Determination of Trace Metals in Petroleum Fractions J. H. KARCHiiER AND E. L . GUNN Humble Oil & Rejining Co., Baytown, Tex. The occurrence of trace metals in petroleum and its products is of considerable significance to the industry. To evaluate the influence of a given metal or its compounds, analytical methods of satisfactory sensitivity and accuracj must be available for testing the oil. The time-honored method whereby combustion of the oil is carried out with subsequent determinations of indhidual components on the ash residue has certain inherent limitations: the possible error through loss of ash matter and the excessive time and effort required for the analysis itself. 4 procedure has been developed in which, by rigid
I T H I S recent years, interest of the petroleum industry in the occurrence of trace metals in various products and substances associated with the industry has increased markedly. This interest derives from the association of metals \Tith geological origin of crudes, the influence of contaminant metals in refming or processing of pet,roleum products, or the effects of metals on utilization or performance characteristics of the finished products. In addition to the organic constituents of petroleum, of which carbon and hydrogen are the chief elemental components, oxygen, nitrogen, sulfur, and various naturally occurring inorganic substances are found in trace amounts. Varying concentrations of the mineral constituents remain wit,h the oil as it flows on its devious course through the refinery. In some petroleum fractions the amount of metal constituents may be lessened t~ssentiallyto zero by virtue of the treatment received en route through the refinery; for other fractions, the amount may be considerably increased by concentration or by metal pickup from pipes or storage facilities. The metal content of an oil alp0 may be enhanced as a consequence of the formation of nietal soaps of d f o n i r or naphthenic acids in treating processes PRESESCE OF 3lETALS IN OILS
Influence of Metals. The influence of metals occurring initially in oils or introduced extraneously in processing or during service may be illustrated by certain examples. I n fluid catalytic cracking the presence of only a few pounds of such trace metals as iron, nickel, and vanadium imparted from the feed stock and acc*urnu]ated in each ton of catalyst may result in expensive c,atalyst replacement because of their adverse effects on qualit). anti distribution of the cracked products (14). Inorganic deposition products from fuels often give rise to refinery problems in heat transfer, equipment deterioration, and premature failures. Metal ernbrittlement, pitting by vanadium compounds, or other costly corrosive effects often are attributable to trace nietals rarried by oils ( 7 ) . F'arious chemical reactions of petroleum ronipounds and their derivatives are believed to be accelerated h y the presence of trace metals. Lubricant breakdown and engine wear may be correlated with the identity and concentration of metals in crankcase oils. Interest of the petroleum industry in trace metals does not, of course, stem entirely from debits incurred by the presence of deleterious metals in oils. On the credit side, properties of certain petroleum products actually may be greatly enhanced hy the presence of added metal compounds. Thus, a number of finished products such as additive-containing heating oils, detergent lubricating oils, leaded gasolines, and inhibitor-containing blends owe much of their desirable characteristics to the presence
control of sampling and of variables in combustion and ignition, a reproducibility of about &lo70 in the ash range of 0.0001 to O.OlC& and &5qc in the range of 0.01 to 0.370 is obtained. Colorimetric and spectroscopic methods have proved adequate for determining individual metals in milligram quantities of ash substance, the relative precision of these methods being zk10 to 20%. Spectroscopicmethods providing an accuracy of zk2O7' also may be applied to the direct determination of trace metals in certain stocks to circumvent some of the problems associated with indirect chemical methods.
of metallic compounds Tyhich purposely are introduced into them. States of Metals in Oils. .4 complete identification of the chemical and physical states or forms of all naturally occurring trace metals in petroleum is difficult to ascertain. Such information would be specifically useful to the industry. In oil exploration, for example, further light would be thrown on the source and geological origin of crudes, TTere full information on the physical and chemical forms of trace metals existing in them available. As another example, a ready method whereby such forms could be easily defined or identified would provide potentially useful data in designing effective treating processes, should the removal of objectionable metal contaminants from certain stocks be deemed desirable. The physical forms in which metals may exist in oils include true solutions of organometallic substances, suspended solid particles in highly divided form, and colloidal or emulsified dispersions of minute bodies containing metal compounds or complexes. The coexistence in an oil of trace metals having different physical forms of dispersion or chemical combinationappear probable; this has been indicated by certain metal-decontamination studies made in oils. For example, in this laboratory the percolation of a petroleum residuum over adsorbent gels or activated carbon was found to be highly effective for the removal of compounds of iron, sodium, and calcium, ~1hereas compounde of magnesium, vanadium, and nickel were not removed to any appreciable extent by the treatment. This suggests that differences in selectivity of removal are attributable to differences in states of division or forms of combination of the respective metals in the residuum stock. Liquid extraction techniques such as water washing of gas oils 1%-erefound to be selective in the removal of trace metals; specifically they are most selective for sodium and other alkaline compounds. Even filtration technique* may be effective for some stocks in removal of a portion of the metal contaminants, presumably that portion consisting of metal-containing aggregates too large to pass the filter pores. Centrifugation also provides a possible means whereby metalcontaining bodies may be segregated from an oil. The form of chemical combination of a metal as it occurs in oils may be classified essentially as organic or inorganic. Inorganic forms include oxides, sulfate., carbonates, chlorides, and phosphates. Organic forms include metal soaps and chelatetype organometallic complexes. The separation and identification of organometallic forms are somewhat more involved and difficult than the separation of inorganic forms, but naturally occurring metal complexes have been separated from oils and their forms of chemical combination identified. A study on the identification of metal complexes in Santa Maria Valley, Cali-
