Rapid Determination of Phosphorus in Gasoline Spectrographic and Colorimetric Methods MARGARET E. GRIFFING, C. T. LEACOCK, W. R. O'NEILL, ADELE L. ROZEK, and G. W. SMITH Research laboratories, Efhyl Corp., Defroit,
b This paper describes a spectrographic and a colorimetric method for determining the phosphorus content of gasolines. Neither method i s affected by the presence of additives such as tetraethyllead, antioxidants, dyes, or metal deactivators. The spectrographic procedure is applicable to phosphorus concentrations from 5 to 75 mg. per liter with a standard deviation of 0.8 mg. per liter. A single determination can b e made in 30 minutes; a two-man team can make 70 determinotions per day routinely. The colorimetric procedure is applicable to concentrations of 5 to 50 mg. per liter with a standard deviation of 0.5 mg. of phosphorus per liter. This procedure requires 1 hour for a single determination, but 24 determinations can b e made per man-day.
T
HE use of organic phosphorus compounds as gasoline additives has created a need for rapid methods of determining phosphorus in gasoline. To be acceptable, the procedure should apply to a wide variety of phosphorus compounds and should give accurate results over the concentration range of 5 to 50 mg. of phosphorus per liter of gasoline. I n addition, it should apply to a variety of base stocks containing additives such as tetraethyllead (TEL), antioxidants, dyes, and metal deactivators. The successful use of emission spectrographic techniques for determining phosphorus in lubricating oils suggested its use for analyzing gasolines (8, 17). Simultaneously, a study was made of colorimetric procedures which might be applicable. SPECTROGRAPHIC METHOD
Apparatus. The equipment consists of a 2-meter grating spectrograph with a reciprocal linear dispersion of 5.2 A. per mm. in the first order (IO), a high precision excitation source unit ( I I ) , a comparator-densitometer, and a solution excitation apparatus (16), all supplied by Applied Research Laboratories, Glendale, Calif. T h e spectrograph was modified by replacing the 5-inch focal length cylindrical field lens with a spherical 374
ANALYTICAL CHEMISTRY
Mich. lens of the same focal length. The solution excitation apparatus was modified to rotate a disk electrode a t a speed of 1.25 r.p.m. The two-piece brass shafts (Figure 1) are used to support the disk electrode. A heavy shaft ensured adequate conduction of heat from the electrode. The aluminum boat (Figure 1) is used as the sample container. This boat, which was designed to conduct heat rapidly from the sample, is supported on an insulated platform. A duct, mounted on one side of the platform, has a series of lateral openings located just above the sample boat, so that argon flowing through the duct floods the surface of the sample (Figure 2).
*4-4DNC
TAP
\
and a hemispherical tip. Eastmnn Kodak Co. Spectrum Analysis No 1 film is used. Reagents and Standards. An internal standard-radiation buffer solution is prepared by dissolving 1.43 grams of triphenylarsine in a mixed solvent containing 400 ml. of isopropyl alcohol and 600 ml. of dimethylformamide. Standard samples are prepared as follows: A master standard containing 0.100 gram of phosphorus per liter is prepared by dissolving 0.5945 gram of o-tolyl phosphate (Eastman Kodak No. 1517) in iso-octane in a 500-ml. volumetric flask, and diluting the solution t o volume with iso-octane. At least
'
__
..
\--"4-40NC
I .
-
THDS
-
Figure 1 . E l e c t r o d e support shaft and aluminum sample boat
BRASS W A F T FOR SUPPORT OF DISC ELECTRODE MODIFIED FROM APPLIED RESEARCH LABORATORIES DESIGN
This duct is connected in parallel with a second duct that consists of a nozzle 0.125 inch in diameter attached to a Lucite tube mounted on an arm extending from the top of the solution excitation apparatus. The nozzle directs an argon flow toward the spark gap and away from the slit. A flowmeter calibrated for 0 to 20 liters per minute a t 30 p.s.i.g. controls the argon flow. The sample electrode consists of a high purity graphite disk 0.5 inch in diameter and 0 200 inch thick. The counter electrode consists of a pure graphite rod 3/16 inch in diameter, having a 15-degree included angle taper
2, \
L-
-
-/,
Ji
Figure 2. Arrangement of argon inlets with solution excitation apparatus A. B.
C. D.
E.
