Molybdenum Determination in Plant Material Modi$cation of Thiocyanate Stannous Chloride Method ISAAC BARSHAD University of California, Berkeley, Calif. HE thiocyanate-stannous chloride method for determination molybdenum is based on the formation of an amber to orange-red colored compound when a reducing agent such as stannous chloride is added to an acid solution of a molybdate salt in the presence of thiocyanate. The colored compound was identified by Hiskey and Meloche (3) as a thiocyanate complex of quinquevdent molybdenum. Hurd and Allen (6) found that unless certain conditions are maintained in the molybdate solution during production of the colored compound, neither the yield nor the stability of the compound will be at a maximum. During an investigation of the molybdenum content of plants ( I ) , the author found that, although the conditions specified by Hurd and Allen were maintained, the addition of ferric and nitrate ion may further increase the yield and stability of the colored compound. Although several investigators (8, 4,Q)have noted that the presence of ferric ion in a molybdate solution intensifies the molybdenum thiocyanate color, no attempt has been made to study the quantitative aspects of the intensification, and no mention has apparently been made of the similar effect of nitrate. The results of the present investigation, summarized in Table I, show that when ferric or nitrate ion is present in molybdate solutions, and the conditions specified by Hurd and Allen are maintained during production of the molybdenum thiocyanate color, the intensity of the color is equal to the intensity obtained in molybdate solutions of twice the concentration. The presence of nitrate stabilizes the color so that practically no fading takes place for at least 30 minutes. If, however, the amount of stannous ion is much smaller than the nitrate, rapid fading indicates the complete oxidation of stannous to stannic ion by the excess nitrate; further addition of stannous ion stops the fading and reintensifies the color. The presence of small amounts of stannous ion as compared with nitrate may possibly account for the increased fading with nitrate reported by Grimaldi and \\'ells ( 2 ) . The intensification of the color with ferric ion also held true when the molybdenum thiocyanate color was extracted with ether (Table 11). The intensity of the color for equal concentrations is the same in aqueous as in ether solution (Table 111), and the intensification of the color by ferric ion is proportional to the amount of ferric ion, provided the amount added is less than the equivalent of the molybdenum present, and reaches its greatest intensity when ferric ion and molybdenum are present in equivalent amounts.
Table I.
tration of bIolybdenurn, P.P.RI.
0.50 1.00 2.00
4.00
Much larger amounts of ferric ion caused no further change in the intensity of the color (Table IV). The intensification of the quinquevalent molybdenum thiocyanate colored compound by ferric ion or nitrate may be explained by the followingconsiderations and experimental evidence. A comparison of the potentials (6) of the F e + + Fe+++ (E" = -0.771), NO Nos- (E" = -0.94) couples, and the potentials ,Of the hlo - Mo++* (E" = ca. 0.2), &To+++ ?rloOl+ ( E = ca. O.O), MoOzf - HZMo04 (E" = ca. - 0.4) couples indicates that ferric ion and nitrate in acid solutions will oxidize molybdenum in a reduced state to the hexavalent state. The reaction between trivalent molybdenum and iron is the basis of a method for determining molybdenum ( I I ) , and the oxidation reaction of nitric acid is utilized in bringing molybdenite (MoSz) into solution (11). .4comparison of the potentials of the Sn++ - S n + + + +(E'" = -0.15) couple and the molybdenum couples indicates that stannous ion may reduce hexavalent molybdenum to a lower valency