Determination of Chromium, Vanadium, and Molybdenum in Silicate Rocks E. B. SANDELL, School of Chemistry, University of Minnesota, Minneapolis, Minn.
IiS
T H E following there is described a method for the d e t e r m i n a t i o n of chromium, vanadium, and molybdenum in silicate rocks, which is less laborious and time-consuming than that of H i l l e b r a n d (6) and which, in spite of the smaller sample required, can be used for the e s t i m a t i o n of s m a l l e r amounts of these elements than is possible by the Hillebrand method. A total sample of 1 gram suffices for the determination of as little as 0.001 per cent of vanadium, 0.001 per cent of chromium, and 0.0001 per cent of molybdenum. The chief novelty in the proposed method lies in the determination of vanadium. The determination of traces of these elements is not of great importance in ordinary rock analysis but is required in special studies, as, for example, those concerned with the geochemical d i s t r i b u t i o n of the elements named.
A colorimetric method is described for the determination of chromium, vanadium, and molybdenum in silicate rocks in amounts as small as 0.001,0.001, and 0.0001 per cent, respectively, all in a 1-gram sample, which is decomposed in the usual way with sodium carbonate. Vanadium is determined with phosphotungstic acid after separation from chromium with 8hydroxyquinoline; in neutral or slightly acid solutions, quinquevalent vanadium reacts with 8-hydroxyquinoline to give a compound that can be extracted from the aqueous solution with chloroform, whereas sexivalent chromium does not react and remains in the aqueous layer. Diphenylcarbazide is used as reagent for chromium, after the separation of the element from vanadium. Molybdenum is determined by the stannous chloride-thiocyanate-ether method without previous separation from the other constituents of the filtered leach of the sodium carbonate melt.
molybdenum thiocyanate w i t h e t h e r , and comparison of the color of the latter with a standard molybdenum thiocyanate solution obtained in a similar manner.
Discussion of the Method
VANADIUM.It has recently been shown by Montequi and Gallego (IO; cf. 8) that very small amounts of vanadium can be detected by adding an acetic acid solution of 8-hydroxyquinoline to a neutral or very slightly acid vanadate solution and shaking with chloroform. Quinquevalent vanadium reacts with 8-hydroxyquinoline to give a compound soluble in chloroform with a Bordeaux red to black color. The strongly c o 1o r e d substance formed in the test appears to have the formula. (CsHeON)4V203. Attempts made in this laboratory to base a direct colorimetric determination of vanadium on the reaction of Montequi and Gallego w e r e u n s u c c e s s f u l . chieflv because of the interference of iron. The filtered aqueous e;tract of the sodium carbonate melt of an igneous rock usually contains traces of iron, which will react with 8-hydroxyquinoline to give a quinolate soluble in chloroform with an intense greenish black color; the sensitivity of this reaction is as great as that for vanadium. No way of preventing the interference of iron could be found that did not simultaneously destroy, or interfere with, the vanadium reaction. Consequently, in the method here described, the reaction between vanadate and 8-hydroxyquinoline has been used for the isolation and concentration of vanadium, which can then be determined colorimetrically by some method in which traces of iron (and other elements) do not interfere. Before vanadium can be extracted from the aqueous extract of the sodium carbonate fusion, it is necessary to render the solution slightly acid. It was found that the extraction of vanadium is complete, or practically so, a t a p H corresponding to the intermediate color of methyl orange-a convenient reference point; the extraction is also complete from a solution neutralized to the intermediate color of bromocresol green. The presence of neutral alkali salts, such as sodium sulfate, does not interfere. Moreover, there is no interference from silica, aluminum, pFsphate, arsenate, fluoride, chloride, borate, or chromate-1. e., from those constituents, all or some of which may be present in the filtrate from the sodium carbonate leach of a rock fusion. Ferric iron, sexivalent molybdenum (in small amounts), and uranium accompany vanadium into the chloroform. Tungsten yields a precipitate with 8-hydroxyquinoline in the slightly acid solution which does not appear to be appreciably soluble in chloroform; the tungsten precipitate gathers a t the chloroform-