1733
1734
fornia, crude has been reported (20) in which comparison of absorption spectra of extracts of the crude with literature spectra of pure compounds showed that vanadium is present in the crude as the porphyrin complex. Analytical Methods. Determination of the mineral content of the various petroleum stocks is necessary, so that some measure of control can be brought to bear to maintain product quality. Some of the known effects of various minerals on product quality have been discussed above and current research is seeking to establish other correlations. In many cases a total mineral content has been acceptable as a control, whereas in other cases it is desirable to pinpoint the correlations by identification and determination of a specific element. Several methods of determining the mineral content have been employed. Most common is the ashing procedure in which the oil is burned and the residue is heated to remove the carbonaceous material and then weighed. In this type of analysis it is assumed that all the mineral components are nonvolatile and are not lost in any way through the burning process. It has been recognized that not all mineral components are nonvolatile. For example, in the determination of lead in petroleum stocks the ashing procedure is modified by the addition of sulfuric acid during the ashing step, so that lead will be retained in the residue as the relatively nonvolatile lead sulfate. Another method of determining the total metallic constituents in petroleum is by wet oxidation of the oil in a Kjeldahl-type digestion with hot sulfuric and nitric acids. This method is usually long and tedious and amounts of mineral residue are usually small as a consequence of the small amount of sample that is necessarily used in this oxidation. Another method of determining the metallic content is byextraction of the oil with a strong acid. This type of technique has been used successfully in the determination of lead in gasoline, and has been investigated as a means of determining metals in other types of stocks (18). This method is not in so widespread use as is the ash procedure, presumably because of its lack of versatility and the incompleteness of the extraction when applied to certain stocks ( I d ) . The total mineral or metallic content can be obtained by a summation of individual components of an oil. The individual metallic components can be obtained either by analysis of the ash obtained on burning the oil or by direct examination of the oil. If the analysis of individual constituents is undertaken after the ash is obtained, the subsequent analytical problems are much like the determination of minerals and rocks, except for the very small amount of solid materials usually on the order of about 10 mg. Consequently, conventional gravimetric procedures cannot be employed. For many minerals colorimetric procedures have proved satisfactory and a number of these have been described ( I S , 19, If,22). The emission spectrograph offers a means of determining the mineral content directly without prior ashing of the sample (4). Because of the extreme sensitivity of this instrument to most metals, only very small amounts of oil are required to make the determination. I n one method described in the literature (IO) a small amount of oil is charged to a specially prepared electrode and the ashing is carried out inside the electrode. The electrode containing the ash is then placed in the emission spectrograph and the analysis is obtained in the usual manner. A method of determining minerals by the use of the spectrograph is discussed below, wherein it is not necessary to carry out this initial microashing step, The electrode is dipped into the oil to be analyzed and sufficient oil is retained on the electrode to carry out the subsequent determinations of a number of metals simultaneously. Although each of the procedures for determining the total ash content has some particular merit, each has certain inherent weaknesses, which cast suspicion upon the accuracy of the results. For example, in the ashing technique there is the possibility that some of the metals are lost by volatilization or by mechanical entrainment during the burning process; the extraction method has been shown to be time-consuming and inadequate in some
ANALYTICAL CHEMISTRY cases; and wet oxidation methods are not practical for routine determinations. Because emission spectrographic methods and the ashing procedure offered the least objections, it was decided to investigate these methods to determine if accurate and reliable determinations could be obtained. INVESTIGATIONAL WORK ON ASHING PROCEDURE
In order to determine the extent to which ashing can be relied upon to give accurate results for the total mineral determination and for use as a preliminary step in the determination of individual metals in an oil, it was necessary t o investigate the following: 1. Stratification of sample on standing. 2. Possible losses through volatilization of inorganic constituents. 3. Possible losses through mechanical entrainment during the burning of the oil. 4. Size of sample. Prom this investigational work, which is subsequently discussed in detail, a modified ashing procedure was developed; the procedural steps are described and the accuracy and reproducibility of the method are evaluated.
Table I.
Sample
a
b
Stratification of Samples i n Gallon Glass Bottles (Petrolatum samples stored in oven) Ash Content, Weight % Sampled Sampled from according to t o p after standing procedurea 8 weeksb
Deviation, %
Average of six determinations. Average of two determinations.