than quinquevalent mol bdenum if present in considerably larger amounts than hexavaint molybdenum. That such a reduction takes place was indicated by the intensity of the molybdenum thiocyanate color in relation to the amount of stannous chloride for a given concentration of molybdenum. Table IV shows, that the intensity is a t a maximum when stannous ion is about equal or in a slight excess to the molybdenum, and a t a minimum when the concentration of stannous ion is ten or more times the concentration of molybdenum; also that the difference between the maximum and minimum intensities is approximately equal to the intensity of a solution one fourth the concentration. I t is believed that nitrate or ferrous ion, possibly as a complex ion with thiocyanate, when present in the solution prevents stannous ion from reducing the quinquevalent molybdenum to a lower state of valency by virtue of their oxidation potentials in relation to the potential of molybdenum in a reduced state. This oxidizing power of nitrate ferrous ion, possibly as a complex ion, may account for the intensification of the molybdenum thiocyanate color brought about when these ions n-ere added to an ammonium molybdate solution in which the molybdenum thiocyanate was produced by an excess of stannous ion (Table V). The additional 2 5 7 increase in intensity when nitrate or ferric ion is added prior to the stannous is believed t o be brought about by an oxidation reaction whereby these ions oxidize reduced molybdenum which seems to be present in molybdate solutions of hydrochloric or sulfuric acid. That reduced molybdenum, possibly in the trivalent state, is present in such solutions was indicated by the following experimental evidence. The intensity of molybdenum thiocyanate color produced by small amounts of stannous ion in a solution of ammonium molybdate in sulfuric acid in which nitric acid was added and then removed by evaporation was 2 5 9 higher than in a solution to which
-
-
-
Comparison of Molybdenum Thiocyanate Color Intensity of Equal Concentrations of Molybdenum in Absence and Presence of Ferric Chloride, Sodium Nitrate, or Both
Bensitivity Range of ' Absorption Size of Colorimetera Cell, Cm.
3 3 3 3
1 1 1
i
F e + + + Or *Os- Absent Immediate 30 min. later
13.0 25.0 44.5
* * t
SS.0 *
1.0 12.0 1.0 23.0 1.0 40.0 2.0 78.0
Per Cent Absorption Using Green Filter Fe + + + b Present Nos-C Present Immediate 30 rnin. later Immediate 30 min. later
25.0 t 1.0 22.0 =t 1.0 25.0 * 1 . 0 24.0 1.0 44.5 f 1.0 38.5 * 1.0 44.5 f 1 . 0 44.0 1.0 85.0 * 2.0 75.0 * 2.0 8 5 .. 0. . . 2 . 0 R 1 . O. 2.0 ....... .......
* 1.0 t
=+
*
f
No. 7-089 Fisher electrophotometer. Concentration of F e + + +equal or greater than concentration of M o . 1 ml. of 5 N NaNOa in 100 ml. of solution. d Average deviations.
*
-
Fe + + + and NOS Present Immediate 30 min. later
1.0 25.0 * 1.0 23.0 * 1.0 44.5 * 1.0 41.5 * 1.0 85.0 ...* . . .2.0 . 80.0 . . . *. . 2.0 .
* 1.0 *. . 2.0 .
a b
____
-
-
1148
...
~
-
__
.
1149
V O L U M E 21, NO. 9, S E P T E M B E R 1 9 4 9 Table 11. Molybdenum Thiocyanate Color (Developed in HC1, extracted with ether,,and read immediately using No. 3 range of colorimeter) Concentration Per Cent Absorption of Molybdenum, Fe+++ Fe+++ P.P.M. absent present
12.0 24.0 35.0 44.5 70.0
0.5 1.0 1.5 2.0 3.0
24.5 45.5 67.0 85.0
..
Table 111. Intensity of Color in Aqueous 31edium and in Ether (Equal concentrations of molybdenum in presence of FeCla, using KO.3 rang? of colorimeter) Concentration Per Cent Absorption of hlolybdenum. Aqueous Ether P.P.M. (1 cm.) (1 cm.)
0.25 0.50 1 .oo 1.50
Table IF'.
12.0 24.0 45.0 67.0
12.0 24.5 45.5 67.0
Intensity of Molybdenum Thiocyanate Color
F~+ + + a in Pro ortion
t o hlolygdenum
(No.3 range of colorimeter used) 70 Sn++b Absorption (1 Cm.)
in Proportion
t o hlolybdenum
70
Absorption (1 Cm.)