Outline of the Method The sample is decomposed in the usual manner by fusion with sodium carbonate, the melt is leached with water, alcohol is added t o reduce the manganate, and the mixture is filtered. The filtrate, containing chromate, vanadate, and molybdate, is diluted to the mark in a volumetric flask. If more than 0.01 or 0.02 per cent of chromium is present, this element can be determined directly in the usual manner by comparing the color intensity of the solution with that of a standard potassium chromate solution. For the determination of vanadium, a suitable aliquot portion of the solution is taken, neutralized t o methyl orange with sulfuric acid, and extracted with chloroform after the addition of an acetic acid solution of 8-hydroxyquinoline. Chromate does not react with P~-hydroxyquinoline,whereas vanadate forms a compound with the reagent which is extracted by the chloroform, thus leaving chromium in the aqueous solution. The chloroform extracts are evaporated to dryness, and the residue is fused with sodium carbonate to convert vanadium into sodium vanadate, which is dissolved in water. The solution so obtained is treated with phosphotungstic acid to form phosphotungstovanadic acid which possesses an intense yellow to orange color and can thus be determined colorimetrically by comparison against a vanadate solution of known concentration similarly treated. If the chromium content of the sample is very small, the colorimetric comparison against chromate leads to inaccurate results in most cases because of the presence of traces of foreign substances, chiefly iron, which impart a slight color to the solution. In such cases, a small aliquot portion of the sodium carbonate leach is neutralized with sulfuric acid, treated with 8-hydroxyquinoline, and the solution extracted with chloroform to remove vanadium. Chromium is then determined in the solution from which vanadium has been removed, by the addition of diphenylcarbazide and sulfuric acid and comparison of the red-violet color (of the diphenylcarbazide oxidation compound) with that of a known chromate solution treated in the same way. Molybdenum is determined in another aliquot portion of the solution by acidification with hydrochloric acid, addition of potassium thiocyanate and stannous chloride, extraction of the 336
SEPTEMBER 15, 1936
ANALYTICAL EDITION
water interface, causing trouble by preventing the coalescence of the chloroform droplets after shaking. The vanadium in the chloroform extract can easily be converted into sodium vanadate by evaporating the solvent and fusing the residue with sodium carbonate. The isolated vanadium can be determined colorimetrically by the use of a number of reagents giving sensitive color reactions with the element. It was found in the present work that phosphotungstic acid is especially suitable for the purpose. (Winogradow, 14, has used phosphotungstic acid for the colorimetric determination of vanadium in ashes of organisms.) The strongly colored yellow compound that this reagent yields with quinquevalent vanadium in acid solution is presumably phosphotungstovanadic acid. Vanadate gives a yellow color with tungstate alone in slightly acid solution, but the color is much intensified by phosphoric acid. The sensitivity of the phosphotungstic acid reaction for vanadate is several times as great as that with hydrogen peroxide. Moreover, it seems that the yellow color given by the former reagent can be more accurately matched in a colorimetric comparison than can the reddish brown given by hydrogen peroxide. With phosphotungstic acid as reagent it is possible to detect 0.005 mg. of vanadium in 10 ml. of solution when the latter is viewed through a thickness of 5 cm.; by using a blank for comparison, 0.002 mg. of vanadium can be detected under the same conditions. The reaction is best carried out by adding in succession to the neutral or slightly acid vanadate solution, 85 per cent phosphoric acid and sodium tungstate solution; the quantities specified in the procedure provide a sufficient excess of reagent to give the maximum color intensity for such quantities of vanadium as can be present for satisfactory color comparison in 10 ml. The color development is immediate, and the color intensity undergoes no change with the lapse of time, It is therefore possible to make the color comparison by colorimetric titration-i. e. , by adding a standard solution of vanadate to the phosphotungstic acid reagent contained in a tube identical with