Stratification of Samples in Containers. The inorganic components of an oil may be present in solution or as solids in suspension. The particle size of the solids is not known. It was SUSpected that erratic answers could be obtained upon ashing the samples if care were not exercised in sampling the oil. In order to determine the effect of stratification of samples in containers upon the accuracy of the ash procedure, five plant samples were analyzed for ash contents immediately after receipt and after standing for 8 weeks without agitation. In Table I are shown some results obtained on samples as received and on portions withdrawn from the top of each sample (contained in a gallon bottle) after standing for 8 weeks. Negligible differences in ash contents were obtained on three of the samples, while two of the samples showed a substantial reduction in ash content on standing. Although these data show that not all samples are affected by settling, they demonstrate that stratification does affect the results obtained on certain samples. Consequently, it was believed advisable to incorporate in any revised procedure a provision that all of the sample in the container be removed for the analysis. This, together with the added stipulation that the containers are to be washed with xylene and the washings added to the original sample, would be likely to eliminate any errors due t o possible stratification. Ash Volatilization. POSSIBLELOSSES DURING IGNITION. To determine the importance of this factor upon the accuracy of the ash determination, a plant sample was first ignited in a muffle oven to a constant weight for 2 hours a t 1150' F., the temperature was then raised in two steps and the sample reweighed after it had attained a constant weight a t each temperature. Temperature, 1150 1500 1800
O
F.
Ash, Wt. % '
0.0086 0.0035 0.0006
Temperature had a very profound effect upon the measured
V O L U M E 24, NO. 11, N O V E M B E R 1 9 5 2
1735
--
Table 11. Volatilit? of Sodium Chloride at Various Temperatures
burned create a larger volume of gas than the volume of olygen consumed, This is shown in the following equation:
(25 ing of sample employed)
Temp
1'
' '
C,H,
Time of Heat, Hours lo
Weight c& Loss after Heating
.^^^
LVVU
1150
2,'O
2:s
2,'s
$.'O
4:0
1300 6.0 8.0 14.5 2 0 1500 31 0 64 0 8 4 . 0 88.0 994 . 0 1800 99.5 .. .. . .. a IO-my. sample instead of 2 3 mg.
8'0
10.0
44.0 ..
.. ..
..
2o
.. .. ,.
--
8.'"
::. . ..
ash content of this particular sample. Tests conducted upon a water extract, obt,ained from another portion of this sample sholyed that the soluble portion of the ash consisted principally of sodium chloride. Sodium chloride was therefore selected as a tl-pica1 inorganic material for study because it is perhaps more volatile than the majority of t,he other inorganic materials commonly occurring in oils. Although the vapor pressure of sodium chloride has been established, its rate of volatilization can be influenced by the shape of the dish, the volume of the muffle furnace, and the rat,e at which the air is displaced from the furnace. Consequently, a series of experiments was conducted to determine the effect of temperature upon volatilization of sodium chloride xhen using the same equipment that was ordinarily used for the ashing determination. This equipment is perhaps typical of that commonly found in manj- refinery laboratories. I n Table I1 are shown results obtained when 25-mg. (and 10mg,) samples of sodium chloride \yere heated from 0.5 t o 24 hours a t temperatures ranging from 1000" t o 1800" F. Essentially all of the sodium chloride had volatilized within 0.5 hour at 1800'F., and only about 4% of the sodium chloride \vas lost through volatilizaiion after -1 hours at 1150' F. Because periods as long as 7 hours may be required to remove all of the carbon in a sample when the temperature is as IOK as 1150' F., and the loss of sodium chloride through voiat'ilization may be appreciable over extended periods of time, even at 1150' F., it was deemed advisable t o conduct the ignition at a temperature of about 1000" F. A 10-mg. sample of sodium chloride lost only 8% through volatilization after 24 hours at 1000" F. From the volume of the muffle furnace, which may be computed from the dimensions of 5.25 inches wide by 3.75 inches high by 11 inches deep, and the vapor pressure of the sodium chloride a t a given temperature, the weight of sodium chloride present as a gas in the muffle oven can be coniputcd. assuming that the perfect gas l a w hold. This value can he compared to t'he actual losses of sodium chloride shoxvn in Table I1 for the respective temperatures. The actual loss divided hy the theoretical weight of sodium chloride required t o saturate the gaseous volume of the muffle a t a given temperature gives a rough indication of the number of times the air in the muffle was displaced. This value divided by the number of hours the sample was maintained at the temperature gives an average air displacement value per hour. In this manner the valurs shown in Table I11 vere obtained. The average displacement per hour increases x i t h increasing temperature, -4lthough the reason for this is not fully understood, this effect may be partially explained by the fact that as the temperature increases the density of the gas decreases, thus creating a greater pressure differential. These values offer means of roughly approximating how much of a given inorganic material will be lost through volatilization a t a given temperature, from a knowledge of its vapor pressure. (hlthough these values are characteristic of the particular furnace used in this study, the values for other furnaces of like size and design would probably be similar.) In actual ashing it should be anticipated that the volatilization loss per hour for the initial few hours ma)- be somewhat higher than indicated, owing to t'he presence of carhonaceous materials which when
+( + TL
3 -
=
0 2
nCO2
+
H20
The number of moles of gas on the left side of the equation is n x/4 while on the right side it is n x/2, which represents a slight increase in volume. Thus the amount'of gas in the muffle is greater than that which would be accounted for by leaks alone. INFLUESCE OF SULFATION.A series of experiments \vas conducted t o determine whether volatilization losses could be reduced by converting the sodium chloride in the original sample t o sodium sulfate, through treatment with sulfuric acid, before igniting the sample. The data in Table IV show that negligible volatilization of sodium sulfate occurred after 20 hours at temperatures as high as 1300"F., and that only 2.4 weight 7 0 is lost through volatilization a t 1800" F. Although it is obvious that errors caused by volatilization of sodium chloride could be essentially eliminated hy converting the chloride to sulfate, this technique has the disadvantages that (1) it is necessary to carry out the additional step of adding sulfuric acid and carefully fuming it off, and (2) erroneously high results will be obtained for the weight per cent ash if the chloride is converted t o the sulfate. Hon-ever, iron (and other related metals) compounds in the ash would not be affected by sulfating, since the subsequent ignition temperature of 1800' F would be sufficiently high t o convert the ferric sulfate back t o ferric ocide.