7.0
'
a Concentration of molybdenum in solution 1.50 p.p.m. FeCja abse,nt. Concentration of molybdenum in solution 2.0 p.p.m.. reading immediate.
nitrate was never added, but equal to the intensity of molybdenum thiocyanate produced in the presence of nitrate or ferric ion. The addition of a large excess of stannous ion reduced the intensity of the molybdenum thiocyanate color in the solutions in which nitrate was absent but not in the solutions in which nitrate was present (Tables I and VI). The actual reduction of nitrate when added to hydrochloric or sulfuric acid solution of ammonium molybdate in the absence of stannous ion and potassium thiocyanate was demonstrated by the gradual production of nitrite as shown by the sulfanilic and OLnaphthylamine test for nitrite (8). A4similar reaction took place when nitrate was added to an acid solution of trivalent molybdenum but not in an acid solution in the absence of molybdate.
(100" C.) plant material (between 0.5 and 25 gramsj and ignite in a porcelain dish. After flaming ceases, heat over a low flame until the greater part of the material is oxidized, then place the dish in a muffle furnace a t 450" to 500" C. for 2 to 5 minutes. The foregoing steps take about 10 to 15 minutes. If the ash contains carbonates (ash from leguminous plants), neutralize with dilute nitric acid; if ash is siliceous (ash from grasses), wet with a 1 N sodium nitrate solution. Dry the treated ash first on a steam bath and then in a Hillebrand-Willard crucible radiator over a Bunsen flame; then return to the muffle for a minute or two to e n s m complete destruction of all organic carbon. The steps described require about 1 hour. Wet cooled ash with 1 to 1 hydrochloric acid, and then dry on a steam bath, To the dried ash add 28 ml. of 1 to 1 hydrochloric acid and let stand on steam bath for a few minutes before filtering through a No. 40 Whatman filter paper. Wash with 1 to 100 hydrochloric acid solution until the total volume of the filtrate is about 80 ml.; this solution is now ready for the determination of molybdenum. To solutions that are not distinctly colored with ferric chloride, add 3 or 4 drops of 0.01 N ferric chloride. This was necessary only occasionally with solutions obtained from the ash of certain grasses. Developing Molybdenum Thiocyanate Color. To the sample solution add 1.0 ml. of 5 N sodium nitrate and shake thoroughly; then add 6.0 ml. of 10% potassium thiocyanate, and shake again; add 1.0 to 6.0 ml. of stannous chloride dihydrate solution made up in 1 to 9 hydrochloric acid (amount to be added depends on the amount of ferric ion present judged by the disappearance of the Fe(SCX)--- color and the absence of fading on further addition of stannous chloride). Finally, make up to volume (100 ml.) with distilled water and shake thoroughly. Determine intensity of color immediately in a colorimeter using a green filter. Before each reading, adjust colorimeter to zero with distilled water. Correct the results for the molybdenum content of the chemicals used. Read the concentration of molybdenum in the sample from standard curves prepared from known molybdenum solutions n-hich contained both ferric chloride and sodium nitrate. The proposed modifications may possibly increase the precision a t the lower range of concentration (0.01to 0.50 p.p.m.) by stabilizing the color, for in this range even a small degree of fading introduces a large percentage error. As ferric iron is always present in solutions of plant ash, if it is absent from the standard solution used for comparison with the unknown, an error of nearly 50% may result. The elimination of extraction of the molybdenum thiocyanate color with a small volume of solvent not only simplifies and speeds up the procedure but also increases the precision, pnr-
Table V. Effect of Ferrous and Ferric Iron" on Intensity of Molybdenum Thiocyanate Color in Presence of Large Excess of Stannous Chloride (Concentration of molybdenum in solution 1.50 p.p.m., No. 3 range o! Colorimeter)
The coriclusion may be drawl that the increase in intensity of the molybdenum thiocyanate color by nitrate and ferric ion is possibly brought about partly by preventing the reduction of quinquevalent molybdenum to a lower state of valency by stannous ion and partly by the oxidation of reduced molybdenum which apparently is present in acid solution of molybdates. However, because conditions in such solutions are rather complex, other reasons may account for the intensification of the molybdenum thiocyanate color by iron or nitrate salts. A method for determining molybdenum in plant material was developed which eliminates the use of a solvent for extracting the molybdenum thiocyanate compound, as is the case in some of the recommended procedures ( 7 , Q ) . If the colorimeter in use contains an absorption cell with a depth of 5 cm., a concentration as low as 0.01 p.p.m. may be measured. If the concentration falls below this limit, the solution must be concentrated by extraction with a very small volume ( 5 ml.) of ether or butyl acetate. PROCEDURE
Ashing. To prevent the smoke, emitted during the early stages of burning plant material, from charring the muffle and smoking the room, add about 1 ml. of ethyl alcohol to the oven-dried
c/O ,
Treatment of Absorption hlolybdate Solution (1 Cm.) Fe + or Fe absent 38 0 Fe + added after color developed with S C S - a n d S n + T 51 0 F e + + added before color developed s i t h SCN - a n d Sn + 51.0 Fe added after color developed with S C N - and S n + + 51 0 F e + T +added before color developed with S C N - a n d S n + . 67.0 * F e + - + or Fe present in largrr amounts than molybdenum +
+
+ +
+
+ +
+
+ + +
Table VI.