F. G. H.
Solution excitation apparatus, synchronous motor Brass electrode supporting shaft Sample boat Multiple-orifice argon inlet 0.1 25-inch nozzle argon inlet 3/16-inch rod electrode 1 /2-inch graphite disk electrode Argon flowmeter
five additional standards covering the concentration range from 0.075 to 0.005 gram per liter are prepared by accurate successive dilutions of the master standard with iso-octane. Procedure. Bring the temperature of the gasoline sample and the internal standard solution t o room temperature (25' C.). Dispense 6 ml. of internal standard with a buret into a 10-ml. volumetric flask. Fill the flask t o the mark with the sample and mix the contents. Mount the graphite disk electrode on the male section of the brass supporting shaft and clamp it firmly in position with the female portion of the shaft. Attach the shaft to the solution excitation apparatus. Mount the counter-electrode rod in the upper electrode holder of the spectrograph spark stand. Space the electrode with a 7-mm. gap centered on the optical path. Mount the sample boat on the platform of the solution excitation apparatus, and fill it with the buffered gasoline sample. Raise the platform until the disk electrode dips beneath the surface of the liquid. Rotate the disk electrode a t 1.25 r.p.m. Regulate the total argon flow across the sample and across the analytical gap to about 15 liters per minute. Set the excitation source to deliver a high voltage spark of two discharges per cycle with a power circuit voltage of 90 volts, a capacitance of 0.014 pf., and an inductance of 720 pl. Use a prespark time of 100 seconds and an exposure time of 80 seconds. Photograph thc spectral region from 2100 to 4700 A. with a 50-micron slit. Process the film in accordance with the ASTM recommended practice (2). With the densitometer, measure the transmittances of the phosphorus line a t 2535.65 A,, the arsenic line at 2456.53 A., and the continuous background adjacent to each line. Correct each line for background, and calculate the intensity ratio of the phosphorus line to the arsenic line in accordance with the ASTM recommended practice (3). Read the phosphorus concentra-
tion from an analytical curve prepared from the standard samples. Phosphorus determinations are frequently reported as theories of phosphorus, a theory being a weight ratio equivalent to two atoms of phosphorus to three atoms of lead. Theories of phosphorus are calculated as follows: Theories of phosphorus = 3 x 3.785 X 207.21 A 2 x 1.057 X 30.98 B
where A is the phosphorus concentration of the sample in grams per liter, and B is the concentration of tetraethyllead in milliliters per gallon. The factor 1.057 converts milliliters of tetraethyllead to grams of lead. The analytical curve can be plotted to give phosphorus concentrations in units of grams per liter, or as theories of phosphorus for a given tetraethyllead concentration, If manganese is also present in the sample, a correction must be applied, because a manganese line interferes with the phosphorus line used. For this correction, measure the transmittances of the manganese line a t 2551.88 A. and the adjacent background, and calculate the intensity ratio of the manganese line to the arsenic internal standard line. Refer this intensity ratio to a n analytical curve which gives an apparent phosphorus concentration corresponding to the manganese present. Subtract this value from the concentration given by the analytical curve for phosphorus. Development of Spectrographic Method. Although the phosphorus content of gasolines containing phosphorus additives usually ranges from 15 to 30 p.p.m., concentrations as low as 5 p.p.m. may be of interest. This is an unfavorable range for spectrographic work, because the limit of detection for phosphorus is usually
-"i c
A
'
1
5,
' o r
75
Figure
m
A B
1
C D E 5 mm
A
0
C D E
6 mm
A B
C D E 7 mm
A
B C D E CHE M I C I L
3. Effect of g a p width on base stock and additive effect A. B. C.
D. E.
Gasoline Gasoline Gasoline Gasoline Gasoline
35.94 A B
=-
W plus dimethyl tolyl phosphate W plus commercial thionophosphate X plus commercial thionophosphate Y plus commercial thionophosphate Z plus commercial thionophosphate
considered to be about 100 p.p.ni. (1). Moreover. the intensity of the phosphorus line may vary considerably for different gasolines containing the same additive, and for different additives in a single gasoline base stock. The problem was further complicated by the necessity to correct for manganese interference for samples containing organomanganese antiknock additives. Studies of the pertinent variables are summarized below. I n general, the' radiation buffer increased the sensitivity and the effects of base stock and additive were minimized by using the optimum combination of radiation buffer, escitation conditions, electrode speed, analytical gap width, and argon atmosphere. RADIATION BUFFER. Because phosphorus lines were enhanced by the addition of dimethylformamide to the gasoline sample, and gasolines could be diluted greatly with it without a serious loss of phosphorus line intensity, dimethylformamide was chosen as a radiation buffer. Dimethylformamide