INDUSTRIAL
ENGINEERING CHEMISTRY
AND
A N A L Y T I C A L E D I T 10 N PUBLISHED
BY
THE
AMERICAN
CHEMICAL
SOCIETY
0
HARRISON
E.
HOWE,
EDITOR
Colorimetric Determination of Phosphorus in Iron Ore E. JOHN CENTER' AND HOBART H. WILLARD U n i v e r s i t y of Michigan, A n n Arbor, Mich.
I
RON interferes indirectly with the colorimetric determination of phosphorus. A sample run by the regular pro-
cedure but without addition of the final reagent is colorless to the eye, and regardless of the percentage of iron in the sample the transmittance at 450 millimicrons in a Coleman spectrophotometer is the same (distilled water is used as a blank). Curve 2, Figure 1, shows the spectral distribution of such a sample. Curve 1, Figure 1, s h o m the spectral distribution of a regular sample containing the yellow phosphorus complex [(NH4)sPO4.NH4VO3.l6MoO3according to Mission ( I ) ] .
spectrophotometer (less light falls on the photoelectric cell) and the critical wave-length setting. Figure 1 also indicates that a larger slit is out of the question. Thus a wave length of 450 millimicrons is employed with a width of 30 millimicrons. Although the iron interference is constant for varying percentages of iron when the color complex is undeveloped, a marked deviation from the correct phosphorus content is noted when the color is developed and the iron value falls below the 50 per cent mark. I n a lean ore or rock (iron below 50 per cent) this phenomenon indicates a low value for the phosphorus content. Figure 2 shows the deviation in the phosphorus value as a function of iron per cent. For an ordinary iron ore the interference is negligible. If the amount of iron in a sample of lean ore or rock is known, the per cent of phosphorus can be obtained from Figure 2 by adding the correction from the plot to the apparent colorimetric value. The ammonium phosphovanadomolybdate solution is appreciably sensitive to temperature changes, and, for the most precise analysis, samples must be run at constant temperature. I n this work 27' C. is arbitrarily selected as a convenient temperature for seasonal work. Distilled water does not change appreciably in transmission with temperature changes. Therefore, it can be substituted for the theoretically correct iron blank thus eliminating the necessity of keeping the blank a t any definite temperature; moreover, distilled water does not vary in transmission over a period of time whereas a colored, highly acid iron blank must be constantly checked against a standard. The use of a dis-
h/AyE LENGTH MILL I M ~ C R ~ N S FIGURE 1. TRANSMITTANCE CURVES 1. Color developed 2. Color undeveloped
The proper width of the monochromatic light band (30 millimicrons) is also indicated on this curve, which shows that a narrower slit, 5 or 10 millimicrons, would not be applicable in eliminating fixed iron interference, and is impractical because of the decreased energy output from the 1
Present address, Battelle Memorial Institute, Columbus, Ohio.
PrRcr;NT f%OJPH0.4?UJ
FIGURE 2. IRON INTERFERENCE WITH COLORIMETRIC PnoePnoRns
Vol. 14, No. 4
INDUSTRIAL AND ENGINEERING CHEMISTRY
288
Summary The colorimetric determination of phosphorus in iron ores by the phosphovanadomolybdate method gives results in good agreement with the usual volumetric method. Iron gives a definite measurable interference which is negligible if the ore is above 50 per cent iron. Temperature must be controlled for the most accurate work.
Literature Cited C%VC€NTR4T/ON8 P € R C f N T
FIGCRE 3. PER CENT TRANSMITTANCE us. PER CENTPHOS-
(1) Mission, G., Chem-Ztg., 32, 633 (1908). ( 2 ) Willard, H. H., and Center, E. J., IND.ESG. CHEM.,A S I L . ED.. 13,81 (1941).