Intensity of LMolybdenumThiocyanate Color in Solution of Ammonium Molybdate
(FeCls absent.
Concentration of Mo in solution 2.0 p.p.m. No. 3 range .of colorimeter)
S n + +in Proportion to Treatment of Ammoniuiri ?6 (l Cm.) Molybdenum Molybdate Solution Immediate 30 min. later 5 87.0 1 I 7 76.5 50:O 200 1 "Or not added 64.5 "Os not added 44.5 40:O 200 HNOs added a n d then removed by repeated evaporation t o near dryness in presence of HzSO,.
ANALYTICAL CHEMISTRY
1150
Table VII. Determination of Molybdenum in Plant Material by Thiocyanate-Stannous Chloride Method without Ether Extraction
Sample
Molybdenum In Original Sample, P.P.M.
Molybdenum Added, P.P.M.
Molybdenum Found, P.P.M.
Molybdenum Recovered, P.P.M. Yo
tained negligible amounts of molybdenum even with material high in silica; the molybdenum thiocyanate complex is determined directly in the hydrochloric acid solution without extraction; and nitrate and ferric ion are added to the unknown solution and to the standards. The method was tested by the addition of known amounts of molybdenum to plant samples before ashing. Table VI1 shows the molybdenum content obtained by the proposed method of the original samples as well as of samples to which molybdenum was added. The amounts recovered were nearly equal to those ad-led. LITERATURE CITED
Barshad, I., Soil Sci., 66, 187 (1948). Grimaldi, F. S., and Wells, R. C., IND. ENG.CHEW,ANAL. ED., 15,315 (1943).
tirularly in plant samples high in molybdenum where the possibility of incomplete extraction exists. The absence of the interfering ions, chromium, vanadium, tungsten, and rhenium ( 4 , IO), in solutions of plant ash also justifies the elimination of extraction. The concentration of molybdenum in solutions of plant ash encountered ranges approximately between 0.02 and 2.0 p.p.m. The sensitivity in the range between 0.02 and 0.20 p.p.m. is approximately 0.001 p.p.m. per 0.5% absorption and between 0.1 and 2.0 p.p.m. is approximately 0.01 p.p.m. per 0.5% absorption. The foregoing method differs from that proposed by RIarmoy ( 7 ) in that the filter paper and residue after filtration are not ignited and resulting ash fused with sodium carbonate, because spectrographic analysis and fusion revealed that the residue con-
Hiskev, C. F.. and Meloche. V. W., J . Am. Chem. Soc., 62, 1565,
isis (1940); 63,964 (1941).
Hoffman, J. I., and Lundell, G. E. F., J . Research Natl. Bur. Standards, 23,497 (1939).
Hurd, L. C., and Allen, H. O., IXD.ENG.CHEM.,A a . 4 ~ ED., . 7, 396 (1935).