has a limited solubility in gasoline, and the addition of either ethyl alcohol or isopropyl alcohol was sometimes necessary to obtain solution. A sample diluted with a solution containing 60% dimethylformamide and 40% isopropyl alcohol showed little loss of phosphorus line intensity until a proportion of two volumes of gasoline to three volumes of diluent was reached. When used as a radiation buffer, a solution of dimethylformamide and isopropyl alcohol improved the detection of phosphorus and somewhat reduced the effects of base stock and additives. Naphthenates of heavy metals such as bismuth, and of light metals such as potassium and lithium, were tried to improve the effectiveness of the buffer. Bismuth and potassium showed no effect when added in concentrations of 8 and 3 grams per liter, respectively. A lithium concentration of 0.7 gram per liter of buffer reduced the effect of additives somewhat, but also substantially reduced phosphorus line intensity. A buffer solution consisting of 60% dimethylformamide and 40% isopropyl alcohol was considered to be satisfactory for gasoline containing the usual phosphorus additives. INTERNAL STANDARD.After both antimony, as triphenylstibine, and arsenic, as triphenylarsine, had been tried as internal standards, the arsenic line a t 2456.5 -4.was selected because of its proximity to the phosphorus line. Arsenic lines of suitable intensity were obtained by adding 0.35 gram of arsenic (1.43 grams of triphenylarsine) per liter of buffer mixture. The analytical curve based on this line pair was free from shifts with time. ELECTRODE SYSTEM. Three electrode VOL. 32, NO. 3, MARCH 1960
375
Table I. Use of Correction Curve for Manganese Interference in Spectrographic Method
Phosphorus
Added, Theory 0.20 0.20 0.20
Phosphorus, MangaTGeoW . nese, ApparCorG./Liter ent rected 1.12 1.1 0.21 1.17 0.16 0.67 0.53 0.23 0.64 0.22 0.43 0.21 0.21
0.20
0.11
0.20
0,042
0.20
0.021
0.20
0.00
0.40
1.1
0.40
0.53
0.40
0.21
0.40
0.11
0.40
0.042
0.40
0.021
0.40
0.00
0.40
0.29 0.28 0.25 0.22 0.21 0.20 0.20 0.21 1.30 1.43 0.85 0.82 0.61 0.57 0.48 0.48 0.45 0.43 0.43 0.45
0.40 0.41
0.20
0.20 0.20 0.22 0.21 0.20
0.19
...
*.. 0.44 0.38 0.42 0.38 0.39 0.39 0.41 0.40 0.41 0.40
0.41 0.42
... ...
systems were considered: the porous cup electrode (6), the rotating platform electrode (18), and the rotating disk electrode (16). A rotating disk electrode 0.200-inch thick with a smooth surface gave the most favorable line intensities. The speed of rotation of the disk influenced line intensities. Intensities were almost doubled by reducing the disk speed from 7.5 to 1.3 r.p.m. Two types of counter electrodes were tested: a graphite rod 1/4 inch in diameter with a 90-degree included angle cone tip, and a rod 3/10 inch in diameter with a 15-degree included angle taper rounded to a hemispherical tip. The different shapes did not appear to affect the results. EXCITATION. Because the nature of both the gasoline and the phosphorus additive may affect phosphorus line intensities, it was necessary to establish excitation conditions that minimized these effects. The relatively low voltage discharges produced by a multisource unit (9) gave lines that were susceptible to changes in sample composition. More satisfactory results were achieved by using a high voltage discharge. The excitation source used (11) gives a spark controlled by a rotary gap synchronous interrupter. Three discharge arrangements are possible: four discharges per cycle with 0.007-pf. Capacitance, two discharges per cycle with 0.007-pf. capacitance, and two discharges per cycle with 0.014-pf. capacitance. The discharge can be
376
ANALYTICAL CHEMISTRY
modified by varying the added inductance in the high voltage circuit up to 720 pl., and by varying the voltage applied to the primary of the transformer (power circuit voltage). Excitation conditions were studied using different gasolines, phosphorus additives, radiation buffers, and internal standards. I n all cases, an atmosphere of either argon or nitrogen was maintained around the sample and analytical gap. The effects of varying capacitance, inductance, power circuit voltage, and gap width were similar for all combinations of gasoline and buffers tested. As the power circuit voltage was increased, the influence of the gasoline base stock (matrix effect) became more pronounced. With a power circuit voltage of 90 volts and a capacitance of 0.014 pf., a single analytical curve served for widely different base stocks. At 100 volts, separate analytical curves were required. As the voltage was increased further, the curves became more widely displaced. For a given voltage and a capacitance of 0.014 pf., one analytical curve appeared to hold for a given base stock regardless of the additive. When the capacitance was reduced to 0.007 pf., dissimilar base stocks gave separate analytical curves. As increasing the inductance gave increased line-background ratios, the maximum inductance (720 pl.) of the source was used. EFFECT OF INERT ATMOSPHERE.An inert atmosphere about the sample was required to prevent ignition of the sample vapors. The highest line intensities were obtained when the sample and