the one holding the unknown until the colors of the two solutions match. This method of comparison is to be preferred to the use of a colorimeter when the amount of vanadium being determined is very small, since the former method is more sensitive. The results given in Table I serve to indicate the performance of the phosphotungstic acid method for vanadium in pure solutions and in the presence of such substances as have a bearing on the application of the procedure in rock analysis. The values recorded in the table were all obtained by colorimetric titration as described in the procedure, the solutions being contained in 30-ml. cylindrical tubes, 15 cm. in height and with flat bottoms. In most cases the final volume was near 10 ml. Silica, alkali metal salts, fluoride, borate, and arsenate do not interfere. Aluminum and especially ferric iron diminish the color of the phosphotungstovanadic acid and lead to low results, whereas molybdate gives high results. The amount of iron going into the filtrate from the sodium carbonate melt of an igneous rock is too small to cause appreciable error. Molybdenum does not occur in sufficiently large quantities in rocks to introduce any error. Table I1 contains the results obtained in the application of the proposed method to a synthetic basic rock with added amounts of vanadium, and also to a few natural rocks to which were added known amounts of the element. The analysis was made as described in the procedure below, the vanadium first being separated with 8-hydroxyquinoline and chloroform. Except in Nos. 11, 19, and 20, the aliquots used corresponded to 0.1 gram of sample. It may be concluded from these results that the method gives values of satisfactory accuracy for Vz03contents ranging from 0.001 to 0.1 per cent. Chromium is without effect. The use of a mixture of sodium carbonate and potassium nitrate, instead of sodium carbonate
331
alone, for the decomposition of the rock appears to be unnecessary. The amount of vanadium retained by the leached residue from the sodium carbonate fusion appears to be small enough to neglect (No. 6). I n a few cases a little difficulty arose from the formation of an emulsion of chloroform and the aqueous solution, this being more prone to occur with the acid rocks. In the absence of appreciable amounts of chromium, vanadium can be determined with fair accuracy without previous separation with 8-hydroxyquinoline and chloroform if the amount present is not too small. The figures in Table I11 (obtained with rocks practically free from chromium) indicate the accuracy that may be expected. The values for the direct determination were obtained by acidifying 10 ml. of filtrate corresponding to 0.1 gram of sample and adding phosphotungstic acid. Generally the hues of the comparison solution and the rock filtrate are not the same, and great exactness in matching is not possible. Although the direct method is not recommended, it may be of some value when an approxi-
TABLEI. DETERMINATION OF VANADIUMBY COLORIMETRIC TITRATION WITH PHOSPHOTUNGSTIC ACID AS REAGENT No:
Addition
1
2
3 4 5
6
7 8 9 10 0.2 gram of Na?SO& 11 0.5 gram of NazSOl 12 1.0 gram of NaCl 13 0.10 gram of Si02 as sodium silicate 1 ml of 6 N Hzs04 14 10 mg. of A1 as AlCls 15 50 mg. of A1 as AlCls 16 0.5 mg. of Fe as FeCls 17 1.0 mg. of Fe as FeCls 18 1.0 mg. of Fe as FeCls 0.5 ml. of S5%
+
+
&Po4 19 10 mg. of N a F 0.1 ml. of 6 N &So4 20 0.1 gram of HsBOs 21 0.5 mg. of Mo as NH4 molybdate 22 1.0 mg. of Mo as NH4 molybdate 23 10.0 mg. of Mo as NH4 molybdate 24 50 mg. of NazHAsOa
+
.
Vena-
Vana-
Taken dium Mg. 0.003 0.005 0.012 0.020 0.022 0.031 0.032 0.040 0.050 0.020 0.025 0.025
Found dium
Mg.
Mg.
0.003 0.004 0.012 0.021 0.021 0.029 0.030 0.041 0.051 0.019 0.025 0.023
0.000 -0.001 0.000 $0.001 -0,001 -0.002 -0.002
0.025 0,025 0.025 0,025 0.025
0.024 0,022 0 017 0.023 0.021
-0.001 -0,003
0,025 0.025 0.025 0.020 0.020 0.020 0.025
0.021 0.025 0.025 0.022 0.023 0.026 0.024
-0.004 0.000 0,000 $0.002 $0.003 $0.006 -0.001
Error
+O.OOl
+0.001 -0,001 0.000 -0.002
-0.00s
-0,002 -0.004
TABLE 11. DETERMINATION OF VANADIUM IN SILICATE. ROCKS BY THE 8-HYDROXYQUINOLINE-PHOSPHOTUNGSTIC ACID METHOD vzos vzoa VrOs Sample
NO.
1 Svnthetic basic rock5
Present
Added
%
%
.... ....
% 0 . OOOb
.... .... ..,.
0.004 0.015 0.042 0.060 0.105 0.105
0.0027 +0:0006 0.005 +0.001 0.014 -0.001 -0.002 0.040 0.059C -0.001 +0.005 0.110 -0.004 0.101
tirzVs
....
0.006
0.005
-0.001
Crz03
....
0.0120 0.0030 0.0066 0.049
0.0115 0.0040 0.007 0.065
-0.0005 -0.0005 -0.001 -0.003
0.033
0.062
.... .... .
