Rapid Cornbustion Method for Determination of Phosphorus in Petroleum Products GEDANSKY, J. E. BOWEN, and 0. I. MILNER
S. J.
Reseorch Deporfment, Poulsboro laboratory, Socony Mobil Oil Co., Inc., Poulsboro, N. J.
,A method has been developed in which organically combined phosphorus i s converted to the ionic form by rapid burning in an oxygen-filled flask and fusion in sodium carbonate. The liberated phosphate i s determined colorimetrically as the reduced phosphomolybdate. A single sample, containing phosphorus in the range of 0.003 to 470,can be analyzed in less than 20 minutes of elapsed time. Under routine conditions 25 to 30 samples can be analyzed in an 8-hour day. The over-all standard deviation of the method i s 0.00 16 for oil samples in the range of from 0.024 to 0.39% phosphorus and 0.028 for additive samples in the 1.2 to 4.070 range.
L
OILS conmionly incorporate organophosphorus-sulfur compounds and their derivatives as antioxidants and detergents. To provide blending production control a rapid method for determining phosphorus has becn investigated. Emission spectroscopy (8, 10-13) has been used to determine phosphorus, but i t is somewhat lacking in precision. Phosphorus is also often determined as the reduced phosphomolybdat,e complex or the phosphomolybdivanado complex (6) following conversion of the organophosphorus compounds to inorganic phosphate. This conversion can be accomplished by wet oxidat’ion ( 1 ) or by nshing in zinc oside (9,10,14). Neither is particularly rapid. More recently, the oxygen-flask technique has been used for the rapid combustion of organic samples for phosphorus determination (2, 3, 5-7). The mcthod reported by Barney, Bergniann, and Tuskan ( 2 ) is most nearly applicable to the present problem. I n this procedurcl. the combustion products are absorbed in nitric acid and the phosphate is determined colorimetrically as t’he phosphomo1ybdit.anado complex. I n the present work! Barney’s procedure has been modified by incorporating a fusion technique to overcome the need for a n oxidizing absorbing solution. This permits the more sensitive reduced phosphomolybdate complex to be used. At the same time, the over-all procedure is shortened considerably. d single UBRICATING
sample, containing phosphorus in the range of 0.003 to 4%, can be analyzed in less than 20 minutes of elapsed time. Under routine conditions 25 to 30 samples can easily be analyzed in a n 8-hour day. For most control laboratory purposes, i t is the short elapsed time for a single sample that will prove most valuable. The range of the method can be increased to about 7% phosphorus by the use of a micro sample. APPARATUS
Combustion Apparatus. This apparatus is shown in Figure 1 . The joints are hand-lapped with fine Carborundum to ensure a good fit. The platinum basket, available from Arthur H. Thomas Co., Philadelphia 5, Pa., as “Platinum Wire Gauge Sample Carrier for Thomas-Schoniger Combustion Apparatus,” can be bent into the desired shape. H A N D L E LONG ENOUGH TO ALLOW P L A T I N U M B A S K E T TO REACH BOTTOM OF FLASK WHEN REDUCING ADAPTER IS REMOVED
’
$ 24/40
REDUCING ADAPTER j 2 9 / 4 2 OUTER j 2 4 / 4 0 INNER
$ 2 9 / 4 2 (TOP FLARED)
‘\-
CYLINDRICAL
( Figure
’ P L A T I N U M BASKET
500 M L F L A S K
1.