electrodes remained cool. These two aims were best accomplished by flooding the surface of the sample with argon; the arrangement used is shown in Figure 2. An inert gas directed a t the analytical gap also was required to produce the desired type of spark. With no gas flow, the spark would not break across the wide gap used. The jet which directed the gas stream a t the analytical gap was connected in parallel to the duct which supplied gas to the sample boat. A total gas flow of 15 liters per minute was optimum. Slight changes in the position of the jet had a marked effect on the line intensities but little effect on intensity ratios. Positioning the jet was complicated by the turbulence caused by the gas striking the walls of the rotating electrode apparatus. The optimum position was determined by trial. Helium, nitrogen, and argon were tried. The phosphorus lines were very light when helium was used. Nitrogen improved the line intensity somewhat, and argon enhanced the phosphorus lines. GAP WIDTH. The effects of variations in additive and base stock were less pronounced when wide spark gaps were
used. Tlie effect of gap width was studied by analyzing a number of gasolines using spark gaps of 5, 6, and 7 mm. One aviation and three automotive gasolines were used, each having a phosphorus concentration of 16.7 mg. per liter. Two different phosphorus additives were used in each of the gasolines. Each sample was analyzed in quadruplicate, using each of the three gap widths. The results showed that the effect of variations in base stock was reduced as the spark gap width was increased (Figure 3). The average deviation from the input was 9.4, 4.3, and 3.8% for gaps of 5, 6, and 7 mm., respectively, as compared with 3.1% for chemical analysis. The 7-mm. gap was selected on this basis. When a 90-volt power circuit voltage was used, the argon atmosphere was needed to enable the spark to break across the 7-mm. gap. Gaps wider than this could not be bridged by the spark, even when argon was used. TIMEOF WAITAND EXPOSURE TIJIE. Moving film studies showed that, with most phosphorus compounds, the intensity of the phosphorus line increased as sparking continued until the disk electrode ran dry. However, the ratio of the intensities of the phosphorus and arsenic lines became constant after about 60 seconds of sparking. An exception occurred when the phosphorus was present as trimethyl phosphate. For this compound, phosphorus line intensities were almost constant during the sparking cycle. As the intensity of the arsenic line increased during the sparking time, trimethyl phosphate gave phosphorus-arsenic intensity ratios that decreased as sparking continued. From these studies, a prespark time of 100 seconds was selected to obtain maximum line intensities during the exposure. The exposure time was selected to give a continuous background intensity that could be easily measured (about 70% transmittance). Because level of background intensity varied with the gasoline being analyzed, it was necessary to take this variation into account. With exposure conditions that gave a light background (above 90% transmittance), intensity ratios calculated without a background correction varied with sample matrix. The effect of sample matrix was minimized by obtaining ratios under conditions which permitted a valid background correction t o be made. CORRECTION FOR MANGANESE.A correction curve was used to correct for manganese when present. Apparent phosphorus values, obtained from standard samples containing a range of manganese concentrations but no phosphorus, were plotted against the ratio of the intensity of the manganese line at 2551.9 A. to that of the arsenic line a t 2456.5 A. I n the analysis of samples,
apparent phosphorus results I\ ei c obtained from the established phosphorus analytical curve. These resuits u. ere then corrected by subtracting the values obtained from the manganese correction curve (‘Table I). PREC~SIOS A X D ACCURACY. At the time the method was established, .a standard deviation of 5.3% was obtained for 208 phosphorus determinations over a concentration range of 0.08 to 0.8 theory (6.7 to 67 mg. per liter). Since then, precision values have been obtained from the standard samples which are included on every film. These results are probably a better indication of the precision obtained in routine analyses. They show an almost constant standard deviation of 0.01 theory (0.8 mg. per liter) for phosphorus concentrations ranging from 9.1 to 0.5 theory (8 to 40 mg. per liter). This corresponds to standard deviations of 3, 6, and 9% for phosphorus concentrations of 0.4, 0.2, and 0.1 theory, respectively. These values vere obtained for gasolines that did not contain manganese. I n daily use, the method does not appear to be biased, and the accuracy should be equivalent to the precision. This does not preclude the possibility that the method may exhibit a bias for certain gasolines or phosphorus additives. COLORIMETRIC METHOD