7 Synthetic basic rock 8 Synthetic basic rock 9 Synth_etio basic rock with 1%
10 Synthetic basic rock with 1%
11 12 13 14 15 16
.... 0.0021
Found
Granite (0.5 gram) 2% F Granite Gabbro (0.005% CrzOa) Gabbro (same as No. 13) Diabase (same as No. 15)
+
.
I
.
0.0015 0.0015 0.019 0,022d 0.029 0.029d 0.044 0.041d
17 Shale 18 Shale (same as No. 17) 19 Magnetite-ilmenite sand (0.01 gram) e 0.245 20 Magnetite-ilmenite sand (same as No. 19) 0.23d
,...
.... ....
....
....
.... .... ....
....
0.000
....
..... ...
0 Percentage composition: SiOz, 55; AlzOs, 20: FezOa, 10; MgO, 6; CaO, 7. Ti?*, 1; PzO1, 0.5; MnO, 0.5. b Senktivitv of method for 0.5-eram s a m d e < 0.0005 Der cent VaOa. 0 kefkion the Mashed residu; from the Brat sodium carbonate fusion yielded an additional 0.001 per cent of VzO.2. d Sample fused n i t h Sa?COs T KSO3 (4 to 1) instead of with NaECOa alone as in other determinations. Fused 1 hour, instead of 20 t o 30 minutes as in other determinations.
of
338
INDUSTRIAL AND ENGINEERING CHEMISTRY
mate figure for vanadium in a rock sensibly free from chromium is desired. CHROMIUM.For the determination of very small amounts of chromium in rocks, the usual method involving the comparison of the color of the filtrate from the sodium carbonate melt with a potassium chromate solution of known concentration is unsatisfactory, not so much because of insufficient sensitivity as on account of interference from traces of foreign substances which impart a color to the solution. It is usually found that the filtered leach of the sodium carbonate melt of a rock possesses a yellow-brown tinge, even when no chromium is preBent. This slight coloration is probably due largely to colloidal hydrous ferric oxide. If nitrate is used in the fusion and the crucible is attacked, a yellowish coloration may be imparted to the solution by the dissolved platinum. Incomplete reduction of manganate, and failure to wash traces of coloring matter from the filter paper with hot strong sodium carbonate solution may also lead to false coloration of the filtrate, but these sources of error are easily avoided. The presence of iron in the filtrate seems to result from the peptization of hydrous ferric oxide by silica. The error may be considerable in the case of acid rocks containing relatively much iron. Thus a rhyolite high in iron showed an apparent Cr20acontent of 0.005 per cent by the usual method, when actually no chromium could be detected by the sensitive diphenylcarbazide reaction described below, and a granite gave an apparent percentage of 0.004 C1-203 when actually only 0.0006 per cent was present. With basic rocks the error may be smaller. For example, a gabbro containing 0.005 per cent of Cr203gave a n apparent value of 0.007 to 0.008 per cent, and the synthetic basic rock of Table I1 showed 0.004 to 0.005 per cent instead of the true value, 0.002 per cent. Therefore, it is desirable to base the colorimetric determination of traces of chromium on the use of a reagent which gives a sensitive specific reaction with the element. Such a reagent is diphenylcarbazide, which, as discovered by Cazeneuve ( I ) , reacts with sexivalent chromium in acid solution to give an intensely colored red-violet oxidation product. A colorimetric
VOL. 8, NO. 5
method for chromium has been based on this reaction by Moulin ( l a ) and others. The method does not appear to have been applied to the determination of chromium in silicates, although the reaction is used for the detection of the element in rocks and minerals (a). In the present work it was found that a fairly satisfactory determination of traces of chromium in silicates could be obtained by the use of diphenylcarbazide. Quinquevalent vanadium interferes by reacting with the reagent in acid solution to give a strongly colored yellow compound. However, vanadium can be removed from the solution as previously described by the addition of 8-hydroxyquinoline and extraction of the vanadium hydroxyquinoline compound with chloroform; sexivalent chromium is not reduced in the process. The excess of 8-hydroxyquinoline is simultaneously extracted by the chloroform from the neutralized solution. If the amount of V Z Odoes ~ not exceed the amount of CrzOB, it is usually unnecessary to remove the vanadium. Although the hue of the chromium-diphenylcarbazide solution is then altered, becoming more red, no appreciable error results. A brief study was first made of the determination of chromate in pure solution with diphenylcarbazide. Without giving the numerical data obtained, the results may be summarized as follows : The stability of the red-violet compound formed in the oxidation of the diphenylcarbazide by chromate is good under the proper conditions. Of chief importance is the acidity of the solution, which must be sufficient to permit the development of the maximum color intensity and to prevent the interference