336
ANALYTICAL CHEMISTRY
Table I.
Measurement of Sulfur Dioxide Concentration Sulfur Dioxide, % ’ Recorder reading Iodine titration
Time Date May 23 May 24
Hour 4:30
P.Y.
8 : l j A.M 8:30 8346 9:35 10 : 00 10:30 11:38 1:30 P.W. 1:45 1:55 2:lO 2:30 4:OO
0 108
0 0 0 0 0 0 0 0 0 0
091 086 089
083 085 084 079 076 073
105
0 116 0 100
0 101
0 107
0 0 0 0 0 0 0 0 0 0
093 087 090 084 087 085 081 078 073 108 0 118 0 101 0 101
trolyte concentration, are insufficient to exceed the required accuracy of 5%. Figure 4 shows a typical 24-hour continuous chart record. Table I lists comparisons of randomly selected recorder readings with corresponding concentrations determined by iodine titration. The vital importance of this continuous method of measuring sulfur dioxide concentration is particularly illustrated by the area on the chart of Figure 4 enclosed by asterisks. Here the abrupt rhanges in sulfur dioxide concentration were readily traced t o inadvertent diversions of process liquor. By extension of this method of measurement to automatic control such variations, nil1 be avoided. ACKNOWLEDGMEZT
the range of 3.8 to 4.2 and the temperature of the liquor is maintained at 125’ to 130”F. The fact that frequent checking over an extended period of time has not revealed significant inaccuracies, indicates that variations in pH level and temperature, together with possible variations in naturally occurring indifferriit elrc-
The awistance of H. E. Gorman and T . G. LIeilleur in this work is gratefully acknou-ledged. LITERATURE CITED
(1) Wilson, L. D., and Smith, R. J., . \ s . i L . CHEM.,25, 218 (1953). RECEIVED for review March 5 , 1952.
Arcepted July 15, 1952.
Colorimetric Determination of Phosphorus Pentoxide in Fertilizers Using a Standard Calibration Plot G. L. BRIDGER, D. R. BOYLAN, AND J . W. M A R K E I Department of Chemical and Mining Engineering, Iowa State College, ilmes, Zowa
A S Y investigators (6, 9, IO) have demonstrated that phosphorus may be determiued in a variety of materials by :t colorimetric method. The method generally used is based on the transmittancy of the yellow colored ammonium phosphomolybdovanadate co~nplex(7, 8). In an exhaustive study Kitson and Mellon (6) showed that the ions likely t o be present in the analysis of fertilizers would not cause interference. Barton ( 2 ) applied the method to the analysis of phosphate rock. Epps ( 3 ) showed the method to be sufficiently accurate for the direct determination of available phosphorus pentoxide in various fcrtilizers, if standard samples \!ere used uith each determination. Hanson (4) described the suitability of the method for the works laboratory. He found occasional recalibration of a standard graph necessary, however. The present study was undertaken to corroborate the abovc u ork and to determine whether a single standard calibration graph could be developed which would be satisfactory for th(1 determination of phosphorus pentoside in a variety of phosphatic inaterials used in the fertilizer industry, including fused phosphates, without the necessity of recalibration or the extreme precautions previously reported. PROCEDURE
Mixed Color Reagent. The mixed color reagent proposed by Barton was used in this investigation. It was prepared as follows: Forty grams of ammonium molybdate were dissolved in about 400 ml. of distilled water; 1 gram of ammonium vanadate was dissolved in about 300 ml. of distilled water, and. 200 ml. of concentrated nitric acid were added. The two solutions were mixed and diluted to 1 li!er. The reagent was allowed t o age 4 to 6 days, so that all solids would settle out before use. Standard Phosphate Solution. Chemically pure potassium dihydrogen phosphate (KH2POr)was dissolved in distilled water and adjusted t o a concentration of 0.1 mg. of phosphorus pentoxide per ml. Instrumentation. A Klett-Summerson photoelectric colorinieter, Model 900-3, with blue filter (approximately 425 mp)
mas used to measure optical density. Standard colorimeter test tubes 14 mm. in diameter, calibrated at 5 and 10 ml., were used in place of rectangular solution cells. Phosphate Solutions. Solutions of the phosphatic materials were prepared according t o the official methods of the Association of Official Agricultural Chemists ( 1 ) for water-soluble, neutral ammonium citrate-soluble, citrate-insoluble, and total phosphorus pentoxide fractions in solution. [All phosphate solutions were diluted to exactly 500 ml.] Water-soluble and citrate-soluble solutions required the addition of about 5 ml. of concentrated nitric acid before final dilution t o avoid cloudiness. Determination. Various aliquots of the standard phosphate solution were taken t o contain from 0 to 3.5 mg. of phosphorus pentoxide, pipetted into a 100-nil. volumetric flask, and diluted to approximat,ely 50 ml. nith distilled water. Exactly 25 ml. of mixed color reagent were pipetted into each flask. The solutions were diluted to volume with distilled water and thoroughly nlised. Five t o 10 minutes were allowed for the yellow color t o develop before the optical density or Nett-Summerson scale reading was measured. As the color reagent itself is colored, a reagent blank was prepared by pipett,ing exactly 25 ml. of color reagent into a 100-ml. volumetric flask, diluting to volume with distilled water, and mixing thoroughly. The colorimeter was allowed t o “warm up” for 30 to 60 minutes before use. The instrument then was “zeroed” with either the color reagent blank or distilled water. When the instrument was zeroed with xater, the scale reading for the reagent blank was subtracted from that of the sample. The net scale reading for each aliquot was plotted linearly against milligrams of phosphorus pentoxide as shown in Figure 1. Phosphate Unknown. An appropriate aliquot of the phosphate solution was selected to contain approximately 2 mg. of phosphorus pentoxide, and the corresponding Klett-Summerson scale readings were obtained as described above. The phosphorus pentoxide content of this aliquot was determined directly from the standard calibration graph shown in Figure 1. The phosphorus pentoside content of the sample was easily calculated by adjusting for the size of aliquot used.