Hoffman .Tone?, Robbins, aiid Alsberg (12) have x h l k h e d z method for ,hosphvrus as the determinatic>r tritolyl ::IIOS~~.AW 1x1 gasoline. dlthough results we good for tritolyl phosphate, they cend to be low for more volatile compounds such as tris(b-ch1oroisopropyl)thionophosphate. Losses occur during sample treatment, which involves waporation of the gasoline and wet oxidation of the residue to form the orthophosphale ion. To correct this, Scafe suggested decomposing the sample by Ignition Jf the gasoline adsorbed on zinc Jxide (19). More recently, Fett and llatsuyama (7) have published a method involving ignition cf the gasoline sample on zinc oxide, separation of the ohosphate from the zinc. and subqequent xeasurement by the colorimetric nethod described by Lueck and Coltz .:GI. I n our xethoc?, the Tlimple is adsorbed on zinc oxide rtcd ignited. The phoFphorus is measured in the presence of the zinc, using tne molybdenum blue method which was studied extensively by Mellon m d Xitson (16). Although iess sensitive than some other modifications, the color obtained is stable and reproducible in the presence of zinc and lead ions. jr’
Apparatus and Reagents. Absorbance is measured with a Beckman
Colorimetric Analysis of Blended Samples Difference from Input Phosphorus, Mg./Liter .idded Found F~iel Mg./liter 70 $1.4 +2.1 Avitit i o i i 66 3 67.7 f 0 . 7 $2.2 $3.3 llotor .\ 66.0 69.1 f 0 . 7 $2.8 $1.8 llotor B 64.0 66.7 f 0 . 8 $1.1 .ivintioii 33.2 34.3 f 1.0 $3.3 -2.0 -0.7 30.2 29.6 f 0.5 35.0 35.0 f 0 . 4 0.0 0.0 $1.5 +4.5 Motor .4 33.4 34.9 f.0.7 $0.3 +0.9 Motor B 32.6 32.9 f 0.3 -1.0 -0.2 Aviation 20.1 19 9 + 0 . 7 -1.6 -0.3 18 4 f 0 . 2 18.7 +1.8 $0.3 16.6 16.9 f 0 . 7 +1.8 +0.3 Motor A 16 7 17.0 f 0 . 7 +1.8 +0.3 Motor B 16.3 16.6f0.1 -0.3 -3.6 Aviation 8.3 8.0 f 0.3 -0.1 -1.2 Motor A 8.4 8 . 3 f0 . 3 0.0 0.0 Motor B 8.1 3.1 f 0.3 Average range of four replicates = 1.1 ing. P per liter. Standard deviation = 0.51 mg. P per liter. Table II.
Model D U spectrophotometer, using matched Corex cells 10.00 cm. thick. The necessary glassware and crucibles are reserved for this work and are washed with cleaning compounds t h a t do not contain phosphorus. REAGENTS.Ammonium Molybdate Solution. Add 150 ml. of sulfuric acid (specific gravity 1.84) to 500 ml. of water. Cool to room temperature, stir in 50 grams of ammonium molybdate ~(NE14)BMo,024.1H*0),and dilute to 1000 ml. Hydroquinone Solution. Dissoive 2.5 grams of hvdroauinoni it1 100 ml. of water. Add 5 ml.*of IN sulfuric acid and diiute to 500 ml. Rith water. Sodium Sulfite Solution. Dissolve 100 grams of sodium sulfite in mater and dilute to 500 ml. of solution. Sulfuric Acid Solution. Add 100 ml. of sulfuric acid (specific gravity 1.84) to 1000 ml. of water. Standard Phosphorus Solution. Dissolve 4.393 grams of dried potassium dihydrogen phosphate in 150 ml. of sulfuric acid solution and dilute to 1000 ml. with water. Dilute 10 ml. of this stock solution to 1000 ml. with nater dontaining spproximately 0.1 gram of potassium permanganate added to prevent mold formation. The phosphorus concentration is 0.01 n g . per ml. Zinc Oxide. TTse reagent grade material as purchased. Preparation of Calibration Curves. Add 2.0 grams of zinc oxide and 25 ml. ,)f sulfuric acid solution t o each of seven 200-ml. Berzelius beaders and heat to oonipiete the bokmc~n. After cooling +he srxtions, add 0.5, 1.0, 2.0, 3.0,4.0, ‘mi 5.0 ml. of the standard phosphorus solution to the separate beakers. The solution in the seventh beaker is carried through as a blank. Transfer each solution into a 100-ml. volumetric flask, and wash its beaker with sufficient water to bring the volume in the flask to about 50 mi. Add 10 ml. of ammonium molybdate solution, 5 ml. of sodium sulfite solution, m d 5 ml. of hydroquinone solution suc.essively to each flask, mixing thormghly after the addition of each
Table Ill. Colorimetric Analysis of Samples Containing Different Phosphorus Additives
;Blends contain 18 mg. of phosphorus per liter) Phosphorus ’ additive Recovered, % Xs@-chloroisopropyl) thionophosphate 102 Trimethyl phosphate 99 Triethyl phosphate 955 Dimethyl m (and p)-tolyl phosphate and methyl di-m (and p)-tolyl phosphate 102 p-Chlorophenyl dimethyl phosphate 103 Ditolyl {chloromethyl) phosphmate 102 Bis [o (and p j-chlorophenyl] methyl phosphate and o (and p b chlorophenyl dimethyl phosphate 99 Bis [chloro-m (and p)-tolyl]methyl phosphate and chloro-m (and p)-tolyl dimethyl phosphate 99 Tritolyl phosphate 98 a Purity of triethyl phosphate used to blend these samples is questioned. reagent, After diluting to the mark with water, mix the solutions again, and let them stand for a t least 0.5 but not more than 2 hours. Transfer a portion of each solution to separate 10-cm. Corex cells and read the absorbances of the solutions a t a wave length of 660 mH, using a slit width of 0.04 mm. Use distilled water in the reference beam. Correct the absorbances of the solutions by subtracting the zbsorbance of the blank solution. Plot the corrected absorbances against milligrams of phosphorus. Procedure. Add 2.0 grams of zinc oxide to a clean and dry No. 0 porcelain crucible. Make a deep depression with u. stirring rod in the center of t h e zinc oxide charge. Record the temperature of the gasoline, and pipet a 1-ml. sample into the depression. VOL. 32, NO. 3, MARCH 1960
377
Table IV.
