ANALYTICAL CHEMISTRY
1568 cause the relative error to be small. A titration timr of 10 minutes would permit a 2-.wcond error in the end point with an rrror of 0.3%. ACXNOWLEDGM EYT
The author wishes to e x p i w s his thanks to W. T . IS. Elmrndorl Tor his help on the design of thc constant, current soiir(:e, t,o H. A . 1H:iys for his work on the int1io;itor spswm, and to his co-workwr:
LITERATURE CITED
and Bawden, A. T.. J . A m . Chrrn. Soc., 48, 2045 (1) Foulk, C. W., (1926). (2) Myers, R. J., and Swift, E. H., Ibid.,70, 1047 (1948). (3) Yease, J. W., Nirmann. C., and Swift, E. H., ANAL.CHEM.,19, 197 (1947). (*) Shaffer, P. A , , Jr., Hriglio. :I.. J r . . mid Hrockman. J. A., Jr., Ihid.. 20, 1009 (l948j. (5) Smhellbdy. I,.. :ind S;oniogyi. %.. %. m n l . ( ' h r m . , 112, 385 (1R:IXI.
Colorimetric Determination of Molybdenum by Means of Nitric and Perchloric Acids 11. J . W I N S . E. K. IDIIHVIS, h V I ) F. E. l%EtN Rutgem nii.ersi1.v. .\ric, R r u n s t r i r k . \, J .
I
Y (;I- or after the complex has been extracted with ether, a4 governed by interfering ions present and the molybdcmim conoen t ration. In an effort to increase the accuracy of Marmoy's method, Sichols and Rogers ( 4 ) recommended the addition of 10 mg. of trivalent iron to each sample, because smaller quantities affect the color density of the molybdenum-thiocyanate complex. The silica residues from soil and plant samples were volatilized as silicon tetrafluoride. According t o Sandell ( 7 ) , silica residues should be fused with sodium carbonate t'o prevent loss of molybdenum. Robinson (6) modified the original method by using isopropyl rather than ethyl ether for extraction of the molybdenum complex, because its lower vapor pressure causes less loss from evaporation. Ellis and Olson ( 3 ) found that acetone was more effective than stannous chloride as a reducing agent when the color density of the molybdenum complex was determined direct>ly in a 20-ml. volume of the unknown. But the evaporation of nitric-perchloric digestates of a sufficient quantity of plant material t o a volume of 20 ml. causes precipitation of salts and a possible loss of molybdenum. Barshad ( 1 ) recommended that sodium nitrate be added in the determination to prevent molybdenum from being reduced to the valence below 5 that is necessary for the formation of the complex. Certain methods for determining molybdenum in biological materials require that organic matter be destroyed by dry ashing. I t was found in this laboratory that nitric and perchloric acid digestion way more conkenient than dry ashing for plant tissues. The silica residues from wet digestions of plant tissues do not retain appreciable quantities of molybdenum and fusion or other treatment is, therefore, unnecessary. I m s than 1 microgram of molybdenum was contained in the silica residues from twelve 6gram digestions of alfalfa when combined and fused with sodium carbonate. In wet digestion, all traces of organic matter must be destroyed. Even though the digestate appears colorless, traces of organic materials induce a yellow color, in many cases, upon addition of ammoni,.im thiocyanat,e and stannous chloride. To avoid this, the digestion mixture should remain overnight on a hot plate at a low temperature. Xhen dryness is reached, hydrogen peroxide and additional quantities of nitric and perchloric acids are added. Ammonium thiocyanate proved more satisfactory a s a reagent, than the geneidly used potassium thiocyanate.
I ligh potassium c:onc.cfintI':ltl(Jll> cause prwipitation 01 Iiotassiuiii perchlorate arid loss of molybdenum through :idsorption or through copreripit :tt ion. The following mcthod employs the additional modifirations found newssary t o adapt the procedure for use on digest,ates of nitric and perchloric acids. This procedure is much more rapid than that of dry ashing, which is reported t,o require a fusion or other treatment of tht. d i c a residues. DETERMINATION OF M0LYBDENU.M IN PLAST T I S S U E
Reagents required. Concentrated nitric and hydrochloric acids. A 70% solution of perchloric acid. A 3Oy0 solution of hydrogen peroxide. A 10% solution of stannous chloride freshly prepared in 1 9 hydrochloric acid. A 10% solution of ferric chloride (49 grams of ferric chloride hexahydrate per liter). Sodium nitrate solution, 5 N . Standard solution containing 100 p.p.m. of molybdenum (0.150 gram of Xo03) in 10 ml. of 0.1 N sodium hydroxide, made slightly acid and made up t o 1 liter with water. Isopropyl ether purified by shaking in a separatory funnel with one tenth its volume of a mixture containing one third each of the stannous chloride solution, ammonium thiocyanate solution, and water. Prepared freshly each day it is used.