+
+
Table 111. Average Number of Air Displacements from Muffle Furnace per Hour Temp., 17. 1000
24
0 7
16
1300 I500
20 2
Table IV.
1 1
1.3 1.9
Volatility of Sodium Sulfate
0.5 T e m p . , ' F, 1800
Average Displacement per Hour
1150
(200-nig. soinple employed.
1150 1300
Period of Measurement, Hours
0 0 0 0 0 5
Samples previously dried 1 hour a t 212' F.) Time of H e a t , Hours 1.0 2 0 4 0 5.0 20.0 Weight % ' Loss after Heating 0 1 0 0 0 2 0 2 0 0 +01 +03 f O 2 0 0 0 2 0 8 1 0 1 1 1 2 2 4
The validity of the conclusions concerning the volatility of the sodium chloride and the ferric oxide was demonstrated using a series of synthetic samples which were prepared by adding t o an oil of an extremely IOIT ash content known amounts of sodium chloride and ferric oxide. The results, shown in Table V, indicate that all the sodium chloride was lost completely after 1 hour at 1800" F., while essentially all the ferric oxide was recovered. In those cases Tvhere the samples were sulfated, the calculated amounts of sodium sulfate plus ferric oxide were obtained. An accuracy of about 401, map. obtained. It mal- be possible t o utilize this technique of sulfating the ash for determining volatile alkaline salts. This could be accomplished by sulfating a portion of a sample and igniting both the sulfated and the nonsulfated samples at 1800" F. for about 1 hour. Because essentially all of the alkaline chlorides and none of the alkaline sulfates will volatilize under these conditions, the weight per cent of the alkaline chlorides in the sample could be determined from the difference in weight of the two samples after the ignition by applying the appropriate conversion factors. The results in Table V illustrate this technique. Losses by Mechanical Entrainment. Mechanical entrainment losses are those sustained when minute particles of the ash are carried upward in the flame by the rising heat current and out of
1736
ANALYTICAL CHEMISTRY
Table V. Results of Ashing Sulfated and Vnsulfated Oil of Know-n Ash Content at Final Ignition Temperature of 1800" F. for One Hour Deviation, 5 Weight Added, Weight of .4sh, Based upon Gram Gram Based upon S-aCl NaCl Fez02 Calcd.a Found total ash content Unsulfate 0.0179 0.1225 0.1225b 0.1230 +O. 4 +2.8 0.0944 0,1229 0.1229b 0.1276 +5.6 t3.6 0.1727 0.1025 0.1025'~ 0.1092 i6.1 +3.9 0.2704 0.1027 0.1027* 0.1115 +8.5 +3.5 0.2218 0.0783 0.0753b 0.0863 i-12.6 +5.0 0.0852 0.1062 0,10626 0.1122 +5.6 +7.0 0.0454 0.2070 0.2070; 0.2106 +1.7 +8.0 Sulfated 0.1103 0.1103 0.1124 0.0 . .. +1.9 0.0956 0.1028 O.218gc 0.2163 -2.2 -1.2 0.0850 0.1062 0.2094c 0.2086 -0.1 -0.4 0.1838 0.1040 0.291OC 0.2933 i1.2 +O. 8 0.1857 0.1004 0,3263C 0.3191 -2.2 -3.2 0.0108 0.1096 0.1227c 0.1231 +3.2 i3.1 a Ash content of oil (0.0010%) not included in calculation, since weight of ash obtained from 50-cc. portions used would be negligible. Complete volatilization of iYaC1 assumed. Calculated as iYa?SO4 plus FetOs.
