ANALYTICAL CHEMISTRY
334 Iiiterestingly enough, back in 1895, Durlin ( I ) , in analyzing several copper mattes of varying copper content, reported satisfactory results with the cyanide titration on the basis of the equation C U ( ? ~ T H(NO& ~)~
+ 4SaCS =
+
S ~ ~ C U ( C S ) 2xaN03 I
+ 2h%
in which the ratio 4NaCN = 1 Cu appears. His procedure involved regulating the amount of free acid in the copper nitrate solution and the excess ammonium hydroxide used. LlTERATURE CITED
(1) Durlin, R. S., J . Am. Chem. Soc., 17, 346 (1895). (2) Glasstone, S., J . Chem. SOC.,1929,702.
(3) Hildehrand, J. H., “Principles of Chemistry,” 4th ed., New
York, Macmillan Co., 1940. (4) Hogness, T. R., and Johnson, W ,C., “Qualitative Analysis and Chemical Equilibrium,” 2nd ed., Sew York, Holt Co., 1940. (5) Kunschert. F., 2. anorg. Chem., 41,351 (1904). ( 6 ) Latimer, W. M., “Oxidation Potentials,” New- York, PrenticeHall, 1938. (7) Latimer, W. R‘f., and Hildehrand, J. H., “Reference Book of Inorganic Chemistry,” rev. ed., New York, Macmillan Co., 1940. (8) Partington, J. R., ”Textbook of Inorganic Chemistry,” 5th ed., New York, Macmillan Co., 1950. (9) Sidgwick, S . V., “Chemical Elements and Their Compounds,” London, Oxford University Press, 1950. (10) Spitser, F., 2. Elektrochcm., 11,346 (1905). (11) Treadwell, F. P., and Girsewald, C. V., 2. anorg. Chsin., 38, 92 (1904). RECEIVED for review Bpril 2, 1952. Accepted October 30, 1952.
Polarographic Analysis Using Flowing Samples C o n t i n u o u s A u t o m a t i c Measurement of Sulfur Dioxide in Corn S t e e p Liquor LYNN D. WILSON’ AND R. J. SMITH George M . Moffett Research Laboratories, Corn Products Rejining Co., Argo, Ill.
HE problem of determination and control of sulfur dioxide Tconcentration in corn steep liquor has been one of the most vexing difficulties in the wet milling industry. Customarily, the concentration of sulfur dioxide is determined by iodine titration of the steep acid. Control has been manual and must be regarded as very crude in the light of modern control concepts, since lags and surges in concentration cannot be followed by other than continuous automatic measurement. The method of continuous measurement described here is immediately applicable to automatic control and thus provides the first known means of interpreting product quality in terms of a specific variable. FTilson and Smith ( 1 ) have shown that quantitative polarographic analysis using flowing samples is possible. It was demonstrated that the diffusion current was least influenced in both magnitude and uniformity of current oscillations when the sample flowed vertically downward past the dropping mercury electrode. This fact is extremely fortunate in application to industrial process liquors containing relatively large amounts of suspensoids, since the necessity for highly efficient filtration required for other conditions of flow is thus obviated. Figure 1 shows the special electrolysis cell developed for the measurement of sulfur dioxide in steep acid using vertical down&-yard flow. Dimensions are not critical except for the diameter of the outlet which is adjusted by grinding to permit a flow rate of 175 to 200 ml. per minute. This flow rate range was chosen as the upper limiting rate above which appreciable distortion of the diffusion current occurs, It is determined by referring to a series of polarograms obtained a t various flow rates with the particular cell in use. A sidearm overflow provides for the disposal of scum and foam which tend to foul the capillary electrode and the cell outlet if carried down into the cell body. A mercury pool is used as the anode, with the entering sample impinging on its surface in a self-cleaning action. Calomel electrodes with various types of salt-bridge surfaces proved unsatisfactory as reference anodes, since sludge in the sample deposited on the surfaces and resulted in a continuous change in the resistance of the half-cell, thus changing calibration. Figure 2 shows the polarizing unit. A fixed electromotive force of 1 volt is applied to the voltage divider which is tapped a t 0.9 of full resistance, thus applying a voltage to the cell which corre1
330
address, Wilson Industrial South Wells St., Chirago 6, Ill.