of other elements that may give a color at low acidities. A too high acidity leads to fairly rapid fading of the chromate-diphenylcarbazide color. Thus, the color of a solution containing the equivalent of 0.010 mg. of Cr203,5 ml. of 6 N sulfuric acid, and 2 ml. of 0.25 per cent diphenylcarbazide in 25 ml. showed a decrease in intensity amounting t o 7 or 8 per cent in 1 hour; solutions containing less chromium are more unstable, other conditions being the same. When the acidity was reduced to approximately 0.2 N (1 ml. of 6 N sulfuric acid in 25 ml. of final volume) the color was more stable. After 1 hour, with the same amount of chromium and reagent as before, the color intensity in this case had decreased by 3 per cent on the average. Moreover, at the lower acidity the color given by low chromate concentrations (0.1 mg. of c1-203per liter) was nearly as stable as that of more concentrated solutions. The excess of diphenylcarbazide has but a small effect on the intensity and stability of the color. Thus, with 0.010 mg. of CrzOs in 25 ml. of 0.2 N sulfuric acid, the color intensity was the same within a few per cent whether the amount of diphenylcarbazide solution used was 2, 1, 0.5, or 0.2 ml. of 0.5 per cent reagent in 1 to 1 acetone, and the color stability was approximately the same in all cases. The reproducibility of the color is not all that might be desired, as illustrated by the following figures (expressing relative color intensities) obtained by adding a mixture of 1 ml. of 0.25 per cent diphenylcarbazide, 1 ml. of 6 N sulfuric acid, and 3 ml. of water to 15 ml. of water containing the designated amounts of chromium, and diluting to 25 ml. 0.005 mg. of CrzOs--1.00, 0.96; 1.00, 1.02 0.010 mg. of CraOs--1.00, 1.025, 1.015; 1.00,1.02, 1.025 0.015 mg. of CrzOa--1.00, 1.01
Beer’s law is followed (Figure 1). The presence of sodium sulfate does not alter the hue or intensity of the chromate-diphenylcarbazide color, even when as much as 5 per cent of the salt is present in the solution.
FIGURE1. RELATION BETWEEN CHROMATE CONCENTRATION AND COLOR INTENSITY IN DETERMINATION OF CHROMIUM BY THE DIPHENYLCARBAZIDE METHOD
The results given in Table IV were obtained by following the directions given in the procedure. Chromium was added to the samples in the form of chromic sulfate. The method proposed gives, on the whole, satisfactory results. There is a distinct tendency for the chromium values to be slightly low. The color of the filtered sodium carbonate leach of the rock, after removal of vanadium and addition of diphenylcarbazide, usually does not have exactly the same hue as that of the standard chromate solution. The difference ismost marked at very low chromium concentrations; then the sample solution generally possesses a distinct brownish tinge, in contrast to the
SEPTEMBER 15, 1936
ANALYTICAL EDITION
339
the stability of the colored product is increased, and at the same time the substance is concentrated in a small volume of solvent, so that the determination of extremely small amounts of molybdenum becomes possible. In this way as little as 0.001 mg. of the element can be determined in 100 ml. of solution. Rhenium gives the same reaction as molybdenum (S),but it may be expected that this element will not be present in detectable amounts in silicate rocks. It is advantageous to make the colorimetric determination of TABLE 111. DETERMIXATION O F VANADIUMI N ROCKS WITHOUT the molybdenum thiocyanate in the ether by colorimetric titraPREVIOUS SEPARATION WITH 8-HYDROXYQUINOLINE AND CHLOROtion: adding a standard ethereal solution of molybdenum thioFORM cyanate to pure ether having nearly the same volume as the unknown solution and contained in an identical tube, until the colors VzOt Found match. By using color-comparison tubes of suitable size, it is %Hydroxyquinoline Direct Difference, possible to determine as little as 0.0001 per cent of molybdenum No. Samde method method VaOa trioxide when a 0.5-gram sample is taken. Ethyl ether is not an % % % ideal solvent for the extraction and for making up the standard -0,006 Synthetic basic rock 0.042 0.036 molybdenum solution, because of its volatility. However, there 0.11 $0.005 0.105 Synthetic basic rock are certain objections to the use of other solvents, such as butyl -0.003 0.023 Diorite 0.026 acetate and cyclohexanol as shown by Hurd and Allen ( 7 ) , and +0.003 0.022 Gabbro (0 005 CrzOs) 0.019 10.005 0 . 06to0.07 0.065 Gabbro (0:0052 CrzOs) therefore eth 1 ether was employed in the present work; it is +0.002 Diabase 0.031 0,029 satisfactory irreasonable care is taken in its use. -0.007 0.055 Diabase 0.062
pure red-violet of the comparison solution. At higher chromium concentrations the difference in hue is very slight and rarely gives any trouble. As little as 0.0005 per cent of CrzOa can be detected when a 0.1-gram sample is taken and vanadium is first removed.