-
*
the dish. This may be caused by too rapid a rate of combustion and/or by surface turbulence which may cause a spray of oil droplets to ignite above the surface of the oil. Traces of water are known to cause this turbulence, as do oils containing a mixture of hydrocarbons of mixed boiling ranges. By carefully controlled combustion these losses can be minimized, To determine the degree of mechanical entrainment of an ash in the smoke during the burning operation, a number of experiments have been performed with both synthetic mixtures and regular plant samples. I n general, the data obtained from these experiments, while not entirely conclusive, have indicated that under careful combustion conditions the systematic error introduced as a result of mechanical entrainment probably does not exceed about 5 to 8% of the ash from those samples. The results cannot be said t o be entirely conclusive because the synthetic samples used in these studies may or may not simulate the normally occurring metallic constituents in the petroleum oils. The evidence upon which these conclusions are based is discussed briefly below.
A plant sample was burned by the normal combustion procedure for a prolonged period and the smoke formed in the burning operation was collected on a cool surface. Analysis of this material showed that it contained an amount of ash equivalent to about 6% of the ash content of the original sample. I n another experiment knoan weights of sodium as sodium naphthenate were dissolved in highly refined white oil and burned. The coke portion of this sample was treated with concentrated sulfuric acid (to convert the sodium to sodium sulfate and thus minimize loss of ash due to volatilization) and ignited to a constant weight. Results obtained from the experiments are tabulated in Table \-I, Part B. The loss experienced during the combustion operation amounted to about SC7, for a sample containing 100 p.p.m. of sodium. A number of synthetic mixtures prepared either by adding solid sodium chloride and/or ferric oxide to oil, or by adding soluble metallo-organic compounds of iron, nickel, chromium, and vanadium to highly refined white oil, have been analyzed by the modified ashing procedure developed during this investigation, with results as shown in table VI. There is no evidence of a systematic error in these determinations. I n a series of experiments with synthetically prepared samples made by using radioactive tracers of compounds of sodium, calcium, and iron, Morgan and Turner (16) shoxed that if the type of additives used in this study adequately represented the additions of a normally occurring metallic contaminant, the losses attributable to entrainment and volatilization of ash components are not of great significance in a properly conducted analysis. Morgan and Turner defme carefully controlled conditions as being a slow rate of combustion and the final ignition temperature not to exceed 550" C. (1022" F.). I n another experiment designed to determine the extent of mechanical entrainment during burning, a series of blends of two oils of widely diverging ash content was repared and each of these was subsequently ashed. One oil h a f a n ash content of 1.6 p.p.m. and the other had an ash content of 74.5 p.p.m. The ash contents of the various blends were determined. It would be expected that if losses by mechanical entrainment were appreciable, the amount of the loss would increase as the ash content of the
blend was increased. However, the data as shown in Table VI1 indicate that the ash values of the blends calculated from the components were in good agreement with the experimentally determined values. Size of Sample. Many samples submitted for ash analyses have an ash content of 0.0017, or less, and for samples of so low an ash content it is especially important to consider the influence of the accuracy of the weight measurements upon the over-all accuracy of the method. Khen an ordinary macrobalance is used under routine conditions, an uncertainty of 0.2 mg. may be expected in each weighing. As two weighings are usually made in an ash determination, it is probable that a total of 0.3 mg. will be experienced in the over-all operation (the major portion of this error is due to the fact that the weight of a large dish can vary as much as 0.1 to 0.2 mg. because of changes in pressure, temperature, and humidity, 11). Assuming that an over-all accuracy of a t least 10% on the ash determination is desired, it follorvs that the ash obtained from the burning operation must weigh a t least as much as 3.0 mg. For instance, if the ash content of a sample is 0.001%, at least 300 grams of sample must be burned.
Table VI.
Accutacy of Alodified Ashing >lethod on Synthetic Samples
Results Obtained by Ashing Oil Containing Known Weights of Solid Feros and IVaCl Weight b d d e d , Gram Total Ash, Veight % NaCl Fez03 Calculated By analysis Deviation, % 0.0806 4-1.0 0.0 0.0399 0.0798 0.4754 -0.7 0.2394 0.4788 0.0 0.0806 t0.3 0.0402 0.0 0.0804 0.1566 -1.1 0.0792 0.0 0.1584 0.1722 +0.6 0.0470 0.0386 0.1712 0.1396 0,3044 -1.0 0.0142 0,3076 0.3408 iO.8 0,0127 0.1563 0.3380 Samples prepared by adding indicated quantities of NaCl and/or Fez08 to 50-gram samples of oil having a determined ash content of 0.0068 weight 70. All synthetic compositions corrected for ash content of base stock. B. Results Obtained Using Or anometallic Compounds Dissolved i n G h i t e Oil Total Ash, Average Weight % Found, B 3' By Weight synthesis analysis % % Error Na naphrhenate in white oil 0.0011 0 , 0 0 0 9 0.0009 -18.2 A.