Present
Hygiene a n d Research
Laboratories,
sponds to a point about midway in the diffusion current region. No provision is made for correcting for the residual current, since extended observation has shown it to be constant within the desired limits of accuracy. A portion of the voltage developed across the fixed resistors, F , G, H , is fed to a Brow1 Electronik potentiometer recorder of 0 to 20-millivolt range. The chart and scale are calibrated in major divisions of millivolts and subdivisions of 0.2 mv. By suitable adjustment of the input with potentiometers, G and H , the scale will read directly in hundredths of a per cent of sulfur dioxide for each millivolt and 0.002% for each 0.2 mv. The condensers across the fixed resistors are for the purpose of reducing voltage oscillations corresponding to growth and fall of the mercury drops. Condenser connections across the resistors are made with single-pole double-throw toggle switches
1
I
F 4 1 m PYREX T U B l f If i
emmPYREx TUBING
SAMPLE INLET
190mn ANODE CONNECTION
FACE ELECTRODE OF DROPPING
\/
--GROUND SQUARE TO DELIVER 175-200m1 PER MINUTE 2
in
Figure 1. Electrolysis Cell
V O L U M E 25, NO. 2, F E B R U A R Y 1 9 5 3
335 so wired that when a condenser is not in use it is short-circuited to maintain high reproducibility of damping capacity. Figure 3 presents a schematic arrangement of the process liquor sampling system now in use in the steep house a t the Argo plant of the Corn Products Refining Co.
The Sarco line strainer is a standard 2-inch model fitted with an internal sleeve containing about 400 perforations per square inch. This provides sufficient filtration to remove coarse particles which might clog the outlet of the cell. Steep acid flov-s continuously through the blow-down side of the strainer in a continuous scrubbing action, thus reducing maintenance requirements to only monthly cleaning. The dropping mercury electrode consists of an 18- to 21-c1n. length of polarograph capillary selected n-ith a drop time of 2.0 to 2.5 seconds. (Suitable capillary may be purchased from E. H. Sargent and Co., Chicago, Ill., under number 5-29351.) The mercury consumption a m o w t s to about 250 ml. per week; it is recovered from the trap below the cell and is refined by the usual nitric acid tower technique.
Figure 2.
Polarizing Unit
A. B.
1.5-volt d r y cell 200-ohm helipot C. Toggle switch D . 3-volt Weston voltmeter E . 100-ohm ohmitemultivolt resistor tapped at 90 ohms F. 1000-ohm resistor
G. 1000-ohm ohmite multivolt resistor tapped at each 200 ohms H . 200-ohm Helipot I . Six-position tap switch J . 1000-pfd. condenser K. 2000-#fd. condenser L. 3000-pfd. condenser M, N, P. Single-pole doublethrow toggle switches
Figure 4. DROPPING ELECTRODE ASSEMBLY
-
8
VALVE
I
LINE STRAINER
I I
1
ELECTROLYSIS CELL
I
CONSTANT LEVEL DEVICE
I
I
I
I
1
TOSUMP
1
I
I Figure 3.
I
I I
Schematic Arrangement of Liquor Sampling System
Typical 24-Hour Continuous Chart Record
The measurement is calibrated by iodine titration. After a 15-minute warm-up period for the polarizing unit and recorder, the applied electromotive force is adjusted to 1 volt, and sufficient damping capacity is switched in to reduce the oscillations of the recorder pen to 0.5 to 0.75 scale division. A sample of steep acid is taken from the outlet of the cell while the recorder concentration is noted. -4filtered 10-ml. portion is titrated Kith standard iodine solution (1 ml. equals 0.01 gram of sulfur dioxide). The recorder is adjusted with potentiometers G and H to the percentage indicated by titration. On startup after periods of shutdon-n, calibration is repeated after about 15 minutes. .it other times only one titration is necessary for checking calibration. Total iodine consumption by a sample of steep liquor is the sum of iodine reactions with (1) free dissolved sulfur dioxide, (2) sulfur dioxide adsorbed by solids present, and (3) other constituents oxidizable by iodine. This can be demonstrated by titrating a sludgy sample for total oxidizables, filtering another portion of the same sample, titrating the filtrate for free sulfur dioxide, and xvashing the precipitate and titrating for adsorbed sulfur dioxide. Since only free sulfur dioxide is reduced at the dropping mercury electrode, it is necessary that filtered samples be uaed in calibration by iodine titration. Factors which might affect the accuracy of the determination are variations in pH level, temperature, and indifferent electrolyte concentration. The p H level of steep liquor is constant within
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 p H 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. n i t h 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.