8 Shale 9 Magnetite-ilmenite sand
0.044 0.245
-0.005
0.039 0.245
0.00
O F CHROMIVM IN ROCKS WITH TBLE Iv. DETERMINATION DIPHENYLCARBAZIDE
CrnOa Present
Sample
NO.
%
1 Synthetic basic rocka
2 Svnthetic basic rock 3 Sjmtlietic basic rock4 4 Synthetic basic rock 5 SyntLetic basic rock* 6 Syntlietic basic rock 7 Synthetic basic rock 8 Synthetic basic rock 0.10% V9O.b 9 Synthetic basic rock $ 0 .
+
Vdhb
10 11 12 13
0
Rh&lite (VnOs = nil)bXc Rhyol!tebfc Rhsoliteb Rhvoliteb
....
CrsOa
CnOs
Added
Found
%
%
....
0.002 0.002 0.002 0I002 0.002 0.002
0.005 0.005 0.010 0.020 0.020 0.050
0.002
0.005
0.002 0.050 0.000 0.0010 0.000 0.0020 0.000 0.0020 0.0050 0,000 0.0050 0.000 0.000 0.002 0,000 0.002 0.000 0.000 0.0006 0.0020 0.005 0.010 0.005 0.000 0.005 0.010
Error
CrnOs
%
.... 0,002 0.0065 -0,0005 0.007 0.000 0,012 0.000 0.023 $0.001 0.000 0.022 0.053 $0.001 0.0065 -0.0005 -0.002 -0.0002 -0.0004 0,0017 -0.0003 0.0044 -0.0006 0.0000 0.0050 0.050
0. nons
0.ooi6 0.001
0.0015 0.000 0.0025 0.014 0.0045 0.0135
-0,001
-0.0005 0.000 0.000 -0,001 -0.0005 -0.0015
For com osition, see Table 11; 0.2-gram sample used in No. 1, 0,I gram
in all other feterminatlons.
Treated with 8-hydroxyquinoline and extracted with chloroform; others not so treated. C Chromium (as chromate) added t o filtrate of leached melt: in all others, chromium added before fusion. 6
MOLYBDENUM.For the colorimetric determination of this element in silicate rocks, a very sensitive reaction must be made the basis of the method, since the amount of Moo3 present may be no more than 0.001 t o 0.0001 per cent. Attempts t o make use of potassium ethyl xanthate (9) as reagent for the purpose resulted unsatisfactorily in the author's hands, as has also been the experience of others. Phenylhydrazine (11) is not well suited for the determination of very minute amounts of molybdenum because of insufficient sensitivity. (von Hevesy and Hobbie, 5, in a paper on the determination of tungsten and molybdenum in rocks have described the determination of molybdenum with phenylhydrazine as colorimetric reagent; these authors used samples weighing from 150 to 270 grams for to 10-3 per cent.) molybdenum contents in the range The familiar potassium thiocyanate-stannous chloride method for molybdenum was, however, found excellent for the minute amounts of the element occurring in igneous rocks. (Stanfield, IS, has applied this method to t'he determination of molybdenum in plant ashes and soils, first isolating the element as the sulfide and finally extracting the molybdenum thiocyanate with butyl acetate. Hauptmann and Balconi, 4, have used both the thiocyanate-stannous chloride method, without an extractant, and the phenylhydrazine method for the determination of small amounts of molybdenum in manganese minerals.) The reaction can be applied directly to the filtered leach of the sodium carbonate melt of the sample. By using ethyl ether to extract the molybdenum thiocyanate compound from the aqueous solution,
The method proposed was tested with synthetic mixtures, simulating acid and basic rocks, t o which known amounts of molybdenum were added, and also with rocks of various types. The results are tabulated in Table V. Other elements do not, in general, interfere. The major rock constituents, including phosphorus, are without effect; chromium, vanadium, uranium, tungsten, tantalum, and fluorine do not interfere, at least in the amounts that these are met with in silicate rocks. Much fluoride leads t o low results. Small amounts of iron, such as may be found in the sodium carbonate filtrate, give no coloration when the directions given below are followed, as was proved