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 Molybdenum i n Nitric and Perchloric Acid Digestates of Plant Material M O
M O
MO
Content P,p.l?l
Added
Mu
samIJ1,le
Found
RPC""Wd
P.P..".
P.P.Wl.
Alfalfa Alfalfa Alialln Alfalfa Alinlis Alinlib Bl"egrh*s Blueeraas BlUtW"S3
1.9 1.9 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 that 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 potmium 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. Melting Curves of Various Materials
Figure 1. Instrument for Measurins Melting Point
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. The initial anomaly observed when measuring the melting point of paraffin demonstrates that 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
ANALYTICAL CHEMISTRY
1570 accurate by the following treatment: The powdered material is placed on the table of the Kofler instrument, melted, allowed to cool, and remelted. The curve of the second melting is taken. Thus the lower rupture of the photocurrent indicates the temperature of the stiffening point. The difference between melting and stiffening point is characteristic for fats-i.e., 7.5" C. for lard and 12.5" C. for beef tallow. The melting points of fats and waxes have been measured in this way. The curve of chloral hydrate is of interest, for here the first maximum means a change of structure. Then passing through a minimum the curve ascends to the maximum a t around 57" C. Measuring in a capillary tube, the melting points of 50.0' to 50.2 C. for naphthylamine purified by recrystallization and of 80" C. for naphthalene purified by sublimation were obtained. Temperature rise occurring during the measuring period is about 3' C. in the case of waxes and 7.7" C. in the case of naphthylamine. This phenomenon indicates the different melting mechanisnls of the two types of material. During the measurement
the regulator resistance of the Kofler instrument was calibrated in a way to keep the speed of temperature increase a t about 1 C. per minute in the vicinity of the melting point. Application of the photocell makes the determination of melting point quicker, more simple, reproducible, and exart. A similar procedure may be applied to determination of evaporation time, particle size, and number of blood cells. ACKNOWLEDGMENT
The valuable help of Thomas Sandor in making the measurements is acknowledged. LITERATURE CITED
(1) Chamot and Mason, "Handbook of Chemical Microscopy," Vol. I, p. 198, New York, John Wiley & Sons, 1940. ( 2 ) Scott, W. W., "Standard Methods of Chemical Analysis," 5th ed.. Vol. 11, p. 2453, New York. D. Van Nostrand Co., 1938. RECEIV D ~June 15, 1949.
Distillation of Antimony Trichloride for Improved Reagent ALEXANDER MUELLER AND SERECK H. FOX with the technical assistance of ROBERT I. DAY, Gelatin Products Division, R . P . Scherer Corporation, Detroit, Mich. UTIMONY trichloride has become well established as a A-chromogenic agent for the photometric analysis of vitafins A and D (1,4,6) and several sterols ( 2 , J ) . It is often difficult to prepare a satisfactory reagent in chloroform, especially from a previously opened stock bottle of antimony trichloride. Exposure of the crystals or lumps to air and moisture causes a discoloration. The addition of drying agents such as acetyl chloride, acetic anhydride, or anhydrous potassium carbonate gives some degree of improvement, but does not give complete assurance of a stable and reproducible reagent.
Table I.
Li2m. Values
Trial 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Average Max. range, % 1Ia.x. deviation, %
on U.S.P. Vitamin A Reference Standard
Undistilled SbCb 12.22 12.42 12.51 12.66 12.27 12.84 12.70 12.80 12.53 12.28 12.81 12.35 12.40 12.39 12.35 12.43 12.34 12.15 12.30 13 06 12 50 7.28 4.48
Distilled SbCla 13.46 13.45 13.20 13.48 13.32 13.27 13.39 13.73 13.20 13.05 13.11 13.09 13.61 13.39 13.49 13.07 13.18 13.05 13.17 13.21 13.296 5.12 3.26
When a stock of antimony trichloride contains impurities or becomes discolored from frequent exposure, it can easily be purified by simple distillation under moderate vacuum. It distills smoothly a t a temperature of 140" to 150" C. and a vacuum of 10 to 15 mm. (obtainable with a water aspirator). The distillate is white and remains colorless for several months. The reagent in chloroform prepared from distilled antimony trichloride is more stable, readily reproducible, and more sensitive.
ANTIMONY TRICHLORIDE DISTILLATIOh
The vacuum still (Figure 1) consists of a 2-liter threenecked round-bottomed flask and a 105" distilling connecting tube mounted for reflux. This is attached to a 30-cm. condenser
Figure 1. Apparatus for Distillation of Antimony Trichloride