Effect of Sample Volume in Colorimetric kethod
Additive Tris (8-chloroisopropyl) thionophosphate
\ oi., MI. I
Phosphors,, ' :i Added .Feud* 2.01
1.70 4.03
3.40 Mixed methyl tolyl phosphates
3 4 i 2 3
6.04
8.05
1.66 3 32 4 97
6.63 a
1.99
1.72
3.8s 2.8.1 5.53
6.27 1.63 3 39 5
I!
6 73
Recovery,
9:
9!?
101 94 84 92 78 102 102
103 10'
Average of two dewrminations.
Foint the tip of the pipet so t h a t the sample flons to t h e bottom of the depression. Gently t a p t h e crucible until the depression is filled and the sample is covered with a layer of zinc oxide. Ignite the gasoline by passing a flame over the top of the crucible. After burning is completed, heat the crucible and contents for a few seconds over h low flame, then with the full heat of t h t burner. While the crucible is red hot, burn the remaining carbon from the toy of the oxide and the side of the crucible with the flame from a second burner or by placing the crucible in a muffle funace at 600" C. After cooling the crucible, place it ia a 20Gml. Herzelius beaker, and add 25 mi. cf sulfuric acid solution. Warm the deaker 011 a hot plate untii t h r oxide coxpletel! dissolve?. Transfe ti,sdution t a a 106-n !. volumetric fiasi:, and rinse the beaker ana crucible with sufficient water to bring the volume i? the flask to about, 60 mi Develop the color and measure it; absorbance by tne procedure described under the preparation of the calibratiorl curve After correcting the absorbance of the sample by subtracting the absorb ance of the blank, read the phosDhorus content of the sample from the calibration curve. Prepare a new blank for each new supply _ _ - of zinc oxide or reagent solution. Calculate the vhosvhorus content a t 60" F. as follows: Phosphorus, mg. per liter = 1000 A [I 0.001 ( t - 15.6)]
+
where A is the phosphorus content of the 1-ml. sample, and t is the temperature of the sample in degrees C., at time of sampling. Analytical Results. T h e accuracy and precision of the method were established by analyzing a series of specially blended samples. Three base stocks were used, one aviation fuel and two motor fuels. Three additives-tris(8-chloroisopropyljthionophosphate, mixed methyl tolyl phosphates, and tritolyl phosphatewere chemically analyzed and were blended with the base stocks at four concentrations, 8.0, 17.0, 32.0, and 65.0 mg. of phosphorus per liter. All samples contained 3 mi. of T E L per 378
ANALYTICAL CHEMISTRY
gallon as Ethyl Corp.'s Motor Mi?: antiknock compound. Each blend was analyzed at least four times (Tabie 11). The average range of results (1.1 mg. of phosphorus per liter) was independent of the phosphorus concentrstion, of the gasoline base stock, and oi the particular additive used. Although the standard deviation was 0.51 m g phosphorus per liter, the accuracy appeared to be a function of the phos. phorus concentration. Further cal-. culations showed that the results wert consistently 3y0 high. This systematic error was traced to the use of 1-mi. transfer pipets for the sampling operation. When the samples were weighed. the accuracy of the procedure was equiva1sr.t t n tlic: precision. aowever, weighing is not :rcoinrr.ended, becausc the additional accuracy toes not justif!. the time required. Lsing teiJ phosphorus compounds witr, varying voiatiiities (Table l l i j : sampies were blended a t concentrations of about i 8 mg. of phosphorus per iitei and aoalyzed by t h t procedure. The resu!ts are within the determined accuracy of the method. The recovery for trimethyi phosphate is equivalent, to that for tritolyl phosphate. Because trimethy! phosphate is more volatile than the currently used commercial additives, the method should give accurate results for any phosphorus compound that will form zinc orthophosphate. Development 04 Colorimetric Method. T h e most difficult problem was conversion of t h e organic phosphorus compound t o the orthophosphate ion. A rapid wet oxidation method similar t o the one described by Hoffman et al. ( l a ) gave iow results when applied to tris(p-chloroisopropy1)thionophosphate. A modified chemical method gave quantitative recovery for this and other experimental additives, In this method, the phosphorus compound was hydrolyzed by refluxing the gasoline with alcoholic potassium hydroxide, followed by controlled evaporation of the sample through an air condenser until the refluxing tempernture was 200" to 210" C. The phosphate ion was then extracted with dilute