+
PROCEDURE
Digest 1 to 10 grams of plant material containing from 1 to 20 micrograms of molybdenum by the nitric-perchloric digestion method described by Piper ( 5 ) . Allow the digestion mixture to evaporate t o dryness on a hot plate a t low heat, then add 5 ml. of nitric acid and 1 ml. of perchloric acid and again evaporate to dryness. After the second evaporation, add 1 ml. of hydrogen peroxide and allow the mixture to go to dryness. When digestion is complete, add 70 ml. of water, boil approximately 1 minute, and add 10 ml. of concentrated hydrochloric acid. Filter and wash the filter with hot, water, then allow to cool and make to 100 ml. Transfer to a separatory funnel and add 5 ml. of the ammonium thiocyanate solution, 1 ml. of the sodium nitrate solution, 1 ml. of the ferric chloride solution, and 5 ml. of the stannous chloride solution. Mix, add exact,ly 10 ml. of ether, and shake 100 times. Allow the two layers to separate; draiv off the aqueous phase and deliver the ether into a glass-stoppered flask or tube. If the sample contains 20 micrograms or more of molybdenum, return the aqueous phase to the separatory funnel and extract with a second portion of ether, and combine it with the first. Ten minutes after separation, determine transmittance with a spectrophotometer or colorimeter, using a covered cuvette. Maximum absorption is a t 475 mp. Wratten filters 47.4 and 45 are satisfactory. Prepare a standard curve, following the same prowdure. R u n 3 blank determination on each nen. group of reagc,nts.
V O L U M E 22, N O . 1 2 , D E C E M B E R 1 9 5 0
1569
Table I. Determination of M o l y b d e n u m i n Nitric and Perchloric Acid Digestates of Plant Material samIJ1,le
Alfalfa Alfalfa Alialln Alfalfa Alinlis Alinlib Bl"egrh*s Blueeraas BlUtW"S3
M O
M O
MO
Added
Mu
Content
Found
RPC""Wd
P,p.l?l 1.9 1.9
P.P..".
P.P.Wl.
%
1.5
1.2
0.9 0.6
inodifientwns of the thiocyanate-st,annous chloride procedure and additional ones that are necessary to adapt it for use on nitric and perchloric acid digestates. The revised procedure eliminates interference from traces of organic matter t h a t may remain after t,he digestates appear colorless. It also utilizes ammonium thiocyanate instead of potassium thiocyanate, which prevents a h e e v precipitate of p o t m i u m perchlorate during the determination.
0.5 0.4
LITERATURE CITEU
0.8
This procedure has been used for the determination of molybdenum in digestates of many types of plant tissues. The accuracy of the method is within 10% (Table I). According bo Sandell (7), no greater accuracy is expected in dealing with such minute traces of elements. SUMMARY
A modified method for the determination of molybdenum in materials digested in nitric and perchloric acids includes proposed
(I1 Marahad. Issae. ANAL.CITEM.. 21,1146-50 (1949). (2) Ellis, R., and Olson, R. V.,Ibid., 22.328-30 (1950). (3) Marmoy, F. B.,J . Soe. Chem. Ind.,58. 275 (1939). (4) Niohols. M.L., and Rogers, 12. H., IND. ENG.Cnm., ANAL.En., 16, 13740 (1944). ( 5 ) Piper. C. S., "Soil and Plant Analysia," University of Adelaide,
Australia, 1942. ( 6 ) Robinson, W. O., Soil Sci.. 66, 317-22 (1946). (7) Sandell, C. B.. "Colorimetric Analysia of Traoe York, Interscience Publishers. 1944.
Metale," New
November 8. 1949. Paper of the Journal Series, New Jersey Agricultural Experiment Station. Rutgers Iinirersity. The State University of New Jersey. Department of Soils. QECEIVE~
Objective Method for Determining Melting Point FERENCKARDOS United Incandescent Lamp and Electrical Company, Ltd., Ujpest near Budapest, Hwngary
Wll the determination of melting point, the increase in the light current caused by the melting of the material under examination is registered by 8 selenium photocell and accepted on a galvanometer with a 2 X 10- ampere sensitivity (Fig"re 1). Industrid nrganic raw materials, intermediates, and end produots are best controlled by determination of their charaeteristic physical constants, which may he chosen to furnish adequate information regarding quality and fitness for certain purposes. It is often desirable to work with small quantit.ies of material having a small heat enpacity, and in many cases Kofler's met,hod (1, 8 ) of measuring melting point is most appropriate. Use of a selenium photocell having a sensitiveness of 500 ma. per lux renders the test less fatiguing than use of a microscope for ocular examination. The material being examined, placed on the glass of the Kofler melting paint rkgistering apparatus, absorbs the light of the light source. The galvanometer hegins t o move when the
melting point is approached and in the vicinity of the meltbig point indicates the change in light intensity. At the ssme time the thermometer shows the temperature of Kofler's apparatus. To render the process even more objeet,ive, an automatic writing device may replace ocular registration. The method Cannot be used with compounds where melting involves no change in light absorption.
60
50 Y
40
3 '
0
30
X o(
PO 10
PO
30
40
50
60
70
80
w
" c. Figure 2.
Figure 1. Instrument for Measurins Melting Point
Melting Curves of Various Materials
The curves (Figure 2) were taken using a galvanometer for measuring the photocurrent and a thermometer for the temperature. The temperature was taken visually a t the right moment indicated by the upper rupture of the photocurrent. T h e initial anomaly observed when measuring the melting point of paraffin demonstrates t h a t this compound will soften before melting and while it diffusesi t blocks the path of the light; only afterward does the real melting tnke place. Measurement oan be made more