by the addition of 0.5 mg. of iron as the chloride. Any separation of silica after acidification of the filtrate leads to low results, presumably because of the adsorption of molybdenum. It was feared that traces of platinum might interfere, since chloroplatinous acid is soluble in ether with a yellow color; however, thiocyanate reduces the sensitivity of this reaction. Blanks showed that there is no significant error due t o slight attack of the platinum crucible by the sodium carbonate melt. Tungsten in appreciable amounts (0.05 per cent) appears t o alter the hue of the ether solution of the molybdenum thiocyanate, making it more yellow, and thus renders the color comparison less exact. The small amounts of tungsten usually encountered in igneous rocks do not interfere. Even in the absence of tungsten, differences in hue between the unknown and standard solutions were sometimes noted. In every case in which there was a difference, the standard possessed a more orange tint. These differences were usually slight, and in no case, is it believed, was a n error greater than * 10 per cent introduced on this account; therefore the cause, or causes, of the difference in hue was not investigated. It is important that the ether used for diluting the standard molybdenum thiocyanate solution be shaken with stannous chloride and potassium thiocyanate before use; ether not so treated is likely to bleach the color of the molybdenum thiocyanate and give a decided reddish hue to the normal yellow-brown color of the standard. The standard solution is not stable; it becomes darker on standing. I n determining minimal quantities of molybdenum by this method, it is important to run a blank on the sodium carbonate used for the decomposition; in the most accurate work only sodium carbonate showing a negligible blank should be employed. The c. P . anhydrous sodium carbonate of one manufacturer gave a blank corresponding to 3.5 * 1 X l o + per cent of hfo03, whereas the product of another gave a barely detectable reaction corresponding to 1 X per cent of MoOa. There appears to be no appreciable retention of molybdenum by the leached residue, a t least for amounts in the range of 0.01 to 0.0001 per cent of hfOO3.
INDUSTRIAL AND ENGINEERING CHEMISTRY
340
TABLEV. DETERMINATION OF MOLYBDENUM IN ROCKSBY THE THIOCYANATE-STANNOUS CHLORIDE-ETHER METHOD NO.
Moos
Sample
Present
% 1 Synthetic acid rock (0.5 gram)' 2 Synthetic acid rock (0.5 gram) 3 Synthetic acid rock (0.1 gram) 4 Synthetic hasic rock (0.5 gram)b 5 Synthetic basic rock (0.5 gram) 6 Synthetic basic rock (0.1 gram) 7 Granite (0.1 gram) 8 Granite 10% PZOS(0.1 gram) 9 Granite 0 5 F (0.1 gram) 1 : 3 7 F (0.1 gram) 10 Granite 0.5k U 1% T a 11 Granite
+++ +
rn.1 pram)
17 Slate 18 Synthetic acid rock (0.5 gram) 19 Synthetic acid rock WO1
+
MoOa Added
%
Found
MOOS
Error, Moos
%
%
0.0000
0.00070 0.00075 +0: 06005 -0.001 0.010 0,009 0.0000 0.0000 0.00040 0.00045 +O:&l005 -0.001 0.006 0 ~ 0 0 0 0 0.007 0.008 +0.001 0.0003 0.007 $0.001 0.0003 0.007 0.008 0.000 0.007 0.0003 0.007 0.0055 -0,002 0.0003 0.007 0.0000 0.0000
0.0003 0.0001 0.0001 0.0001 0.00015 0.00015 0.00005
+ 0.5% F 0.0000 + 0.05% 0.0000
0.007 0.0075 0.00055 G O , 0006 0.0002 0.00025 0.0002 0.00035 0.0003 0 0,0005 0.0007 0.00075 0.00055C0.00055
0.000 -0.00005 -0.00005 $0,00005 -0.00005 -0.0001 -0.00005
0.0007
0.0008
+O.OOOl
0.0007
0.0009 +0.0002
a Percentage composition: 6101, 77; AbOa, 17; FelO8, 2.1; MgO, 1.3; CaO, 2.2; TiOi,,O.2; PzOS,0.1: MnO, 0.1. b For composition see Table 11. c Molybdenum added to filtrate of sodium carbonate melt; in all other determinations element added to sample before fuslon.