nitric aLid a?ca riissoive 3 q i : m m eeriai was oxidized nit!. C:PC a? sJ"cri0 ncails This r n 4 i o j J. i s > f 1-r deiermimtlsn o, L - & L ( ~ qdaii phospnorus but is too tme-ror,suinir,, for geiieral >cafe (191 suggested that tb- c w , bustinn of a 2-ml. sample 0; gasolin' absorbed on ~1 2-gram bed of zinc ouiw provided a maid and convenient metho . for converting organli. phospharua COII' D o m u - tc, tbie pnosptiarc I:) I . Th. application cd this rechnioue t o gasoi contaiiiine t r i s ( ~ - c h l o r o i s o p r o ~ ~ ~ ~ ~ ~ ~ nophosDhatr and tnmethyi phosniia rpuultei in some 1?s3 of ~kiospno i1.3
reaaced to nu. recover>. u 'r Lonsequentlj , in? proce studied to aetermine the -!fecrs and oE ~ 1 1 0 5 1 10 - 1 ed 60 contain app mate;? 19 mg. of Dhosphorus ?er
rwoven
01;
mixed
*%:ti: dXere- . '1 lit tnalyses 'T a 1m;l t n 2 c o n t l u i i o i tnat c c,f p h o s p h n r ~ is ~~inae~tnc~rli~ c~oicentratiorrin tnf ~drige0 Ji, :JS,
idrid
stocka.
tive Subsequent studies showed tnat equally satisiactori results were obtained by using a 2-m!. sample ign on a 3-gram charge of zinc oxide The yelioir color of molybdivanadophosphate ( I S ) was used t o measure thr phosphate ion when the alkaline drolysis procedure mas used for composition of the samnle When zinc oxide ignition procedure adopted, zinc interfered. A mol. ? denum blue method of H d t ~ a n Melion (5) was compared to t h ? A 0 (4) method, studied by hlellor R Kitson (151 The -4QA.C metho-l x adopted because, ic the presence zinc, it yields better precislo;, is mol convenient for handnng multipi samples, and requires less tune ne' sample
Method variables were studied ai;$ the results generally confirmed the wor of Mellon and Kitson (15). Calinration data prepared over the range of 0.00; to 0 12a mg. of phosphorus ir, aqueous solution oheled Beer's law The zinc
oxide decomposition procecluw iiitrot l i i c ~ d:i large concentration of zinc: ion which appeared to change the slope of tlie calibration curve and decrease the stnhility of the colored sJxtein. ‘I’he zinc intcrfcrcnce resulted from the cffmt of the zinc ion on the p H of the sJ.stcin. Therefore, the quantity of acid \vas increased to bring the p H back to approximately 1. The recommended procedure automatically provides the correct p H for 2.0 grams of zinc oxide. If the quantity of zinc is altered, the pH of the final system should be adjusted. Experience Jvith a large number of gasolines containing a variety of antioxidants, dyes, and metal deactivators has shown that these additives do not interfere. Also, manganese added t o gasolines as (methylcyclopentadieny1)manganese tricarbonyl does not interfere with either the decomposition or colorimetric measurement. Xlthough the analysis of a single sample requires 1 hour of elapsed time, 24 determinations can be made per man-day. DISCUSSION
The spectrographic and colorimet,ric
lrocedures have bccn routinely ap’plied by a number of analysts to hundreds of samples, including gasolines blended for road tests, special fuels blended for laboratory tests, and field samples collected from service stations, Keithcr method is affected by the presence of tetraethyllead, scavengers, antioxidants, metal deactivators, or sulfur. Manganese antiknock compounds do not affect the colorimetric method. With the spectrographic method a correction for manganese is required. The spectrographic method can be applied to any fuel composition. However, it does give low results for samples containing trimethyl phosphate, triethyl phosphate, or dibutyl hydrogen phosphate unless special calibration curves are used. On the other hand, the colorimetric method provides accurate results for any gasoline-additive composition. LITERATURE CITED
(1) Ahrens.
L.
“Spectrochemical Analysis,” pp. 204-5, Addison-Wesley Press, Cambridge, Mass., 1950. ( 2 ) Am. Soc. Testing Materials, Philadelphia, “Methods for Emission Spectrochemical Analysis,” pp. 1-11, 1957. H.,
(3) Ibid., pp, 12-36. (4) Assoc. Offic. Agr. Chemists, “Methods of Analysis,” Sections 6.38, 6.39, 6.40, ,nen
lY3W.