Combustion
apparatus
Colorimeter. X Klett-Suninierson colorimeter with a S o . 69 filter (maximum transmission a t about 700 mp) was suitable. A 0.25-, 1-, 2-, or 4-em. light path was used, depending on the phosphorus level. llethylcellulose Capsules. Xo. 4 small, Arthur H. Thomas Co., Catalog NO. 6471-G. Sodium Carbonate Scoop. Fabricate a scoop from glass tubing 4 mm. in
outside diameter and mark i t to contain 50 f 10 mg. of sodium carbonate, anhydrous powder. REAGENTS
Ammonium Molybdate Solution. Dissolve 40 grams of reagent grade ammonium molybdate [ (NHJJIolOza. 4Hz0] in a cooled mixture of 450 ml. of concentrated sulfuric acid and 1 liter of water. Dilute to 2 liters with water. Hydrazine Sulfate Solution. 1.5 grams per liter. hl o l y b d a t e - H y d r a z i n e Reagent. Dilute 50 ml. of ammonium molybdate solution with 130 ml. of water: Add 20 ml. of hydrazine sulfate solution and mix well. Use 50 ml. for each determination. Prepare no earlier than 1 hour before use because this mixture is unstable. Stock Standard Phoswhorus Solution (1.60 mg. per ml,). *Dissolve 4.393 grams of dried reagent grade potassium dihydrogen phosphate (KHaPO4) in 150 ml. of 1 t o 10 sulfuric acid and dilute to 1 liter with water. Dilute as needed for working standard solutions (0.01 and 0.1 mg. per ml.). PROCEDURE
Add 10 ml. of 1 t o 10 sulfuric acid t o the combustion flask and fill with oxygen. Place 1 scoopful of sodium carbonate in the bottom half of a methylcellulose capsule. Weigh a sample of not more than 50 mg., estimated to contain between 0.0013 and 0.4 mg. of phosphorus, directly onto the sodium carbonate bed. A small cork serves as a handy support for the capsule. Cover the sample Ivith another scoopful of sodium carbonate. Insert a filter paper fuse, approximately 11/* X inches between the top and bottom halves of the capsule. Place the capsule in the platinum basket with the fuse toward the glass stopper. Moisten the ground glass surfaces of the reducing adapter and seat it firmly in the flask. Ignite the filter paper fuse and, rrithout delay, insert the sample into the flask and seat the joints firmly. Immediately place the flask behind a safety shield and tilt it at a 135’ angle so that the absorbing solution forms a liquid seal a t the neck of the flask. (Avoid inverting the flask completely as this may cause the capsule to fall from the holder before combustion is complete.) Hold the stopper firmly in place during the combustion. After the combustion allow the flask t o cool for 10 to 15 VOL. 32, NO. 1 1, OCTOBER 1960
1447
seconds, then shake the flask for about 30 seconds, holding the stopper in place. Momentarily remove the , platinum basket holder and rime the inner and outer surfaces of the reducing adapter with 40 ml. of distilled mater, collecting the washings in the combustion flask. Lower the platinum basket into the solution. Add 50 ml. of molybdatehydrazine reagent. Heat the solution rapidly t o boiling and boil for 2 t o 3 minutes. Cool the solution to room temperature in a n ice water bath, transfer it to a 100-ml. volumetric flask, and adjust to volume with water. Measure the absorbance against distilled water in a n appropriately sized cell. Convert the net absorbance to milligrams of phosphorus by use of the corresponding calibration curve. (Separate calibrations are prepared for each size cell by adding portions of the working standard phosphorus solutions to 10 ml. of 1 to 10 sulfuric acid and developing the color as described above.)
of the two methods in the 1.2 to 4.0% range. A t test (95% confidence level) showed no bias between the methods. DISCUSSION
In preliminary tests, with conventional Schoniger-type ignition, nitric acid was used as a n absorbing solution. With the use of the phosphomolybdivanado color, the method lacked sensitivity for our purposes. I n attempts t,o use the more sensitive phosphomolybdo color, it was necessary first to eliminate the nitric acid; this required too much time. Nonoxidizing absorbents permitted the direct use of the reduced phosphomolybdate complex, which has the added advantage of forming more rapidly than the phosphomolybdivanadate, but recoveries were low, in agreement with a previous report (S). A t tempts to use solutions of sodium peroxide, sodium hypobromite, and sodium persulfate as absorbents, and to destroy the oxidant prior to reduction of the phosphomolybdate complex were also unsuccessful. A new technique, which uses potassium pyrosulfate (with or without added potassium chlorate) as a n oxidizing and fusing medium in the combustion zone gave results that were slightly low but encouraging. In this technique the oil samples !vert’ impregnated on the salt and burned in methylcellulose capsules; recoveries were 90 to 967,. As there was little evidence of unburned sample, it was believed that some of the SO3, liberated in the combustion, dissociated to yield SO2 which reduced some of the phosphate, possibly in the vapor phase. Combustion in sodium carbonate proved successful.
RESULTS
A series of representative lubricating oil and additive samples was analyzed by both the conventional zinc oxide (IO,14) and the proposed oxygen-flask procedure (Table 11). The over-all standard deviation of the oxygen-flask procedure was 0.0016 for oil samples in the range of 0.024 to 0.39%, and 0.028 for additive samples in the 1.2 to 4.0% range. The corresponding standard deviations for the zinc oxide procedure are 0.0035 and 0.038. Application of the statistical F test (95% confidence level) to these data indicates that the increase in precision is significant in the 0.024 to 0.39% range. There is no statistical difference in the precisions
Table
hIetals Present
...
Zn Ba, % t i
...
Zn Ba, Zn Ba, Zn Ba, Zn ... Ba
Ba Ba Ba
...
Zn
I.