337
V O L U M E 25, N O . 2, F E B R U A R Y 1 9 5 3 Reference Analysis. Volumetric determinations of phosphorus pentoxide were made according t o the official method (1) as checks on the colorimetric determinations from the standard curve. EXPERIMENTAL R E S U L T S
The colorimetric method was applied in routine manner to the determination of phosphorus pentoxide in approximately 350 fertilizer samples including normal and triple superphosphate. To obtain consistently reproducible results it was essential that the color reagent be allowed to age 4 to 6 days. During this time considerable amount of yellow crystals deposited on the bottom of the reagent bottle.
,030
a w
t -I
i
= 020 a W
n
: I)
required for each individual instrument.) I t is evident that the Beer Lambert law does not hold, and a straight line cannot hr inferred, as previously reported. The standard calibration graph has been used satisfactorily to determine phosphorus pentoxide in all the different fractions of various fertilizer materials produced from different phosphate rocks, including Florida, Tennessee, and western rocks. Frre acid phosphorus pentoxide, water-soluble phosphorus pentoxide, neutral ammonium citrate-soluble phosphorus pentoside, citric acid-soluble phosphorus pentoxide, citrate-insoluble phosphorus pentoxide and total phosphorus pentoxide were determined. Typical results are reported in Table 11. The color caused by various impurities in the different phosphate rocks (particularly Hestern rock) was found to be negligible. The largest source of error in the determination is the difficulty iri accurately measuring the small aliquots required, which are as small as 1 or 2 ml. from 500 ml. of solution for the water-soluble and total phosphorus pentoxide in triple superphosphate. Calibrated pipets minimize this error. The method provides a means of accurately determining small amounts of phosphorus pentoxide, such as those found in the citrate-insoluble fractions, xith much greater facility than does the volumetric method. The colorimetric method was found to be no less precise than the volumetric method in a statistical comparison of a largc, number of samples. Regression technique was used (data not reported) n-ith the per cent phosphorus prntoxide by the volumetric
g ,010 4
a
Table 11. Comparison of Colorimetric with Volumetric Determination of Phosphorus Pentoxide in Various Phosphatic Materials
2 i
z
.ooo-,
,
,
,
0
,
,
,
,
,
,
,
200
100 SCALE
.
,
,
I
300
READING
Figure 1. Colorimeter Calibration Graph
Material Phosphate rock (Florida) R R-2
B
Although the transmittancy is temperaturesensitive, the net scale reading due to phosphorus pentoxide coloration-i.e., with blank deducted-does not vary significantly for teniprratures from 20' to 32" C., as shown in Table I. It was also found that the phosphorus pentoxide coloration was stable from 15 minutes to as long as 3 days after development, if the solution was kept in a closed flask. The blank for the color reagent varied from d a j to day and from one lot to another, but the net reading for the phosphorus pentoxide coloration remainrd constant. The standard calibration graph presented in Figure 1 has remained constant for 2 years. I n some measure this may be attributed to the dual photoelectric cells of the Klett-Summerson colorimeter, which makes possible measurements that are independent of voltage variation and age or replacement of the light source. (-4separate calibration curve, however, mould be
Table I.
Effect of Temperature on Net Scale Keading Due to Phosphorus Pentoxide Coloration
Sormal superphosphate (Florida rock) I