PHORUS
DLstilled water used as a blank
Color compared at 27' C
tilled water blank changes the concentration vs. transmittance curve (Figure 3). For increased accuracy and economy of time the original colorimetric method for phosphorus ( 2 ) has been slightly modified. A 1-gram sample is used instead of the original 0.5 gram to allow the determination of silica and manganese on the same sample.
Procedure Weigh out a 1-gram sample of iron ore into a 150-ml. beaker, add approximately 10 ml. of concentrated hydrochloric acid, and heat on a hot plate until the sample is in solution, adding more hydrochloric acid if necessary. Khen the ore is in solution add 0.5 ml. of concentrated nitric acid, evaporate the solution to dryness, and carefully bake. Allow the beaker to cool somewhat, add 15.0 ml. of 70 to 72 per cent perchloric acid, and fume on a hot plate until the dark brown ferric solution has changed to a light straw color. The color change requires from 3 to 6 minutes, depending on the temperature of the hot plate. Extreme care should be taken to see that no organic matter contacts the hot fuming perchloric acid. Allow the sample to cool slight1 add 10.0 ml. of ammonium vanadate solution, place the saxnpyi back on the hot plate, and bring it to a boil.
A Filtering Device for Viscosity Tubes STANFORD J . HETZEL S u n Oil Company, Norwood, Penna.
C
OYSIDERABLE difficulty is often encountered in preparing an uncontaminated sample of oil or other liquid for a viscosity determination. A. S. T. M. Designation D-445-39T requires that the sample be filtered through sintered glass or a fine wire screen into a small beaker or bottle. It is somewhat difficult to keep all the apparatus clean and free from solid particles or lint and, unless great care is exercised, the sample may pick up lint from the air before being transferred to the viscometer pipet.
TABLE I. DETERMINATION O F PHOSPHORUS Sample No.
888 GG 889 GG 7106 GB 7107 G B 7108 G B 866 M B 867 M B 868 M B 869 M B 870 M B 871 h l B
Volumetric
Phosphorua
Colorimetric
%
%
0.058 0.066 0.082 0.089 0.093 0.104 0.105 0.129 0.128 0,082
0.058 0.064 0.081 0.088 0.092 0.104 0.105 0.128 0.127 0.082
0.099
0.099
Remove the sample from the hot plate and wash the cover glass with a little distilled water; filter into a 100-ml. volumetric flask. Scrub the beaker and wash the contents onto a filter paper. Wash the filter paper five times with distilled water. The insoluble residue on the paper may be used for a silica determination. Fill the volumetric flask to the mark with distilled water, and mix by repeatedly inverting the stoppered flask. Measure out 50.0 ml. to use for a manganese determination if required. T o the remaining aliquot in the flask add 7.5 ml. of ammonium molybdate solution. Invert and shake the flask to mix the reagents and dissolve the precipitate that first forms. Place the sample in a water bath maintained at 27.0' C. When the sam le is at bath temperature measure the transmittance at 450 m i b microns in a Coleman spectrophotometer. Read the per cent of phosphorus from the concentration 08. transmittance curve (Figure 3). Table I shows a comparison of the colorimetric and volumetric methods for determining phosphorus.
FEMALE GRINDING SINTERED GLASS DISC
2 0 X 2 MM,
The filtering device illustrated is a much more convenient and rapid means for ensuring an uncontaminated sample for viscosity measurement. It consists of a small interchangeable filtering cap made by sealing a sintered-glass disk 20 mm. in diameter and 2 mm. thick in a glass tube fitted with a standard '/la female joint. A male joint is ground on the end of the capillary arm of the viscometer. With the cap in place the pipet is inverted and immersed in the beaker or wide-mouthed bottle containing the sample, and filled by suction in the usual manner. Two porosities of sintered-glass disks, medium or coarse, depending on the viscosity of the sample, may be used. The filtering cap holds about 6 to 7 cc. of sample. Cleaning is accomplished by drawing petroleum ether or benzene through the cap in order to retain any foreign particles on the outside and drying in a stream of air or in an oven.