Comparison of Analytical Results 70
Phosphorl1s
Zinc Oxide ( 1 4 )
Oils 0.0028, 0.0027 0.024, 0.023 0.032, 0.037, 0.040, 0.035, 0.034, 0.033 0.040, 0.041 0.052, 0.051, 0.050 0.062, 0.060 0.075, 0.074 0.083, 0.087, 0.086, 0.086 0.092, 0.091 0.130, 0.133 0.150, 0.147 0.211, 0.224 0.241, 0.243 0.319, 0.312, 0,311, 0.314, 0.310 0.392, 0.380, 0.386, 0.394, 0.391
Proposed Method
0,0029, 0.0027 0.023, 0.024 0.032, 0.033 0.036, 0.037, 0.037, 0.041 0.052, 0.052 0.060, 0.060, 0.060 0.072, 0.073, 0.072 0.086, 0 087 0.092, 0.092 0.134, 0.135 0.143, 0.145 0.221, 0.223 0.238, 0.242 0.310, 0.309 0.384, 0.390
Additives Ba Ba Ca
1.22, 1.26, 1.27 1.35, 1.37, 1.35 2.42, 2.36, 2.32 3.75, 3.70, 3.71 3.94; 4.04
... ...
1448
e
ANALYTICAL CHEMISTRY
1.22, 1.27 1.34, 1.34 2.40, 2.34, 2.37 3.71, 3.68 3.93, 3.99, 3 . 9 8
The phosphorus pentoxide produced in the combustion should be extremely soluble in the moist atmosphere present within the flask. It should, therefore, not be necessary to allow vapors t o condense for long periods as is customarily done. Experiments showed that shaking the flask for only 30 seconds after combustion gave the same results as allowing vapors to condense for 20 minutes in a n ice water bath. Many of the samples normally analyzed contain organobarium salts. I n conventional colorimetric procedures, i t is usually necessary to filter off the barium sulfate. However, a small enough sample is taken in the present procedure, that, a t most, only a slight barium sulfate haze is produced; this can almost invariably be ignored as the error has never been found to exceed 2y0 relative. To ensure accuracy greater than this for samples high in barium, the solution may be centrifuged for several minutes just prior to the color measurement. This v a s not done in this study. To develop the reduced phosphomolybdate color, it is customary to heat the solution just below the boiling point for 20 to 2.5 minutes (4, 1 5 ) . I n the present work, a stable color having the same intensity was obtained by boiling the solution for 2 to 3 minutes. The values obtained for blanks containing 30 to 40 mg. of Decalin (decahydronaphthalene) in place of the sample were reasonably constant (about 0,0024 mg. of phosphorus). Therefore an arerage blank value was determined nnd checked occasionally. Results obtained for samples containing more than 0.4 mg. of phosphorus nere consistently low and erratic. As the color system is linear in this region, it n-as felt that these low results were attributable to the combustion and/ or fusion process. Burning the samples in a flask twice as large gave improved results, although still somellhhat low (Table 11). Best results were obtained n ith the use of a proportionately smaller sample weighed on a microbalance. Homever, even the very small samples tended to give low results as the phosphorus level increased (Table 11). Apparently a factor in low recoveries on high phosphorus samples is a deficiency of oxygen in the neighborhood of the burning sample, resulting in incomplete oxidation of the phosphorus. K i t h the 500-ml. capacity flask, the largest sample that can be conveniently burned is about 30 mg. This corresponds to an estimated lower quantitative limit of about 0.002% phosphorus, if a 4-em. light path and the apparatus described are used. With a 1-liter combustion flask, a 100-mg. sample can be burned and the lower limit decreased by a factcr of two. As some special oils may contain less than 0.002% phos-
Tuskan, W. G., ANAL. CHEM.31, 1394 Table II.
Effect of Sample Size and Total Oxygen Volume
Zinc Oxide Sample A B C
D
E
(14)
4.72, 4.77 5.60, 5.59 5.81, 5.88 6.88, 6.90 7.20, 7.10
% Phosphorus 10-15 mg. 10-15 mg. sample 500-ml. sample 1000-ml. oxygen flask oxygen flask 4.28 4.94, 4.40 4.94 5.25, 4.62
4.72 5.41, 5 . 4 6 5.81 6.56 6.18
...