Latimer, M .W., “Oxidation Potentials,” Sew York, PrenticeHall, New York, 1938. Marmoy, F. B., J . SOC.Chem. Ind.,58, 275 (1939). Rider and Mellon, IND.ENG.CHEM.,ANAL.ED., 18, 96 (1946). Sandell, E. B., “Colorimetric Determination of Traces of Metals,” New York, Interscience Publishers, 1944. Sandell, E. B., IND.ENG.CHEM.,ANAL.ED.,8, 336 (1936). Scott, W. W., “Standard Methods of Chemical A4nalysis,”New York, D. Van Nostrand Co., 1939. RECEIVED .4pril 26, 1948.
Use of Tributyl Phosphate for Separating Acetic Acid from Hydrochloric Acid H. ARMIN PAGEL, PAUL E. TOREN,
AND
FRED W. MCLAFFERTY
University of Nebraska, Lincoln, Nebr.
N .W earlier paper it was shown that the fraction of acetic Iphosphrtte acid extracted from aqueous solution by means of n-tributyl is essentially independent of the acid concentration. (1)
Hydrochloric acid, however, showed a pronounced increase of extraction with increased concentration; hence a reasonably complete separation of acetic acid from hydrochloric by this method appeared questionable if the concentration of the latter were fairly high. Work recently completed shows, hoxever, that the fraction of the hydrochloric acid extracted by the ester phase can be expressed by the equation:
This relation holds for mixtures of hydrochloric acid and sodium chloride as well as for hydrochloric acid alone, as shown in Tables I and 11. Therefore, if the hydrochloric acid in a mixture of the ti?-oacids is partially neutralized to a pH approximately the same as that of a comparable concentration of acetic acid alone, a satisfactory separation should take place. This wa- found to he true. PROCEDURE
In the work reported earlier, the amount of hydrochloric acid extracted into the ester phase was determined by draining the aqueous layer (lower) from a separatory funnel and then determining the acid in the ester by titrating with standard base. Because the hydrochloric acid concentration in the aqueous phase is always very high compared to that in the caster, significant errors resulted from the aqueous film left in the separatory funnel. The procedure was modified as follows: Large volumes (80 to 100 ml.) of ester and of aqueous solution were put into a 250-ml. glass-stoppered flask, which was placed
in a constant temperature bath (25.00’ * 0.05” C.) and shaken vigorously for about 1 minute a t 15-minute intervals for several hours, without being removed from the bath. Complete phase separation took place after about 10 hours a t constant temperature, after which pipetted portions of each phase were analyzed. The hydrochloric acid was removed from the ester sample by extracting three times with about 10 ml. of water each time, followed with a fourth portion containing a small amount of base. The hydrochloric acid was then determined gravimetrically as silver chloride. The acid in the aqueous samples was determined by titration with carbonate-free sodium hydroxide. Where mixtures of hydrochloric acid and sodium chloride were
Table I. Distribution of Hydrochloric Acid in Two-Phase System Watern-Tributyl Phosphate Hydrochloric Acid, .lf Ester phase Water phase
=
0,035 0.035 0.034 0.034 0.036 0.037 0.038
0,00390 0,00124
0.187 0 334 0.631 0,930 1.211 1.445 1.808
(CHCl)eater ( C H CCI-)wster
0.0134 0,0292 0.0518 0.0771 0,1257
Table 11. Distribution of Hydrochloric Acid in Two-Phase System Watern-Tributyl Phosphate, in Presence of Sodium Chloride Water Phase (Cl-), -21
(H+), M 0.0822 0.0970 0.442 0,446 0,874 0.879 0.884 0.886 0.890
1.078 1,105 0,688 1.460 1.877 1,126 1.918 1.398 1,017
Ester Phase (HCL), M ’
(CHCl)eiter = K ( c H + CCI-)water
0.00288 0,00348 0.00992 0,0203 0.0577 0.0331 0.0596 0.0418 0,0301
0.033 0,033 0.033 0.031 0,035 0.034 0.035 0.034 0.033