0.0009
Xa naphthenate in white oil
0.0100
0,0091
0.0092
-8.0
0.0092
0.0093 Saphthenates and oleates of Fe, S i , V, and Cr in white oil
0.0534
0.0531 0.0547 0.0549
0.0542
4-1.4
Naphthenates and oleates of Fe, S i , V, and Cr in white oil
0 0017
0.0019 0 0020
0 00195
t14.5
The conclusions regarding size of sample necessary to obtain the desired degree of accuracy in the ashing tests are based upon the use of a conventional macrobalance having a normal uncertainty of about 0.2 mg. under routine operating conditions. For balances of greater precision, the above conclusions do not apply, as the size of the sample necessary t o obtain a suffrcient quantity of ash for accurate weighing will be less than indicated above. B limited amount of work has been performed to ascertain the feasibility of utilizing a microtechnique in the ash determination, so as to take advantage of the small sample requirements t o reduce the over-all time of analysis. I n Table VI11 are shown the results obtained by the modified ash technique (described below in detail) and by the microtechnique on portions of the same sample. Except for sizes of sample and equipment involved, the micromethod is essentially the same as the macromethod. The results obtained by the two methods on a sample of oil which contained approximately 20 p.p.m. of ash arecomparable, even though the size of the sample required by the micromethod is only about 10 grams as compared with about 1500 grams required by the macromethod. Blank runs on platinum dishes (about 7 grams in weight) have indicated that the average
1737
V O L U M E 24, N O . 11, N O V E M B E R 1 9 5 2
to place the dish in the oven to remove the accumulated coke beTable VII.
Effect of Ash Content on Losses by Mechanical Entrainment
Composition of Blend, wt. Heating oil
Petrolatum 0
100
Table VIII.
~~~~~t of Blend Burned, Grams 307
-ksh Formed, Mg.
Ash Content of B1end,P.P.l\l. Found Calcd.
.
0.65
~
Comparison of nlicro- and Macromethods for Determining Total Ash Macro RIicro -
Size of sample, grams Net weightof a s h , g r a m Ash, 70
1533 0,0299 0,00195
1646 0.0326 0.00198
9.49 0.000205 0.00216
9.79 0.000208 0.00212
deviation from the mean experienced in a series of weighings of these dishes amounts to about 0.026 mg. for a new dish and 0.030 mg. for a used dish. On a percentage basis, this error is of about the same order as that experienced in the macromethod for the sample sizes required. In all probability this error in the microprocedure could be reduced considerably by employing tared weights. As a microdetermination can be carried out with accuracy equivalent t o the macroprocedure, it is recommended that whenever limited amounts of sample are available, this procedure be employed. Modified Ashing Procedure. SAMPLING. The entire content of the bottle in which the sample is submitted should be used for the analysis, to minimize the effect of stratification. It is recommended that the quantity of sample submitted be governed by the approximate ash content according ta the following schedule: Approximate Ash Content, P.P.M.
Quantity t o Be Poured into Clear Sample Bottle, Grams6
2-10 10-100 100 or more
1500-300 300-50 50-20
a A clear bottle is preferred, SO that particles clinging to the walls of the container can be readily detected.
If a larger quantity of sample than specified is submitted, the analyst should obtain as representative a portion as possible by placing the container in a large oven t o liquefy the contents completely and to lower the viscosity, then thoroughly mixing and pouring an appropriate amount into a sample bottle.
PROCEDURE. Weigh the sample bottle and transfer the contents to a weighed platinum dish. (-4 porcelain dish may be sub-
stituted for a platinum dish in most cases where the sample upon ignition does not stain or attack the glazed surface of the dish. The ash can be weighed, after complete ignition, by transferring it quantitatively to a tared a a t c h glass.) The dish should never be filled to more than tao-thirds of its capacity; hence it may be necessary to conduct a burning operation in several steps. Rinse out the last traces of sample from the bottle with xylene, taking care to remove all particles clinging to the walls. A4ddthe xylene washings to the dish along with the sample. Reweigh the dried sample bottle; the difference between this weight and that of the full bottle represents the weight of the sample. Cautiously apply a flame of low intensity to the bottom of the dish until the contents start to burn slowly. (If the sample contains appreciable amounts of water, as indicated by sputtering when heating, place the dish overnight in an oven maintained a t 212" F.) When the sample begins to burn, remove the flame and allow the combustion to proceed. If combustion ceases, reignite with the flame and allow the burning to pfoceed until no liquid remains in the dish. Toward the end of the initial combustion place the dish on an electric hot plate so as to ensure complete burning of any tarry material that may be present. I n some cases it may be necessary to apply a flame again to remove the tarry matter completely. When the combustion is complete, place the dish in a'muffle oven maintained a t 1000° & 50" F. Continue ignition until successive weighings agree within 0.3 mg. Two to 7 hours may be necessary for this operation, according to the amount and type of carbonaceous material present. When a large amount of sample is used, it may be necessary after a portion of the sample is burned
fore burning additional portions. Subtract the weight of the dish from the weight of the dish plus contents after constant weights have been obtained. The difference represents the weight of ash in the sample. Accuracy and Reproducibility of Modified Ashing Procedure. Because of the lack of information concerning the composition of ashes in petroleum, it was not possible to prepare synthetic miutures that were known to simulate such samples exactly in composition. However, the method was evaluated for accuracy using tTTo types of synthetic samples: The first type was prepared by adding known amounts of ferric oxide and/or sodium chloride to an oil of low and measured ash content, and the second type was prepared b;- adding knoll-n amounts of metalloorganic compounds to a highly refined nhite oil. The results obtained from the analyses of synthetic mixtures prepared in this manner are shonn in Table VI. For the first type of synthetic mixture, an average error of about j=l%was obtained over the concentration range of about 0.08 to 0.34 Tyeight % ash. For samples of the second type, the percentage error ranged from about 18 t o 2% over the concentration range of 0.001 t o 0.05 weight % ash. I n addition t o the limited data on synthetic mixtures, the modified method has been further evaluated by a number of reproducibility studies conducted on typical plant samples. Results obtained from a number of multiple determinations on various samples are shown in Table IX. The relative per cent standard deviation of individual stocks varied from 1 to 10% over the concentration range of 0.02 to 0.25 Jveight %ash.