phorus, it may be necessary to increase the range of the method even further. The use of a spectrophotometer set a t the absorption maximum of 830 mp will effect an additional %fold increase in senbitivity . When the amount of phosphorus present is high enough to require the use of less than 0.03 gram of sample, it has been the practice to add sufficient Decalin to provide a total of 0.03 to 0.05 gram of organic material for combustion. Several hundred combustions have been carried out without mishap. However, all combustions are performed behind a safety shield as a precaution. By having apparatus, reagents, etc.,
5-6 mg. sample 500-ml. oxygen flask 4.73, 4.76 5.65, 5.68 5 . 8 7 , 5.86 6 . 7 3 , 6.77 6.87, 6.91
in readiness, a result has been calculated in 16 minutes after receipt of the sample. Total elapsed time could be shortened even further if a reducing reagent could be found that would form a stable color in 5 minutes or less without boiling. Preliminary experiments with l-amino2-naphthol-Psulfonic acid (16) have shown some promise. LITERATURE CITED
(1) Am. Soc. Testing Materials, Phil-
adelvhia. Pa.. “ASTM Standards ..__ nn . . Petrbleum Prbducts and -Lubricants,” Method D 1091-58T, 1958. (2) Barney, J. E., 11, Bergmann, J. G.,
(1959). (3) Belcher, R., MacDonald, A. >I. G., Talanta 1 , 185 (1958). (4) Boltz, D. F., “Colorimetric Determination of Nonmetals,” Interscience, pp. 32 ff., New York, 1958. (5) Cohen, L. E., Czech, F. W., C‘hemistAnalyst 47,86 (1958). (6) Corner, M., Analyst 84,41(1959). (7) Fleischer, K. D., Southworth, B. C.,
Hodecker, J. H., Tuckermann, M. M., ANAL.CHEM.30,152 (1958). (8) Gambrill, C. M., Gassmann, A. G., O’Neill, W. R., Zbid., 23, 1365 (1951). (9) Gerhardt, P. B., Dryoff, G. V., Ibid., 28,1725 (1956). (10) Griffing, ILL E., Leacock, C. T., O’Neill, W. R., Rozek, A. L., Smith, G. W., Ibid., 32, 374 (1960). (11) Pagliassotti, J. P., Porsche, F. W., Ibid., 23,198 (1951). (12) Ibid., p. 1820. (13) Rappold, W. A., Ramsey, R. E., Am. SOC.Testang Materials, Spec. Tech. Publ. N O .,214, pp. 109-13 (1957). (14) Socony Mobil Oil Co., Tnc., Research Department, Paulsboro Laboratory, Paulsboro, N. J., Mobil Method 71-56. (15) Welcher, F. J., “Organic Analytical Reagents,” Vol. I, p. 229, Van Nostrand, New York, 1947. RECEIVEDfor review April 1, 1960. Accepted July 22, 1960. Third Delaware Valley Regional Meeting, ACS, February 25, 1960.
Elemental interaction Effects in the Spectrochemical Determination of Additive Elements in Lubricating Oils E. 1. GUNN Humble Oil and Refining Co., Boyfown, Tex.
b A study of the effects of interaction in the spectrochemical determination of additive elements in lubricating oils has been made in which a statistical design was employed. The direction and magnitude of interaction effects are defined, and methods of correcting for them are illustrated. The influence on interaction effects of such variables as physical properties of the oil, additive type, excitation characteristics, and electrode type are exemplified.
T
precision of the direct spectrochemical analysis of processed lubricating oils provide a powerful tool for controlling the additive content in industrial blending operations. Both the porous cup and rotating disk or platform techniques have been employed by petroleum laboratories in the direct determination of additive elements in lubricating oils (2-6),and each has been proved efficient for this purpose. The experience in the present HE SPEED and
author’s laboratory has been that the porous cup is more flexible than the rotating types where a variety of analytical operations are required, as well as being more rapid in a given series of sample excitations. I n the spectrochemical determination of additives in lubricating oils by either technique the intensity of an elemental spectral line measured in the analysis is influenced by the presence of other coexisting elements. This mutual influence, or interaction, has the effect of producing an erroneous answer for an element unless proper correction or compensation is made. This usually is accomplished by the use of calibration standards which simulate very closely the composition of the oils to be analyzed. The additive elements often determined are calcium, barium, zinc, and phosphorus. Hence, the interactions of these elements in lubricating oila are those of most concern in a study of the mutual interaction effect. Evidence, subsequently presented,
shows that the compound type or identity of the metallo-organic substance may also influence the emission of t h e analytical line. Various means have been devised to avoid or minimize the matrix effect so as to achieve a more nearly universal method. One is through the use of buffers which moderate the spark gap discharge so as to equalize its energy condition from sample to sample. In this laboratory, lead in the form of the naphthenate was added to several synthetic blends so that each contained 1% lead. No observable lessening of i n t e r action was achieved in porous cup excitation. The use of magnesium as a spectroscopic buffer with the rotating disk has been reported to be effective in overcoming elemental interaction (4). The present author has not applied this buffer in porous cup analysis, however. Ash-inorganic matrix techniques are another approach. Such procedures are both time-consuming and laborious in comparison with a direct sparking techVOL. 32, NO 11, OCTOBER 1940
@
1449