Procedure DECOMPOSITION OF SAMPLE. Weigh 1 gram of 100-mesh rock powder into a platinum crucible and mix with four to five times as much powdered anhydrous sodium carbonate (reagent quality). Cover the crucible, heat to fusion, and keep at the full heat of a Meker burner for 20 to 30 minutes, or longer if relatively much chromite or magnetite is present. Allow the crucible to cool to room temperature, add a few milliliters of water to cover the melt, and then warm with a small flame to loosen the cake. Transfer the cake to a small Pyrex beaker, rinse the crucible with hot water, add 2 to 5 drops of ethyl alcohol (de ending upon the amount of manganate present) and 30 or 40 my of water to the beaker, and heat on the steam bath. During the digestion aid the disintegration of the cake by crushing and grinding with a glass rod flattened at one end. When the residue has been well disintegrated and the manganate reduced, filter the liquid through a small fine-grained filter paper which has been previously washed with hot 20 per cent sodium carbonate solution to remove traces of coloring matter. Wash the insoluble material in the beaker and on the paper with hot 1 per cent sodium carbonate solution (four or five 5-ml. portions will usually suffice).
If the filtrate has a distinct yellow color, determine chromium in the combined filtrate and washings by the usual method of comparison against a standard solution of potassium chromate containing sodium carbonate, using a colorimeter or Nessler tubes, depending on the strength of color. For chromium contents less than 0.01 or 0.02 per cent of Crz03, the value thus obtained may be appreciably in error and it is then better to determine chromium with diphenylcarbaeide as described below. VANADIUM.Special solutions are prepared as follows : 8-Hydroxyquinoline, 2.5 per cent. Dissolve 2.5 rams of finely powdered 8-hydroxyquinoline in 100 ml. of 2 # ( l to 8) acetic acid. Sodium tungstate, 5 per cent. Dissolve 5 grams of Na2W042Hz0 (reagent quality) in 100 ml. of water. Standard vanadium solution. Prepare a solution of ammonium metavanadate (or other alkali vanadate) containing the equivalent of approximately 0.01 mg. of V2OSper ml. by diluting a stronger solution. Standardize the stronger solution in the usual manner by reducing the acid solution with sulfur dioxide and titrating with standard potassium permanganate after expulsion of sulfur dioxide. Make up the combined filtrate and washings from above to 100 ml. in a volumetric flask. For the determination of vanadium, transfer 10 to 25 ml. of this solution, depending upon the probable vanadium content (for basic and intermediate rocks such as diabases, gabbros, and diorites, 10 ml., corresponding to 0.1 gram of sample, will usually suffice, whereas for acid rockse. g., granites-25 ml., or better 50 ml., should be taken) to a 50-
VOL. 8, NO. 5
or 100-ml. Erlenmeyer flask, and add one drop of methyl orange indicator solution. Then add 4 N sulfuric acid carefully from a buret until the solution assumes the intermediate color of the indicator; make a note of the volume of acid required if chromium is to be determined later with diphenylcarbazide. Swirl the liquid to liberate carbon dioxide present in supersaturated solution, and transfer the solution to a small separatory funnel. Add 2 ml. of chloroform (analytical reagent) and 0.1 ml. of 8hydroxyquinoline solution. Shake moderately vigorously for 1 minute, allow the layers to separate, and draw off the chloroform into a platinum crucible; add 1 ml. of chloroform to wash out the stem of the funnel. Then add 2 ml. of chloroform and 0.1 ml. of 8-hydroxyquinoline to the solution in the funnel, shake as before, and transfer the chloroform to the platinum crucible. Repeat the extraction once more with chloroform and 8-hydroxyquinoline. A total of three extractions should suffice for the majority of rocks. The last chloroform extract should show only a faint yellowish coloration due to 8-hydroxyquinoline itself. To the crucible containing the combined chloroform extracts add 0.10 gram of anhydrous sodium carbonate, and eva orate to dryness a t a low temperature. Then heat the crucibg with a flame to destroy organic matter finally applying the full heat of the burner for 1 or 2 minutes. bool, add 3 or 4 ml. of water, let stand until solution is complete (warming, if desired, to hasten dissolution), and transfer the solution to a color-comparison tube. It is convenient to use a tube having an internal diameter of 1.5 cm. and a length of 15 or 16 cm.; if the vanadium content of the sample is very small (