Table IX.
Reproducibility Studies Using RIodified Method for Determining Total Ash Ash Content,
Sample Gas oil
W t . 7c
Standard Deviation, s-
Arithmetical Mean, W t . 70
Wt. %
mean
".
ua nf /"
0.0197 0.0214 0.0174
0.0207 0.0213 0.0216
0.0203
0.0016
7.9
Still bottoms No. 3
0.092 0.093 0.096
0.105 0.098 0.097
0.097
0 0046
4 7
Still bottoms
0.042 0.041 0.041
0.053 0.045 0.044
0.044
0 0046
10 4
Still hottoms No. 10
0.062 0.061 0.061
0.066 0.061 0.066
0.063
0 00?3
4 0
Still bottoms
0.073 0.071 0.066
0.074 0.062 0.066
0.069
0 0047
6.8
Fuel oil No. 24 Fuel oil
0.124 0.121
0.123 0.124
0.123
0.0014
1.1
0.170 0.172 0.174 0.167
0.170 0.168 0.177 0.169
0.171
0.0035
2.0
s o . 33
Fuel oil No. 25
0.250 0.255 0.268
0,238 0.262 0.246
0.253
0.0109
4.3
Fuel oil No, 15
0.240 0.264 0.228
0.242 0.23: 0.232
0.239
0.0090
3.8
so. "0
h-0. 90
The accuracy of the modified ash determination was further demonstrated by an experiment in n-hich a portion of a sample of filtered petrolatum was ashed by the modified ashing procedure and this result compared n-ith the results obtained by a n e t oxidation of another portion of the same sample. The portion of the sample that was oxidized by concentrated sulfuric and nitric acids was analyzed for the individual metallic components, which summed u p to a total ash content of 60.6 p.p.m. This may be compared t o the ash content obtained by the modified ashing procedure of 58.7 p.p.m. which is an average obtained on six determinations. The data for this experiment are shown in Table X.
1738
ANALYTICAL CHEMISTRY Table X.
T o t a l Ash by W e t Oxidation
Elements Determined after Digestion in HzSO4 and HKOi Sodium Silicon Iron Sickel Vanadium Chromium
Calculated as
Total Total ash by modified ashing procedure a
P.P.M. in Oil (after Blank Subtraction)
__
60 6 58
i o
Average of 6 determinations
COLORIMETRIC METHODS FOR DETERMINING INDIVIDUAL COMPONENTS
After the sample has been put into solution using the mixed acids, a n aliquot may be withdrawn for the sodium and potassium determinations. The remaining portion is poured into a solution of sodium hydroxide containing sodium peroxide, which precipitates the iron, nickel, and titanium as the hydroxides, while the vanadium and the chromium are converted to water-soluble vanadates and chromates. It is necessary to redissolve the iron, nickel, and titanium hydrosides and then to reprecipitate them in order to remove possible occlusions of chromium and vanadium. The residue containing the hydroxides of nickel, titanium, and iron is put' into solution by treatment xith acid; the nickel is isolated by adding sodium citrate, ammonium hydroxide, and dimethylglyoxime to precipitate nickelous dimethylglyoxime, which is then extracted with chloroform. Iron and titanium are determined on separate aliquots after the nickel has been reinoved. Chromium and vanadium, in the alkaline portion, are separated by the reaction of vanadium with 8-quinolinol and extraction with chloroform a t a definite pH. An alternative procedure, which was used in t,his laboratory until it was noticed that most of the ash samples went into s o h tion readily with hydrochloric and sulfuric acids, consists of fusing the sample with eutectic mixture of sodium and potassium carbonates plus a small amount of sodium nitrate. This fusion mixture makes the vanadium and chromium wat'er-soluble, converting the iron, nickel, and titanium to insoluble oxides, which may be brought into solution by subsequent fusion with potassium pgrosulfate. The separation and determination of chroniium a.iid vanadium and of nickel, iron, and titanium are carried aut as described above, after the initial separation is made with tile sodium hydroxide-sodium peroxide solution.
