaction temperature, the water formed was driven off before each addition of acid. The perchloric acid was not added until all of the easily oxidizable material was eliminated. After the wet-ashing and the volatilization of the excess acids, the procedure for determining vanadium in steel was used. Results and Discussion, The data given in Table VI show the results of the analyses of some crude and residual oils. Considering the fact that dry-ashing procedures were used for the comparative values, it is apparent that the results are in good agreement except in the case of sample 6. This discrepancy has not been explained. The sensitivity of the vanadium determination permits the use of relatively small samples, thus aiding the speed of decomposition. If iron(1II) is known to be absent or present only in small amounts, the electrolysis and subsequent oxidation with peroxydisulfate may be omitted. The treatment with nitric and perchloric acids leaves the vanadium in the quinquivalent state. The wet-ashing procedure (7’) is comparatively rapid, requires little attention, and does not involve the larger sample and possible physical losses that dry-ashing does. Thus the combination
1395 of the wet-ashing and the benzohydroxamic acid method provides a reliable and convenient approach t o the determination of a wide range of concentrations of vanadium in petroleum products. Recommended Procedure. Place the sample (1 to 2 grams) in a round-bottomed flask and attach a condenser by means of a ground-glass connection. Add 5 ml. each of concentrated sulfuric acid and concentrated nitric acid and a few glass beads. Heat a t reflux, adding additional 5-ml. portions of nitric acid as the reaction subsides. Before each addition of nitric acid remove the water from the flask by stopping the flow of water through the condenser. Continue the nitric acid treatment (two to 3 additions are usually sufficient) until all particles of carbon are absent, and cool the solution. Add cautiously 3 to 5 ml. of 60 or 727’ perchloric acid, slowly heat, reflux until the solution is a light yellow or orange color ( 5 t o 15 minutes), and cool. Remove the condenser and fume off the remaining acids until the volume is about 1 ml. Cool, transfer to an electrolytic beaker, and dilute to 40 ml. Complete the determination as for steel. CONCLUSIONS
The method is a satisfactory and versatile one for determining microgram quantities of vanadium. It is sensitive and precise. The system is stable for prolonged periods. The procedure does not require an evceedingly large excess of reagent, and the common variables such as pH, volume of the aqueous layer, and ionic strength can vary within wide limits without affecting the results. Because almost all of the complex is extracted with the first 20.0-ml. portion of 1-hexanol, only one extraction is required. Therefore, the amount of extractant used is the only exact measurement of volume that must be made during the entire procedure. The method as given can be satisfactorily applied to steels ranging from 0.03 to 0.5% vanadium. However, by altering the procedure it can easily be extended outside of these limits. The authors feel that with the proper modifications this method can be used for determining minute amounts of vanadium in carnotite ores, petroleum products, and biological materials. ACKNOWLEDGMENT
The authors wish to express their appreciation for the financial support of this work by the Office of Ordnance Research, U. S. iirmy. LITERATURE CITED
(1) B h a d u r i , A. S., a n d R a y , P., Science and Culture (India),18,97-8 (1952). (2) B l a t t , A. H., “Organic S y n t h e s e s , ” 001. 11, p. 67, Wiley, Sew Y o r k , 1944. (3) C a i n , J. R . , J . Ind. Eng. Chem., 3,476-81 (1911). (4) D a s G u p t a , A. K., a n d Singh, hf. hI., J . Sci. Ind. Research (India), 11B,268-73 (1952). (5) P a r k s , T. D., a n d Agazei, E. J., Anal. Chem., 22, 1180 (1950). (6) S i m p s o n , T. H., Chemist Analyst, 27, 44 (1938). (7) S m i t h , G . F., Bnal. Chim. dcta, 8,417 (1953). (8) W r i g h t s o n , F. LI., ANAL.CHEM.,21, 1643-5 (1949). RECEIVEDfor review October 13, 1954. Arcepted May 23, 1955. Presented before the Division of Analytical Chemistry a t the 126th Meeting of the AYERICAS CHEJIICAL SOCIETY, Xew York. September 1954.