(5) Boltz, D. F., Mellon, M. G., AKAL. CHEM.19,873-7 (1947). (6) Feldman, C., Ibid., 21, 1041 (1949). (7) Fett, R. E., Matsuyama, G., Chem. Analyst 47, 32-4 (1958). (8) Gambrill, C. M., Gassmann, A. G., O’Neill, W.R., ANAL.CHEM.23, 1365911951). (9) Hasler, M. F., Dietert, H. W., J. Opt. SOC.Am. 33,212-28 (1943). (10) Zbid., 35, 802 (1945). (11) Hasler, M. F., Kemp, J. W., Miller, W. H., Zbid., 37, 990 (1947). (12) Hoffman, F. F., Jones, L. C., Jr.,
Robbins, 0. E., Jr., Alsberg, F. R.,
ANAL.CHEM.30, 1334-6 (1958). (13) Kitson, R. E., Mellon, M. G., IND. ENG.CHEM.,ANAL.ED. 16, 379 (1944). (14) Lueck, C. H., Boltz, D. F., ANAL. CHEM.28, 1168 (1956). (15) Mellon, M. G., Kitson, R. E., IND. ENG.CHEM.,ANAL.ED. 16, 466 (1944). (16) Pagliassotti, J. P., Porsche, F. K., ANAL. CHEM:.23, 198-200 (1951). (17) Ibid., pp. 1820-3. (18) Rozsa, J. T., Zeeb, L. E., Petrol. Processing (November 1953). (19) Scafe, E. T., Socony Mobil Oil Co., Inc., Research Department, Paulsboro,
N. J., Socony Mobil hnalytical Method 71-56.
RECEIVEDfor review July 22, 1959. Accepted December 1, 1959.
Thiomalic Acid as Reagent for Zirconium SUSEELA 8. SANT’ Carr Laborufory, Mount Holyoke College, South Hadley, Mass. 8HARAT
R. SANT’
Coates Chemrcul laboratories, Louisiana State Universify, Baton Rouge, l a .
b Thiomalic acid i s shown i o b e o suitobie reagent for the quantitative precipitation of zirconium from dilute nitric or hydrochloric acid solution. Common anions Dike chloride, sulfate, and nitrate and most o f the cations d o not interfere. Interferences due to mercuryill) and thorium a r e easily eliminated. Bismuth i s coprecipitated and is the only interfering element; its prior separation i s therefore recommended. The composition o f the zirconium compound varies somewhat but corresponds approximately t o zir{CHSH. COO\
cony1 thiomalate,
S
(
1
\CH,.coo
yro.
organic precipitants for zirconium have bcen proposed in the past few years ( I ) . By far the most Present address, Department of Chemi s h r , University of Toronto, Toronto, Ontario, Canada. ETERAL
useful is mandelic acid and some of its derivatives. The formation of tetramandelat,es and t’he possibility of a titrimetric finish are distinct advantages. Recently, Sarit and Sant (3) reported the use of thiodiglycnlic acid, S(CH2COOH)2,as a precipitant for zirconium. Preliminary experiments showed that an aqueous soiution of thiomalic acid, HOOC . CHSH. CH2.COOH, on boiling with a zirconyl salt solution gives a white precipitate suitable for gravimetric analysis. The reagent is almost specific for zirconium but has hitherto not been investigated. Most elements do not interfere and the few which do can be easily eliminated. Only bismuth causes serious interference, but a prior separation of zirconium and bismuth can be readily accomplished, for instance, by sulfide. This paper describes optimum conditions for using :hiomalic acid as a reagent for zirconium.
EXPERIMENTAL
A. D. Rlackay’s reagent grade zirconyl nitrate was used to prepare a n aqueous solution. To prevent hydroslyis, the stock solution was acidified with nitric acid so that the over-all acidity was about 0 . 2 N . The zirconium content was determined bv the m-nitrobenxoic acid method (1). A pure sample of thiomalic acid, HOOC.CHSH.CH2.COOH, was used and its 2% solution was made in water. All other chemicals were of reagent grade, Procedure for Zirconium in Its Pure Salt Solution. T o a n aliquot of zirconyl nitrate solution diluted t o about 150 ml., enough nitric acid \vas added t o maintain its total acidity in the system at 0 . 2 N . When using hydrochloric acid, the over-all acidity should not evceed 0.1N. Fifteen t o 50 ml. of 2y0 thiomalic acid solution were added and the system was heated on a hot plate at 80’ t o 90” C. for about 0.5 hour. The white precipitate formed n-as set aside for 1 hour. filVOL. 32, NO. 3, MARCH 1960
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