The determination of individual components in the ash is different from the conventional analysis of minerals only in that the amount of sample available is usually euceedingly small and the usual gravimetric or volumetric procedures are not sufficiently sensitive. Hence, colorimetric methods which are sufficiently sensitive may be employed advantageously. Many colorimetric methods allow satisfactory determinations t o be made when only a few hundredths of a milligram of the component is present. Colorimetric techniques have been applied t o the determination ilfter the above steps, each element may be determined by of iron, nickel, vanadium, titanium, and chromium; sodium and colorimetric methods ( I S , 19, 21, :?2) using specific reagents-for potassium have been determined by flame photometry, which is example, iron with ammonium thiocyanate or with o-phenanthroperhaps equally eensitive. Calriuni, silicon, and magnesium line, nickel with dimethylglyoxime, chromium with diphenyl have been determined by usual gravimetric determinations; howcarbazide, titanium with hydrogen peroside, and vanadium with ever, colorimetric or other procedures suitable for the deterniinaeither hydrogen peroxide or phosphotungstic acid. The concention of micro amounts are available for these elements and could trations are determined by measuring the respective color intensiperhaps be readily adapted to this service. ties with a photoelectric colorimeter or a spectrophotometer and In the determination of the constituents of an ash it is necescomparing these readings n-ith previously prepared calibration sary that the entire sample be put into solution and that a semicurves. The optimum concentration range, the absorbance of micro chemical separation be made in those cases where the presthe color complexes, and a fern critical factors for several elements ence of one element interferes with the accurate determination are presented in Table S I . As these methods (13, 19, 21, 2 2 ) of the otheIs Sodium and potassium can be determined rapidly have been adequately reported in the literature, no further deand satisfactorily on an aliquot portion of the solution by using the scription of details are presented here. flame photometer, provided the analyst is cognizant of possihle mutual interferences of the elements listed ( 2 ) . The ash may be put into Table XI. S u m m a r y of Colorimetric Procedures solution by fusion n ith potassium pyrosulfate or sodium carbonate-sodium nitrate, or by treatment with hydrochloric and sulfuric acids. While theie Iron Thiocyanate (19,p. 363) 0.2 to 5 0 540 Time of standing kind of (21, p. 307) acid, concentration of SCK, has been no attempt t o stand( f 3 ,p. 409) light, presence of other ardize upon either method of ( 2 2 . pp 218, 237) anions putting the sample into soluo-Phenanthroline 119, p. 375) 0 4 to 8 0 500 Acid range, p H 2-9, color very stable, interference of (81, p. 314) tion, the acid treatment is pre(IS, p. 318) other ions can be corrected ferred when it can be used a5 Nickel Dimethylglyoxime (19,p. 470) 0.1 t o l . 5 325 Color stable 1 5 minutes; hue of color depends on acidity; (21, p. 345) it does not introduce into the 0.25-0.50 N HC1 produces ( I S , p. 502) sample a large amount of alkano change line salts which prevent alkaYanrtdium Hydrogen peroxide (19, p. 609) 3 to 60 470 Acid not critical b u t peroxide concentrations should be in( S f ,p. 453) line metal analyses from being creased as acid increases. Stable 24 hours, fading due carried out. t o loss of peroxide In most of the cases of ashes analyzed in this laboratory, solution was readily effected by use of hydrochloric and sulfuric acids. Any silica in the sample (appearing as an insoluble residue after the acid treatment) is separated and decomposed with hydrofluoric acid. Following the treatment with sulfuric acid to remove fluorides, all soluble portions are combined.
Phosphotungstate
( 1 9 , p. 607) (21, p. 45.5)
0.5 t o l l
400
Titanium
Hydrogen peroxide
(19, p. 572) ( 2 1 , p. 431) ( 2 2 , p. 384)
3
420
Chromium
Diphenylcarbazide
(19, p. 260) (81, p. 274) (IS, p. 316)
0 02 t o 0 8
(22,p. 167)
TO
30
540
S o t applicable in presence of large amounts of K. Mole ratio of phosphoric acid t o sodium tungstate lies in rangeof 3:l t o 2 0 : l Alkaline sulfate causes bleaching which can be compensated by increased acidity. Peroxide should be increased as acidity increased Although acidity is not critical 0.2 ,\' is optimum; Fe and large amounts of V interfere
V O L U M E 2 4 , N O . 11, N O V E M B E R 1 9 5 2 Table XII.
Identification of Geological Source of Crude Oils"
Chemical, P.P.1zI. _ __~ __ Geological Spectroscopich, P.P.11. Ash l'orniation Sa Fe A1 V Xi residue Sa Fe Devonian 1.1 0.4 0 7 3.1 3.6 26 3.0 0.3 Silurian 1 4 0.4 < 0 . 2 41 1.1 3.1 0 6 0.9 Ellenburger 0 . 7 0.3 < O 2
